Amorphous

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Amorphous StructuresCh.

High Temperature Materials

Course KGP003

By

Docent. N. Menad

Dept. of Chemical Engineering

and Geosciences Div. Of

process metallurgy

Luleå University of Technology( Sweden )

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An amorphous solid is a solid in which there is no long-range order of the positions

of the atoms. (Solids in which there is long-range atomic order are called crystalline solids.) Most classes of solid materials can be found or prepared in an amorphous

form. Ex:

common window glass is an amorphous ceramic, many polymers (such as

polystyrene) are amorphous, and even foods such as cotton candy are amorphous

solids.

Amorphous materials are often prepared by rapidly cooling molten material. The

cooling reduces the mobility of the material's molecules before they can pack into a

more thermodynamically favourable crystalline state. Amorphous materials can also

be produced by additives which interfere with the ability of the primary constituent to

crystallize. For example addition of soda to silicon dioxide results in window glass

and the addition of glycols to water results in a vitrified solid.

Some materials, such as metals, are difficult to prepare in an amorphous state. Unlessa material has a high melting temperature (as ceramics do) or a low crystallization

energy (as polymers tend to), cooling must be done extremely rapidly.

Amorphous solids can exist in two distinct states, the 'rubbery' state and the 'glassy'

state. The temperature at which the transition between the glassy and rubbery states is

called their glass transit ion temperature orT

g.

Amorphous Materials

http://en.wikipedia.org/wiki/Solid

http://en.wikipedia.org/wiki/Long-range_order

http://en.wikipedia.org/wiki/Atom

http://en.wikipedia.org/wiki/Crystal





http://en.wikipedia.org/wiki/Polymer

http://en.wikipedia.org/wiki/Polystyrene

http://en.wikipedia.org/wiki/Cotton_candy

http://en.wikipedia.org/wiki/Thermodynamics

http://en.wikipedia.org/wiki/Soda

http://en.wikipedia.org/wiki/Silicon_dioxide

http://en.wikipedia.org/wiki/Diol

http://en.wikipedia.org/wiki/Water

http://en.wikipedia.org/wiki/Vitrification

http://en.wikipedia.org/wiki/Glass_transition_temperature

http://en.wikipedia.org/wiki/Glass_transition_temperature

http://en.wikipedia.org/wiki/Vitrification

http://en.wikipedia.org/wiki/Water

http://en.wikipedia.org/wiki/Diol

http://en.wikipedia.org/wiki/Silicon_dioxide

http://en.wikipedia.org/wiki/Soda

http://en.wikipedia.org/wiki/Thermodynamics

http://en.wikipedia.org/wiki/Cotton_candy

http://en.wikipedia.org/wiki/Polystyrene

http://en.wikipedia.org/wiki/Polymer




http://en.wikipedia.org/wiki/Atom

http://en.wikipedia.org/wiki/Long-range_order

http://en.wikipedia.org/wiki/Solid

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2. Amorphous Structures


Amorphous structures of inorganic glass, even called vitreous state, can bedefined as follows:

An isotropic material that does not have long range 3-dimensional atomicperiodicity (after 20 Å) and has a viscosity greater than 1014 poise.

The difference between a glass and its corresponding liquid

can be demonstrated by the volume/temperature relationship

during cooling shown in the diagram to the right. Duringcooling of a melt to its crystallization temperature (melting

temperature) Tm, the volume follows line “a”. When the

crystallization doesn’t occur the volume follows line “b” and

a super-cooled liquid is formed. After further cooling, line

“b” changes to line “c” with an inflection point at the glasstransition temperature, Tg, resulting in the formation of a

glass. Due to the high viscosity of super-cooled melts,

cooling must occur extremely slowly if the cooling curve is to

show a distinct inflection point. Otherwise, the

volume/temperature relationship follows a curve like line “d”.

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When a crystalline material is heated, the melting temperature

(TM) is well defined. But on cooling, it is often possible to

undercool the liquid so that nucleation and growth do not occur.

At a temperature below TM, the material can become rigid and

act like a solid, while preserving the amorphous atomarrangement.

The Glass Transition Temperature

This is called the glass transition temperature Tg. A plot of volume of density is often used to

illustrate this, since the expansion coefficient of the liquid, crystalline solid, and amorphous solid are

all generally different.

In real materials, Tg depends on cooling rate, and many other

secondary factors, and may not show such a sharp transition

as indicated here

2. Amorphous Structures2. Amorphous Structures

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Glass can be classified as a super-cooled liquid in which

crystallization has not occurred. There are quite many similarities

between glass and liquids.

Glass structure is however much more complicated than crystal

structure. Glass has properties that are difficult or impossible to

simulate in crystalline material. These properties are alsocontinuously variable in a way that is impossible for a crystalline

material as well.

