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CHAPTER ONE
General Introduction to PorousMaterialsPorous materials widely exist around us and play a role in many aspects of
our daily lives; among the fields they can be found in are energy manage-
ment, vibration suppression, heat insulation, sound absorption, and fluid fil-
tration. Highly porous solids have relatively high structural rigidity and low
density of mass, so porous solids often serve as structural bodies in nature,
including in wood and bones [1,2]; but human beings use porous materials
more functionally than structurally, and develop many structural and func-
tional integrative applications that use these materials fully [3,4]. This chap-
ter will introduce the elementary concepts and features of this kind of
material.
1.1 ELEMENTARY CONCEPTS FOR POROUS MATERIALS
Just as their name implies, porous materials contain many pores.
Porous solids are made of a continuously solid phase that forms the basic
porous frame and a fluid phase that forms the pores in the solid. The latter
can consist of gas, when there is a gaseous medium in the pore, or of liquid,
when there is a liquid medium in the pore.
In that case, can all materials with pores be referred to as porous?
Perhaps surprisingly, the answer is “no.” For instance, holes and crannies
that are the result of defects will lower a material’s performance. This result
is not what designers want, and so these materials cannot be termed porous.
So-called porous materials must possess two essential characteristics: one is
that the material contains a lot of pores, and the other is that the pores
are designed specifically to achieve the expectant index of the material’s
performance. Thus, the pore of porous materials may be thought as a func-
tional phase what designers and users hope to come forth within the mate-
rial, and it supplies an optimizing action for the performance of the
material.
Porous Materials Copyright © 2014 Tsinghua University Press Limited.Published by Elsevier Inc. All rights reserved.
1
2 Porous Materials
1.2 MAIN GROUPS OF POROUS MATERIALS
The number of pores (i.e., porosity) will vary for different porous
materials. Porous materials can be classified as low porosity, middle porosity,
or high porosity based on the number of pores. Generally, porous materials
with low and middle porosity have closed pores (Figure 1.1) which behave
like a phase of impurity. For porous materials with high porosity
(Figures 1.2–1.4), there are two different cases according to various mor-
phologies of the pore and the continuous solid phase. In the first case, the
continuous solid constructs a two-dimensional array of polygons; the pore
is isolated in space, taking on polygonal columniations accordingly; and
the cross-sectional shape of the pore is commonly triangle, quadrangle, or
hexagon (Figure 1.2). This structure looks similar to the hexagonal cell of
a honeycomb, and such two-dimensional porous materials are called honey-
comb materials. Porous materials with directional pores [5], which are called
lotus-type porous materials, have a similar structure as honeycomb materials,
but the cross-sectional shape of the pores for these materials is circular or
elliptic, and the pore often cannot run through it, resulting in less uniformity
of distribution and a lower density of the array. In the second case, the con-
tinuous solid presents a three-dimensional reticulated structure (Figure 1.3),
and such porous materials can be termed three-dimensional reticulated foamed
Figure 1.1 Porous composite oxide ceramics, which is a low-porosity material, shown asa cross-sectional image.
Figure 1.2 Two-dimensional honeycomb materials: (a) conductive honeycomb TiCceramics with quasi-square pores [6]; (b) thermal storage of honeycomb ceramics withsquare pores (with dimensions of 100 mm�100 mm�100 mm, cell-wall thickness of1 mm, and square-pore side length of 2.5 mm) [7].
Figure 1.3 Three-dimensional reticulated foamed materials: (a) nickel foam; (b)iron foam.
Figure 1.4 Bubblelike foamed materials: (a) a closed-cell bubblelike foamed material ofaluminum foam [8]; (b) an open-cell bubblelike foamed material of iron foam.
