Porous Materials || Making Porous Metals

CHAPTER TWO

Making Porous MetalsThe making of porous metals has a long history. The first preparation of

porous metals by the powder metallurgy process was reported at the begin-

ning of the twentieth century. With the progress of technology and the

emergence of new methods and processes, metals with porosity of 98%

or even more can be obtained today. However, metals prepared at the

beginning of the twentieth century only had porosity as low as about

30%. Currently, a number of other porous metal preparation methods are

available [1–5], such as sintering metal powders for the filter and melt

foaming for the light porous aluminum. In practice, porous metals can be

prepared by different processes, including powder metallurgy, melt foaming,

electrical deposition, and infiltration. All these methods will be described in

detail throughout this chapter.

2.1 POWDER METALLURGY

Porous metals were first prepared in the form of powder by sintering

or other similar processes, and these metal powders maintain their solid state

during the process. The sintered porous metals have either an isolated closed

structure with low porosity or a connected open structure with high poros-

ity. The framework is constructed by more or less individual spherical par-

ticles through connection of the sintered necks of particles. Sintering metal

powders is the earliest approach tomaking porous metals, and it also has been

the general production method used in the powder metallurgy industry.

Powder metallurgy is a process through which porous metals, compos-

ites, and other materials can be prepared by mixing powders, molding, and

sintering [6,7]. Porous products created by powder metallurgy were first

mentioned in a patent in 1909, and similar patents concerning the prepara-

tion of porous filters by powder metallurgy were released in the late 1920s

and early 1930s. The pore ratio, radius, and distribution of the porous mate-

rials prepared by powder metallurgy can be controlled effectively. For

instance, there are near-dense materials, with porosity of less than 1–2%;

semi-dense materials, with porosity of around 10%; porous materials, with

porosity of >15%; and more porous materials with porosity as high as 98%.

Spherical powders are widely used to make porous materials through the

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21

22 Porous Materials

typical powder metallurgy process, which has the advantages of easy control

of the pore radius and good permeation. Accordingly, for the preparation of

porous materials with high pore radius and permeation requirements, irreg-

ular shaped powders other than spheric powders shall be removed. How-

ever, for the preparation of porous materials with powders of a

nonspheric shape, pore-forming agents like ammonium acid carbonate,

urea, and methyl cellulose shall be used in order to increase porosity and

permeation.

2.1.1 Preparation of Metal PowdersIn general, preparing metal powders means to transform metals, alloys, or

compounds that are in a solid, liquid, or gaseous state into powder. Metals

and alloys in the solid state can be made into powders by mechanical

crushing, electrochemical corrosion, and reduction of metal oxides or chlo-

ride. For metals and alloys in the liquid state, atomization, permutation

reduction, and electrolytic methods can be applied. The condensation of

gaseous metals, thermal dissociation of gaseous metal carbonyl compounds,

and gas phase reduction of halide can be used to change gaseous metals to

powder. The general methods are summarized in Table 2.1, the most widely

used of which are atomization, reduction, mechanical pulverization, and

vapor phase.

The general methods for the preparation of spheric powders are atom-

ization, the carbonyl method, and gas deposition. For nonspheric powder

preparation, in addition to the alloy ingot crushing and ball milling pro-

cesses, nonspheric metal powder mixing followed by alloying and crushing

processes can be used. The refractory metals and alloys are not easy to make

into spheric powders, and the spheroidizing treatment can be applied if

necessary.

The following are brief discussions of atomization, mechanical crushing,

reduction, vapor phase, and liquid phase methods [6,7].

AtomizationAtomization, also called the spraying method, is a process in which molten

metals are broken into small drops of liquid by high-speed fluids (gas as

air or inert gas; liquid as water) or fluids with centrifugal force, and then

solidified into powder. The schematic diagram for the spraying process is

shown in Figure 2.1 [7]. Pb, Sn, Al, Zn, Cu, Ni, Fe metal powders, Cu-Zn,

Cu-Sn, alloyed steels, and stainless steels (Figure 2.2 [6]), and bronze and Ni

spherical powders can be made by the spraying process.

Table 2.1 Preparation Methods for Metal Powders [6,7]

Methods

Original State ofthe RawMaterialsin Preparation

Example of MetalPowders

Atomization Gas atomization Liquid metals

and alloys

Sn, Pb, Al, Cu, Fe, Ni,

brass, bronze, alloyed

steels, stainless steels

Liquid atomization

(main water

atomization)

Liquid metals

and alloys

Cu, Fe, Ag, Au, Ni,

brass, bronze, alloyed

steels, stainless steels

Rotational disk

atomization

Liquid metals

and alloys

Cu, Fe, brass, bronze,

alloyed steels

Rotational

electrode

atomization

Liquid metals

and alloys

Fe, Ni, Co, Ti,

refractory metals,

oxygen-free Cu, Al

alloys, Ti alloys,

stainless steels,

superalloys

Rotational crucible

atomization

Liquid metals

and alloys

Fe, Ni, Co, Ti, Al

Arc spraying

atomization

Liquid metals

and alloys

Ni, Fe, Monel metal,

stainless steel

Mechanical

crushing

Rotational grinding Solid metals and

alloys

Fe, Al, Ni, Cu, Pb,

Fe-Ni alloy, alloyed

steels

Smashing by cold

airflow spraying

Solid metals and

alloys

Fe, stainless steels,

superalloys

Mechanical

grinding and

crushing

Brittle metals and

alloys, artificial

brittle metals,

and alloys

Sb, Cr, Mn, high-C

iron, Fe-Al alloys, Fe-Si

alloys, Fe-Cr alloys

Reduction

method

Carbon reduction Solid metal

oxides

Fe, W

Gas reduction Solid metal

oxides and salts

W, Mo, Fe, Co, Ni,

Cu, Fe-Mo alloys,

W-Re alloys

Gaseous hydrogen

reduction

Gaseous metal

halides

W, Mo, Co-W alloys,

W-Mo or Co-W

coating graphite

Thermal reduction

of gaseous metals

Gaseous metal

halides

Ta, Nb, Ti, Zr

Thermal reduction

of metals

Solid metal

oxides

Ta, Nb, Ti, Zr, Th, Hf,

U, Cr-Ni alloys

Continued

23Making Porous Metals

Table 2.1 Preparation Methods for Metal Powders [6,7]—cont'd

Methods

Original State ofthe RawMaterialsin Preparation

Example of MetalPowders

Vapor method Metal vapor

condensation

Gaseous metals Zn, Cd, Mg, Pb, Sb

Caroxide thermal

dissociation

Gaseous metal

carboxides

Fe, Co, Ni, Fe-Ni

alloys

Arc evaporation Gaseous metals Zn, Pb, Sb

Liquid phase

method

Metal replacement Solution of metal

salts

Cu, Sn, Ag

Hydrogen

reduction in

solution

Solution of metal

salts

Cu, Ni, Co

Precipitation from

molten salts

Molten metal

salts

Zr, Be

Electrolysis Aqueous solution

electrolysis

Solution of metal

salts

Fe, Cu, Ni, Ag, Fe-Ni

alloys

Molten salt

electrolysis

Molten metal

salts

Ta, Nb, Ti, Zr, Th, Be,

Ta-Nb alloys

Electrochemical

corrosion

Intergranular

corrosion

Stainless steels stainless steels

Electrocorrosion Metals and alloys Metals and alloys

Spherization Melting in an inert

filler

Irregular metal

and alloy

particles

Fe, Co, Ni, bronze

spheres

Plasma spherization Irregular metal

and alloy

particles

Ta, W, Mo spheres

Vertical furnace

spherization

Irregular metal

and alloy

particles

Ti, Mo, WC, stainless

steel spheres

24 Porous Materials

Mechanical CrushingMechanical crushing is not just an independent powder preparation process;

it also is a supplementary procedure in some other powder preparation pro-

cesses. It uses mechanical forces like crushing (pulverizing, rolling, and

jawing), striking (with hammer or similar tools), grinding (with ball and

rod), and then breaking the large blocks and particles into powder. The pul-

verizer, double-roller, and jaw crusher can make large particles, and then a

further fine-down process is required to make the powders into porous metal.

Much finer powders can be produced by hammer mills, rod mills, normal

ball mills (Figure 2.3), vibration ball mills, or stirring ball mills [7]. In the ball

Powder

Gas GasMolten metal

Figure 2.1 Schematic diagram of the molten metal atomization process.

Figure 2.2 Stainless steel spheric powder created by gas atomization (� 300).

Figure 2.3 Materials in a ball miller at different rotation speeds: (a) low speed; (b) appro-priate speed; (c) critical speed.


26 Porous Materials

milling process, the balls are generally made of corundum, with great hard-

ness and strength, and it takes place in air or in water, alcohol, gasoline, or

acetone liquid.

ReductionReduction is a widely used method to generate powder by reducing metal

oxides or chlorides. As the reducing agent, solid carbon can be used to pre-

pare Fe and W powders. H, H2+N2, or both are used to produce W, Mo,

Fe, Cu, Co, and Ni powders. Transformed natural gas (H2 or CO) can be

used for the preparation of Fe powders. And Na, Ca, and Mg

metals are used for the preparation of rare metal powders like Ta, Nb, Ti,

Zr, Th, and U.

Vapor Phase DepositionThe following methods can be used to prepare the metal powders:

1. Metal vapor condensation: This method is used with alloys with low melt-

ing points and high vapor pressures to produce Zn and Cd powder.

2. Thermal decomposition of carbonyl: In this process, metal powders can be

created by decomposing a metal’s carbonyl compounds.

3. Gas reduction: This method includes the gaseous H reduction and the

gaseous metal thermal reduction. In fact, it also can be part of the

second method, because thermal decomposition of carbonyl is one

important way of obtaining the raw powders (like Ni, Fe, and Co)

to prepare porous metals, particularly for microporous filter/separation

products. These transition metals can react with CO to form metal car-

bonyl compounds [like Me(CO)n] that are either in the liquid state

(which tend to evaporate), or in the solid state (which are easy to sub-

limate). For instance, Ni(CO)4 is a colorless liquid with melting point

of 43 �C, Fe(CO)5 is an amber liquid with melting point of 103 �C, andCo2(CO)8, Cr(CO)6, W(CO)6, and Mo(CO)6 are all crystals of easy

sublimation. Also, these carbonyl compounds have the tendency to

decompose into metal powders and CO. The reaction of carbonyl

compounds is

Me+ nCO!Me COð Þn (2-1)

For instance, nickel carbonyl can be formed by

Ni+ 4CO!Ni COð Þ4 (2-2)


The decomposition of carbonyl compound is

Me COð Þn!Me+ nCO (2-3)

and nickel carbonyl can decompose into

Ni COð Þ4!Ni+ 4CO (2-4)

This decomposition is an endothermic reaction. In the decomposition

temperature range, the higher the temperature is and the higher the decom-

position rates are, the more crystal nuclei form and the finer the particles

will be. The gas released from the thermal decomposition is toxic, in that

CO can be absorbed by the cuprammonium solutions and then purified

for recycling.

Liquid Phase DepositionLiquid phase deposition, like metal replacement, gas reduction in solution,

and thermal reduction in molten salts, can be performed in different ways.

Metal replacement is a process in which one metal takes the place of another

in a water solution. And thermodynamically, only metals with higher neg-

ative potentials can replace metals with higher positive potentials, and the

reaction is

Me2+� �

1+Me2!Me1 + Me2+

� �2:

(2-5)

For instance,

Cu2+ +Zn!Cu+Zn2+ : (2-6)

In this way, Cu, Pb, Sn, Ag, and Au powders can be prepared.

CO, SO2, H2S, and H2 can be used as the reductant in solution in the gas

reduction method, in which H2 is more popularly used. The reaction is

Men + + 1=2ð ÞnH2!Me+ nH+ : (2-7)

For example,

Ni NH3ð ÞnSO4 +H2!Ni+ NH4ð Þ2SO4 + n�2ð ÞNH3: (2-8)

In this way, Cu, Ni, Co, and Ni-Co powders can be prepared.

Sedimentation in molten salts achieves a thermal reduction of the metals.

For example, Zr powders can be reduced and broken down after cooling

through mixing ZrCl4 and KCl and adding Mg and increasing the temper-

ature to 750 �C, and then they are treated with water and HCl.

28 Porous Materials

Spherization of PowderAt times, the further spherization of nonspheric powders is necessary to

make porous materials. Spherization processes include vertical spherization,

plasma spherization, and inert liner spherization. In the vertical spherization

process, metal particles are heated to temperatures higher than their melting

points, melted in a furnace, and then allowed to fall freely due to surface ten-

sion. The molten drops are spherized and form spheric powders after

cooling.

In plasma spherization, nonspheric powders are melted in the plasma

beam and then sprayed into the water trough to form spheric particles. Gen-

erally, N2 was used to transport the powder. Due to the much higher plasma

arc temperature, it is more practical to use this process to produce metal

powders with higher melting points.

For the inert liner spherization process, metal or alloy powders are mixed

with an inert filler (like Al2O3 powder) and then heated in a nonoxidative

temperature until they melt. The spheric particles form due to surface ten-

sion and then are separated from the inert filler after cooling. Various fillers

shall be applied to the different metal powders. For instance, Cu uses carbon

black as the filler, whereas Fr and Ni will take MgO and Al2O3 as fillers.

The properties of porous metals produced by powder metallurgy are

related to the size and shape of the particles to a high degree. The grading

of particle sizes can be realized by vibrating screens, compressed gas flows,

powder sedimentation rate interaction, and gas discharge enrichment sepa-

ration due to specific surface charges. The spheric and nonspheric powders

can be separated by the discharge on the different specific surface areas

(higher specific surface areas with the nonspheric particles), as well as the

centrifugal separation process due to the different friction forces (smaller

friction forces for the spheric particles). More details will not be given here;

sources with further information are given in the “References” section at the

end of this chapter.

2.1.2 Molding of the Porous BodyThere are three types of molding of the porous body [6]:

1. Pressure molding: The powders can be molded under pressure, and they

are deformed into green bodies under high pressure by pressing, extrud-

ing, and rolling.

2. Non-pressure molding: The powders can be molded without pressure via

various methods, like powder slurry pouring and loose sintering.


3. Other molding: This category includes methods like spraying, vacuum

deposition, and other forming processes.

The selection of which molding method to employ depends on the shape

and size of the final product and the property of raw materials. Mold pressing

can be applied to small parts with simple shapes to be produced in large num-

bers. Extrusion molding is more suitable for the continuous production of

tubes, bars, and rods with uniform pore ratios. Isostatic pressing can be used

to obtain the green body of a uniform structure with binders as additives, and

then the green body can be subjected to machining and finally made into

complex-shaped and large products. The rolling of powders can make porous

plates and belts continuously, and the various products can be formed with

further rolling, welding, and clipping. Slurry pouring is used for molding

when the rawmaterials are metal fiber, finer spheric, and nonspheric powders.

Complex-shaped, large products with uniform pore ratios can be produced by

slurry pouring, and loose sintering is used to mold spheric powders.

The sintering process is the key procedure of making porous products,

and it should be strictly controlled. The green body with a pore-forming

agent shall be heated slowly during the sintering process to avoid cracking

from the volatilization of the agent. If the sintered porous parts need further

machining, metals, alloys, plastics, or resins are immersed into the porous

body for ease of processing and achieving precise control of the size. The

metals and alloys shall have low melting points and are insoluble and not

chemically reactive with the porous body. The immersed metals, alloys,

plastics, or resins shall be removed during heating to avoid blocking the

pores. For the economical preparation of porous parts with corrosion resis-

tance, porous irons or low-carbon steel is prepared and then subjected to Sn,

Cu, Ni, and Cr plating or the vapor-phase Cr plating process. Alternatively,

they can be made using the obtained plated powders.

Press MoldingPorous tubes and sheets can be made by press molding, and the main equip-

ment required for this process is the pressing machine. During molding, the

additives have the following features: (1) the proper viscosity to achieve the

required strength; (2) lubrication for the demolding process; (3) sufficient

pore-forming ability to meet the requirements of the pore ratio; (4) no

harmful residues are left after sintering; (5) being in liquid or solid state with

a lowmelting point, or with the ability for solvents to mix with powders; (6)

lack of reaction with the powders and lack of damage to the facility’s com-

ponents in the heated atmosphere. Depending on their roles in the molding,

30 Porous Materials

the additives can be lubricants, binders, pore-forming agents, or plasticizers.

The lubricants in general use are oil, glycerin, petroleum jelly, stearate, sul-

fate, oxide, and graphite. The binders are resin, amylum, and polythene

alcohol. The pore-forming agents include ammonium acid carbonate,

ammonium carbonate, sodium carbonate, organic fibers, granule of carbon,

naphthaline, urea, fiber, plastic, and sawdust (like TiH2, ZrH2, CaCO3,

Na2CO3, and K2CO3). The plasticizers are olefin and beeswax. The addi-

tives are selected based on the characteristics of the powders and the pressing

requirements, and they normally are dissolved in an organic solution (such as

gasoline, benzene, acetone, alcohol, or carbon tetrachloride) and thenmixed

with the powders.

Isostatic PressingThere can be both cold and hot isostatic pressings. For cold pressing, water or

oil is used as the pressure medium, while Ar gas is used as the pressure medium

for hot pressing. Usually, the preparation of porous materials is conducted by

cold isostatic pressing. The fluidmedium is pressed into a sealed steel container

with high-pressure resistance by using a high-pressure pump. The resulting

high pressure will be applied to the powders in the elastic mold isotropically

at the same time. The friction from powder/powder, powders/mold wall is

small enough and then the green bodywith uniform density will be produced.

Molds used in isostatic pressing shall meet the following requirements: (1) the

original geometrical shape must be maintained in loading powders, with high

strength and certain elasticity; (2) there must be high abrasion resistance and

easy machinability; and (3) there must be no reaction with the powders phys-

ically and chemically. Natural and synthetic rubbers are generally used as mold

material, and they are now gradually substituted by the plastic due to the prob-

lem of deformation and wrinkling after contacting with the mineral oil for the

rubber. Thermal plastic soft resin is one of the important materials for mold

application; its softness and hardness can be adjusted by the composition and

content of the plasticizer. A typical recipe used to make molds in China is as

follows: 100 portions of PVC resin (in weight), 100 di-octylphthalate (or

dibutyl phthalate), 3–5 tribasic lead sulfate, and 0.3 stearic acid [8]. The loaded

sealed mold is sheathed by the porous metallic tube and then put in the high-

pressure container. Next, the pressure is increased slowly to avoid creating

internal soft parts in the green body. The applied pressure cannot be reduced

quickly, or else the green body will crack due to the expansion of compressed

gas in the green product.


Rolling of PowdersGreen bodies can be prepared when the metal powder continuously passes

between a pair of rollers in contrarotation and undergo pressure from them.

The final porous sheet products will be obtained after the rolled green body

completes the pre-sintering and sintering, rolling, and heat-treatment pro-

cesses. Essentially, the metal powders with rolling capability are loaded in a

specially designed funnel to the required height and then are fed into the

rollers continuously due to the action of external and internal friction

between powder and roller and powder and powder. Three zones can be

seen during the movement of the powders (Figure 2.4) [7]:

• Zone I: A free zone from the gravity effect on the powders

• Zone II: A feeding zone from the interaction of powder and rollers

• Zone III: A rolling zone at which a green belt with a certain density and

strength is produced from the loose individual powders

Zone III covers the feeding of powder into the roller and the production of

the green belt from the rollers. Usually, the porous belt is produced by cold

rolling, as demonstrated in Figure 2.5 [6]. During rolling, the rolling speed and

feeding speed must be compatible to prevent damage to the final product.

Figure 2.4 Schematic diagram of the powder-rolling process: I—Free zone; II—feedingzone; III—rolling zone.

Sieving

Metal powder Liquid agent

Porous metal belt

Mixing Rolling Sintering Flatting Porous metal sheet

Figure 2.5 Porous belt-rolling process.

32 Porous Materials

The rolling properties for these powders include the plasticity, mol-

dability, and fluidity, which have great effect on the rolling process. The

density and thickness of the final green belt product decrease with the

low fluidity of the powders, increase with the high apparent density of

the powders, and significantly increase with the height and the applied force

on the powders. However, as the apparent density of powders increases, the

bending property of the final green belt product will be reduced. This is

because the mechanical engagement strength between the powders will

decrease if the size of the powders decrease and the roundness of the powders

increase for the high apparent density of the powders.With the fixed feeding

speed and roller gap, an increased rolling speed will reduce the green body’s

density and thickness.Moreover, if a low-viscosity gas like H2 is applied dur-

ing rolling, the density and thickness of the green body increase. Sintering of

the green body can be performed in a protective atmosphere like H2, in an

inert atmosphere like Ar or He, or in a vacuum.