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A kinetic theory which describes the transition of a melt to a

glass is based on the slow nucleation of crystal grains and

subsequent polycrystalline growth. The rate of nucleation is, as

well as crystal growth rate, temperature dependent but the rate

controlling functions contain conflicting terms that lead to the

different functions having distinct rate maxima, see adjacent

diagram..

Tendency to glass formation increases with:

Increased cooling rate

Increased surface tension melt/crystalline phase

Increased transformation temperatureDecreased melt volume

Decreased grain density

As soon as a nucleation has begun, a crystal will start to grow at

a certain rate depending on the rate of atomic diffusion to the

crystal surface. Crystal growth is therefore dependent on the

viscosity of the melt. Crystal growth rate reaches a maximum,

which for glass, often lies at a different temperature than the

maximum nucleation rate. Also noteworthy, is that the activation

energy for nucleation as well as crystal growth within glass

systems is higher than the Gibbs free activation energy for

viscous flow or self-diffusion

This phenomenon can be explained by the

fact that bonds within a molecular unit

must be broken to allow for crystallization

to occur

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Some amorphous metallic alloys can be prepared under special processing conditions (such as rapidsolidification, thin-film deposition, or ion implantation), but the term "metallic glass" refers only to

rapidly solidified materials.

Even with special equipment, such rapid cooling is required that, for most metals, only a thin wire or

ribbon can be made amorphous. This is enough for many magnetic applications, but thicker sectionsare required for most structural applications such as scalpel blades, golf clubs, and cases for

consumer electronics.

Recent efforts have made it possible to increase the maximum thickness of glassy castings, by

finding alloys where kinetic barriers to crystallization are greater. Such alloy systems tend to have thefollowing inter-related properties:

Metallic Glass

Many different solid phases are present in the equilibrium solid, so that any potential crystal will find that

most of the nearby atoms are of the wrong type to join in crystallization.

The composition is near a deep eutectic, so that low melting temperatures can be achieved without

sacrificing the slow diffusion and high liquid viscosity seen in alloys with high-melting pure

components

Atoms with a wide variety of sizes are present, so that "wrong-sized" atoms interfere with the

crystallization process by binding to atom clusters as they form. One such alloy is thecommercial "Liquidmetal", which can be cast in amorphous sections up to an inch thick

.

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- Si – O – Si- + M2O -Si-O-M+

M+

O- - Si-

With the help of X-ray diffraction studies, the structure of glass

is shown to be based on SiO4 tetrahedral units that are bound toeach other in a random network with so called modified atom

species spread throughout void spaces, as seen in the adjacent

schematic. Here one notices that certain Si-O-Si bonds are

broken and replaced by free O-endings and metal cations

according to the following formula:

A requirement in oxide glass formation is the possibility to

form a 3-dimensional structural network with a comparable

energy to that needed for a corresponding crystalline structural

network. This requires that the primary coordinate number for

every atom in the glass should be nearly identical to those in

the crystalline material. Furthermore, the secondary coordinate

should have only a very small contribution to the total

structural energy. Consider SiO2 for example, the structural

energy difference between Cristobalite and glassy SiO2 (Quartz

glass) is only ~1%

GLASS STRUCTURE

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Structures found from the glass beads demonstration. A perfect crystal (a),

an amorphous structure (b), and a crystal with a vacancy (c) is shown



2 Amorphous Structures

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The amorphous stateobtained by a rapid

quench from a random

start to a temperature

just above T= 0

The crystalline stateobtained after a long

slow cooling with

occasional heating

An intermediate statein the cooling process,

which after reheating

eventually resulted in

a picture similar to B.

A B C


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To fulfill these requirements, Zachariassen has proposed the following

necessities for oxide glass formation.

1. Every oxygen atom should not be bound to more than two cations.

2. The coordinate number for oxygen ions positioned around the

central cation should be small, i.e. less that 4.

3. Oxygen polyhedra can share a corner with each other in order to

form a 3-dimensional network. However, the polyhedra can not share

common edges or surfaces.

4. At least 3 corners of every polyhedron should be shared.

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2. Amorphous Structures2. Amorphous Structures VISCOSITY OF GLASS

The viscosity is a crucial factor in glassformation.

Glass formation is favorable in a material

when:

1. The viscosity is high at the melting point.

2. The viscosity below the melting point

increases with decreasing temperature.

The glass transition temperature, Tg, and the melting temperature, Tm, were

introduced and which are also present in the given diagram. Normally, the

two temperatures have the following relationship

Tg 2/3*TmTg

2/3*Tm

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The viscosity is, as mentioned, also very dependenton composition. In silicate glass, the viscosity

decreases with increasing content of modified

cations. In many cases the change is very

pronounced. For example, for a quartz glass (pure

SiO2) at 1700°C the viscosity decreases 104 poise

with an addition of as little as 2.5 mol% K 2O.

This effect can be attributed to the presence of

oxygen atoms that only have one bond and cause aweak link in the Si-O- network. In the adjacent

diagram, the effect of replacing 8% of SiO2 in a

74SiO2-10CaO-16Na2O glass with different divalent

oxides to the viscosity at 1400°C is shown.