3General Introduction to Porous Materials
4 Porous Materials
materials. These materials have connective pores that are of a typical open-
cell structure. In the third case, the continuous solid shows the cell wall
structure of pores of sphericity, elliptical sphericity, or polyhedron shape
(Figure 1.4), and such three-dimensional porous materials can be called
bubblelike foamed materials. Within these materials, the cell wall may separate
many isolated closed pores or cells, forming a closed-cell, bubblelike foamed
substance (Figure 1.4a). The cell wall may make up open-cell, bubblelike
foamed material as well (Figure 1.4b). In the literature, three-dimensional,
reticulated foamed materials are referred to as “open-cell foamed materials,”
closed-cell, bubblelike foamed materials are called “closed-cell foamed
materials,” and open-cell, bubblelike foamed materials are “half open-cell
foamed materials.”
Porous solids include two types of porous bodies (i.e., natural and arti-
ficial). Natural porous solids can be found universally [1], such as bones that
support the bodies and limbs of animals and human beings (see Figure 1.5),
plant leaves, wood, sponge, coral (Figure 1.6), pumice (Figure 1.7), and lava
(Figure 1.8). Lava is a sort of natural porous material that can be used in con-
struction or for creating artwork (Figure 1.9). It is not accurate to refer to the
natural, porous solids of living animal bones and tree trunks as “natural
porous materials.” However, when a tree is cut down tomakematerials used
by human beings to make things like furniture, it becomes natural porous
materials. The fluid phase contained in the pores of plant leaves and
living tree trunks always consists of liquid (namely sap), while that within
Figure 1.5 Cross-sectional view of a reticulated porous bone of a whale.
Figure 1.6 An optical photograph showing the porous morphology of coral.
Figure 1.7 An image showing the porous morphology of pumice.
Figure 1.8 Cross-sectional view of the porous morphology of lava.
Figure 1.9 A vase made of lava.
6 Porous Materials
artificial porous materials is mostly gas. Artificial porous materials can be sub-
classified further into porous metals, porous ceramics, and polymer foams.
1.3 POROUS METALS
Porousmetals are a relatively newclass of engineeringmaterials that can
serve functional and structural purposes [9–11]. They have undergone rapid
development over the last thirty years. These lightweight materials not only
have the typical characteristics of metals (weldability, electrical conductivity,
and ductibility), but also possess other useful characteristics, such as low bulk
density, great specific surface area, low thermal conductivity, good penetra-
bility, energy management, mechanical damping, vibration suppression,
sound absorption, noise attenuation, and electromagnetic shielding. Conse-
quently, these materials have increasing applications, and have emerged as a
focus of great attention in the international material field [12]. The next sec-
tions describe the main characteristics of these types of metals [11,13–15].
1.3.1 Powder-Sintering TypeThe powder-sintering type of porous metallic material is commonly made
from metal or alloy powder with spherical or irregular shapes via molding
and sintering. The porous bodies obtained in this manner will have various
porosities, pore sizes, and pore-size distribution due to differences of the
selected raw materials or technological systems. However, all of them have
7General Introduction to Porous Materials
the characteristics of good penetrability, controllable pore sizes and levels of
porosity, and great specific surface area, as well as endurance under high or
low temperatures and resistance to heat fluctuation.
Powder-sintering porous metals were developed early, with pore size usu-
ally less than 0.3 mm and porosity mostly less than 30%. However, the
production with porosity much higher than 30% can be prepared by using
special technological processes, e.g, the space-holdermethod. In themetallurgy
and chemical engineering fields, high-temperature and high-pressure environ-
ments are frequent, and accordingly, filtration and separation materials are
needed; during catalysis reactions, catalyzer materials with great specific surface
area are needed to supply the reactive interface area; and many types of oils and
working gasesmust be filtered strictly to guarantee that the aviation and hydrau-
lic pressure systems work safely. The areas of aviation and rockets demand that
porous materials with great heat endurance and heat fluctuation resistance and
well-proportioned pore structures be used as the basic structural material for
volatilization cooling. In general, porous polymer or ceramic bodies are difficult
to adapt to these conditions, which require great strength, plasticity, and high
temperature tolerance at the same time, but powder-sintering type porous
metallic materials can do this well, and therefore scientists worked to develop
them speedily.