Plastification ExtrudingExtrusion molding (also known as plastification extruding) is a process that the

stack of powders or the green body in die is pushed out to assume another

form of green body or other final product under pressure. The cold extrud-

ing process is applied to mixtures of metal powders and organic binders, and

extruding is performed at low temperatures (40–200 �C) to form the green

body. The processes include material preparation, preprocessing, extruding,

cutting, and reforming. The porous products can be obtained after drying,

pre-sintering, and sintering of the extruded green body. It is an effective way

to produce a long porous tube with a small diameter.

The pretreatment of a mixture under pressure involves making full contact

between the plasticizer and particle surfaces, to remove the gas inclusion and

finally to ensure uniform density. Plasticizers have a large effect on a material’s

properties. Therefore, certain requirements are needed for them, including

that there should not be any reaction with porous materials during sintering,

and that they should be removable, sticky, and have great pore-forming abil-

ity. The plasticizers in common use are olefin, amylum, and polythene alco-

hol. Powders will be subjected to pressure from the side wall, friction from

either the powder and the wall or the extrusion shaft and the wall, in addition

to the normal compression from the extrusion shaft. The key factors affecting

the properties of green body extrusion are the types of powders, the particle

shape and size, the plasticizer type and content, the precision of the mold, the

pressure from extrusion, the extrusion speed, and the preheating temperatures.


The selection of the preheating temperature depends on the optimal plastic

used for the plasticizer at the selected temperature. The extrusion speed

can be determined experimentally, and it is closely related to the particle size,

shape, extrusion ratio, fluidity, extrusion force, and plasticizer. Higher extru-

sion speeds may cause the green body to crack.

Slurry PouringThe slurry pouring process were better suited to porous products with com-

plex shapes. It requires simple facilities with low cost, needs a long produc-

tion cycle, and has low productivity. The powders or fibers are first prepared

as slurry (suspension), poured into a plaster mold, dehydrated for some time,

and finally dried and sintered to obtain the porous product [7]. The slurry-

pouring process is illustrated in Figure 2.6.

The slurry is composed of the metal powders and a liquid solution of

water mixed with additives. These additives are binder, dispersant (stabi-

lizer), degasifier, and titrant. The presence of a binder contributes to the vis-

cosity of slurry as well as the binding of the powders after drying. The binder

shall not react to the powders and plaster, and less residue shall be left after

sintering. The binders commonly used in this process are alginic acid sodium

and poluthene alcohol. Dispersants can prevent the agglomeration of the

particles and help form the stable suspension to improve the wetting of pow-

ders with liquid and to control the sedimentation speed. A certain amount of

ammonium hydroxide, hydrochloric acid, ferric chloride, and sodium sili-

cate mixed in water can be a perfect dispersant. Alginic acid sodium is

another common dispersant. The titrant is used to control the pH value

and the viscosity of slurry. Caustic soda, ammonia water, hydrochloric acid,

and ferric chloride can be used as titrants. Degasifiers can remove the

absorbed gas on the powder surface, and n-caprylic alcohol is widely used

for that purpose. In addition, the gas can be removed in a time-controlled

Figure 2.6 Diagram of slurry pouring: (a) Plaster mold; (b) pouring of slurry;(c) dehydrating of slurry; (d) molding.

34 Porous Materials

method, in which the stirred slurry is put aside for some time and the gas

escapes due to the air density difference. The vacuum treatment is also a

good way to remove the gas in slurry.

The sedimentation speed of powders in the slurry, its liquid/solid ratio,

the dehydrating rate of plaster, and the viscosity, pH value, and stability of

slurry are all factors that influence the quality of the poured product. The

liquid/solid ratio of slurry is the weight ratio for the water versus metal pow-

der, and it determines the viscosity of slurry and the sedimentation speed.

The smaller the liquid/solid ratio is, the higher the viscosity is and the slower

the sedimentation speed is. However, if the liquid/solid ratio is too small, the

slurry is more difficult to pour. If the pH value of the slurry falls below a

certain value, good fluidity will be obtained and the agglomeration of par-

ticles is prevented. The lower sedimentation speed of slurry is well suited to

the preparation of parts with complex shapes and small cross-section areas.

The metal powders and foaming agents can be mixed to form slurry and

then poured into the mold [8]. The metal powders can be Ni, Fe, Al, Cu,

brass, and stainless steel; and the foaming agents can be hydrochloric acid,

hydrafil, and orthophosphate.

2.1.3 Sintering of the Porous BodyThe purpose of sintering is to control the microstructure and property of a

product. Technically, it can be regarded as a heat treatment—that is, the

semifinished product is heated to the temperature below the melting point

of its main materials for a period of time and then cooled down to room

temperature. After sintering, the agglomeration of particles will change to

the agglomeration of crystals. At last, materials or products with the desired

physical and mechanical properties are obtained. Sintering is different from

the solid reaction since sintering may have some chemical reaction or even

have no chemical reaction at all. Many types of migration processes exist.

The following migration phases during sintering of a pressed green body

can be described in terms of the pore change in the porous materials: the

initial combination between particles (adhesion and linkage of unsaturated

bonds on the particle surface), the growth of a sintered neck, and the shrink-

ing and coarsing of pores. Based on the appearance of the liquid- and

sintered-phase compositions, the process can be divided into single-phase

sintering, multiphase solid sintering, and multiphase liquid sintering

(sintering at temperatures above the low melting point of the elements).

Single-phase sintering can be regarded as a solid-state reaction that is

determined by the change in the system energy state. Multiphase sintering


is influenced by the alloy’s thermodynamics. Both sintering processes exhibit

the free energy reduction of the system as the driving force, including (1) the

reduction of the total surface area and the total free energy of the surface led

by the increased powder reaction area (sintering neck) and the flat powder

surface; (2) the reduction of the total volume and total surface area of the

pores in the sintered body; and (3) the elimination of lattice distortion in

the powder. The grain boundaries might move through recrystallization

or polycrystallization, and the number of grain boundaries will decrease.

The total surface area of the pores tends to decrease due to the cylindrication

of the metal frame or spheroidization of the pores, regardless of the change of

the total pores. The closed pores stop shrinking when the inner pressure

exceeds the surface tension.

At the early stage of sintering, the required activation energy is low

because van derWaals forces exist among the powders and no obvious atom

displacements are required. Other migration processes such as diffusion,

evaporization and agglomeration, and flowing can occur only at high

enough temperatures or under high enough external forces because the

required activation energy is high.

In general, the sintering temperature is the highest one that will be

maintained during the sintering process.

Migration Mechanism During SinteringThe migration mechanism of sintering involves several elements, as follows:

1. Viscous flow: According to this model, the sintering process includes two

stages: increase of the contact surface area between the adjacent particles,

and gradual reduction of the size of the closed pores formed. Atoms and

interstices in the crystal preferably will move along the direction of the

surface tension, and the migration volume is in proportion to the surface

tension.

2. Evaporation and agglomeration: Inside the powder, the vapor pressure at

the convex area is large, while it is small in the concave area. The atoms

evaporate from the convex surface and agglomerate again at the con-

cave surface, such as the sintering neck, due to the pressure difference.

The vapor pressure at the convex and concave surface has the following

relationship with the radius of curvature (Kelvin equation):

lnp

p0¼ MgrRT

1

r1+

1

r2

� �(2-9)

36 Porous Materials

where p is the vapor pressure at the convex and concave surface; p0 is vapor

pressure at the flat surface; g is the surface tension; r1 and r2 are the two prin-cipal radii of curvature surfaces (positive at convex, negative at concave,

infinite at flat); r is the solid density; M is the molecular weight; and R is

the Mol gas constant.

3. Volume diffusion: With high density of the interstices at the contact sur-

face of the particles, the atoms migrate toward the contact surface by

exchanging positions with the interstices to make the sintering neck

grows. At the specified temperature, the interstices density is in propor-

tion to the surface tension.

4. Surface diffusion: The migration of the atoms on the particle surface will

expand the contact surface, and the concave surface will be flattened.

Essentially, the sintering of powders is a thermodynamic phenomenon

due to the extremely high surface area and high surface energy. At

low or mid-level sintering temperatures, the surface diffusion dominates,

while at higher temperatures, the volume diffusion is preeminent. The

smoothness and roundness of the closed pore will be promoted due to

surface diffusion. The diffusion of atoms along the surface of particles

or pores is mainly the vacancy mechanism, since its activation energy

is much lower than that of the interstice and transposition of atoms.

The vacancy will migrate from the concave to the convex area, while

the atoms migrate to the concave area and the sintering neck due to

the vacancy intensity and chemical potential differences on the surface

with different curvatures.

5. Grain boundary diffusion:The grain boundary can “trap” the vacancy dur-

ing vacancy migration. The activation energy for the grain boundary dif-

fusion is only half that of volume diffusion, and it will be much less with

decreasing temperatures. The pores close to the grain boundary always

disappear or reduce in number, and the grain growth for the metals dur-

ing sintering is accompanied by the movement of grain boundaries and

pore disappearance. The grain boundary moves from the concave sur-

face, with high energy, to the center of curvature, with low energy.

The surplus surface energy at the grain boundary is also the driving force

for the grain growth.

6. Plastic flow: A row of atoms will move or the crystal planes will slide with

the generation of dislocations in the crystal caused by surface tension.

The sintering is analogous to metal diffusional creep. High-temperature

creep is a process of the continuous microdeformation for metals under

constant low stress (driving force). The surface tension (driving force)


will decrease during sintering, and then the sintering rate will slow

accordingly.

7. Combined theory of sintering: In fact, the abovementioned mechanisms will

play simultaneously or alternately in the same sintering process. The

sintering of powders with high vapor pressure is conducted through

the vaporation and agglomeration mechanisms. The surface and grain

boundary diffusion mechanism is popular for sintering at lower temper-

atures or for sintering of ultrafine powders. For isothermal sintering, sur-

face diffusion contributes only to the formation and growth of the

sintering neck and pore spheroidization, not to the shrinkage of the

sintering body. The grain boundary diffusion always accompanies vol-

ume diffusion and helps the densification of the sintering body. At much

higher temperatures, the volume diffusion is predominant for most metal

and compound crystal powders. The distinct shrinkage of the sintered

body is the result of volume diffusion.

There are many sintering mechanisms, and the driving forces always

come from surface tension. The main barrier for the grain growth and

movement of the grain boundary in sintering are the presence of pores,

and other barriers include the secondary phases and grain boundary groove.

Influential Factors in SinteringThe influential factors involved in sintering include the following:

1. Metal powder type: The intial sintering temperature will increase with the

reduction of crystal lattice symmetry.

2. Powder activity: The diffusion (grain boundary) is promoted with much

smaller and finer grains. The higher activity for the ball-milled particles is

due to the generation of crystal defects, reduced particle size, and

increased total surface area.

3. Oxides on the powder surface: When a thin layer of oxides (smaller than a

certain thickness) is formed on the surface, it is prone to sintering due to

the quick reduction of the oxides to the metals and the increased activity.

In addition, the diffusion and sintering will be hindered with the thicker

layer of oxides or the lack of reduction in the oxides.

4. Additives: Diffusion and sintering can be accelerated if the additives can

form a solid solution with the powders to reduce the sintering temper-

ature due to activation by crystal lattice distortion.

5. Sintering atmosphere: Vacuum sintering can be done with most metals;

however, it will cause more loss of metals due to the volatilization

and deformation of the final products. Some additives may be introduced

38 Porous Materials

to activate the powders in the sintering atmosphere. The physical effect

of the atmosphere is that the gas compositions and properties in the pores

are different and demonstrate different diffusivities and solubilities in

solids in different sintering atmospheres. The chemical effect of the

atmosphere refers to the chemical reaction between the gas and the

sintering matter. In a sintering process controlled by positive ion diffu-

sion, for instance, it is advantageous that it take place in an oxygen atmo-

sphere or under higher oxygen partial pressure; this is because of the

increased positive ion vacancy from the excessive negative oxygen ions.

It is favorable for sintering with any contributions to the diffusion.

Features of Porous Material SinteringPorous materials require a certain porosity and strength. Therefore, powders

with narrow size ranges and spherical or near-spherical shapes shall be used

to prepare them, and pore-forming agents are usually added to the powders.

No shrinkage is expected for the loosely compacted or premolded green

body after sintering; that is, there are no changes of porosity or pore sizes

after sintering. As indicated in the sintering model of porous materials by

powders, the atoms at the contact area of powders will leave their crystal

lattices and then diffuse to form the intial bonding at the temperature of

0.4 Tm (i.e., the melting point of the metal powders). At a temperature

of 0.5 Tm, the atoms on the free surface at the convex area will migrate

toward the neighboring powders to form the sintering neck. The growth

of the neck needs more atoms to migrate without affecting the porosity (that

is, no shrinkage of the sintered body occurs). The connections of the pores

continue to exist and the growth of the neck leads only to the smoothness of

the pore channels. Finally, the pore channels will become stable with the

growth of the neck and the progress of sintering. It is then known from this

model that the ideal porous body with round channels can be obtained at

low temperatures (about 0.5 Tm) and over a long period of sintering.

Sintering Methods for Porous MaterialsThere are several methods of sintering porous materials:

1. Sintering of molded powders: In this commonly used sintering process, the

mixture of metal powders and pore-forming agents is loaded in the pref-

ormed body and then heated in a reductive atmosphere. During sin-

terting, the organic materials decompose and the atoms diffuse and

combine to form the porous metals. The metal powders can be Al,

Mo, Mo alloy, W, W alloys, or mixtures of these materials.


2. Loose sintering: The powders are loaded into the mold for sintering with-

out any other pressing (though shaking may be applied). They make

contact with one another from the effect of capillarity and the surface

tension during sintering. It is mostly used in the production of porous

filter materials with more permeation and low purification, sound and

thermal insulation porous materials, and sealing materials. The mold

materials used for this process shall not react with the powders and have

enough high-temperature strength and stiffness. The thermal expansion

coefficient of the mold materials is also as close as that of the powder

materials for sintering.

Bronze (Cu-Sn alloy) filters are usually produced by this route. It can

also be used to make brass (Cu-Zn alloy) filters, as well as the Ni dia-

phragm used as the electrode of alkaline batteries and fuel cells, which

has a porosity of 40%–60%. The higher porosity can be achieved with

the addition of pore-forming agents. The filter of Fe, Ni, Cu, and their

alloys. In some cases, porosity of 70%–90% can be obtained if ammo-

nium chloride and methyl cellulose are used as pore-forming agents [8].

3. Activated sintering: For the sintering of metal powders with high melting

points, much higher temperatures and longer periods of time are needed.

If the activator is used or the activating treatment is applied, the sintering

temperature can be reduced and the sintering time can be shortened.

Essentially, activated sintering can reduce the activation energy in the

flow, diffusion, vaporization, and condensation processes thermody-

namically and then increase the reaction rate.

Activated sintering can be conducted physically and chemically. In

the physical method, the alternating magnetic field, high-energy particle

radiation, static loading, ultrasound vibration, and periodical sintering

around the allotropic transformation temperature are applied to consol-

idate the sintering. In the chemical way, the hydride, the reactive gas, the

trace elements, preoxidation, and periodical oxidation and reduction are

applied to consolidate the sintering. Sintering of porous materials is

mostly prepared chemically and it is based on reduction and dissociation

in the chemical reaction. The newly formed atoms of Ti and Zr will be

dissociated from Ti and Zr hydrides in sintering. These Ti and Zr atoms

are more effective in sintering than pure Ti and Zr. The most effective

way is to introduce halide (HCl) vapor in the sintering atmosphere for

the chemical method. However, it may cause corrosion to the products

and the equipments. It is necessary to eliminate halides thoroughly with

hydrogen after sintering. Moreover, it is also effective to add a small

40 Porous Materials

amount of alloying elements. For instance, the addition of less than 1% of

elements from iron family members (like Ni) or platinum family mem-

bers to W and Mo powders (or fibers) will lower the sintering temper-

ature considerably. However, it is not a good way to mix mechanically

with Ni or Co as an activator since an activating layer cannot be formed

on the metal particle surface. Therefore, these additives of the alloying

elements shall be in the state of their chlorides or other substances and

then introduced into the solution. After that, the metal powders are

mixed into the solution and dried and a thin layer of oxide will form

on the particle surface.

For the mechanism of activated sintering, it is generally believed that

volume diffusion is predominant, but the grain boundary and surface dif-

fusion also act in the process. Due to the different diffusion coefficients

for the metal elements, the vacancy defects left in the surface area of par-

ticles may contribute to the migration of atoms.

In order to increase the permeation property for the iron-base filter,

the Cu, Fe, and Ni chlorides and phosphates are usually used in activated

sintering, and the halides also can be added and dissociated to activate the

sintering. Metal powders with low melting points like P, B, Ag, Cu, and

Sn are used in the activated sintering of stainless steels in the industrial

hydrogen atmosphere. For the activated sintering of TiC, WC, ZrB2,

and TiB2 spheric powders, 5% polyethylene resin in alcohol is used as

the plasticizer and 3% CoCl3 is used as the activator.

The sintering temperature of refractory metal fibers can reach 0.95

Tm, so activated sintering is more meaningful for the refactory metal.

4. Electic spark sintering: The spark discharge occurs among powders while

applying a mid- or high-frequency alternating current (AC) and direct

current (DC) to the powders, and then increasing to a high tempera-

ture. The spark discharge could last for 15 seconds, and the sintering

process can be completed within a few minutes. This method can be

used to make porous metals and refractory metals with high melting

points.

5. Liquid phase sintering: The migration of atoms in the liquid phase is faster

than that in the solid phase, if the components with low melting points

melt, or form the eutectic phase with low melting points at the sintering

temperature. The driving force of liquid phase sintering is surface tension

in the liquid phase and interface tension in the solid-liquid phase. The

typical application of this method for making porous materials is the

Cu-Sn porous body.

Figu


Sintered bronze is the earliest porous material used for anti-friction

purposes, and it contains 10% Sn. Sometimes 1%–3% graphite or less

than 3% Pb are added to it to enhance its anti-adhension and – friction

characteristics [7]. The mixture of the powders or atomized pre-alloyed

powders is first pressed and then sintered in the protective atmoshphere

(reductive gas or solid carbon stuffing) at 800 �C–850 �C to form a prod-

uct with 20%–30% porosity. At the final stage of sintering, the Cu-Sn

liquid phase disappears. Cu and Sn are mutually soluble to form a series

of transional phases (electron compound) and the corresponding finite

solid solutions, see Figure 2.7. For example, in the sintering process with

an alloy with 10% Sn, the Sn powders start to melt when the temperature

increases to 232 �C; furthermore, the melted alloy flows and fills in the

interstices of the Cu green body. Cuwill dissolve in the liquid Sn to form

the Z phase through a eutectic reaction (–60% Sn). as temperature

increases further, Cu continues dissolving until the e phase (38% Sn)

forms at 415 �C by the peritectic reaction, and the liquid phase increases

0

T/C˚

150

200

300

400

500

600

700

800

900

1000

1100

10

798

520

586

350

189227

415

640

755

186

20 30 40

α + L

β + L

β + γα +

β

α + ε

ε + η′

ε + η

ε + L

γ + L

γ + ε

ξ + ε

γ + ξ

η

η′η + Sn

η′ + Sn

η + L

α + γ

γ + δ

δ +

ε γ ′

50 60 70 80 90 100

wt /%

0 10 20 30 40 50 60 70 80 90 100

SrCu atm /%

α γ

ξ

δ

β

ε

re 2.7 The phase diagram of Cu-Sn.

42 Porous Materials

correspondingly. Therefore, Cu keeps dissolving with increasing tem-

perature until the remelting reaction temperature (640 �C) is reached.The e phase changes to the g phase, and then the liquid phases reduce

significantly. When the temperature reachs 755 �C, the g phase will

change to the b phase by the peritectic reaction and the liquid phase

will reappear. When the sintering temperature is higher than another

peritectic reaction temperature (755 �C), the b phase will decompose

again and finally form the Cu-base a solid solution. From the critical

temperature point in the phase diagram, the stable liquid phase can be

formed only at temperatures above 850 �C for alloy powders with

10% Sn. The stable liquid phase also can be formed at lower temperatures

with higher content of Sn (>10% Sn). The a and e phases form the equi-

librium structure after cooling. In fact, the phases at room temperature

could be the unhomogenized a phase and a small amount of high-

temperature d phase when the mixture of powders are used without

enough diffusion.

