phy351 ch 7

CHAPTER 7

CERAMICS & GLASSES

PHY351

Ceramics

Ceramics materials are inorganic and nonmetallic

materials that consist of metallic and nonmetallic

elements bonded together primarily by ionic and/or

covalent bonds.

Good electrical and heat insulation property.

Hard, brittle, and lesser ductility and toughness than

metals.

High chemical stability and high melting temperature.

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Traditional Ceramics: Basic components (Clay and

Silica).

Example:

- Glass

- Bricks

- Tiles

Engineering Ceramics: Pure compounds (Al2O3, SiC).

Example:

- Gas turbine engine (Silicon Carbide, SiC)

- Spark plug insulator material(Alumina, Al2O3)

Note:

Depends on electronegativity difference.

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Mixture of Ionic and Covalent Types.

Processing of Ceramics

Produced by compacting powder or particles into shapes and heated to bond particles together.

The basic steps in the processing of ceramics are:

1. Material preparation:

- Particles and binders and lubricants are (sometimes ground) and blend wet or dry.

2. Forming or casting:

- Formed in dry, plastic or liquid conditions.

- Cold forming process is predominant.

- Pressing, slipcasting and extrusion are the common forming processes.

3. Thermal treatment

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PRESSING

Dry Pressing:

- Simultaneous uniaxial compaction and shaping of power along

with binder.

- Wide variety of shapes can be formed rapidly and accurately.

Forming Or Casting6

Isolatic pressing:

- Ceramic powder is loaded into a flexible chamber and pressure is

applied outside the chamber with hydraulic fluid.

Examples: Spark plug insulators, carbide tools.

Hot pressing:

-Ceramics parts of high density and improved mechanical

properties are produced by combining the pressing and firing

operations.

- Both unaxial and isostatic methods are used.

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SLIP CASTING

Powdered ceramic material and a liquid mixed to prepare a stable

suspension (slip).

Slip is poured into porous mold and liquid portion is partially

absorbed by mold.

Layer of semi-hard material is formed against mold surface.

Excess slip is poured out of cavity or cast as solid.

Alternatively, a solid shape maybe made by allowing the casting to

continue until the whole mold cavity is filled. This called s solid

casting.

The material in mold is allowed to dry and then fired.

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Figure 11.25 Slip casting of ceramic shapes:

a) Drain casting in porous plaster of paris mold

b) Solid casting9

EXTRUSION

Single cross sections and hollow shapes of ceramics can be

produced by extrusion.

Plastic ceramic material is forced through a hard steel or alloy die

by a motor driven augur.

Examples:

- refractory brick

- sewer pipe

- hollow tubes.

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Drying:

- Parts are dried before firing to remove water from ceramic body.

- Usually carried out at or below 1000C.

Sintering:

- Small particles are bonded together by solid state diffusion

producing dense coherent product.

- Carried out at higher temperature but below MP. Longer the

sintering time, larger the particles are.

Vitrification:

- During firing, glass phase liquefies and fills the pores.

- Upon cooling liquid phase of glass solidifies and a glass matrix

that bonds the particles is formed.

Thermal Treatment11

Mechanical Properties of Ceramics

Strength of ceramics vary greatly but they are generally

brittle.

Tensile strength is lower than compressive strength.

Many ceramic materials are hard and have low impact

resistance due to their ionic-covalent bonding.

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Ceramics Deformation

Covalently bonded ceramics:

- Exhibit brittle fracture due to separation of electron-pair bonds

without their subsequent reformation.

Ionically bonded ceramics:

- Single crystal show considerable plastic deformation.

- Polycrystalline ceramics are brittle.

Example:

NaCl crystal slip in {100} family of planes is rarely observed as

same charges come into contact. Cracking occurs at grain

boundaries

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Ceramics Fatigue Failure

Fatigue fracture in ceramics is RARE due to absence of plastic

deformation.

Straight fatigue crack in has been reported in alumina after 79,000

compression cycles.

Ceramics are hard and can be used as abrasives.

Examples:- Al2O3, SiC.

By combining ceramics, improved abrasives can be developed.

Example:- 25% ZrO2 + 75% Al2O3

Figure 11.37:

Optical micrograph of fatigue cracking of polycrystalline

alumina under cyclic compression.

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Question 1

a. What are factors affecting the strength of the ceramic materials?

b. What are important properties for industrial abrasives?

Thermal Properties of Ceramics

Low thermal conductivity and high heat resistance.

Many compounds are used as industrial refractories

which are materials that resist the action of hot

environment.

For insulating refractories, porosity is desirable.

Dense refractories have low porosity and high

resistance to corrosion and errosion.

Aluminum oxide and MgO are expensive and difficult to

form and hence not used as refractories.

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Question 2

Define the following defect terms:

a. Ceramic creep

b. Ceramic thermal shock

c. Ceramic static fatigue

Glass

Glass material is a ceramic material that is made from

inorganic materials at high temperature and

distinguished from other ceramics in that its constituents

are heated to fusion and then cooled to a rigid condition

without crystallization.

Up on cooling, it transforms from rubbery material to

rigid glass.

Some of the glass properties are transparency, strength,

hardness and corrosion resistance.

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Figure 11.41:

Solidification of crystalline and glassy (amorphous) materials showing

changes in specific volume.

Tg is the glass transition temperature of the glassy material.