The first patents mentioning powder-sintering porous components were
approved as early as 1909, and patents dealing with the techniques to make
powder-sintering filters were developed until the early 1930s. During
WorldWar II, powder-sintering porous materials underwent rapid develop-
ment for military applications. Powder-sintering filters were applied to air-
planes and tanks, porous nickel was adopted to make radar switches, porous
iron was employed to make cannonball hoops instead of dense metallic
copper, and iron filters were used as flame extinguisher. In themid-twentieth
century, porous materials with oxidation resistance were applied to the
fireboxes and blades of jet engines for volatilization cooling to heighten the
efficiency of engines. In response to developments in chemical engineering,
metallurgy, atomic energy, aviation, and rocketry, many types of powder-
sintering porous materials with high penetrability and resistance to corrosion,
high temperatures, and high pressure were created. Some more advanced
porous materials were produced in the 1960s, including the corrosion- and
heat-resistant porous materials of Hastelloy, Inconel, titanium, stainless steel,
tungsten, tantalum, and other refractory metals and alloys. At present,
powder-sintering porous materials of bronze, stainless steel, nickel, titanium,
and aluminium alloys have been mass-produced and employed. Figure 1.10
shows a powder-sintering type of porous titanium alloy.
Figure 1.10 SEM image of the porous TiNiFe alloy fabricated by powder sintering [16].
8 Porous Materials
1.3.2 Fiber-Sintering TypeThe fiber-sintering type of porous metal is an improvement over powder-
sintering porous metals for the above mentioned purpose. Porous materials
made of metallic fiber may be superior to that of metallic powder in some
ways. For example, filtration materials fabricated of metallic fiber will have
a much greater degree of penetrability than those made of metallic powder
with the same diameter as the metallic fiber. In addition, they have a higher
mechanical strength, corrosion resistance, and thermal stability. These mate-
rials can reach a porosity of over 90%, with all through pores, good plasticity
and impact toughness, and a high dust retention capacity. Known as second-
generation porous metallic filtration materials, theymay be used bymany businesses
under rigorous filtration conditions. Figure 1.11 shows a porous structure
crafted by metallic fiber sintering.
1.3.3 Melt-Casting TypeThe melt-casting type of porous metal is formed via cooling molten metals or
alloys, which can include a very wide range of porosities and have diversely
shaped pores with different castingmanners. One example of this is aluminum
foam produced by melt-foaming and infiltration-casting processes. Materials
made from melt foaming are mostly closed-cell or half open-cell porous
Figure 1.11 Micrograph of a porous material fabricated by metallic fiber sintering [11].
Figure 1.12 An aluminum foam produced by melt foaming [17].
9General Introduction to Porous Materials
materials (Figure 1.12), and those made from infiltration casting commonly
take the form of three-dimensional, reticulated, open-cell ones with high
porosity.
1.3.4 Metal-Deposition TypeThe metal-deposition type of porous metal is created via depositing atomic
metal on open-cell polymer foam, followed by eliminating polymers and
sintering. The main features of such metals include connective pores, high
Figure 1.13 SEM images of nickel foam samples of various thicknesses made by themetal deposition process: (a) a thinner nickel layer; (b) a thicker nickel layer.
10 Porous Materials
porosity, and a three-dimensional, reticulated structure. This porous mate-
rial, a new type of functionally and structurally integrative substance with
excellent properties, is a very important class of porous metals. When used
in certain settings, its merits include low density, high porosity, great specific
surface area, good pore connectivity, and uniform structure, which is diffi-
cult to achieve for other types of porous metals. However, the feature also
results in some limits to the strength of metal-deposition type porous metals.
These materials first were manufactured and utilized in the 1970s, and then,
during the 1980s, they were speedily developed for a wide variety of appli-
cations and demands. At present, these porous materials are produced on a
large scale in many countries, with the products of nickel and copper foams
typically made by the electrodeposition process. Suchmetal foams are shown
in Figure 1.13.