Cu can be dissolved quickly in the Sn liquid phase. Specifically, Cu

can reach its satuation state in the molten Sn when the size of the Cu

powders is very small (< 15 μm). With increasing temperatures, the gphase forms and then the liquid phase reduces or disappears. Before

the disappearance of the liquid phase, the sintering progresses quickly

and the density increases due to the dissolution of Cu. With the forma-

tion of the g phase, sintering occurs in the solid phases. For the sinteringthat takes place above the peritectic reaction temperature (798 �C), thehomogenization of the a phase will be finished via a liquid phase and leadto expansion until the temperature reaches 820 �C. Then it shrinks as thetemperature increases further. The liquid phase diffuses into the a phase

and disappears after the peritectic reaction (b!a+liquid), and the dis-

solved gas (H2 in Cu) in the liquid phase during the solidification process

is expelled and then the pores are left in the alloy. Therefore, the heat

preservation shall be conducted at the peritectic reaction temperature

to facilitate the diffusion and solidify the liquid phase slowly. After that,

sintering at temperatures above the peritectic point will not lead to

expansion.

6. Slurry foaming sintering [2,3]: Slurry is prepared by mixing the metal pow-

der, foaming agent, and organics; after that, the finished slurry is heated

and foamed, and finally the solid porous materials are obtained. This pro-

cess is used to prepare Be, Ni, Fe, Cu, Al, stainless steel, and bronze

porous materials.


Sintering Process1. Sintering temperature and holding time: The determination of the sintering

temperature is related to the compositions, particle size, surface state, and

the property required for the product. As mentioned previously, the

ideal sintering temperature is 0.5 Tm and a long sintering period is

needed. Considering the strength, hardness, toughness, ductility, poros-

ity, and particularly mechanical strength, the sintering temperature shall

be above 0.6–0.8 Tm. For powders that are prone to oxide formation on

the surface (e.g., chromia and titania on stainless steel powders), it can be

reduced only in pure hydrogen at a high temperature.

For pure metal solid solutions, the sintering temperature is 2/3 – 3/4

Tm. For example, the sintering of Fe takes place at 1000 �C–1200 �C,and for Cu at 700 �C–900 �C. For sintering of mixed metal powders,

the sintering temperature is generally lower than that of the main com-

position, or a little above the lowmelting eutectic temperature according

to the phase diagram. The finer the powders are, the more active the

powder surface is, and then the lower the sintering temperature is.

The sintering temperature and holding time varies with the different

physical and mechanical requirements for the product.

The holding time during the sintering process depends on the tem-

perature, the required porosity, and the pore shape [6]. With the

required porosity, the holding time is short if sintering occurs at higher

temperatures, and it is longer at low temperatures. In practice, the

sintering temperature and holding time can be determined by experi-

mentation. Genenrally, low sintering temperatures and short holding

times are preferred for sintering in order to reduce the requirements

and increase the productivity of the sintering facility.

It is less important for the calculation of shrinkage of porous materials

than that of powder metallurgy alloys. Sintering is always controlled to

minimize the amount of shrinkage as a porous body was produced.

Sometimes pore-forming agents are used in sintering.

2. Sintering atmosphere: The proper sintering atmosphere (reductive, neu-

tral, inert, vacuum, or air) needs to be in place to achieve the required

physical and mechanical properties. No oxidation of the powders hap-

pens and the oxides in the mixtures must be reduced during sintering.

Gas desorbtion, removal of impurities, reduction and dissociation of

oxides, migration of gaseous metal, interaction of gas and sintering mate-

rials (formation of stable and unstable compunds), and surface diffusion

will be influenced by the atmosphere.

44 Porous Materials

The sintering atmosphere controls the chemical reaction between

the powders and gas and the removal of decomposed lubricants. For

example, it is used to prevent or reduce oxidation and decarbonization

of the porous body; to remove the absorbed gas, surface oxides, and

inclusions; and to maintain or change the effective compositions in

the sintered body, such as the carbon control, nitriding, and preoxidation

sintering of steels. Depending on their function, sintering atmospheres

can be divided into five types:

1. Oxidative atmosphere: Pure oxygen, air, and water steam

2. Reductive atmosphere: Pure hydrogen, decomposed ammonia, CO,

and transformed gas (mixture of H2 and CO) from carbon hydrides

3. Inert or neutral atmosphere: N2, Ar, He, and vacuum

4. Carburization atmosphere: CO, CH4, and other carbon hydrides

5. Nitriding atmosphere: NH3 and N2 for sintering of stainless steels and

Cr-steels

The same atmosphere might be neutral, reductive, oxidative, carbu-

rizing, or neutral/decarburizing depending on the metal involved. For

instance, CO2 and water steam is neutral to Cu, but oxidative and dec-

arburizing to carbon steel; H2 is decarburizing to carbon steel, while CO

and CH4 are carburizing; and N2 is neutral to most metals, but nitriding

to Cr, V, Ti, and Ta. The most widely used gases in the sintering atmo-

sphere are reductive or protective gases containing H2 and CO since

they are reductive to most metals.

The sintering of porous materials is conducted most frequently in the

reductive atmosphere or in vacuum to prevent the oxidation of metals

and to purify the sintered body through removing the absorbed gas,

oxides, and impurities. If chemical heat treatment and sintering are com-

bined, the sintering atmosphere can realize the alloying, carburization,

and nitrding processes simultaneously.

Different sintering atmospheres are needed for different materials.

The noble metal powders and stable oxide coated powders can be

sintered in air, while Cu, Fe, Co, Ni, W, and Mo metals/alloys must

be sintered in a reductive atmosphere to reduce the surface oxides [7].

The most common reductive gases are transformed coal gas,

decomposed ammonia, and H2. Co, H2, and transformed coal gas can

be used to sinter Cu base alloys. No oxygen or water steam is allowed

in the sintering atmosphere for the sintering of metals/alloys with high

affinities to oxygen (e.g., Cr, Be, Mn, and stainless steel). The small pres-

ence of oxygen or steam can react with the sintered body to form oxides


and then impede the sintering process and reduce the plasticity of the

sintered body. The sintering of these metals and alloys must be con-

ducted in a highly pure and protective atmosphere. Vacuum sintering

is applied to metal products that are prone to absorbtion or dissolution

of the gas in the atmosphere, such as Ta, Nb, Ti, and Zr. It also contrib-

utes to the vaporization and decomposition of the inpurities in Ti and Zr

powders in a vacuum. In addition, the impurities of C and H will be

vaporized in a vacuum and is helpful to the reduction of oxides at high

temperatures. The Si, Al,Mg, and Ca impurities and their oxides can also

be removed in a vacuum and their materials purified. However, it is also

a concern that the vaporization loss of the metals during liquid-phase

sintering in a vacuum will change the final compositions and

micristructures of the sintered alloys; meanwhile, vaporization could

impede the sintering process.

The metallic Ti can react with H, O, N, and C easily; therefore, Ti is

usually sintered in a vacuum or in a highly pure Ar atmosphere. Under

vacuum conditions, the degree of the vacuum shall be 10-3 – 10-4 mmHg

and the sintering temperature shall be 800 �C–1350 �C. Ta and Nb can

easily absorb H, O, N, and C and then become brittle. So they are

sintered in vacuums of higher than 10-3 –10-4 mmHg.

3. Filler in sintering: A high content of lubricants or pore-forming agents is

included in the pressed green body for the preparation of porous mate-

rials and a large amount of gases or evaporized materials will be released.

Therefore, fillers are added to sustain the green body, reduce the release

rate of the gases or evaporized materials, and absorb the fluid with a low

melting point. Otherwise, collapsing, cracking, bubbling, and other

defects will be generated during sintering [6]. Another function of fillers

is to prevent the infiltration of air into the furnace to oxidize the prod-

ucts. Generally, burned Al2O3, MgO and graphite particles are used as

fillers to cover the green body. In addition, the fillers can contribute to

the uniform heating of the sintering body and prevent bonding of the

sintering bodies.

The first criterion for the selection of filler materials is that there be no

reaction between the fillers and the sintering body or the sintering boat. Sec-

ond, there is no deformation at the sintering temperature and the fillers have

a certain range of particle sizes. Graphite and charcoal can be used as fillers for

the sintering of iron and copper products, while electically melted magnesia

and alumina are used as the fillers for Ni, Monel alloy, and stainless steel

products. The particle sizes of the fillers depend on the powder size for

46 Porous Materials

the sintering of porous products: filler sizes are a little larger than for powders

so that the fillers cannot fill the gap of the powders, but they cannot be so

large as to release the volatile materials in the green body slowly.

If the oxides of some elements in the sintered products are very difficult

to reduce, some additional activators will be used to activate the sintering

atmosphere. For instance, titanium hydride can be added to fillers for the

sintering of stainless steels to reduce the oxides by the hydrogen atoms.

Ammonia chloride in the fillers can consolidate the sintering of iron

products.

Preparation of Materials with High Porosity1. Addition of a pore-forming agent: This step connects the pores and increases

porosity. There should be no water absorption, no decomposition at

room temperature, and no chemical reaction with the metal powders.

Pore-forming agents are decomposable when heated, and there is no

harmful residue after vaporization in the base metals. They can be inor-

ganic compounds, salts with a low melting point, such as camphor, urea,

salvolatile, ammonium acid carbonate, and stearic acid [6]; and ammonia

chloride and methyl cellulose [9]. Figure 2.8 shows an Al foam product

with open cells prepared by the space-holder method [10].

2. Addition of pore-forming enhancers: In this step, the enhancers can be

reduced by hydrogen or decomposed into metallic salts. Unmetallic

Figure 2.8 The macrostructure (a) and a scanning electron microscope (SEM) micro-graph (b) of Al foam prepared by the space-holder method [10].


compositions form an evaporated gas, which then creates the pores,

while metallic compositions can form compounds with the base metals.

If the melting point of the compounds is lower than that of the base

metals, then the compounds will be melted, and the materials are

strengthened by liquid phase sintering.

3. Natural cellulose: Natural cellulose is dipped into a solution of one or

moremetallic salts that are thermally decomposable. Afterward, the dried

cellulose is heated and burned in the reductive atmosphere, and the

metallic salts can be decomposed into metals or alloys. The gases

decomposed from the cellulose and salts can be used to form the pores,

and in the end, materials of high porosity with interconnected pores

can form.

The abovementioned metallic salts can be decomposed completely, and

no stable oxides are formed. Under these conditions, the salts can be

decomposed and foster the sintering of metals in a reductive atmosphere.

It is applicable to Ni, Mo, Fe, Cu, and their alloys, as well as W, Mo,

Au, and Ag noble metals. The natural cellulose can absorb the dipped solu-

tion, while synthetic fiber and high-molecular polymers cannot.

It is reported [11] that porous metals with a porosity of>90% have been

prepared by mixing carbonyl fine Fe powders, Ti alloy coarse powders, and

the double polyhydric alcohol-isocyanate with pore-forming agents. These

porous bodies have a reticulated structure with pore size of 100–200 μm and

are applicable to catalysis, biomaterials, and composite materials.

ExamplesPorous Al has been prepared successfully by powder metallurgy at the

Fraunhofer Institute of Applied Materials (IFAM) in Bremen, Germany

[12]. The base metal, alloy powders, or the mixtures of powders were mixed

with the pore-forming agents, and then the semi-products of the powders

were created by densification. During densification through uniaxial com-

pression or extruding or rolling of powders, the pore-forming agents were

buried in the base metals and no residual open pores appeared. Afterward, it

was subjected to heat treatment at temperatures close to the melting point of

the base metals. During this process, the uniformly distributed pore-forming

agents in the dense base metals are decomposed. The released gases make the

densified powders expand to form the porous materials. Before the forma-

tion of the pores, the preformed materials can be manufactured in flakes,

rods, or other forms by rolling, forging, or extruding in order to improve

fluidity during the pore-forming process. The density of the porous metals

48 Porous Materials

can be controlled by adjusting the content of pore-forming agents, temper-

atures, and heating rate. If the hydrides act as pore-forming agents, the con-

tent of hydrides is less than 1% in most cases.

This process is widely used to prepare porous Al, Sn, Zn, brass, bronze,

and Pb by using the proper pore-forming agents like pure Al, 2��, and

6��Al alloys, together with the proper processing parameters. The cast

AlSi12 alloy is also widely used due to its low melting point and good pore-

forming ability. In principle, any kind of Al alloys can be used as the agents

with the proper adjustment of processing parameters.

The porous metals can take any irregular form after sintering if the pref-

ormed product is put in the furnace without restriction. Therefore, in order

to obtain the designed shape of porous metals, the preformed product is put

into the hollow die for sintering. The core/shell structured sandwich plates

can be prepared by sticking the sheet and porous metals together. If the pure

metals are needed for sticking to occur, the metal sheet can be rolled onto the

green body of the porous metals, the composite is deformed via a deep draw-

ing process, and finally, heat treatment is applied to the drawn body.

Additionally, a dence structured body can be produced by putting the

plate slides into the container; these plates are produced by extruding the

mixtures of Al powders and hydride particles [13,14]. A closed-cell porous

core can be obtained when it is heated to the same temperature as the solid.

This porous core is isotropical, and some large pores can be seen occasion-

ally. It is easy to form the structure that is filled by the porous body, and the

related products are the sandwiched panel and the tube with a foamed filler.

It is particularly advantageous to the application of porous materials that are

insensitive to mechanical properties.

For instance, a dense preformed product was prepared by the axial com-

pression of the mixture of Al powders and TiH2 at a certain temperature, and

then heated to release the gas to force the preformed product to expand into

the Al foam [15].Mixing, pressing, and foaming are the three important pro-

cedures in the foaming of powders. The sintering pressure is 130–150 MPa

at the temperature of 400 �C–450 �C. Different structured Al foams can be

prepared by adjusting the content of the pore-forming agent (about 1%) at a

temperature range of 600 �C – 720 �C for 3–15 min.

In another example, the shape memory alloy of Ti50Ni48Fe2 is prepared

by powder metallurgy [16]. First, the commercial pure powders of TiH2, Ni,

and Fe, as well as the pore forming agent of NH4HCO3, were blended

according to a certain ratio and made into green compacts, and then placed

in the furnace in a vacuum. After the NH4HCO3 powders have been

Figure 2.9 Sintered porous TiNiFe alloys after adding different amounts of NH4HCO3

[16]: (a) 0%, (b) 12.5%, (c) 25%, and (d) 37.5%.


decomposed at 200 �C for 2 h and then the TiH2 powders have been

dehydrogenating at 800 �C for 1 h, the compacts were heated to sinter at

1,000 �C for 5 h. Figure 2.9 shows the sintered porous TiNiFe alloys.

Common Porous Filter Metallic MaterialsThere are two steps in the process of preparing porous filter materials by pow-

der metallurgy [17]: the densification/packing step and the sintering step. If

the binder is used in the molding, it should be removed before or during

sintering. Today, most porous products are prepared by one of the following

processes: (1) loose loading/gravity sintering (bronze); (2) axial/isostatic press

densification and vacuum sintering (stainless steel, carbon steel, superalloy, Ti,

and Al); (3) asymmetrical designed filter (i.e., the AS (asymmetrical) method).

Later, the developed asymmetrical filter is a powder/powder composite

made of the supporting coarse metal powders and the thin active filter layer

of the same alloy (< 200 μm). The plate-type supporting materials are pre-

pared by axial pressing, while the filter tube is made by isostatic pressing.

A thin metal film is used during the separation. The diffusional combination

can be created between the supporting materials and the active filter during

50 Porous Materials

sintering. The next developed AS method is a process for the porous struc-

tural product and the porous coating preparation, and ultrafine metal pow-

ders can be used to do this.

The most widely used areas that employ sintered porous metal parts

are process engineering and chemical engineering. The porous filter can

undertake deep filtering by using the total volume of the pores due to the

synthetic physical effects. This is the main difference between deep filtering

and surface filtering, and it enhances porous products made by powder met-

allurgy. The involved physical effects are the reduction of the particle flow

rate and the adherence particle to the pore wall. The decision of whether

deep or surface filtering should be used depends on the particle distribution

and the range of pore sizes in the fluid.

Powder metallurgical filters have the following characteristics:

1. Shape stability: Self supporting can be realized via a high pressure differ-

ence in the fluid.

2. Good fatigue property: Higher impact and shock resistance are demon-

strated compared to other filters (such as those made of paper, plastic,

or ceramic).

3. High-temperature and thermal-shock resistance: Bronze filters can be used at a

temperature of 400 �C, and highly alloyed steels can be used at 600 �C,and filters made of a special alloy can withstand temperatures of 950 �Cor even higher. Metallic filters are better than organic filters in this respect.

Under certain conditions, thermal shock resistance is required, and this

characteristic is better for these filters than those made of ceramic.

4. High reliability for separation during deep filtering:This characteristic is favor-

able for these filters compared to fabric, paper, and silk screen filters.

5. Good back pressure flow: These filters can be cleaned extremely well using

high-pressure steam, chemicals, or burning.

With more stringent requirements imposed by recent environmental

laws for recycling and environmentally friendly substitution for the trash,

there are many advantages of powder metallurgical metal products with high

porosity. It is more important for the selective competition of technical

products.

2.2 FIBER SINTERING

In powder metallurgy, metal fibers are substituted, partly or totally,

for metal powders, and then metal fiber porous materials can be prepared.


Metal fiber sintering is very similar to metal powder sintering, but it has

some distinctive features.

2.2.1 Preparation of Metal FibersThemethods used for preparation of metal fibers are cold drawing, spinning,

cutting, and plating [2,18,19]. They are described in the next sections.

Cold DrawingIn cold drawing, multiple drawings of a single wire obtain ultrafine fibers

with the optimal cross-sectional shape (with a precise diameter) and the sur-

face state (which is smooth). However, the productivity of this process is low

and the cost of die is high, which are distinct disadvantages. With cluster

drawing, where tens or even hundreds of wires are drawn simultaneously

through the die, productivity can be improved significantly and the cost

can be reduced as well. The metal wires are wrapped with copper and drawn

several times with annealing, and then cut when a certain diameter is

reached. Further drawing is performed with the bundled cut wires in the

wrap until the required fiber diameter is reached. The wrapped material

can be dissolved in acid (like nitric acid). Finally, the metal fibers are

obtained.

Spinning MethodIn the spinning method, metal fibers can be prepared from liquid metal at

low cost, but special equipment is needed. For example, the molten metals

can flow out through small holes in the bottom of the container from

mechanical force or gas pressure, and then they solidify in a proper atmo-

sphere. The connection between the metal fiber crystals is weak after solid-

ification, so the substance is so thermally brittle that the short fiber can be cut

easily with only a little bit of force. Therefore, uniformmetal short fibers can

be produced with shear force during the dropping of the solidified metal

fibers. The shear force can be applied by being struck with a metal plate with

the metal fibers somewhat tilted. It is simple to manufacture and has a high

production rate. In addition, there is no oil, water pollution, or residual

stress.

There are three types of spinning methods:

1. Melt spinning: This method is widely used to prepare glass fibers and syn-

thesized fibers. In addition, it is applied to the development of Al, Sn, and

Pb long fibers with diameters of 25–250 μm and a low melting point.

However, traditional melt spinning cannot be used on metals with high

52 Porous Materials

melting points due to the high surface tension on these liquid metals.

The tension may lead to the breaking of the metal wires into balls; in

such a case, long metal fibers cannot be produced. The following mea-

sures can be taken to overcome these problems: (a) stabilize the injection

with indirect physical methods, (b) change the surface state of the liquid

injection, and (c) accelerate the heat transfer of the injected metal to

solidify the metal wire before breaking.

2. Pendant-drop melt-extraction: This process involves two parts: the heater

and the quenching wheel. The metal wires are first put into the heater

and melted. The molten liquid drops fall onto the quenching wheel,

which has a high speed of rotation, and then they are spinned off cen-

trifugally and solidified with a cooling rate of 105 �C/s. The cross-

sectional shape of the metal fibers with small diameters (25 – 75 μm)

is round, while the shape is a crescent for fibers with large diameters.

3. Glass-coated melt-spinning: In this process, liquid glass has a high viscosity

and can be made easily into fibers as follows: Ametal rod is inserted into a

glass tube and then passed through a high-frequency induction coil and

melted together. Then the molten metal covered with the glass is cooled

at a speed of 105–106 �C/s to form into long, round thread and then

spinned onto the reel. After removing the outer layer of glass, the metal

fiber with diameter of 1–100 μm can be produced with a fine-grained or

an amorphous surface and thickness of 500–2,000 nm. Due to the

quenching effect, the thermal stress and deformation from the drawing

are presented in the metal fibers; therefore, the fibers demonstrate great

strength. For example, the tensile strength of IN865 stainless steel fiber

with a diameter of 2 μm is 14,500 MPa. Au, Ag, Ni, Co, Fe, Ti, V, Pt, Ir,

Cu, Al, and intermetallic fibers can be produced by this method.