VISCOSITY OF GLASS

For Borsilicate glass, the change in viscosity with an addition of alkali oxides is more

complicated. At high temperatures the viscosity decreases with increased alkali addition,

whilst at lower temperatures, the viscosity increases with increased alkali addition. This phenomenon has yet to be explained theoretic.

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Thermal Stress in Glass

It is understandable that thermal stresses occur between

materials with different thermal expansion behavior during

heating and cooling. When considering glass materials,

thermal stresses can occur when different parts of the glass,having different specific volumes, cool at different rates.

This is the case when a glass product contains segments

with diverse cross sections. Naturally, a thick material will

cool faster than a thin material. A glass material that is

cooled rapidly at the surface will suffer thermal stress

between the surface and interior.

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When reheating the glass the volume curve will

follow the same curve as mentioned above but in

the opposite direction.

2 Amorphous Structures

2 A h S

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The next diagram illustrates the behavior of 3 different

glasses; A, B and C , with different specific volumes (i.e.

different cooling rates) in terms of temperature vs.

physical expansion. Glass A is a rapidly cooled glass, B

a moderately cooled glass, C a slowly cooled glass.

The diagram demonstrates that the thermal expansioncoefficient is the same for all 3 glasses at temperatures

up to 400°C, i.e. the curves are parallel. Above 400°C,

glass A contracts up to roughly 560°C until the glass

structure has reached equilibrium, i.e. the specificvolume of the slowly cooled glass. On the other hand,

glass C follows the “normal” glass cooling curve from

the opposite direction which maintains that the structure

in this glass is in equilibrium during the entire course of

cooling.

Viscosity of glass atdifferent temperature

Annealing temperature

which is a characteristicproperty for every glass

It is through the use of thermal treatment at temperatures near the annealing temperature

that “thermal” stresses in glass can be eliminated. This is a standard practice in

manufacturing of glassware with varying thickness, for example; drinking glass, glassbottles and so on.

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GLASS’ CHEMICAL COMPOSITIONS

(Crystal glass)

SiO2 Al2O3 CaO MgO Na2O K2O PbO B2O3

Type of Glass

Chemical composition , Wt.%

Window glass 72 1.3 8 4 14 0.3

Packing glass 72 0.1 5 4 15

Borsilicate glass 80 2 4 13

Lead glass 54 1 8 37

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TYPES OF COMMERCIAL GLASS

WINDOW GLASS

PLATE GLASS

BOTTLES AND CONTAINERS

OPTICAL GLASS

PHOTOSENSITIVE GLASS

GLASS CERAMICS

GLASS FIBERS

MISCELLANEOUS TYPES OF GLASS

RECYCLING GLASS

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RECYCLING GLASS

Scrap glass taken from the glass manufacturing process, called cullet, has been

internally recycled for years. The scrap glass is economical to use as a raw material because it melts at lower temperatures than other raw materials, thus saving fuel and

operating costs.

Glass that is to be recycled must be relatively free from impurities and sorted by color.

Glass containers such as bottles and jars are the most commonly recycled form of

glass, and their colors are flint (clear), amber (brown), and green. Other types of glass,

such as window glass, pottery, and cooking utensils, are considered contaminants

because they have different compositions than glass used in containers. The recycled

glass is melted in a furnace and formed into new products.

Glass containers make up 90 percent of the total recycled glass used in the United

States. The recycling rate for glass in 2000 was about 23 percent. Other uses for

recycled glass include glass art and decorative tiles. Cullet mixed with asphalt forms a paving material called glassphalt.

2 A h S

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The structural evolution in mechanically

alloyed binary aluminum-iron powder

mixtures containing 1, 4, 7.3, 10.7, and 25 at.

pct Fe has been investigated using x-raydiffraction and electron microscopic

techniques. The constitution (number and

identity of phases present), microstructure

(crystal size, particle size) and transformation behavior of the powders on annealing have

been investigated. The solid solubility of Fe

in Al has been extended up to at least 4.5 at.

pct. compared to the equilibrium value of

0.025 at. pct Fe at room temperature. A fully

amorphous phase plus solid solution in the

Al-10.7 at. pct Fe alloy; agreeing well with

the predictions made using the semi-

empirical Miedema model.

Structural Evolution in Mechanically Al loyed Al-Fe Powder Mixtures

Debkumar Mukhopadhyay

2. Amorphous Structures2. Amorphous Structures Example

2 A h St t

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X-Ray Diffractometry

TS-1 and VS-1 have a good crystallinity and show the

typical reflexes of the orthorhombic MFI-structure

Amorphous cogel with 3% titanium TS-1 with 3% titanium

2. Amorphous Structures2. Amorphous Structures Example

2 A h St t

E l

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Heat flow at heating 10K/min (solid line) and

subsequent cooling with 10K/min

Example

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Amorphous