1.3.5 Directional-Solidification TypeThe directional-solidification type of porous metal forms via dissolved gas in
molten metal releasing in the course of directional cooling [5,18], namely by
GASAR. The resultant products have a very similar structure to plant lotus
roots (Figure 1.14), so they are called lotus-type porous metals, porous metals
with directional pores, or Gasarite.
1.3.6 Composite TypeComposite-type porous metals are porous metal composite materials. They
can be obtained by compositing different metal species or metal species and
Figure 1.14 A lotus-type porous metal formed by gas-metal eutectic directional solid-ification [18].
11General Introduction to Porous Materials
nonmetal species to form a porous body. Examples of this type of metal
include graphite-nickel composite porous material created by electroplating
a nickel layer onto a graphite felt, and a composite of aluminum alloy and
nickel foam made by pouring a melted aluminum alloy into a three-
dimensional, reticulated nickel foam. Such materials also can be fabricated
by using porous metals as a core to form a metallic composite porous “sand-
wich”; for example, by putting together stainless steel fiber felt and wire
netting or by integrating aluminum foam and metallic panels. Compositing
makes the materials acquire the respective merits of these different ingredi-
ents and improved their properties; the result is a completely new synthetic
material that better meets the demands placed on products made from this
substance.
In addition, certain porous metallic materials are prepared by particular
routes, some of which can be ascribed to those of the above mentioned
types, and others can be those of new types.
1.4 POROUS CERAMICS
Porous ceramics, also known as cellular ceramics, began developing in
the 1970s. They are comprised of a kind of heat-resistant porous material
with many gaseous pores. Their pore size mostly ranges between the ang-
strom and millimeter levels, the porosity usually spans from 20% to 95%,
and the serving temperature varies from room temperature to 1,600 C
[19,20].
12 Porous Materials
1.4.1 Classifying Porous CeramicsIn general, porous ceramics may be divided into two main classes [20–22]:
honeycomb ceramics (Figure 1.15) [23] and ceramic foam (Figure 1.16).
The former has polygonal columnar pores that form a two-dimensional
array (see Figure 1.2), and the latter has hollow polyhedron pores that form
a three-dimensional array. Figure 1.16 shows two ceramic foams with dif-
ferent pore structures, both of which were made from compounded oxides.
There are two sorts of ceramic foam: the open-cell, reticulated ceramic
foam (Figure 1.16a) and the closed-cell, bubblelike ceramic foam
(Figure 1.16b).When the solid species constituting the foamed body is com-
prised only of pore struts, the connective pores will generate reticulated
structures, resulting in open-cell ceramic foams. When pores are separated
by solid cell walls, the closed-cell ceramic foam will be achieved. Such dif-
ferences can be clearly seen by comparing the fluid penetrability of these two
sorts of foamed bodies. The distinction between the two types depends on
Figure 1.15 An optical photograph showing two-dimensional honeycomb ceramicproducts [23].
Figure 1.16 Three-dimensional ceramic foams: (a) an open-cell reticulated ceramicfoam, (b) a closed-cell bubblelike ceramic foam.
13General Introduction to Porous Materials
whether the pore is enveloped by solid cell walls or not [20–22]. In addition,
there are half open-cell ceramic foams.
Apparently, some ceramic foams have both open and closed pores.
These porous structures take on a relatively low level of bulk density
and thermal conductivity, as well as varying levels of fluid penetrability
which is high for the open-cell body. By properly matching the ceramic
raw material to the preparation technique, porous ceramics may be created
that have relatively high levels of mechanical strength, corrosion resistance,
and stability under high temperatures that can satisfy the demands of severe
conditions [21].
Porous ceramics also can be classified according to the size of their pores,
as follows [24]:
• Microporous material, for pore sizes of less than 2 nm
• Mesoporous material, for pore sizes of 2–50 nm
• Macroporous material, for pore sizes over 50 nm
This classification standard has not been adopted abroad because the rules
about using porous materials vary widely from country to country.
In light of the differences among their materials, there are several types of
porous ceramics: silicate; aluminosilicate; diatomite; carbon; corundum; silicon
carbide; and ocordierite [25].