4. Free-flight melt-spinning:Here, a hole for the adjustment of the flow speed

is made at the bottom of the liquid metal container. The liquid metal

flows out under pressure, and a tough film forms on the fiber surface with

the effect of the chemical active chilling agent, or the liquid fiber will be

encouraged to solidify by the introduction of a magnetic field. A fiber

of diameter of 25–1,000 μm can be produced at a cooling rate of

1–103 �C/s. Continuous long or short fibers can be produced by this

method. Be, Al, B, stainless steel, and superalloy fibers are prepared

by this method.

5. Melt-dropping: In this process, liquid metal flows out of a hole on the side

of the container and onto a high-speed rotatingmetal drum, and then it is

spinned off during solidification to form a metal thread. Al and Al alloys,


steels, and bronze fibers are prepared by this method with a large cross-

sectional area. For example, the sectional area of the Al iron fiber is

0.2 mm�2 mm.

GrindingThe metals are ground in a grinder containing abrasive material, and the

metal fibers with the required diameters will be obtained by adjusting the

feeding speed and the size of the abrasive materials. The size of the metal

fibers is influenced by many factors. For example, the finer metal fibers

can be produced with higher content of feeding, while the coarse metal

fibers are made with coarse abrasive materials. Tough metal fibers of Ni

and Ni alloy threads and NiFe alloys can be prepared by this method.

Plated Metal SinteringIn the plated metal sintering process, organic fibers are coated with metals by

chemcial plating, vacuum evaporation, and slurry dipping, or electric plating

after conductive treatment of the fibers. Afterward, they are either sintered

in a reductive atmosphere or burned in the air to remove the organics, and

then the oxides are removed using a reduction treatment. Finally, hollow

metal fibers are obtained.

Other MethodsFiber scraps can be produced by cutting solid metals by chatter machining,

shaving, or slitting. These actions are simple to performwith short production

cycles at low cost. However, it is difficult to obtain fibers with uniform section

and smooth surfaces; therefore, these processes are used mainly to produce

short metal fibers. Metal fibers also can be obtained by preparing the slurry

of powders of metals or metal oxides with an organic binder, extruding the

slurry into the fibers through a spinneret, removing the binder at high tem-

peratures, sintering in a reductive atmosphere, or removing the binder directly

in a reductive atmosphere. For example, Ni fibers can be produced by pre-

paring cream of Ni(OH)2 powders and a binder, extruding, and sintering.

2.2.2 Preparation of Porous BodiesThe normal processes used to prepare porous materials by metal fiber

sintering are threading, felting, and sintering [2,3,20]. The metal fibers with

certain ranges of lengths, diameters, and length-to-diameter ratios are

aligned to felt (also using suspension) and then sintered in a reductive

54 Porous Materials

atmosphere to obtain metal porous fibers. It is applicable to the preparation

of Cu, Ni, and Ni-Cr alloys and stainless steel with a wide range of

porosities.

Long or short fibers can be selected according to the following require-

ments: short fibers are used in metal molding, and long fibers are for knitting.

Then porous metals are obtained after sintering. The fibers are combined in

a three-dimensional (3-D) reticulated way to achieve a porosity of 98%. The

strength of porous metals prepared from fibers is better than that from pow-

ders with the same porosity.

This kind of porous metal fiber has several advantages, including tough-

ness, elasticity, and tension/compression resistance. All porous fibers are

composed of a single fiber, except porous bodies prepared by sintering plated

metals on organic felt. Short fibers can be distributed uniformly, while long

fibers cannot. Porous bodies with long fibers have better mechanical

strength. Therefore, this kind of porous material has disadvatnages as well,

such as large pores and nonuniform pore size distribution.With this method,

it is easy to prepare products with high porosity and interconnected pores.

Powder metallurgical porous stainless steels have properties of gas perme-

ation, noise reduction, and corrosion resistance, and then are used widely in

the aviation, chemical, andmechanical fields. If they are reinforced by stainless

steel fibers, much better mechanical properties will result. A porous stainless

steel with fiber reinforcement can be developed by adding stainless steel fiber

of F0.15 mm�5.00 mm to 0Cr18Ni9 with a particle size of 0.10–0.15 mm,

molding in the polythene achohol solution, and sintering in a vacuum at

1160 �C for 2 h with a heating rate of 10 �C/min [21]. The sintering process

is controlled by atom diffusion, and the diffusional coefficient is constant at the

sintering temperature. The number of diffusional atoms increases by exten-

ding the sintering time and then the conditions are favorable to the growth

of sintering neck and an increase in strength. Due to the exponential relation-

ship between the diffusion coefficient and the temperature, neck growth is

promoted by increasing the temperature to between 1,070 �C–1170 �C,and the strength increases accordingly, as opposed to what happens when only

the sintering time is extended. However, as the sintering temperature

increases, the porosity and permeation coefficient are reduced. The porosity

and permeation coefficient decrease somewhat by increasing the stainless steel

fibers while keeping the sintering temperature and time constant. The reason

for this is that stainless steel fibers are bigger than the particles and the perme-

ation coefficient decreases with the reduced porosity by adding fibers to the

stainless steel powders.

Fig. 2.10 SEM image of a novel porous metal fiber–sintered sheet with a 3-D reticulatedstructure [22,23].


Several studies [22,23] have fabricated a novel porous metal fiber–sintered

sheet with a 3-D reticulated structure (Figure 2.10) by using the solid-state

sintering method on copper fibers. They found that the stress-strain plots

of the uniaxial compressive test showed no obvious yield stage in the uniaxial

compressive process. Additionally, the results showed that the obtained

porous body with higher porosity exhibited greater strain under the given

level of compressive stress, therefore producing less effective stiffness [22].

2.2.3 Electrode Plate with Porous Metal FibersThe porous metal fiber electrode widely used in the battery industry is the

nickel base plate, which is prepared by the following methods [24,25]:

1. The fiber felt is uniformly mixed by the metal fibers with a certain

diameter-to-length ratio, and then sintered to obtain the porous body

in a reductive atmospohere. The fibers are produced by drawing, cut-

ting, and sintering the metal-plated organic fiber.

2. The porous fiber body is obtained through the reductive sintering of

metal-plated synthesized fibers (like polypropylene fibers) after the ther-

mal decomposition of the organics, or direct thermal decomposition of

the metal-plated fibers.

3. The base plate of metal fibers is produced continuously by the thermal

decompositionof themetal carbonyl compounds.Ahigh-quality base plate

can be obtained by this method, but at high cost and with size limitations.

56 Porous Materials

4. Using the conventional facility and technique, like using the nickel

slurry of Ni fiber, Ni powders, a binder, and pore-forming agents, the

Ni fiber porous base plate can be produced by dipping it in the slurry

and then drying and sintering it.

The physical and mechanical properties of the porous metal plate are

influenced by the optimal combination of porosity, pore size distribution,

and structure. The mechanical strength, capacity of the filler, content of the

active agents, and electric conductivity shall be considered together. The

porosity shall be increased with the goal of achieving the required strength

and conductivity. With a certain porosity, the pore size shall be determined

by considering the effect of both ohm resistance and concentration polari-

zation impedance on the electrode properties and tensile and compressive

performace. The availability of active agents will decrease, and the concen-

tration polarization will increase with reduced pore size, increased pore

numbers, and decreased ohm resistance. In all, the uniform pore sizes,

appropriate pore size distribution, regular structure, great strength, and good

ductility are the basis for the high porosity and large capacity of the

base plate.

The sintered composite substance (Ni.C.E.) was prepared by pressure

sintering Ni-plated graphite fibers with high elasticity in the early 1980s,

and it also had greater resistance than the sintered powders. The results in

China have shown that the tensile strength of composite electrodes is much

lower than that of sintered powders, and the pore shape is also less regular

than that of sintered powders. However, this substrate material has a weight-

specific capacity that is much higher than for sintered material and result in

lower consumption of Ni.

In the late 1980s, the porous body was developed by sintering interlaced

metal (i.e., stainless steel) and carbon fibers. The electrode capacity is closely

related to sintering conditions, like temperature and time, which affect the

connection points and connection state. The flexible materials can be used as

the electrode base plate in the normal battery and fuel cells.

The porous Ni plate was prepared by sintering ultrathin Ni fibers (diam-

eter of 2–10 μm, length of 1 mm) overlaid onto the Ni-plated perforated

steel belt or the other base net at 1,000 �C–1,200 �C in the H2 atmosphere.

An integrated effect was achieved with improved flexibility, specific surface

area, and strength of the base plate materials.

The size of the fiber-type base plate is not stable when loading the active

agents due to the large pore size and the low mechanical strength of these

materials. The application of Ni fiber and powders to the sintered porous

body can overcome these problems.


2.3 METALLIC MELT FOAMING

2.3.1 Preparation of Porous Bodies

Thi

6

Additto pu

Figure

The gas-releasing, pore-forming agent is introduced into the metallic melt

with adjusted viscosity, and then it is decomposed thermally. The released

gas from decomposition expands and drives the foaming of the melt, and

finally, the metal foam is produced after cooling [26,27]. Al, Al alloy, Pb,

Sn, and Zn with low melting points can be prepared by this method, and

the common pore-forming agents are TiH2, ZrH2, CaH2, MgH2, and

ErH2 metal hydride powders [8,28,29]. TiH2, ZrH2, and CaH2 are used

to produce Al foam, while MgH2 and ErH2 are used for Zn and Pb foams

[8,9,29]. TiH2 will release H2 when heated to above 400 �C [28]. Once

making contact with themoltenmetal, the pore-forming agents will decom-

pose quickly. Therefore, the gas-releasing powders should be distributed

uniformly in a very short time.

The introduction of ultrafine ceramic powders or alloying elements to

form the stabilized particles increases the viscosity of the molten metal.

The foaming of Al, Mg, Zn, and their alloys can be realized in this way.

Metal foam is one important part of porous metals and it has a long his-

tory. Al foam was developed in 1948 by the evaporization of Hg in the mol-

ten Al (U.S. patent 2434775), and further developed in 1956 (U.S. patent

2751289) [29]. In 1960s, Ethyl Inc. in Richmond, VA, became the research

and development (R&D) center of Al foam. Up to now, many technical

patents have been released concerning the production of Al foam in the

United States, Japan, the United Kingdom, Germany, China, and Canada,

most of which are related to melt foaming.

Figure 2.11 illustrated the technical process for the small-scale commer-

cial production of metal foams by the melt foaming method. Metallic Ca is

added to the Al melt at 680 �C and stirred for several minutes. Due to the

ckening

80 °C680 °C

ion of 1.5% Care A1

Addition of 1.6% TiH2

Foaming Cooling Foamed block Slicing

2.11 Technical process of melt foaming during the production of metal foam [5].

58 Porous Materials

formation of CaO, CaAl2O4, and even Al4Ca, the liquid Al became five

times thicker [5]. In the actual production of the foam, 1.5–3 wt% of Ca

is usually added. TiH2 (1.6 wt%) was added as the pore-forming agent when

the required viscosity was reached, and H2 was released in the hot, viscid

liquid. The melt will expand slowly, eventually filling the container. The

foaming of melt takes place at a constant pressure. The liquid foamwill trans-

form into solid when the temperature is below the melting point, and then

the solid foam is taken from the mold after further treatment. The foaming

time is about 15 min for the batch production in a large furnace. With care-

ful regulation of the processing parameters, foam with a uniform structure

can be obtained, such as Alporas Al foam. ZrH2 also can be used as the

pore-forming agent with the recommended content of 0.5%–0.6% (wt%)

and foaming temperature of 670 �C–750 �C.There will be problems when metal hydrides (like MgH2) are added

to the Al melt: a eutectic alloy (Al-Mg) with a low melting point will be

formed, and then the pore-forming agent will be combined with the eutec-

tic alloy and do not decompose (the system temperature is lower than the

foaming temperature of the pore-forming agent), and foaming can happen

only with the pure Al. The pore-forming agent is added to the liquidmetal at

temperatures that are above the soildius but below the decomposition tem-

perature of the vesicant. The metal solidifies in the designed mold after stir-

ring. Only when the composite is heated above the decomposition

temperature of the pore-forming agent can foaming truly begin. The

released gas generates the bubble and increases the volume.

The general requirements for the pore-forming agents are minimal

decomposition before the mixing of agents and melt, complete decompo-

sition afterward, and enough gas released before solidification [15]. Cur-

rently, TiH2 or ZrH2 is used as the pore-forming agent, and sometimes

eruption is used because its gas-releasing temperature is lower than TiH2,

less gas is generated, and the cost is lower.

2.3.2 Technical Problems and SolutionsMelt foaming is applicable to most industrial mass production of metal foams

due to its simple process and low cost [30]. The Al foam bulks in the market

are also produced by this process. The selection of proper metal pore-

forming agents is one of the technical difficulties of this method, however,

and the basic requirement is quick foaming around themetal’s melting point.

Melt foaming can be used to prepare closed-cell metal foam, but it is hard

to control the pore size. Therefore, it is difficult to obtain uniform porous


materials. There are a few possible solutions, including (1) high-speed stir-

ring of the pore-forming agent particles and then uniform distribution of the

particles in the molten metal; and (2) preventing the escape of gas and the

coalescence and growth of bubbles with increased viscosity of molten metal

[5]. The other problem is the short interval between the addition of the

pore-forming agent and the foam formation, which complicates the process

of cast operation. The solutions to this are to thicken the cast layer in order to

maintain the foamed metal temperature and to lengthen the flowing time or

to apply the continuous casting process.

In this process, the quick foaming of the pore-forming agent makes it

hard to distribute it uniformly in the melt. An oxide-wrapped, pore-forming

agent was invented at the Institute of Solid State Physics, Chinese Academy

of Sciences in Beijing, and it can delay foaming so that uniform foaming of

the agent can be realized [30].

Viscosity shall be controlled carefully to make sure that the pore structure

has a uniform size and shape in the melt foaming process. The melt viscosity

can be controlled by adjusting the temperature, and also the great temper-

ature difference for the alloys between solidus and liquidus. In addition, a

tackifier (which can be gas, liquid, or solid) can be used [5,8,30] and it works

in a more practical way in this process. Tackifiers can be added in several

ways, including melt oxidation, the addition of alloying elements, and the

dispersion of nonmetal particles [8,15]. Melt oxidation is a process that

air, oxygen, or water steam is blown into the molten metal and then stirred;

the oxides will formed a short time afterward. This method is highly efficient

and can achieve great viscosity. Solid oxidant particles are also used in the

melt, like MnO2, with a particle size of 20 μm in the Al melt and the for-

mation of Al2O3. The Al2O3 particles will form the nucleus of the foam, and

then the foamed body with uniform pore size, distribution, and shape will be

obtained [5,8]. The most commonly used method is the addition of alloying

elements like Ca to form fine solid particles in the melt and then increase the

viscosity. It is simpler than the melt oxidation process [15].

Tackifiers can be nonmetal Si polymers, alumina powders, SiC, Al scum,

N2, Ar, and other metals [5,8]. With the addition of a tackifier, the viscosity

of the melt increases, the foam wall thickens, the foam size decreases, and

then the uniformity of the density improves. More uniform foam is obtained

with the degassing treatment if a gas tackifier is used. However, the addition

of a tackifier may pollute the master materials to some degree, as well as

increasing the cost. In a Chinese patent (96117125.1), a special tackifier

was disclosed with advantages like no pollution, no cost, and ease of perfor-

mance [31]. Good results were demonstrated in Zn foam production by

60 Porous Materials

using cyclic foaming [8]. Numerous kinds of tackifiers are used in the cyclic

foaming process, and the broken foam will be foamed again during the stir-

ring process or to form intermediate products.

2.3.3 Case Studies on Porous Aluminum PreparationThe foaming will be triggered when the hydride powders are mixed into the

Al melt [27]. The addition of Ca will help attain higher viscosity in the tem-

perature range of solidus and liquidus. The growth of pores in the liquid can

be controlled by the overpressurized H. Large bulk products with uniform

interspaces and the isotropical property can be prepared by this method, and

uniformity results from the high viscosity of melt and the overpressurized H

in the foaming process.

The technical process for the preparation of Al foam is shown in

Figure 2.12 [20]. Based on the application, the Al alloys are first selected,

and the foaming conditions vary depending on the different Al alloys. Tack-

ifying is the most important step in the preparation of Al foam. The tacki-

fying methods include particle dispersion (dispersion of nonmetal particles in

the liquid metal), alloying (addition of alloying elements) and oxidation of

liquid metal (dispersion of formed oxides in the liquid metals), in which the

addition of active Ca to the liquid Al and a short period of stirring can tackify

the Al melt effectively. The optimal viscosity for liquid Al is about

8.6 mPa � s; overtackifying may cause the bubbles to escape from the liquid,

while undertackifying may lead to nonuniform distribution and the irregular

shape of pores with low porosity.

The following procedure is used to add and mix the pore-forming agent.

The pore-forming agents for the Al foam are TiH2, ZrH2, NH4Cl, NH4I,

(NH4)2SO4, BaCl2, Bi2(SO4)3, CaCO3, CaH2, CaSO4, and NaNO3. The

decomposition of the pore-forming agents shall be a little higher than the

melting point of the metals, and CaH2 is the best pore-forming agent for

the Al alloys. The pore-forming agents shall take the form of particles,

Thickening

Ca TiH2

A1 ingotmelting

Mixing Foaming Cooling

Figure 2.12 The technical process of the preparation of Al foam with melt foaming.


and they are stirred during the addition to the liquid metal for even disper-

sion. Delay release technology is used to extend the stirring time and then

mix in the pore-forming agent more evenly. After these procedures, the Al

liquid is ready for foaming upon standing. The foaming temperature and

time shall be decided by the decomposition termperature and rate of the

pore-forming agent. Higher foaming termperatures lead to irregular pore

shapes, while lower temperatures are not favorable to the growth of the

pores. At certain foaming temperatures, a too-long or too-short foaming

time may lead to pores having smaller diameters or irregular pore shapes.

Therefore, the pore structure can be regulated by the foaming temperature

and time. Finally, the required foam will be obtained after quick cooling of

the designed foaming state. The cooling methods can be air cooling, wind

blowing and oil or water cooling. Different foam states can be obtained using

various cooling methods. Due to the shrinkage of the solidified metal, the

porosity is generally lower than that of the liquid metal. Al foam has the fol-

lowing features: lightness, noncombustion, stiffness, sound absorbility, and

dampening. Therefore, it is one kind of noncombustible, nontoxic, and light

material that is useful in construction. Open-cell Al foam is a good sound-

absorbing material.

In one study [32], the preparation of Al foam with the industrial pure Al

and the additive was reported, in which TiH2 was mainly used as the pore-

forming agent. The processes involved are as follows:

1. Melting and stirring the pure Al and alloys to control the viscosity of

the melt

2. Addition of pore-forming agents and the even dispersion in the melt, and

the decomposition of TiH2 into Ti and H2, which form bubbles in the

liquid Al

3. Maintaining the temperature to control the formation of pores and

growth

The pores obtained are mostly of the equiaxial closed-cell type, as shown

in Figure 2.13. The addition of more pore-forming agent (> 3%) may cause

the formation of large pores in Al foam, while less of the agent may lead to

low porosity, along with the the formation ot foam with irregular structures.

2.4 GAS INJECTION INTO THE METALLIC MELT

Precise control of the foaming termperature range and processing time

is needed for the melt foaming method, while the gas injection method,

introduced in this section, is easier to implement and also has a low cost

Figure 2.13 The cross section of the Al foam with melt foaming [32]: (a) pore size of2.44 mm, porosity of 85.2%; (b) pore size of 2.50 mm, porosity of 70.7%; (c) pore sizeof 2.48 mm, porosity of 57.0%. The content of the pore-forming agent and the timeof maintaining temperature decrease at the same foaming temperatures from (a) to (c).

62 Porous Materials

[8,33]. The metal foams used in gas injection have a broad range of pore sizes

and very high porosity (up to>90%). Gas from the outside is injected into

the bottom of the molten metal, which produces bubbles, and the used gases

can be air, steam, oxygen, CO2, and inert gas. The key technical points

behind this procedure are the proper viscosity for the melt and the large tem-

perature range for the foaming. The formed foams must be stable and cannot

break during the process [8]. The mixture, composed of metal and solid sta-

bilizer particles, is heated to the temperature above the liquidus of the metal,

and then gas is injected to create the closed-cell bubbles. After cooling the

temperature below the solidus, the metal foamwith a large number of closed

cells is obtained [33]. The stabilizing materials can be alumina, Ti, ZrO2,

SiC, and silcon nitride, and the metal gases that can be injected are Al, steel,

Zn, Pb, Ni, Mg, Cu, and their alloys. The particle size and the volume ratio

for the stabilizing materials shall be selected carefully. The small size of par-

ticles causes the problem of mixing, and the high-volume ratio leads to low

stability of the foam, while a low-volume ratio leads to excess viscosity. The

pore size can be regulated through the gas flow rate.