Tm is the melting temperature of the crystalline material.

Viscous Deformation of Glass

Viscous above Tg and viscosity decreases with increase

in temperature.

Where;

Q = Activation energy

η* = Viscocity of glass (PaS)

η0 = preexponential constant (PaS)

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η* = η0eQ/RT

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Question 3

A 96 percent silica glass has a viscosity of 1013 P at its annealing

point of 9400C and a viscosity of 108 P at its softening point of

14700C. Calculate the activation energy in kilojoules per mole for the

viscous flow of this glass in this temperature range.

(Answer : 382 kJ/mol)

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1. Working point: 103 PaS – glass fabrication can be carried out

2. Softening point: 107 PaS – glass flows under its own weight.

3. Annealing point: 1012 PaS – Internal stresses can be relieved..

4. Strain point: 10 13.5 PaS – glass is rigid below this point.

Figure 11.44:

Effect of temperature on the viscosities of various types of glasses. Number

of curves refer to different compositions.

Glass Forming Method

Forming sheet and plate glass:

- Ribbon of glass moves out of

furnace and floats on a bath of

molten tin.

- Glass is cooled by molten tin.

- After it is hard, it is removed

and passed through a long

annealing furnace.

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Blowing, Pressing and Casting

Blowing:

Air blown to force molten glass into molds.

Pressing:

Optical and sealed beam lenses are pressed by a plunger into a

mold containing molten glass.

Casting:

Molten glass is cast in open mold.

Centrifugal casting:

Glass globs are dropped into spinning mold.

Glass first flows outward towards wall of mold and then upward

against the mold wall.

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Figure 11.46:

(a) Reheat and

(b) Final blow stages of a glass blowing machine process.

Tempered Glass

Glass is heated into near softening point and rapidly

cooled.

Surface cools first and contracts.

Interior cools next and contracts causing tensile stresses

in the interior and compressive stress on the surface.

Tempering strengthens the glass.

Examples:

Auto side windows and safety glasses.

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Figure 11.47:

Cross section of tempered glass.

(a) After surface has cooled from high temperature near glass-softening temperature

(b) After center has cooled.

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Question 4

Define the following terms:

a. Annealing glass material

b. Tempering glass material

Optical properties of Glass

Refractive index

Reflectance

Transparency

Translucency

Opticity

Colour

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Transparency & Refractive Index30

When photons are transmitted through a transparent material,

they loose some energy and speed and the direction changes.

Refractive Index = C (Velocity of light in vacuum)

V (velocity of light in a medium)

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If light passes through one media to another:

Total internal reflection if angle φ > φc

If light passes from media of high refractive index to a media of

low refractive index, φ’ = 900 at φ = φc

Sin

Sin

n

n '

'

Reflectance of Light32

For a particular wavelength λ :

(Reflected fraction) λ +(Absorbed fraction) λ + (transmitted fraction) λ = 1

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Reflection of light from a glass surface:

Fraction of light reflected = R =

R = reflectivity (φi=900)

n = refractive index.

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1

1

n

n

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Question 5

a. Calculate the reflectivity of ordinary incident light from the polished

flat surface of a silicate glass with a reflective index of 1.46.

(Answer: 3.5%)

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Absorption of light by glass plate: Light intensity decreases as

light path decreases.

I = Fraction of light exiting

I0 = Fraction of light entering

α = linear absorption coefficient.

t = thickness

teI

I

0

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Question 6

a. Ordinary incident light strikes a polished glass plate 0.5 cm thick

that has a refractive index of 1.5 . What fraction of light is absorbed

by the glass as the light passes between the surfaces of the plate?

Given = 0.03cm-1.

(Answer: 1.5%)

Translucency

Translucency also called translucence or translucidity.

Translucency is a super-set of transparency, allows light to pass

through but does not necessarily follow Snell's law.

The photons can be scattered at either of the two interfaces where

there is a change in index of refraction or internally.

In other words, a translucent medium allows the transport of light

while a transparent medium not only allows the transport of light but

allows for the image formation.

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Opacity

Opacity is the degree to which light is not allowed to travel through

which the opposite property of translucency.

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Colour39

Visible light: Electromagnetic radiation with wavelength 0.4 to 0.75 micrometers.

Ultraviolet : 0.01 – 0.4 micrometers

Infrared: 0.75 – 1000 micrometers

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Light is in form of waves and consist of particles called photons.

ΔE = hν = hC/λ

ΔE = Energy

λ = wavelength

ν = frequency

C = speed of light = 3 x108 m/s

H = plank’s constant = 6.62 x 10-34 J.s

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Question 7

a. A photon in a ZnS semiconductor drop from an impurity energy

level at 1.38 eV below its conduction band to its valence band.

What is the wavelength of the radiation given off by the photon in

the transition? If visible, what is the colour of the radiation? ZnS

has an energy band gap of 3.54 eV.

(Answer: 574.7nm)

References

A.G. Guy (1972) Introduction to Material Science,

McGraw Hill.

J.F. Shackelford (2000). Introduction to Material

Science for Engineers, (5th Edition), Prentice Hall.

W.F. Smith (1996). Priciple to Material Science and

Engineering, (3rd Edition), McGraw Hill.

W.D. Callister Jr. (1997) Material Science and

Engineering: An Introduction, (4th Edition) John

Wiley.

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