Ceramic foam is an important part of porous ceramics, and the open-cell
type of ceramic foam, which is a new type of highly porous ceramics, has a
three-dimensional, reticulated structure with connective pores, resulting in
great specific surface area, high fluid contact efficiency, and a small loss of
fluid pressure [26,27]. In particular, these materials have many connective
pores and capillary holes and have high specific surface energy on the inside,
14 Porous Materials
so they perform well in terms of filtration and adsorption under low fluid
resistance loss conditions. They can be used in many fields, including met-
allurgy, chemical engineering, environment protection, energy, and biol-
ogy, for such applications as metal melt filtration, high-temperature gas
purification, and catalyst support [26]. Moreover, the porosity, density, fluid
resistance loss, and penetrability of these materials can be modulated by
various processing techniques, and the commonly used material species
includes alumina and cordierite. Cordierite is used as a raw material with
the primary purpose of improving the heat fluctuation resistance of products,
and alumina is used to increase a material’s strength and thermal stability. As
the demand of thermal stability heightens for such products, porous silicon
nitride and silicon carbide ceramics also have been developed [19].
The research on porous ceramics has been expansively attended, and lots
of technological applications have become possible for these materials in
practice. In some areas (such as energy and environmental protection),
the applications of porous ceramics can have enormous economic and soci-
etal benefits [25].
1.4.2 Characteristics of Porous CeramicsPorous ceramics have several common characteristics [25]:
1. Good chemical stability. Choosing the appropriate material species and
techniques can make porous products suitable for various corrosive con-
ditions in which the products are expected to function.
2. Great specific strength and rigidity. The shape and size of pores in porous
ceramics will not change under gas pressure, liquid pressure, and other
stress loadings.
3. Fine thermal stability. Porous products made of heat-resistant ceramics can
filtrate molten steel or high-temperature burning gas.
These excellent characteristics promise a great future for porous ceramics
being used in a wide variety of applications, and make such materials adapt-
able in many areas, including chemical engineering, environment protec-
tion, energy source, metallurgy, and electronic industry. The specific
cases for which porous ceramics are suitable depend on both the composi-
tion and structure of the products. At first, porous ceramics were used as fil-
tration materials to filtrate bacteria belonging to the microorganism. Once
the level of controlling the fine pores of porous ceramics was increased, the
resulting products gradually became used in more and more applications,
including separation, dispersion, and adsorption; and they are presently
15General Introduction to Porous Materials
being used in many industrial areas, including the chemical engineering,
metal smelting, petroleum, textile, pharmaceutical, and foodstuff machinery
industries. Also, these porous ceramics have been used increasingly in
sound-absorbing materials, sensitive components, artificial bones, and tooth
root materials.
1.5 POLYMER FOAMS
Polymer foams, also called plastic foams, are porous plastics filled with
bubblelike pores, but products with a reticulated structure also can be seen
frequently in this category [28,29]. These materials contain many pores
filled with gas, so they may be regarded as polymer composites or composite
plastics in which the gas is stuffed. In general, all the thermoset plastics, general
plastics, engineering plastics, and heat-resistant plastics can be made into
foamed plastics. Such porous bodies are one kind of plastic products that are
used on a large scale, and assume an important role in the plastics industry [28].
The density of plastic foams is determined by the volume ratio of gaseous
pores to solid polymer. This ratio is about 9:1 for low-density plastic foams
and about 1.5:1 for high-density ones [30].
1.5.1 Classifying Polymer FoamsThere are a variety of polymer foams. They are classified as follows [28,29]:
1. Open- and closed-cell polymer foams can be defined based on the pore
structure of the foamed body. Open-cell polymer foams have mutually
connected pores, with gaseous and solid phases, which are each contin-
uously distributed (Figure 1.17a) [31]. The penetrability of fluids
Figure 1.17 Three-dimensional porous polymer foams: (a) an open-cell polyurethane(PU) foam [31]; (b) a closed-cell polyolefin foam [32].