The gas injection method for Al and Al alloy foaming was developed at

HYDRO in Norway and Cymat Technologies in Toronto, Canada [5].

SiC, Al2O3, and MgO particles are used to raise the viscosity of the melt,

as shown in Figure 2.14. In the first step, the Al melt, like cast AlSi10Mg

(A359) or precision cast 1060, 3003, 6016, 6061 alloys with these particles,

shall be prepared, and the wetting of the melt to the particles and the uni-

form distribution of the particles shall be resolved. The second step is the

Figure 2.14 Schematic illustration of the manufacture of metal foam by gas injection:(a) mode 1 [5]; (b) mode 2 [34].


injection of gas into the melt through a specially designed rotating propeller

or vibrational muzzle, which creates the foams. The function of the pro-

peller or nozzle is to generate smaller, uniformly distributed bubbles in the

melt. With the presence of these finer bubbles, high-quality foam is pro-

duced. During the drainage of the melt, the thick mixture of bubble and

melt will float above the melt and then become liquid metal foam. Due to

the presence of ceramic particles, this kind of liquid metal foam is quite

stable and can be drawn from the liquid, cooled, and solidified. Before

the complete solidification occurs, the semi-solid foam can be flattened

by rollers or a belt. In principle, the length of the foam belt can be as long

as needed, however, the width depends on the allowed width for a liquid

metal container with normal thickness of about 10 cm. Figure 2.15 shows

two metal foam products.

Figure 2.15 Metal foam prepared by gas injection: (a) two foam plates with differentdensities and pore sizes [5]; (b) an Al foam column [34].

64 Porous Materials

The volume fraction of particles is about 10%–20%, with an average size

of 5–20 μm [5,33]. The particle size and content are selected based on past

experience. The particles on the pore wall play a key role in the stabilization

process. First, the particles increase the surface viscosity and then delay the

exhaust. Second, the particles are partly wetted by the melt, and the con-

tacting angle must be within a specified range to ensure the stability of

the bubble/particle interface and the energy reduction of particles at the

interface compared to the total energy of bubble and particles. Very good

(with a much lower contacting angle) or poor (with a large contact angle)

wetting do not achieve stabilization effectively.

The Al foam prepared by this approach has the porosity of 80%–98%, the

density of 0.069–0.54 g/cm3 with an average pore size of 3–25 mm and wall

thickness of 50–85 μm [5]. The average pore size has an inverse relationship

with wall thickness and density, and it is influenced by the gas flow, propeller

speed, vibration frequency, and other parameters [5,35]. The density, pore

size, and pore extension gradients are present in the foamed panel, which is

the result of gravity-induced exhaust [35]. Additionally, the shear force of

the conveyer belt will cause diagonal distorted pores in the final product

and has a notable effect on its mechanical properties. This effect can be

improved by the vertical drawing of the foam.

The advantages of this method are the capability of large volume produc-

tion of foamed materials, as well as low density. Therefore, metal-based

composite foams are less expensive than porous metals. One disadvantage,

however, is the existence of open pores after the final cutting of the foam.

In addition, the presence of particles for the reinforcement will cause the


metal-based composite foams to be brittle, which is the unexpected side

effect of the foaming process.

This process also can be conducted with zero gravity [20]. The problems

of bubbles floating and increased viscosity, the need of foaming agents can-

not be encountered in zero gravity. The foam can be produced with the

injection of Ar into the liquid metal.

The foam can take one of two shapes: sphere or polyhedron [36]. Pol-

yhedrical pores are usually formed during the gas injection process, while

spheric pores are formed in the early stage of gas generation within the melt

and a mixture of spheres and polyhedrons are presented in the late stage. The

ratio of spheres to polyhedrons is related to the foaming time. Spheric pores

may change to polyhedrical pores in some conditions. In order to obtain

metal foam with high porosity, the key processing parameters in addition

to the size and dispersion of foaming agents are composition, cooling time,

and cooling rate. The wall effect of the container has an influence on the

stability of polyhedrical foam, whereas it exerts no influence on the stability

of spheric foam. The wall effect on the formation and stability of the poly-

hedrical foams reflects the formation rate of the dispersed bubbles and the

free surface generated when the foam breaks. The breaking rate of bubbles

in the foam layer is related to the free surface and the height of the foam

layer, since the drainage rates of the isolated bubbles vary with the height

of the foam layers.

2.5 INFILTRATION CASTING

In infiltration casting, inorganic/organic particles or low-density hol-

low balls are piled up in the mold or the preformed porous product is put in

the mold, and then the molten metal is infiltrated in the interspaces created

therein. The porous metals are obtained after removing the the pileups or

the preformed body [1,5,8,9], and their removal is realized by the dissolution

in solvent or by a heat treatment process. Inorganic particles with heat resis-

tance and solubility can be used for this kind of pileup, like NaCl particles.

Inorganic materials like puffing clay particles, fired clay balls, sand balls, glass

ball foams, and alumina hollow balls can also be used as the pileups, and the

porous composites will be obtained by using them. If the infiltration and

solidification of the melt is fast enough, the polymer balls can be accepted,

and positive or negative pressure may be applied.

The porous Al,Mg, Zn, Pb, Sn, and cast iron can be prepared in the form

of sponges by this method [1,5]. The parts with the designed shape can be

Solid press head

Gas injection

Gasexhaust

Figure 2.16 Schematic diagram for infiltration casting.

66 Porous Materials

manufactured by using the mold of the defined geometry. However, with

the presence of surface tension on the liquid metal (which is particular to

liquid aluminum), quick infiltration of the molten metal into the interspaces

cannot be attained. The wetting problems may result in unfilled interspaces

as well. In order to prevent this phenomenon, a vacuum state in the inter-

spaces may be produced to have a negative pressure, or pressure is applied to

the melt to facilitate the infiltration of the molten metal. Moreover, in order

to prevent the early solidification of melt, the preheating of pileups or the

overheated melt may be used, particularly under the conditions that the

pileups have a higher specific heat capacity or the infiltration pressure is low.

Infiltration casting can be pressurized by the solid press head

(Figure 2.16), gas, differential pressure, and vacuum suction casting [15].

The quality of the metal foams prepared by differential pressure and vacuum

suction is high, the products have good mechanical properties due to the

long infiltration distance of metal liquid and the dense metal frame.

The salt particles can be washed away with water, the sands are removed

during the thermal decomposition of the binder, and the polymer balls are

eliminated by the pyrogenation reaction. A higher pileup density can be

achieved with vibration. A sandwich plate can be obtained by the infiltration

of the melt in the preformed part inserted between two metal sheets and by

the formation of the metallurgical combination due to shell surface melting.

The presintered products are put into the mold and the porous core will be

produced together with the casting of the out-shell structure.

Figure 2.17 Porous Al prepared by infiltration casting: (a) open-cell Al foam with den-sity of 1.1 g/cm3 [5]; (b) porous Al with porosity of 76.0% and 84.3% [37].


One of the advantages of using preformed parts is the precise control of

the pore size distribution. The distribution will be derived from the size of

the filler particles. Figure 2.17 shows the morphology of porous Al prepared

by infiltration casting, in which the fillers have been removed totally.

Water-soluble sodium chloride (NaCl) particles are generally used to

prepare the preformed mold due to the consideration of the source, cost,

and dissolution from water [38]. NaCl is a type of white crystal with a den-

sity of 2.16 g/cm3, melting point of 804 �C, and boiling point of 1,413 �C,and it is water soluble as well. Before application, it must be pretreated to

remove the crystal water; otherwise it will explode and crumble when

heated to a certain temperature. Hence, the crumbling NaCl blocks the

interspaces in the preformed mold and makes the mold change shape.

Therefore, the dehydrated NaCl particles are kept in the desiccator before

using. Potassium phosphate (K2HPO4) is another kind of salt with water

solubility, a density of 2.564 g/cm3, and a melting point of 1,340 �C. Aswith NaCl, the pretreatment is applied to K2HPO4, but at a much higher

temperature. Therefore, the melting point of the alloys for the infiltration

with K2HPO4 is also higher than that of NaCl. The combustible particles

(like charcoal scraps with the strength of cast iron) shall be used in the pro-

tective atmosphere. The preformed particles are not easy to deform and

combust with high-speed infiltration. The particles can be removed via

the high-temperature treatment after infiltration and solidification, and

then the open-cell metal foams can be obtained. Additionally, in order

to remove the preform with subsequent treatments, the processing must

be continuous [15].

If insoluble particles are used to prepare the preformed mold instead of

water-soluble particles, the particles are sealed in the solidified metals and the

68 Porous Materials

hollow pores will become isolated [20]. If the liquid metal is die cast in 3-D

reticulated ceramics with a hollow framework, the pores in the metal-

ceramic porous composites are mainly composed of the original hollow parts

in the ceramic frameworks.

The preparation of porous Al and Al alloys by infiltration casting has been

reported [31,38,39]. The 3-D reticulated porous Al alloys with different

pore sizes and porosity can be prepared by the pressure adding cast process

[39]. During this process, molten metals are poured into a preformed mold

and pressure is applied to force the liquid metal to infiltrate the interspaces,

and a metal-preformed particle composite is obtained after the metal

solidifies. Al-12%Si alloy foam with maximum dimensions of

Ф100 mm�100 mm, pore size of 0.5 – 1.6 mm, and porosity of 60% –

80% is obtained. The preformed part is prepared by mixing the composite

salt particles with a binder and water, pressing into the graphite mold and

sintering. Figure 2.18 shows the pressure adding infiltration casting facility.

The preformed part is put at the bottom of the metal cylinder and heated to

450 �C by resistance heating. The Al-12%Si alloy is melted, refined,

deslagged, and finally poured into the metal mold. After that, the punch head

is pressed quickly to force the liquid metal into the interspaces and then kept

at a constant pressure for 15 min. After the metal has solidified, the metal-

1

2

3

4567

8 9 10

11

12

13 14

Figure 2.18 Schematic diagram of pressure-adding infiltration casting. 1—oil cylinder;2—pillar; 3—upper table; 4—upper template; 5—punch; 6—metal mold; 7—prefab;8—porous baseboard; 9—bottom table; 10—holding furnace; 11—manual handle;12—hydraulic pressure gauge; 13—temperature controller; 14—melting furnace.


preformed cast part composite is obtained. At last, the porous metals are

formed after the salt particle has dissolved in the preformed part.

The preformed particles are shaped like triangles with limited contact

areas, so the pores have more edges and corners. With further baking, the

edges and corners of the pores disappear and the contact areas increase.

These conditions are favorable to the preparation of porous Al with a

smooth pore surface and interconnected pores. The melted binders gather

and flow to the contact area of the particles, creating surface tension and

leading to a smooth connection. Moreover, the binders react with the salt

particles to form compounds with low melting points, further increasing the

contacting area. It is reported that the content of the binder needs to be in

the range of 7% – 10% to achieve this effect.

It is advantageous to prepare the open-cell Al foam by high-pressure

infiltration rather than low-pressure infiltration, even when preforming

flammable particles that cannot deform and burn during infiltration. The

processing of Al foam can be simplified by vacuum infiltration rather than

high-pressure infiltration.

2.6 METAL DEPOSITION

Porous metals can be prepared with gaseous metals or metal com-

pounds in a metal ion solution [5]. A solid, preformed structure is needed

to determine the geometrical shape of the porous materials. For example,

a polyurethane porous plastic is used to prefabricate the substrate.

2.6.1 Vapor DepositionVacuum Vapor DepositionIn vacuum vapor deposition, materials in a vacuum is heated by the electron

beam, electric arc, and resistance heating, and then they are evaporized

and deposited onto the cold porous substrate. The metal vapor finally solid-

ifies and covers the surface of the polymer porous base to form a metal

film. The thickness of the film depends on the vapor density and deposition

time [5,40].

The vacuum-plated film is quite thin, particularly when the synthesized

resin substrate is melted in a vacuum. Thin film (with a thickness of only

0.1–1.0 μm) can be deposited due to the heating of the substrate from

the radiation of the molten metals in the vacuum deposition process.

A �30 �C cooling medium can be introduced, and then the temperature

70 Porous Materials

in a vacuum can be reduced. The temperature of the organic net belt or the

porous material belt can be brought below 50 �C by cooling the transit

roller. Therefore, a thick film can be deposited on any kind of substrate, with

any kinds of metal. The obtained pores are not prone to deformation in the

porous metals [40]. The porous metals can be obtained by the thermal

decomposition of the porous substrate in the H2 reductive atmosphere

and the sintering treatment. The base can be made of synthesized resins,

such as polyester, polypropylene, and poluurethane, and natural organic

materials like natural fabric and cellulose. For the preparation of the porous

composite body, inorganic materials like glass, ceramic, carbon, and min-

eral can be used. Cu, Ni, Zn, Sn, Pd, Pb, Co, Al, Mo, Ti, Fe, SUS304,

SUS430, 30Cr, and Bs metals can be deposited. After the vacuum depo-

sition occurs, Cu-Sn, Cu-Ni, Ni-Cr, Fe-Zn, Mo-Pb, and Ti-Pd compos-

ite film can also be deposited. The organic base can be removed in the H2

reductive atmosphere and then followed by sintering. Meanwhile, the

strength and ductility of the porous metal can be improved. The sintering

temperature is 300 �C–1,200 �C, and sometimes the sintering process can

be omitted depending on the application.

Ambient Vapor DepositionAnother way to prepare porous metals is to evaporize the metal vapor in an

inert atmosphere. The vapor coagulates to form the porous structure, like

the physical vapor deposition after resistance evaporization [41]. In this

way, the metal evaporizes slowly in the inert atmosphere (102 – 104 Pa)

and the evaporized metal atoms collide with the inert gas molecules, scatter,

and lose their kinetic energy. This fact is demonstrated by the temperature

decrease seen in the metal vapors. The metal atoms will coagulate into clus-

ters before arriving at the base, and the “metal smoke” is observed for these

clusters. The reduced-temperature clusters are deposited on the base by the

carrier of the inert gas from the effect of gravity. The metal smoke particles

pile up loosely due to the difficulty in migration and diffusion from the low

temperature, and then a porous metal foam forms.

This preparation method gives the metal foam a high porosity and a sub-

micro structure for the effective restriction of the superheated electrons in

the experiment of “effectively restrain inertial confinement fusion (ICF)

laser.” In this experiment, the superheated electrons were transformed from

laser energy via the radiation of a high-intensity laser on the target. The elec-

trons heated the target materials to reduce the ablated compression quality,


affecting the conversion rate of energy. Therefore, the low-density metal

foams with large numbers of atoms are accepted as the target to reduce

the preheating depth effectively and increase the conversion efficiency. It

has an extremely low density (only about 1% of the solid metal, reaching

0.5% at its lowest point). It is composed of a large number of sub-micro

metal particles and pores that is similar to the foam.

Flat targets of Au and Pb foams with relative density of 1%–10% are pre-

pared in the United Kingdom by this method. In the United States, the tar-

get of Au foam was also prepared, and then it was deposited on the organic

micro balls and applied to the ICF experiment successfully.

The working principle of the machine for the preparation of Au, Cu, and

Al foams at the Chinese Academy of Engineering Physics in Mianyang is

shown in Figure 2.19. The main body for the facility is composed of a

Diffusional pump

Voltageregulator

Transformer

V

AK1 K2

K3 K5

K6

K4

Resistance monitor

Ionizing vacuumgauge

Evaporationsource

Electrode

Platform

Vacuum chamber

Ar

Gas duct

Wide rangevacuum gauge

220 V

Mechanicalpump

Figure 2.19 Principle of the facility for sub-micro metal foam preparation withevaporization and coagulation.

72 Porous Materials

JK-300 high-vacuum unit with an evaporization and coagulation system in a

glass cover, together with other parts like the 500A/10 V output trans-

former, the 5 kW booster (which controls the current), the mutual induc-

tance, and the ammeter. The pilot gas system consists of the stainless steel gas

tube, barometer, flowmeter, and micro-adjustable valve (which regulates

the gas flow). The inert gas pressure in the vacuum chamber is measured

by the digital vacuometer with a range of 10-1–105 Pa.

The influencial factors for the formation of metal foams include the metal

types, heating power, inert gas pressure/flow, evaporization source, heater

type, distance between heating source and substrate, and substrate materials.

The heating power, inert gas pressure, and flow are the most important of

these. Sub-micro, low-density metal foams can be prepared only with the

matching parameters.

The metal foam structure is formed by the nonequilibrium solidification

of the metal vapors. The dendrite will be formed via nonequilibrium solid-

ification with the abrupt thermal gradient. The growth of dentrites with

radial symmetry can form a fluffy structure in the shape of a snowflower.

Much fluffier structured foams are formed from the direct transformation

of gas to solid, together with the higher thermal gradient, concentration gra-

dient, and much lower crystallization rate.

The Al foam is considered stable since there is no presence of alumina or

absorbed oxygen in the Al foam by the spectrum and electron microscopy

examination. The Al foam can be formed only with an inert gas (Ar) pressure

of 102–104 Pa. There is an analogously inverse relationship between the

foam density and the pressure in the range of 102 – 103 Pa.

The cooling rate of the metal vapor depends on the inert gas pressure

when a certain heating power is fixed. The porosity increases with the flow

rate due to the increased thermal gradient around the vapor source, and the

metal foam of lower density can be obtained. While increasing the inert gas

pressure above 104 Pa, the Al metal smoke overflows from the sealed area of

the bell cover and then leads to increased density and decreased film thick-

ness. No “metal smoke” will be formed when the inert gas pressure is lower

than 102 Pa.

The electrical resistivity and the optical absorption coefficient can be

increased more for the metal foams prepared by this method than for solid

metals. Therefore, the absorbability and chemical activity increase and then

are widely used as gas-sensitive materials, temperature-sensitive materials,

molecule sifter, catalyst carriers, wave absorption materials, and electron

emission materials.


2.6.2 ElectrodepositionPrinciple and ProcessingThe metals are plated electrically on the open-cell polymer foam substrate

from the metal ions in the electrolyte, and porous metals are finally obtained

by removing the polymer [5]. Therefore, similar to the investment casting

process, there is no real foaming of the metals.

Currently, the large scale of metals with high porosity can be prepared by

using this process with features like high porosity (80%–99%), uniform pore

distribution, and interconnection of the pores. It takes the open structure as

the base, and usually 3-D reticulated organic foams are used. For example,

organic foams could be polyurethane (including polyether and polyester

series), polyester, vinyl polymer (such as polypropylene or polythene), vinyl

and styrene polymers and polyamide, and fiberfelt materials. The major pro-

cesses include pretreatment of the base, electric conduction treatment, elec-

tric plating, and reductive sintering (see Figure 2.20).

The pretreatment is performed with the alkali (acid) solution to remove

the oil, roughen the surface, eliminate the closed pores, and then clean the

surface before the electric deposition. The electric conduction treatment is

conducted on the organic foam substrate. If the substrate is electrically con-

ductive, this procedure is omitted. The electric conduction can be treated by

evaporization plating (resistance heating), ion plating (arc ion plating),

sputtering (magnetron sputtering), chemical plating (Cu, Ni, Co, Pd, Sn),

conductive gluing (graphite colloid, carbonlolloid), conductive resin coat-

ing (polypyrrole, polythiophene), metal powders (Cu, Ag), and slurry coat-

ing [42–44]. The mostly used treatments are chemical plating and coating

with conductive glue. The flat micro-carbon particles used in the glue

can be overlaid on the synthesized resin frameworks to ensure that the plane

maintains contact between each particle and then forms a conductive layer.

Polymer Coating Metal

Polymer foam ElectrodepositionRemoval ofpolymer

Addition ofconductivecoating

Figure 2.20 The processes of using electrodeposition for making metal foams [5].

74 Porous Materials

The plated metal with little defects on this overlaid surface is smooth and has

a uniform thickness. Therefore, the porous metals of the 3-D reticulated

structure can be obtained with high tensile strength and bending strength.

If chemical plating is used, the oil removal, roughening, sensitizing, activat-

ing and reduction processes are performed. More detailed information shall

be referred to related documents about plastic electroplating technology.

There are some special advantages for the conductive treatment of the 3-

D reticulated substrate by painting the metal micro-powders with a binder.

The resistance is small, and a large current can be applied in the plating pro-

cess. Moreover, the plated layer cannot be burned out in the sintering

process, while it composes the main part of the porous metal frames. The

presence of this layer not only reduces the plating thickness but also saves

plating time. The metal micro-powders can be Ni, Cu, Ag, Al, Au, Fe,

Zn, Sn, P, Cr, Pb, or mixtures of these. The activation is applied to the

metals that are easily oxidized to form nonconductive oxide film. Themetals

with intrinsic high resistance need to be substituted to reduce resistance.

Another treatment that can be used instead of activation and substitution

is to mix the metal powders with more conductive, softer powders like

Au, Ag, or Cu, in the ball miller and then increase the conductivity of

the metal powders.

A conductive macromolecule layer can be formed on the surface of the

porous polymer by the chemical oxidation polymerization process, and then

it is electroplated. The monomer for the chemical oxidation polymerization

can be pyrrole, thiophene, and furan pentacyclic compounds and their

derivatives. The inorganic acids or metal compounds can be used as the oxi-

dants in the chemical oxidation polymerization. The inorganic acids are

hydrochloric, sulfuric, and nitric acids, and the metal compounds are chlo-

rides, sulphates, and nitrates. If there is no special restricition for the solvent

(i.e., the solvent is not seriously corrosive to the macromolecule materials),

water is generally used. In chemical oxidation polymerization, the porous

macromolecule materials make contact with the oxidants in solution, and

then the compounds are provided for the chemical oxidation polymeriza-

tion. Due to the contact with oxidants, a conductive macromolecule layer

with thickness of 1 μm to tens of micrometers is formed on the resin parts.

The ester polyurethane foam is dipped in the iron chloride solution, and a

pyrrole polymer layer is formed on the resin, which makes contact with the

pyrrole vapor. Compared to chemical plating, the electric plating has advan-

tages like no pretreatment before plating, fast growth rate for the plated

layer, easy fabrication, and greater mechanical strength of the coating.


Polymerization can also be used [45]. In the abovementioned method,

the organic mcaromolecules are dipped in the monomer solution or vapor

to form the conductive polymer with catalysis; whereas in the polymeriza-

tion method, a conductive polymer suspension is prepared and then organic

macromolecules are dipped in the suspension to finish the conduction treat-

ment. For the conductive polymer, there are no other restrictions except

that it can be removed in heating and that it can be those like polyaniline,

polypyrrole, polythiophane, polyfuran, and their alkyl-, alkoxy-, and

phenyl- derivatives.

Electric plating can be conducted via the traditional electroplating pro-

cess. However, for electric plating of porous bodies, there will be a shortage

of metal ions in the inner layer from the polarization with increasing the cur-

rent density. The pulse current can be used to reduce the polarization. Dur-

ing plating, the solution in the pores is the same as that the outer solution in

the plating bath due to the diffusional effect when the current is off. When

the current is switched on again, the concentration of ions in the pores will

decrease to the point where it impedes the plating process. If a pulse current

is applied, the consumed ions are supplied again during the interval and then

efficiency improves. The polarization is reduced to form a uniformly plated

layer. The spraying of electroplating liquid can also reduce polarization by

decreasing the concentration difference both inside and outside the pores.

The fresh porous materials can be put on the plated porous materials to make

the pores in the fresh one lack tension and deformation, as well as increasing

the current density.

Continuous eletroplating is used by companies with large production

capacities. Different porous metals or alloys can be prepared by changing

the liquid metals, such as Ni, Cr, Zn, Cu, SN, Pb, Fe, Au, Ag, Pt, Pd,

Al, Cd, Co, In, Hg, V, Tl, and Ga [43], and metal alloys, such as brass, bro-

nze, Co-Ni alloys, Ni-Cr alloys, Cu-Zn alloys, and others. A special solution

can be used for themetals that cannot use the aqueous solution. For example,

the plating of Al and Ge can be electrolyzed by using organic solutions or

molten salt solutions.

The porous metals can be either decomposed and sintered in the reduc-

tive atmosphere with electroplated porous composites, or reductively

sintered in a reductive atmosphere after burning off the organic solution

in air. The thermal decomposition temperatures are determined by different

organic substrates while considering the upper limit of the melting point of

the metals for plating. The reduction temperatures are selected based on the

oxide types and the annealing for the plated metals, also considering the

Figure 2.21 The electroplated Ni layer:* (a) cross-section morphology; (b) surfacemorphology.

76 Porous Materials

upper limit of the melting point of the metals. For polyurethane-based

porous materials, the decomposition temperature is in the range of

400 �C–700 �C (with the optimal range being 600 �C–650 �C), and the

reduction temperature is 700 �C–1100 �C [45]. The warping increases if

the substrate thickness is smaller than 3 mm, so the superposition of the sin-

gle substrate is needed to reduce warping.

The Ni foams for electrode application were prepared using porous

polyurethane plastics as the substrate via the electrodeposition method

[46]. A carbon-based conductive glue was used to conduct electricity with

the conventional Ni plating process. Ni plating with a fine and regularly lay-

ered structure was obtained (see Figure 2.21).

In the two-step process of burning organics and sintering, an Ni-plated

body is pre-heated at 600 �C in air for 4 min and then a thin layer of NiO is

formed on the surface. A coarser layer is left behind due to the outward dif-

fusion growth of Ni in NiO (NiO is a negative semiconductor oxide with

metal deficiency), as shown in Figure 2.22b. The NiO layer will be reduced

to Ni after sintering in the amminia-decomposed reductive atmosphere at

850 �C–980 �C after the organic base has burned off. A Ni foam product

with increased grain size, dense structure, smooth surface, and no oxide res-

idue forms after 40 min of heat treatment (Figure 2.23).

In the one-step process of sintering after plating, there is no formation

and reduction of NiO, but thermal decomposition (formation of gas

CH4, H2O) and the reduction of carbon (formation of gas CH4, C2H6) do

take place. The others are the same as that of the two-step process, and an

Ni layer with similar structure and morphology is formed (Figure 2.24).

Figure 2.23 The Ni layer after reduction and sintering of preheated: (a) cross section;(b) surface morphology.

Figure 2.24 The Ni layer formed after reduction and sintering of electroplated Ni:(a) cross section; (b) surface morphology.

Figure 2.22 The Ni layer after preheating at 600 �C for 4 min: (a) cross section; (b) sur-face morphology.


Figure 2.25 Ni foam prepared by electroplating: (a) the whole 3-D reticulated structure;(b) hollow cross-section structure.

78 Porous Materials

The final product is the 3-D reticulated porous body after these two processes

(Figure 2.25a) with a triangle hollow cross section (Figure 2.25b), in which

the hollow is formed by organic decomposition.

The electroplated Ni layer on the organic porous body has a fine struc-

ture, but it has some obvious defects. The fine structure does not change

with the formation of a NiO oxide file at 600 �C for 2 min. A dense, smooth

Ni layer forms with grown grains and a stable structure after sintering in an

ammonia atmosphere at 980 �C for 40 min. The Ni layer with a similar

structure can be formed after sintering at 850 �C, and it is enough to obtain

the Ni layer by sintering at 850 �C for 40 min. The 3-D reticulated Ni foam

products with a hollow structure have good physical and mechanical

properties.

One study [47] used a self-made delicate noncontact-type extensometer

to measure accurately the ultimate tensile strength, yield strength, and

Young’s modulus for an open-cell nickel foam with an average pore size

of 600 μm. This kind of extensometer can completely avoid any minor

deformation that might be caused by the attachment of a conventional

extensometer to the sample’s surface prior to testing, and this function is

based on the use of a laser camera that detects and records the dimensional

changes as soon as the load is applied.

The Ni-Cr alloy foam can be prepared by the alternate plating of Ni and

Cr and followed by heat treatment with diffusion of Ni and Cr in the plated

layers. The obtained Ni and NiCr foams have plate thicknesses of 2–20 mm,

and densities of .4–0.65 g/cm3 that are independent of the average pore size

of the foam [5].


Some plated porous bodies can be overlapped to achieve the required

thickness and then electroplated again to increase pore uniformity. The

porous metals with uniform structures can also be prepared by overlapping,

then sticking the individual porous body together, finally by vacuum plating

or by spraying after the conduction treatment [43–45]. The sticking

methods include fusion of the surface with a flame, with a binder and a

hot adhesive. Porosity may be impaired by the film formed by the binder

or adhesive, and so flame fusion is the best way to achieve good porosity.

Ni Foam PreparationThe preparation and application of Ni foam was reported 40 years ago

in the United States, and further research and development was conducted

in Japan. With the successful application of Ni foam to the electrodes

in alkaline batteries in the 1980s, its industrialization was accelerated [44].

It is the most common method used to prepare Ni foam by electrode-

position, achieving high quality at a reasonable cost [48,49]. Polyurethane is

used as the substrate for the plating, and it needs to be degreased in a chem-

ical way; that is, cleaned in a solution of NaOH (30–40 g/L), Na2CO3 (15–

20 g/L), Na3PO4 (30–40 g/L). A small amount of detergent is added to the

solution, and then it is heated to 40 �C–50 �C to facilitate emulsification.

Another way is to soak the substrate in the water solution of acetone

(1:4) for 5 min to eliminate surface tension and degrease and flatten the

surface.

A degreasing process was recommended [48] in which the polyurethane

substrate was soaked in a 20% xylol solution for 30 min and then degreased

in a solution of NaOH (25 g/L), Na2CO3 (25 g/L), Na3PO4 (25 g/L), and

OP emulsifier (25 g/L) at 60 �C– 80 �C for 10–30 min. Some closed pores

will be opened by degreasing, and the hydrophilic groups can be formed on

the roughened pore surface that is advantageous to plating. Different

roughing solutions need to be used for different plastics, and the most effec-

tive factors are the composition, concentration, roughing temperature, and

time [48]. A polyether polyurethane base can be corroded with a strong oxi-

dant in acidic conditions to wet the surface and increase the adherence of the

plating through the micro-traces on the surface [49]. For example, roughing

can be achieved with a solution of CrO3 (30 g/L) and H2SO4 (20 mL/L) at

30 �C for 3 min. Water cleaning is performed after roughing for the next

sensitization.

The purpose of sensitization is to absorb a reductive layer of metal ions

(Sn2+) on the surface [48]. There are SnCl2, hydrochloric acid (HCl) and Sn

80 Porous Materials

bars in the sensitizing solution. The presence of HCl is to maintain the sta-

bility of Sn2+ and control the hydrolyzation of SnCl2:

SnCl2 +H2O! Sn OHð ÞCl # +HCl (2-10)

SnCl2 + 2H2O! Sn OHð Þ2 # +2HCl (2-11)

The undissolved resultants after hydrolyzation deposit on the surface and

act as the absorbing layer for the next activation process. The presence of Sn

bars can prevent the oxidation of SnCl2 to Sn4+ in air by this reaction:

Sn0 + Sn4+ ! 2Sn2+ (2-12)

The sensitizing condition in [49] is as follows: a solution of (SnCl2 8 g/L)+

(HCl 20 mL/L) for 4 min. In [47], it is (SnCl2 15 g/L)+(HCl 20 mL/L)+

(Sn bar) at 40 �C for 3–5 min.

The activation is a process that forms a catalysis metal layer of Ag, Au, Pt,

and Pd on the surface. Ag and Pd are the most widely used metals, and they

have the following reactions [48]:

2 Ag NH3ð Þ2� �+

+ Sn2+ ! 2Ag+ Sn4+ + 4NH3 Ag for the activation centerð Þ(2-13)

PdCl4ð Þ2� +Sn2+ ! Pd+ Sn4+ + 4Cl� PdCl4ð Þ2� for the Pdnucleus� �

(2-14)

The Pd activation process is conducted in a solution of 0.4 g/L+5 mL/L

HCl for 2–5 min at 25–40 �C.If the sensitization and activation is conducted simultaneously, the pro-

cess can take place in a solution of [(PdCl2 0.25 g/L)+(NaCl 250 mL/L)+

(SnCl2 0.5 – 5 g/L)+(Na2SnO3 0.5 g/L)+(HCl 10 mL/L)+(Urea50 g/L):

(pH 0.7 – 0.8)] for 6 min [45].

The formed Pd has colloid features and can absorb the Sn ions. Then

peptization is conducted to facilitate the Pd deposition on the surface by

cleaning in 100 ml/L HCl or soaking in 50 g/L NaOH for 1 min. If cleaned

in 3% sodium hypophosphite, the water-cleaning process need not be con-

ducted before the chemical Ni plating [48]. Some other results [49] indicated

that the substrate can be chemically plated without the Pd activation process.

The chemically plated Ni is actually an amorphous or multicrystalline

Ni-P alloy, and the plating liquid can be Ni2SO4 20 g/L+Na2PO5


(30 g/L), sodium citrate (10 g/L)+ammonium chloride (30 g/L). The pH

value is adjusted to be 8.5–9.5, the plating is conducted in the liquid at

35 �C–45 �C for 3–5 min. The liquid can also be [(CuSO4 �5H2O 10 g/

L)+(NiCl2 2 g/L)+(C4H4KNaO6 50 g/L)+(NaOH 8 g/L)], and the plat-

ing time is 30 min.

The electroplating then can be conducted after the conduction treatment,

and no brightening agent is used in the plating liquid. The liquid is [Ni2SO4

(250 g/L)+Na2SO4 (30 g/L)+NaCl (10 g/L)+Mg SO4 (40 g/L)+HB

(35 g/L)] with pH value of 5–5.5. The plating is conducted at 20 �C–35 �Cwith a current density of 0.8–1.5 A/dm2 and the time depends on the required

areal density. The electroplate liquid with composition of [NiSO4 (250 g/L)

+NiCl2 (40 g/L)+H3BO3 (40 g/L)+C12H25 –<benzene ring>– SO3Na]

also can be used. The plating temperature is 55 �C, and the current density is2.0 A/dm2 [49]. The anode is Ni sheet or plate. The liquid should be filtered

regularly, and the compositions also should be adjusted.

Cu Foam PreparationCu foam has good electrical conductivity and ductility, so it is used as elec-

trode materials in batteries. However, the application of Cu foam is limited

due to its poor corrosion resistance compared to Ni foam. The preparation

process is as follows: (1) soaking in polyurethane (with a thickness of

2.4 mm); (2) cleansing; (3) roughing; (4) a second roughing; (5) sensitization;

(6) activation; (7) chemical deposition; (8) electrodeposition; (9) burning;

(10) thermal reduction; (11) Cu foam is created [50]. The two-step roughing

process, with the first step involving [(KMnO4 8.0 g/L)+ (H2SO4(d¼1.84)

5.0 mL/L)] for 10 min and the second step involving [(CrO3 3 g/L)+

(H2SO4(d¼1.83) 4 mL/L)] for 24 h, is effective. A water film can be

absorbed easily on the rough surface.

The SnCl2 solution is used as the sensitizer after roughing with the com-

positions of [(SnCl2 �2H2O 20 g/L)+(HCl(36%) 40 mL/L)]+Sn powder.

The porous body is soaked in the sensitizer for 5 min and then rinsed with

flowing water. The resulting reactions are:

SnCl2 +H2O! Sn OHð ÞCl # +HCl (2-15)

SnCl2 +H2O! Sn OHð Þ2 #HCl (2-16)

Sn OHð Þ2 + Sn OHð ÞCl! Sn OHð Þ3Cl # (2-17)

82 Porous Materials

The Sn2(OH)3Cl is in the form of gel, which has little solubility in water,

and an absorbed layer of Sn2(OH)3Cl on the porous body plays a key role in

the chemical deposition within this layer.

The sensitized surface can absorb a layer of metal particles with a cata-

lyzing effect, and then it can be the catalysis activation center for the chem-

ical deposition. The activating liquid is [(PdCl2 0.2 g/L)+(HCl(36%)

1.0 mL/L)], and it takes 5 min to produce the following reaction:

Pd2+ + Sn2 OHð Þ3Cl!Pd # +Sn OHð Þ3Cl # +Sn2+ (2-18)

Sn(OH)3Cl is still a gel, and the formed simple Pd is absorbed into the gel

layer. The activating liquid can be used repeatedly.

Pd is covered by the gel layer, and this layer should be removed

before the chemical deposition to expose the Pd atoms. A 3 mL/L HCHO3

(pH 8–9) HCHO water solution can be used to remove the gel at room

temperature in 5 min.

HCHOcan be a reducer on the catalysis-activated surface, and a Cu layer

is then deposited on the surface of the porous body:

Cu2+ + 2HCHO+4OH� ¼Cu # +H2 + 2HCOO� +2H2O (2-19)

Due to the catalysis effect of the newly formed Cu by the reaction in

Eq. (2-19), the deposition can be continued until the required thickness

is reached. The liquid is prone to decomposition, and then the stabilizer

need to be used; CuSO4 and HCHO are added after the stabilizer has

been used 10 times. The optimal liquid is [(CuSO4 �5H2O 12.0 g/L)

+(EDTA 42.0 g/L)+(Na2SO4 20.0 g/L)+(stabilizer 4.0 g/L)+(HCHO

20.0 mL/L)], with a pH of 12.5–13.0. The deposition time is 10 min,

and the temperature is 25 �C.The solution of [(CuSO4 �5H2O 70 g/L)+(NaCl 0.60 g/L)+(polyeth-

ylene glycol 0.03 g/L)+(sodium dodecylsulfate 0.05 g/L)+(H2SO4

25 mL/L)] is used to deposit the Cu layer with a uniform structure and duc-

tility. The current increases while the voltage decreases within the intial

2–3 min of plating due to the depositon of the Cu layer with poor electrical

conductivity, followed by improved electrical conductivity and the forma-

tion of crystallized Cu.

Therefore, a short period of pre-plating until stable electrical conduction

occurs is necessary for the preparation of the uniformly structured products.

For example, preplating is conducted for 3 min with 5 V to reach an opti-

mum current density of 0.3 A/cm2 for 25 min of electrical deposition of Cu.


If the current density is higher, H2 will be released to reduce efficiency;

while if the current density is lower, the needed deposition time will be lon-

ger. The deposition temperature has some influence on the internal stress of

the layer, the dispersivity of liquid, and the deposition rate. Increasing the

temperature will lead to reduced internal stress, no cracking, and an

increased deposition rate, but low dispersivity. Therefore, the temperature

of 40 �C–50 �C is selected for the deposition of Cu.

Based on this discussion, the best parameters for the electrodeposition are

pre-plating at 5 V for 3 min, followed by deposition with a current density

of 0.3 A/cm2 at 40 �C–50 �C for 25 min. The organic foam in the Cu foam

can be eliminated by burning it off after drying. The surface color of Cu

foam will become dark with the formation of CuO after burning off, and

the Cu foam is then reduced in N2/H2(1:3) at 700�C to increase the

strength and ductility. With increasing the reduction temperature till

700 �C, the tensile strength increase accordingly, while it will decrease withcontinue increasing temperature above 700 �C.

2.6.3 Reaction DepositionIn reaction deposition, an open-cell porous body is put into a container with

the gaseous metal compounds and then heated to the decomposition tem-

perature of the metal compounds. At that temperature, the decomposed

metals will be deposited on the porous body to form a layer of metal. Finally,

a porous metal is produced by sintering the open-cell metal coated porous

body. For example, Ni foam with hollow sectional threads can be prepared

by using nickel carbonyl [51].

This process can be realized by an effective thermal decomposition reac-

tion at low temperatures. Nickel carbonyl is one kind of gas, and the reaction

formed is Ni+4CO!Ni(CO)4. Ni and CO can be decomposed at tem-

peratures of above 120 �C. A solid Ni layer is plated when porous plastics

pass through nickel carbonyl gas. If the process is repeated, the Ni layer will

become thicker. If heated by the infrared ray, the polymer base will be stable

at the decomposition temperature of the nickel carbonyl. A hollow reticu-

lated metal is obtained after the substrate is removed by heat treatment or

chemical reatment.

2.7 HOLLOW BALL SINTERING

Porous metals prepared by the hollow ball method have the following

features: low density, good energy absorption, thermal exchange capability,

84 Porous Materials

and a high ratio of strength to weight [52]. These kind of porous materials

are different from other traditional open- or closed-cell metal foams, in that

the closed pores take a certain volume fraction and there are gap pores

between each sintered ball. Therefore, these metals feature a mixture of

pores with open and closed cells.

2.7.1 Preparation of Hollow BallsThe hollow balls made from different kinds of metals and alloys, like the Ti

alloy, Ni alloy, and stainless steel, are prepared by the slurry and gas atom-

ization methods. Strict controls can be executed in the process.