16 Porous Materials
through the porous body is related to both open-cell porosity and
polymer characteristics. Closed-cell polymer foams have pores that
are separate from one another, and the solid polymer phase presents
a continuous distribution, but the gaseous phase occurs inside the indi-
vidual isolated pores (Figure 1.17b [32]). Actually, both structures of
pores exist simultaneously in real polymer foams; that is, open-cell
polymer foams contain some closed-cell pores, and closed-cell polymer
foams contain some open-cell pores. In general, open-cell structures
make up approximately 90%–95% in so-called open-cell polymer
foams.
2. Polymer foams can be divided into three categories based conversely on
their density: low foaming, moderate foaming, and high foaming. Low-
foamed or high-density polymer foams have a density of more than
0.4 g/cm3 and a gas/solid expansion ratio (a ratio of the density of dense
plastic to the apparent density of foamed plastic with the same polymer
species) of less than 1.5. Moderate-foamed or middle-density foams have
a density of 0.1–0.4 g/cm3 and an expansion ratio of 1.5 – 9.0. High-
foamed or low-density foams have a density of less than 0.1 g/cm3
and an expansion ratio of more than 9.0. Another way of classifying these
materials is to label products with an expansion ratio of less than 4 or 5 as
low-foamed polymer foams, and those with a ratio of more than 4 or 5
as high-foamed. On occasion, the density with the value of 0.4 g/cm3 is
adopted to bound the high- or low-foamed porous plastics. Products that
commonly use polymer foams, such as mattresses, cushions, and packag-
ing liners, mostly are the high-foamed types; other products, like frothed
plastic plates, pipes, and abnormal components, fall into the low-foamed
category.
3. Polymer foams may be grouped into three types based on their rigidity:
rigid, semi-rigid, and flexible. With rigid foams, the polymer takes a
crystal form at room temperature or has a glass transition temperature
higher than room temperature, and it is quite rigid at room temperature.
With flexible polymers, the melting point of the polymeric crystal or the
glass transition temperature of the amorphous polymer is lower than
room temperature. Semi-rigid foams fall between these two types. Based
on these criteria, phenol formaldehyde resin (PF), epoxy resin (ER),
polystyrene (PS), polycarbonate (PC), rigid polyvinyl chloride (PVC),
and numerous polyolefin foams are rigid polymers, and porous rubber,
elastic polyurethane (PU), flexible polyvinyl chloride (PVC), and a part
of polyolefin foams are flexible [29].
17General Introduction to Porous Materials
From the viewpoint of modulus, rigid foamed plastics are characterized
by porous polymers, of which the elastic modulus is more than 700 MPa at a
temperature of 23 �C and relative humidity of 50%. With flexible foamed
plastics, the elastic modulus is less than 70 MPa at the same temperature
and relative humidity, and with semi-rigid foamed plastics, the elastic mod-
ulus is between 70 MPa and 700 MPa [28].
The resin species most frequently used to make foamed plastics are poly-
styrene (PS), polyurethane (PU), polyvinyl chloride (PVC), polyethylene
(PE), and urea formaldehyde (UF). Other commonly used varieties include
phenol formaldehyde resin (PF), epoxy resin (ER), organosilicon resin (OS),
polyethylene formaldehyde, cellulose acetate, and polymethyl methacrylate
(PMMA). In recent years, some material species have begun to be used to
produce polymer foams, such as polypropylene (PP), polycarbonate (PC),
polytetrafluoroethylene (PTFE), and polyamide (PA; i.e., nylon).
1.5.2 Characteristics of Polymer FoamsAlthough there are many kinds of polymer foams, all of them contain a lot of
pores. Therefore, they have several common characteristics, including low
density, low thermal conductivity, good thermal barrier effect, effective
impact energy absorption, excellent sound insulation, and great specific
strength [28,29]. These characteristics are described in the next sections.
Low Relative DensityThere are lots of pores in polymer foams, and correspondingly, the density of
porous products is only a small percentage of that of dense products. Addi-
tionally, the polymer itself is a class of low-density material species, so the
products of polymer foams may have a very low density, which is the lowest
of all the porous materials. (Note that polymers consist of light atoms, and
the molecules inside are linked by a weak Van der Waals force, causing it to
have a constitution without compactness, with low density and rigidity.)