Slurry MethodSlurry injection for the preparation of metals, intermetallics, and metal

hydride powders was performed by the coaxial nozzle-injecting process

by taking advantage of surface tension to form the balls. Alternatively, the

polystyrene balls were sprayed and coated with the metal hydride slurry.

The semi-dense hollow balls can be obtained by either process, and metal

hollow balls then are obtained after heat treatment [52]. The nominal

particle sizes of the hollow balls are 1 μm for both processes, and the

semi-finished products become fully dense metals by reduction in the H

atmosphere at a high temperature. The metal balls then were sintered

together, either after or during reduction.

When the polymer balls are taken as the carrier, the slurry can be a sus-

pension of the binder and metal powders, or the balls are deposited with any

kind of metals by chemical and electic deposition. Finally, the balls are bur-

ned to obtain a dense metal shell by removing the polymer [5].

The microballs of metals, intermetallics, and metal hydrides prepared by

the coaxial nozzle injection can be dried in the burette and then sintered [5].

A low-oxygen partial pressure is needed in the annealing process for the

transformation of iron oxide to stainless steel balls [14].

AtomizationThe hollow balls can also be formed during molten metal atomization by

using the proper parameters. Ar can be captured in the liquid drops during

the atomization of the metals and alloys. During the throwing process of the

liquid drops, Ar will expand untill the drops solidify, and then the hollow

powders are obtained [14]. The hollow balls can be separated from the pow-

ders by the floatation method.

Coating

Green spheres

Fluidized bed coating

Styrofoam spheres

Metal powderand binder suspension

Shaping Debindingsintering

Figure 2.26 Preparation process for hollow-ball porous metals [5].


2.7.2 Preparation of Porous BodiesThe hollow balls can be combined by isostatic or liquid sintering [14]. This

process is very important since the property of the porous body is sensitive to

the amount of the contact area for each ball.

The hollow balls of Cu, Ni, and Ti metals and their alloys with high

porosity can be obtained by the sintering process shown in Figure 2.26

[5]. The light porous materials made by the hollow balls have pores with

open and closed cells; porous materials with open-cell pores are obtained

by sintering the agglomerated hollow balls, and the sintered neck can be

generated on the neighboring balls. When the force is applied to the balls

during sintering, the balls can deform into the shape of polyhedrons lead

to the increased contacting surface area and also reduced open-cell porosity.

The contact of the balls can be improved by using a binding slurry.

The closed-cell pores can be obtained by filling the gap between the balls

with metal powder and then sintering. Liquid metals also can be used to fill

the gap, and it is better to exert force and preheat the balls to make sure that

the gap is totally filled. For the preparation of the closed-cell pores, it is

enough to use thin-walled balls. The sandwiched structure can be obtained

by sintering a hollow ball between two plates; both the combination of the

balls and the combination of balls and plates are included.

One of the advantages for the hollow structure is the nonrandom distri-

bution of the pore sizes, and the pore sizes can be adjusted by proper selction

of the hollow balls. The mechanical and other physical properties of the hol-

low balls are more predictable than those of “actually obtained porous

86 Porous Materials

materials” with randomly distributed pore sizes. Another advantage is that

the hollow ball process is applicable to all materials prepared by the powder

metallurgy process, like superalloy, Ti alloys, and intermetallics. Therefore,

the hollow balls can be used at high temperatures.

2.7.3 Fe-Cr Alloy Porous ProductsThe balls with an outer diameter of 2 mm are coaxial nozzle-injected with

metal hydride slurry by using Fe- and Cr oxide powders, and then returned

to a semi-dense state as they fall [52]. After that, the balls turn into FeCr

base alloys with a porosity of less than 2% after being reduced in hydrogen.

The obtained metals actually are 405 ferritic stainless steel (Fe-12Cr) after-

reduction, with a grain size of 10–20 μm. The grain size is much smaller than

the wall thickness (0.1 mm), and then it can be completely reduced. Due to

the effect of gravity and inertial forces in the coaxial nozzle injection process,

the variation of the wall thickness is about 50%.

The reduced balls were then poured into the mold vibrationally and

sintered at 1,350 �C for 48 h until full bonding from the diffusional effect

is reached. The density of the porous body is 1.4 g/cm3 and corresponds

to a relative density of 0.16.

2.8 PREPARATION OF THE DIRECTIONALPOROUS METAL

2.8.1 Solid-Gas Eutectic Solidification
Solid-gas eutectic solidification, also known as GASAR, is a newly devel-
oped technology for the preparation of porous metals. It is a casting

method [5,53]. In this process, the liquid metal with the dissolved gas

(H2) solidifies at the eutectic temperature (with a low co-melting point),

and H2 is separated due to low solubility of solidification. Then the metal

solidifies and the pore’s nucleus forms simultaneously at the eutectic tem-

perature. The eutectic temperature depends on the system pressure, and

the porosity can be adjusted by controlling the H2 pressure in the cast cav-

ity. The temperature has a great effect on the H solubility in the liquid

metal, so the melting temperature and the H pressure before solidification

must be adjusted to match the dissolved H in the melt at the eutectic tem-

perature. If the process is not performed with the proper temperature and

pressure, and not at a eutectic temperature, some deputy eutectic phases

will be formed with nonuniform microstructures. The pore size can be


adjusted by the cooling rate, and small pores will be formed with an

increasing cooling rate to reduce the H diffusional distance. GASAR

has been used to produce porous metals like Ni, Cu, Mg, Al, Mo, Be,

Co, Cr, W, bronze, steel, and stainless steel with a pore size of 5 μm–

10 mm and porosity of 0.05–0.75. The porosity depends on the solubility

of H in the melt, and the pore size depends on the diffusional coefficient of

H in the melt. A single cast can be produced with alternate overlapping of

dense and porous layers by GASAR. Sandwich-structured metals with

honeycombs or foam cores can be produced without bonding, and they

can be used as transportation parts due to its high shock resistance and

energy absorbing ability.

GASAR can be performed in the sealed pressure vessel [8,53]. The gas can

dissolve into the molten metal to some degree, and the solubility will increase

with increased pressure and temperature. The metals and gas will go through

the eutectic solidification process and then form metal foam when the gas

reaches its solubility limit. The isotropical and anisotropical metals with high

porosity and different pore structures can be developed by controlling the sys-

tem pressure, cooling rate, and thermal gradient direction (radiation direction)

[8,54]. For example, the pores with longitudinal or radial arrangement can be

prepared with cooling of the mold [54] from the bottom or side. The porous

honeycomb structure, similar towood, can be obtained by arranging cylinder-

like pores with a high shape ratio (i.e., the ratio of height to width, which is

about 10 in this case); see Figure 2.27. Its bending rigidity is as high as that of

the highest-quality engineering materials.

A uniform melt filled with hydrogen can be obtained when the eutectic

metal system is melted in a hydrogen atmosphere with a pressure of 50 atm

[5,55]. The melt can transform into the heterogeneous two-phase (solid+

gas) system through the eutectic reaction that takes place when the temper-

ature decreases. The precipitation reaction occurs at a certain temperature

when the system compositions are close to the eutectic position. The outside

pressure must be compatible with the H content since the eutectic compo-

sitions depend on system pressure. If the heat is removed in the designed

solidification direction, directionally solidified materials can be obtained.

The advancement of the solidified frontier has a rate of 0.05–5 mm/s,

and bubbles will be produced just before the frontier (when hydrogen goes

out of the solidified metal). The selection of the processing parameters is

done to ensure that the bubbles float out of the liquid and are kept in the

solidified metal [56]. The pore morphology strongly depends on the H con-

tent, pressure above the melt, radiation direction, rate, and the chemical

Figure 2.27 Cross section of a porous product with cylindrical pores prepared byGASAR and the natural porous body [54]: (a) transverse sectional porous Cu product(with relative density of 0.84); (b) longitudinal section of porous Cu; (c) transverse sec-tion of Norway spruce (with relative density of 0.3); (d) longitudinal section of Norwayspruce.

88 Porous Materials

compositions. Generally, the elongated pores depend on the radiation direc-

tion and show the shape of round in the vertical direction. The pore size is

10 μm–10 mm, the pore length is 100 μm–300 mm, the morphology ratio is

1–300, and the porosity is 5%–75% [55]. The pore size range is so wide due

to the merge of small and big pores. Sometimes the pores can take the shape

of a taper or ripple.

A practical GASAR facility is shown in Figure 2.28 [5]. Hydrogen is

filled in the pressure jar for the melt, and directional solidification is realized

at the last stage of preparation.

The hydrogen tends to diffuse in the pores before the frontier of solid-

ified metal, and the pores are elongated in the direction of the solidification

[57]. In a single-directional cooling process, ball-like pores can be produced

through the pulse of pressure in the cast cavity. Materials with gradient

porosity and alternating solid/porous layers can be produced by changing

the processing parameters. The pores that may be produced are shown in

1

2

3

4

5

6

7

8

9

10

11

Figure 2.28 Schematic showing of the GASAR facility [5]: 1—gas supply; 2,4—melt; 3—mold; 5—hot sink; 6—solid/gas eutectic solidification; 7—outer wall for compression;8—internal cavity; 9—heating part; 10—insulator; 11—funnel.


Figure 2.29, which demonstrates that pores can be in the grains and along the

grain boundaries. The pores occupy more than 50% of the surface of the

grain boundaries and are not used to evaluate porosity. The elongated grains

and pores are typical in the directional cooled metals. Moreover, the grain

sizes for porous copper are notably smaller than that of solid copper.

2.8.2 Directional SolidificationThe directionally solidified porous metals were prepared successfully with

H2, O2, and N2 as foaming gases in the 1990s [58–60]. The pore distribution

is quite uniform, and the pores have a radius of 10 μm–10 mm with length

not above 80 mm and porosity of not over 80%. It has special properties that

are different from that of sintered bodies and foamed metals.

The principle for the directional solidification is similar to that of the solid-

gas solidification, and it is basedon thegas solubilitydifference in themelt and in

the solidmetals. Themain difference is that there is no requirement for a eutec-

tic system in directional solidification and no precipitation at eutectic temper-

atures during solidification compared to solid-gas eutectic solidification. The

gas will exert pressure on the molten metals due to the solubility difference

of gas, and this will increase the solubility of gas atoms in the melt. Finally,

cooling is applied to make the melt solidify in the designated direction. The

gas atomswill beoversaturatedwith reduced solubility in themelt during solid-

ification, and then they escape in the formof bubbles.During the growth of the

(b) (c)

(d) (e) (f)

(a)

Figure 2.29 Different pore morphologies by GASAR [57]: (a) spheric pores; (b) radialpores; (c) cylindrical pores; (d–f) overlayer of solid/porous body/solid (left to right,the spheric pore, the radial pore, and the cylindrical pore).

90 Porous Materials

bubbles, the surface area and interface energy at the bubble/liquid phase

increase. The contact area (interface energy) between the gas bubbles and

the liquidphasedoesnot change if the solidification rate is the sameas thebubble

growth rate. Hence, the bubbles stop growing or even float up from the solid

and can grow only in the solidification direction. That is, the bubbles are elon-

gated and form the cylindrical pores along the solidification [58]. In order to

control the pore shape and number, the proper selection of the melting tem-

perature, the mixture ratio of foaming gas to inert gas, gas pressure, and the

solidification rate is needed. Furthermore, the thermal gradient and impurities

shall be controlled carefully, and the convection of melt must be restricted in

order to avoid the separation and escape of gas bubbles from the solid during

growth, as well as the connection of the bubbles.

For the preparation of porous copper, highly pure copper is melted in the

high-frequency induction furnace with careful control of the pressure of H

and Ar (both partial pressure of H and Ar is in the range of 0–1.0 MPa)

[59,60]. After H is dissolved in the molten copper at 1,523 K for 1,800 s,

the copper melt is poured into the mold with the bottom water cooling

(Figure 2.30), and the melt solidifies upright, in a single direction. H has

Water cooling

Solidification

Mold

Figure 2.30 Directional solidification facility for the preparation of porous metals [59].


a much lower solubility in solids than in liquids, and most of the H in the

copper melt cannot dissolve in solid copper. H cannot stay in the solid-liquid

interface at a constant temperature and then form elongated pores in the

direction of solidification. The obtained cast has a diameter of 30–35 mm

with a maximum height of 80 mm. Figure 2.31 shows the transverse and

longitudinal section structure of porous copper with the shape of lotus root

in the transverse section (by electric spark cutting). The porosity is deter-

mined by measuring the weight and the volume of the sample, while the

average pore radius is measured with the photo analysis system. The porosity

decreases with the increase of the Ar partial pressure under constant H partial

pressure, whereas the porocity increases and then decreases under more H

pressure with constant Ar partial pressure. Pore size, direction, morphology,

and porosity can be influenced by the melting temperature, H and Ar partial

pressure, the pressure ratio of H to Ar, and the solidification rate. Ther-

eofore, all kinds of porous metals can be prepared by directional solidifica-

tion through controlling these parameters.

The strength of porous metals when the pores are aligned in a specified

direction is better than the strength of metals with randomly located pores

[59]. If N is used as the foaming gas and the pore wall is nitrided during the

formation, the metal is stronger than if it is prepared with H or O as the

foaming gas [58]. Additionally, a good dampening effect is demonstrated

for porous metals due to the increased friction and internal deformation

of metals accompanying gas atom diffusion.

Figure 2.31 Transverse (top) and longitudinal (down) sections of directionally solidifiedporous copper [59]: (a) hydrogen with a partial pressure of 0.8 MPa, argon with a partialpressure of 0, porosity 32.6%; (b) hydrogenwith a partial pressure of 0.4 MPa, argonwitha partial pressure of 0, porosity 44.7%.

92 Porous Materials

A further surface nitriding process may be applied to increase the tough-

ness, hardness, and wear resistance for the directionally solidified porous

metals [58]. If N is used as the foaming gas to prepare the porous metals,

good properties will be achieved due to the self-nitriding. Moreover, nitrid-

ing can increase the wear resistance of the porous body with good chemical

stability. Therefore, directionally solidified porous metals can meet the

requirements for use as human bone joints.

2.9 OTHER METHODS

2.9.1 Powder Melting Foaming
The powder melting foaming process is similar to metal powder sintering in
the solid sintering process. The only difference is that the heating


temperature is above the melting point of metals for liquid sintering, while

the solid process is under the melting point of metals for solid sintering.

In this process, metal powders (single metal, alloy powder, or a mixture

of metal powders) are mixed with the particles of the foaming agent and

transformed into a near-dense semi-product. The obtained semi-product

is heated to a temperature above, but close to, the melting point of the

related alloys, and the foaming agent decomposes and releases gas to expand

the semi-product and form the porous materials [5,61]. The final product is

usually a closed-cell foamed body, and the porosity mainly depends on a

couple of key factors, including the content of the foaming agent, heat-

treatment temperature, and heating rate. The metal powders and foaming

agent are mixed with the rolling mixer, and the gas-releasing agents can be

distributed uniformly in the mixture [62]. Densification can be performed

with powder extrusion, axial thermal pressing, powder rolling, or isothermal

static pressing depending on the required shape. It is economical for the

extrusion process, and the sheet can be rolled. The foaming agent particles

must be buried in airtight base metals to prevent the released gas from escap-

ing the connected pores before the expansion, so it has no effect on pore

generation and growth. The foaming agents can also be poured into the hol-

low die in an appropriate shape and then heated to the required temperature,

and the final parts can be manufactured in various shapes.

The time needed to reach full expansion depends on the temperature and

the size of the preformed part and ranges from several seconds to a number of

minutes. TiH2 and ZrH2 can be employed as the foaming agents for Zn and

Al alloys, while SrCO3 is used for steel. The metal hydrides are used as the

foaming agents, and less than 1% should be enough.

Besides the preparation of Al and Al alloys, it can be used for the prep-

aration of Sn, Zn, brass, Pb, Au, and other metals and alloys with the proper

selection of the foaming agents and the process parameters. The commonly

used foaming agent is pure Al or a precision cast alloy such as the 2��or

6�� alloy. Cast AlSi7Mg(A356) and AlSi12 have low melting points

and are also used as the foaming agents. Generally, the pore size distribution

and shape of the obtained products are random.

Compared to Al foam, steel foams have the following advantage [61]:

(1) high strength and high ratio of stiffness to density; (2) low raw material

cost; and (3) compatibility of the melting points to the structural steels. It

can be prepared by the following processes: mixing the commercial steel

powders (Fe-2.5Cr) with the particles of foaming agent, densification,

and melting the densified part at 1,300 �C, which leads to the expansion

of the foaming agent at the heating rate of 30 �C/min. The total heating

94 Porous Materials

time will be 5 min. Due to the big difference in density between the steel

and the foaming agent, it is significantly important to distribute the mix-

tures of powders uniformly. The foaming agent (SrCO3 or MgCO3) of

0.2 wt% and carbon of 2.5 wt% are added to the steel powder to achieve

a better sintered result.

The top priority in this task is to select the proper foaming agent for the

steel foams [61]. The foaming agents shall be decomposed at temperatures

close to Tm (the melting point of the alloy) and release enough gas to ensure

that the foaming pressure is higher than the environmental pressure. TD

(decomposition temperature) shall be between the solidus and liquidus

(1,250 �C–1,350 �C). SrCO3 andMgCO3 can be used as the foaming agents

and they are decomposed as follows:

SrCO3 TD ¼ 1,290∘Cð ÞSrCO3! SrO sð Þ+CO2 gð Þ (2-20)

MgCO3 TD¼ 1,310∘Cð ÞMgCO3!MgO sð Þ+CO2 gð Þ (2-21)

The ideal decomposition temperature TD of the foaming agent shall be

compatible with the melting temperature of the alloyed steels. IfTD is higher

than the melting temperature of the steel, the foaming agent dissolves into

the melt or floats on the surface of the melt, while the preformed part breaks

from the high internal pressure so long as TD is lower than the melting point.

When the requirements are met, steel foam with the required porosity will

be produced by the careful control of heating and cooling.

The final density and quality of the foamed body will be strongly

influenced by the compositions of the mixtures, including the content of

the foaming agent and the carbon. The carbon content has a notable influ-

ence on the foaming behavior and the mechanical property of the foam

body. The addition of 2%–3% carbon improves the foaming property and

the base metal strength, as well as decreasing the melting and foaming

temperatures.

Expansion of the foam after melting is the key procedure in the process,

and the mixing procedures also strongly affect the foam density and pore dis-

tribution. On the other hand, it has a negligible effect on the densification of

the foam compared to pressure. At the melting point, the viscosity of the

alloyed steel melt is relatively small, and the coarsening of pore comes

quickly with the expansion of the foam. Therefore, the duration at the peak

temperature is controlled over several minutes to reduce the possibility of

Figure 2.32 Pore structure of foamed steel [61]: (a) MgCO3 as foaming agent; (b) SrCO3

as foaming agent.


pore coarsening while allowing the expansion of the foamed body. The rel-

ative density of the final foamed steel (Figure 2.32) is 0.41–0.45 with an

average pore size of 1–1.3 mm.

2.9.2 Investment CastingThe investment casting process is illustrated in Figure 2.33. The plastic foam

(polyurethane) is poured into the container with the designated shape,

followed by the refractory slurry. The foam sponge is burned and removed

after drying and hardening, and a preformed mold is formed with the orig-

inal designed 3-D reticulated plastic [1,5,8,9,15]. The molten metals are

then poured into the inlet of the preformed mold, the mold is removed after

the solidification (with pressurized water), and finally the foamed metals can

be obtained, representing the original polymer sponge structure. If the gap is

not big enough for the liquid metal to flow just from gravity, pressure and

heating may be applied [5].

Porous metals with low melting points like Al, Cu, Mg, Pb, Sn, and Zn

and their alloys can be developed by this method. However, it is difficult to

fill the filaments completely, to control the directional solidification, and to

remove the mold materials without damaging the microstructure [5]. The

obtained porosity is in the range of 2–16 /cm (5–40 ppi). The complex parts

can be prepared with the premolded polymer foam. The density and mor-

phology of the porous metal products with the porosity of 80%–97% is

determined by the premolded polymer.

Space holder

Preparing bulkof space holder

Polymer Filler

(b)

(a)

Metal

Polymer foamInfiltrating slurryand drying

RemovingPolymer

Infiltratingmelt

Removingmold

Infiltrating meltRemoving space

holder

Metal Pore

Figure 2.33 The process for investment casting of porous metals [5]: (a) polymer foamas precursor; (b) particle stacking as precursor.

96 Porous Materials

2.9.3 Self-Propagating, High-Temperature Synthesis (SHS)Self-propagating, high-temperature synthesis (SHS), also known as combus-

tion synthesis, has developed as a technology for material preparation over the

past 30 years [3,63,64]. Intermetallics and composite materials can be pre-

pared by this method. The working principle behind this method is that

the synthesis of the materials is maintained by self-made heat from the chem-

ical reaction. The reactants change into the resultant during the burning that

takes place after the reaction begins. Due to its high reaction rate and the

high thermal gradient, a great density of defects in crystal lattice will be gen-

erated and then the porous frameworks are formed easily in a large surface

area. It has the advantages of short production cycle, low energy consump-

tion, simple process, and low cost.

The porous TiNi alloys have good potential applications in the medical

field due to their superelastic and shape-memory properties, and they also


have excellent biocompatibility, high strength, good shock resistance, and

antiwear/anticorrosion property [65,66]. A TiNi alloy with a porosity

higher than 35% cannot be prepared by the traditional casting process or

by powder metallurgy [65,67,68]. On the other hand, SHS in the Ar atmo-

sphere can be used to prepare TiNi alloys with higher porosity by mixing

pure Ti and Ni powders with atom ratios of 1:1 and pressing into a green

body of 65% density [65]. The green body after the SHS process will be

maintained, but with a length extension of 70%, an apparent density of

3.15 g/cm3 (whereas the density of dense TiNi is 6.45 g/cm3), and a poros-

ity of 51%, which is a significant increase over 35% in the pressed green body

before SHS. The maximum pore radius is 100–150 μm, and the relative

permeability coefficient is 1750 m3/(h.kPa.m2). It is indicated that the

TiNi porous alloys prepared by SHS has good interconnection of pores.

The pore structures are more complex than that of sintered porous metals.

The pore shapes are irregular, and there are two types of pores: large, open-

cell pores with a size of hundreds of micrometers and small, closed-cell pores

of size less than 10 μm. The closed cells are mostly on the walls of the pores

(Figure 2.34). The pore wall is mainly the collective body of the small par-

ticles with closed cells. The main phase is TiNi, and there are also small

transition phases of Ti2Ni and Ti3Ni4. No pure Ti and Ni single phases

are found in the X-ray diffraction (XRD) results. This shows that Ti and

Ni powders can combine within several seconds of reaction time.

There is a big difference between the counterdiffusion of Ti and Ni ele-

ments at high temperatures, and the unidirectional migration is demonstrated

since the diffusion of Ni in Ti is much higher than vice versa [65]. Based on

that fact, porous TiNi alloys can be prepared by SHS: the vacancy is left with

Figure 2.34 SEM morphology of the porous TiNi pore structure [65]: (a) cross section;(b) fractured surface.

98 Porous Materials

the Ni diffusion away and the compound of Ni and Ti will be formed, leading

to volume and porosity increases, along with the release of heat.

The effect of themain processing parameter (i.e., preheating temperature

on the uniformity of pores in TiNi alloys) was investigated [66]. Ti (99.5%

purity) was mixed with Ni (99.6% purity) with sizes of<50 μm at an atom

ratio of 1:1. Then the mixture is dried in the oven in vacuum at 100 �C for

6 h and mixed for another 6 h. The mixed powders were put into the steel

mold and pressed into a cylindrical green body ofF20 mm�20–30 mm in a

thermal press set at 7 MPa. The density of the green body is 45% of the solid

density; at that point, it is put into the graphite mold. After that, the mold is

put in the SHS reactor with flowing Ar gas of 1 atm and heated to different

preheating temperatures(0–600 �C) at a heating rate of 100 �C/min. The

SHS process was ignited by a W wire, and the preheating temperature

(To) and the maximum temperature (Tc) are recorded by the X-Y recorder

and the W-Re thermocouple. It is found that the pores in the resultants are

beltlike and distributed uniformly when the preheating temperature is lower

than 250 �C. The resultants are in a solid state or a semisolid state with small

amount of liquid phases, and they have low porosity due to the fact that the

gas does not expand fully.

The porous body is not ideal since the beltlike pores have a low bearing

capability. When the preheating is conducted at 250 �C –400 �C, the poreswill be spheric and uniformly distributed. Many irregular or spheric particles

are found on the inner wall of the pores by SEM examination. It is indicated

that the results produced at the maximum reaction temperature are in the

coexisting zone of the liquid and solid phases, with moderate content in

the liquid phase. The viscosity is low, and the gas expands more completely

and leads to the formation of spheric pores at this temperature.With increas-

ing the preheating temperature, the reaction temperature also increases and

pore sizes enlarge. However, at this time, there are still more solid phases,

and the viscosity is still high. The gas expansion cannot break the walls of

the pores, but it restricts the floating and agglomeration of pores, leading

to the uniform distribution of the pores. The liquid phase fraction is about

20%–35% at the reaction temperature; and when the preheating tempera-

ture is 400 �C, the resultants have a porosity of 70%. When preheating is

conducted between 400 �C and close to 600 �C, the pores in the resultants

are still spheric but no longer uniformly distributed. The pores are mostly in

the upper part of the product and have smooth inner walls. The liquid phases

are dominant at this reaction temperature with low viscosity, and the resul-

tant cannot restrict the floating and agglomeration of pores to generate

Figure 2.35 Morphologies of porous TiNiFe shape memory alloys made by SHS withdifferent porosities [16]: (a) 56.8%, (b) 59.6%, and (c) 62.3%.


nonuniform distribution of the pores. In the meanwhile, the porosity also

decreases due to the floating and expelling of the gas.

Figure 2.35 shows several porous Ti50Ni48Fe2 (at%) shapememory alloys

fabricated through combustion synthesis, which can be promising porous

implant candidates.

2.10 PREPARATION OF POROUS METAL COMPOSITES

The preparation of porous metal composites is subject to reprocessing

or combined processing based on the abovementioned methods. Examples

of this include the redepositing or filling (casting) of a porous metal with

other metals, alloys, or nonmetals; welding and bonding of porous metals

with other structural metal parts; or making porous bodies by mixing metal

powders, fibers, and other materials (like composite porous electrode mate-

rials with metal and carbon fibers); and sintering of Ni powder and fiber with

addition of a pore-forming agent (NH4)2CO3).

The porous Al and Al alloy composites with metal reinforcements were

prepared by die-casting and squeeze casting in 1980s, and they were used as

the pistol materials in internal combustion engines [69]. Pistols are produced

as follows: The die is heated to 200 �C–400 �C and a metal reinforce-

ment [(1.0–20.0)Cr-(4.0–30.0)Ni-(0–3.0)Mo-(0–3.0)C-(0–8.0)Cu -(0–3.0)

Si-(0–9.0)Mn-Fe, with minimum pore sizes of<3.0 μm] is heated to

400 �C–750 �C. Next, molten Al alloy of 680 �C–820 �C is poured into

the die. Then it is pressed with a pistol-like punch head. The punch head is

pressed with its own weight until the Al alloy solidifies, and then the pressure

is increased to 2,000 bar (1 bar¼0.1 MPa�1 atm). At last, themoltenAl alloy

is pressed into the designated structurewith the pore reinforcement.TheNi-Al

alloy is formed at the interface. The composite is taken out from the die after

100 Porous Materials

complete solidification and thenmachined into the final product. The thermal

andmechanical loadingproperty is greatly improved for theAl alloycomposite.

In the 1990s, a method [70] was invented to prepare porous metal com-

posite material used for gas sensors, electrodes in fuel cells, and chromatog-

raphy separators with the purpose of reducing costs by plating a noble metal

layer onto low-cost porous base materials, while not reducing the activity of

the metal. It includes the processes of metallization on the porous base mate-

rials and the oxidation and reduction of the metal layer. The oxidation of the

metal layer is realized by the oxidative plasma and the reduction is achieved

by reductive plasma in the cold state. The oxidation and the reduction pro-

cesses for the metal layer can increase the numbers of pores in the layer, the

microroughness, and the active surface area. The vacancies are created by

removing the original oxygen atoms from oxides in the reduction process

and the O atoms react with the H atoms. The ceramic or polymer base mate-

rials that are not reactive to the plasma can also be metalized in gas, and the

plated metals can be Pt, Pd, Ag, Ni, and their alloys. The ratio of metal to

base materials must be less than 1:1, and preferably less than 1:100. Oxygen

plasma is used for the oxidation, while ydrogen plasma is used for reduction.

Inorganic film is well suited to the processes of microfiltration, ultrafiltra-

tion, gas separation, and film reaction due to its good heat resistance and

chemical stability [71]. The traditional inorganic porous film has a ceramic

supporting base, and it is prone to be damaged during use. Moreover, the

sealing and joining of the ceramic composites are very difficult to accomplish

at high temperature and under high pressure. These difficulties can be easily

overcome by using a porousmetal substrate. A SiO2membranewas prepared

on the porous Ti base by the sol-gel method using TEOS. The membrane

has been found to crack easily during preparation or use due to the differences

in temperature and large thermal expansion coefficient (TEC) between the

base metal and membrane materials. The porous metal-SiO2 composite

membrane can be prepared by filling the pores with gel particles and by for-

mingmuch smaller pores in a controlled depth of the base metal surface filled

with sol. The whole sol-gel, drying, and buring process is repeated 8–

10 times.

The key technical points for the membrane filling process are as follows:

(1) the sol with different particles can be prepared by the sol-gel method

under different conditions bymultiple fillings of the micropores; (2) the pen-

etration depth can be controlled by using the proper organic solvent; (3)

membrane cracking can be prevented by improved aging and burning pro-

cesses. The SiO2 sol is prepared by the hydrolysis and polycondensation of

TEOS in aqueous alcohol solution with the addition of a certain amount of


diethanol amine andCMC. The hydrolysis of TEOS can be accelerated with

catalyzers (acidic or alkalescence). A linear molecular polymer with low

molecular weight tends to be formed with the acidic catalyzer. Common

acidic catalyzers are HCl, HNO3, HAc, and HF. The pH value shall be

in the range of 7.8–8.0 if HCl is used. The overfiltration of sol into the sub-

strate may affect the flux of gas through the membrane. Therefore, the sub-

strate can be immersed in an organic solvent (heavy hydrocarbon or

haloogenated hydrocarbon) to prefill the pores in the substrate, and then

the substrate with membrane is immersed into the sol with the proper

immersion depth and time. A gel layer is formed in the pores on the surface

of the base through the change of sol to gel by the dissolution of ethanol into

the organic solvent. In order to prevent the gel layer from cracking, a steam

bathing process at a constant temperature can be performed by putting the

substrate with membrane above the bath with a water temperature of 50 �Cfor 10 h, and then drying in air at room temperature for 24 h. For the burn-

ing process, the substrate with membrane is heated to 773 K at a heating rate

of 1 K/min and then held for 300 min to make the gel layer change into

SiO2 film. Finally, the mixture is cooled to room temperature at the same

rate. The process may need to be repeated to obtain a membrane without

defects, and the ratio of water to ester is adjusted to match the gel particle

size and pore size in the substrate.

A porous metal oxide layer was prepared on the inner surface of porous

iron-base materials by the sol-gel, dipping, or perfusing method to improve

the catalyzing activity on the inner surface of the carrier. The oxides can be

alumina, silica, and titania. These oxides can be the carrier of the active cat-

alyzers and used for the heterogeneous multiphase catalysis in the gas reac-

tion. If an alumina layer is needed for the Al-containing porous metal

substrate, high-temperature oxidation can be conducted to obtain the alu-

mina layer, and then another layer of alumina is produced by the sol-gel pro-

cess to improve the combined strength of the different layers.

Other than the sol-gel method, the anode oxidation, chemical vapor

deposition, and nanoparticle deposition methods can be used to deposit

an internal oxide layer in the surface of porous metal substrate [72]. This

is characterized by the wide range of composition selections, pore factors,

and easy surface designs for the sol-gel method, and it can be adjusted by

sol composition and processing.

The open-cell metal-organic composite materials can be used in catalysis,

separation, and gas storage [73]. It is reported that a metal-organic reticulated

composite materials can be synthesized with good stability at a heating tem-

perature of 300 �C [74]. The 3-D reticulated structure with a higher


observable surface area and larger pore volume than that of most porous

crystal zeolite can be obtained through binding the two-carboxyl coated

with single-carboxylate to form the super-tetrahedron composite structure

via the metal carboxylate chemical process.

The biomaterials used to make synthetic human bone joint can have a

lifetime of 25 years or more by maintainng lubrication through the porous

metal composite yielding layer [75]. The sintered gradient stainless steel sub-

strate with a porosity range of 10%–35% is prepared to combine the yielding

layer with the cuplike metallic body. The gradient structure can be obtained

using different sizes of particles and pressing processes. A composite layer

with a higher torsion bearing capability can be prepared by combining

the polymer and the porous metal substrate by the traditional impregnating

and pouring technology. The composite structure by the mechanical bond-

ing of polymer in the gap of metal substrate can effectively increase the inter-

face bonding strength in the composite and prevent the disastrous fracture of

polymer after the interface failure. Therefore, the strain, loading, and the

adhesion property for the porous substrate at failure is higher than that of

the dense materials. The total porosity and related permission rate for a

316 L porous sintered body are influenced by the particle size, pressure,

and sintering temperature. If products are sintered in an Ar atmosphere,

the corrosion resistance is higher than that of products sintered in a vacuum

or in 75% H2: 25% N2 atmosphere, but they will be less hard.

Several unique properties can be demonstrated by the porous metal com-

posite, and the sandwiched structure is an example of a simple, porous metal

composite [76]. A composite structural material combining a dense shell

with porous metals can have the optimized mechanical properties under a

certain load [77–79]. It is well suited to applications in the automotive

and aerospace industries due to its lightness, specific high level of stiffness,

and good dampening performance [77,80,81].

It is easy to obtain a sandwiched panel by binding two plates of metal

sheets to a porous metal core. A real metallic binding can be realized by

rolling the Al or steel sheet onto foamable preformed materials, and a further

deep drawing of the composite can be applied to deform the composite.

Finally, the core is foamed and expanded, and the panel is maintained its

dense state during heat treatment (see Figure 2.36). Porous Al can be made

into the composite with steel or Ti and Al sheets. Al sheet melting during

foaming can be avoided by selecting core materials and sheet metal with dif-

ferent melting points (e.g., the melting point for the sheet is higher than that

of foaming materials).

Figure 2.36 Sandwiched panel with a porous Al core (12 mm in thickness) and two steelsheets [5].


A tube or cylinder with a random shape can be filled with porous Al in

different ways [5]: (1) the preformed rod for foaming can be inserted into the

cylinder and then heated in the furnace to foam and fill it; (2) the hollow

foaming materials are inserted into the cylinder while contacting the inner

surface and then expanded centripetally. Another way to prepare the sand-

wiched composite is via thermal spraying of Al on the premolded porous Al,

and then preparing the porous Al part with a dense outer shell. Of course,

the application of this process is not limited to the tube parts.

A sandwiched plate with area of 2,500�1,200 mm2 and thickness of

130 mm was produced by a German company in a recent study [82]. A flat

sandwiched plate can be made into different shapes of products based on the

requirements. The development is focused on the sandwiched structure

with Al foam as the core.

With the progress of porous metal development, porous iron and stain-

less steel have attracted the most attention due to their low cost, high com-

pression resistance, low TEC, and high thermal stability. It is also

advantageous compared to foamed metals with low melting points because

they have higher strength, improved energy absorption, and high-

temperature capability, and they have great potential for applications in

the automotive, shipbuilding, bridge construction, and transportation indus-

tries [77,83–86]. Figures 2.37–2.39 show examples of sandwich-structured

porous iron and stainless steel created with a simple process, and metallur-

gical bonding between the sheet and the core is demonstrated [87].

Iron or stainless steel foam is preferable to Al foam and its sandwich-

structured products in terms of strength and weldability [84]. Iron foam,

Figure 2.37 Sandwiched structure with porous iron and 304 sheets [87]: (a) iron foam;(b) bending sample of an iron foam sandwich structure; (c) plane sample of an iron foamsandwich structure.


with its highly porous sandwiched structure, can be used as light and highly

functional material in transportation, machining, and construction of struc-

tural parts [79,83–86,88,89]. The reports on iron foam is not so popular due

to the difficulty in processing, and no related sandwiched iron foam material

has been reported until now.

2.11 SPECIAL PROCESSING OF POROUS METALS

The cutting process for the porous metal products needs to vary due to

the required size and shape of the application. The traditional sawing, grind-

ing, and drilling processes can be applied to porous metal products, but they

have some problems, such as they distort the material to some degree and

also damage metal foam with low density [90,91]. For the highly required

smooth surface of metal foam, electric spark cutting, chemical polishing,

water jet cutting, or high-speed cutting can be applied.

What constitutes proper processing of metal foam depends on the quality

requirements in place [91]. For example, a diamond saw is used to cut the

intrinsically hard or hard-phase intensified metal foams, while electric spark

Figure 2.38 Cross section of the interface at a plane/core of sandwiched iron foam [87]:(a) cutting-edge area; (b) cross-sectional part; (c) high magnification of (b).

Figure 2.39 Stainless steel foam and the related sandwiched structure [87]: (a) 304foam; (b) 304 foam/sheet sandwich structure.



cutting or chemical polishing is generally used for normal metal foams

because other tools may damage the foam surface. The following sections

describe the special machining process used for metal foams [92].

Numerical control (NC) Electric Spark CuttingElectric spark cutting is applicable to metal foams made by powder metal-

lurgy. The size of the material in this process is highly adjustable, and the

surface quality is easily controlled. It has special advantages in the processing

of small, ultrathin metal foams. It is indicated that pulse width is the main

factor that influences surface roughness depending on the scale of the dis-

charge pit. Therefore, reducing the discharge energy by narrowing the pulse

width can lead to improved surface quality.

Water Jet ProcessingWire-electrode cutting is not applicable to the composites of metal foam and

nonmetals. In this case, sawing, milling, and water cutting are mostly used.

Blade cutting may lead to damage like fracture, collapse, and stripping of the

framework from the metal foam. High-pressure water cutting can prevent

these issues. In high-pressure water jet cutting, water is pressurized and then

jetted through a nozzle of very small diameter to produce a high-speed jet

stream. Sand introduced into the water jet improve the cutting force. It is an

ideal process for machining metal foam with a large area and resin-bonded

metal foam. Water-jet cutting of metal foam is illustrated in Figure 2.40.

In this process, the surface quality is proportionally related to the cutting

speed. A second or third cutting may be needed to achieve the desired

Figure 2.40 Water-jet cutting of metal foam [92].

Figure 2.41 Illustration of laser-cut metal foam [92].


surface quality. An obvious taper may be generated after a thick product has

been cut, due to the expansion of the water flow, and the existence of taper-

ing mostly affects the bottom of the product.

Laser ProcessingThe desired shape of products made by dense materials can be obtained with

precision cutting with lasers. Specific process parameters are required to cut

the metal foam due to limitations in the process. The surface of the metal

foam is rougher than that of the dense metals after laser cutting, and uneven

surfaces may be generated, as occurs with spherical cutting. A laser cutting of

metal foam is illustrated in Figure 2.41.

It can be seen from Figure 2.41 that the surface is rough and uneven after

laser cutting, and this is caused by a nonuniform thermal distribution. A layer

of oxides forms on the pore wall in heating, and the oxides have a higher

melting point than that of metals. Spherical cutting occurs due to the differ-

ent melting rates for the surface and the interior of the foam framework.

Metal foam with low density is easier to melt, so it also is much easier to

cut into metal foam with small pores than large pores.

2.12 CONCLUDING REMARKS

There are many ways to make porous metals. The tendency is to create

porous metals with high porosity, uniform structure, and good mechanical

properties, and then expand their application areas.Most applications of porous


metals demand higher porosities and higher specific surface areas based on the

strength requirements, except for sandwiched structural materials and thermal

insulation materials, which need a closed-cell structure. Therefore, large-scale

productionof 3-D reticulated porousmetals is promoted.Currently, the appli-

cationsof 3-Dreticulatedhighly porousmetals cover nearly all the applied areas

for porousmetals and evenhave expandeda littlemore, suchaswith filters; fluid

mixers; heat exchangers; soundabsorbers; electromagnetic shieldmaterials; cat-

alysts and their carriers; electrodes forNiCd,NiH,Li and fuel cells; cathodes for

electrical synthesis and recycling of heavy metals; composite materials; and

structural materials in the aerospace industry. The methods used to make

porous metals are all applicable to the preparation of highly porous metallic

materials except for metal deposition (electrodeposition), special powder met-

allurgy, and infiltration casting. It is clear that further development and prepa-

rationof highlyporousmetals are needed to explore theprospects of using these

high-quality engineering materials.

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