Excellent Performance of Heat InsulationThe thermal conductivity of foamed polymers is greatly reduced compared
to the corresponding dense plastics due to the fact that porous products have
so many pores, and the gas in these pores has a thermal conductivity with an
order of magnitude less than that of dense solid plastics. Furthermore, the
gaseous phase in pores is separate for closed-cell foamed bodies, which
reduces the convection heat transfer of gas. As a result, the thermal barrier
effect for polymer foams is improved.
18 Porous Materials
Good Impact Energy AbsorptionGas in the pores of polymer foams under impact loading will be compressed,
resulting in hesitation. Such compression, springback, and hesitation will
consume the energy from the impact load. Moreover, the foamed body also
can terminate the impact load step by step with a small deceleration, so it will
acquire an excellent damping ability.
Excellent Sound InsulationThe sound insulation effect of polymer foams comes into play in the follow-
ing two ways: (1) the porous body absorbs sound wave energy to terminate
the reflection and transferal of the sound waves; (2) the porous body elim-
inates resonance and decreases noise. When the sound wave arrives at the
cell wall of a pore in polymer foams, it will strike the pore and make the
gas within it to be compressed. This causes hesitation, so the impact energy
of the sound wave will dissipate. In addition, increasing the rigidity of the
polymer foams can eliminate or decrease the resonance and noise caused
by the sound wave hitting the pores.
Great Specific StrengthSpecific strength is the ratio of material strength to relative density. The
mechanical strength of polymer foams will decrease when porosity increases,
but the specific strength as a whole will be much higher than that of porous
metals or porous ceramics with equivalent porosities.
Polymer foams that are made from hollow globular stuffing and resin
matrix have a very great specific strength of compression, and they can be
used for such applications as the elastic material on the hulls of ships serving
in deep seawater [32]. Usually, the stuffing may employ hollow or porous
granules of glass and ceramics, as well as thermoset plastics or thermoplastic
resins. The tiny ball stuffing also may be used in fiber-reinforced plastics and
enhances the toughness of fiber-reinforced resins.
Strengthening polymer foams advances the potential development in
material sciences. The exploitation and application insufficiencies make
the virtue not adequately utilized yet, but the reinforced thermoplastic
materials have some advantages both in economy and in technology. In
many cases when specific strength is demanded, these recent applications
of reinforced plastics may come in handy. Also, using the reinforcement
technique and the other materials can give some of the composite porous
materials a number of outstanding properties which integrate the low den-
sity, low combustibility, low cost, and great specific strength.
19General Introduction to Porous Materials
Of course, all of the abovementioned porous metals, porous ceramics,
and polymer foams can be incorporated with other materials to form excel-
lent porous composites, whose combined properties can be well suited to
more demanding purposes.
1.6 CONCLUSIONS
Making a densematerial porous endows it with brand-new, very useful
properties. These additional properties make porous materials suitable for
many applications for which dense ones are not well suited. This enhances
thedegreeof creativity that is possibleusingporousmaterials andgreatlyopens
up the range that these materials will be applied in engineering. There are
many varieties of porous material, but all the types have some common
characteristics, including low relative density, large specific surface area, high
specific strength, small thermal conductivity, and good energy absorption
compared to the dense version of the same materials. Low-density porous
materials may be used to design lightweight rigid components, large portable
structural frames, and various flotages. Low-thermal-conductivity products
canbe applied to simple andconvenient formsofheat insulation, and the effect
is just a little inferior to that of more expensive and difficult varieties. Low-
rigidity foamed bodies serve as the perfect material for mechanical damping.
For example, elastic foams are standard materials used to install machinery
bases. In addition, the large compressive strain of these materials make them
quite attractive for energy absorption applications, and there is a hugemarket
for porous materials to protect articles. This book mainly discusses artificial
porous materials, their production, application, and characteristics, as well
as the results of relevant research on these substances in recent years.
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