Ceramics and Glass

7/28/2019 Ceramics and Glass

1/26

CERAMICS& GLASS

REFF:

Materials Science & Engineering; An Introduction

Callister, W. D, Jr, 2007, John Wiley & Sons Fundamental of Ceramics, Barsoum, M. W., 2003,McGraw-Hill

Engineering Materials 2; An Introduction toMicrostructures, Processing and Design, Ashby, M. F

and Jones, D. R. H, 1986, Pergamon Press


2/26


3/26

Introduction

CERAMICS: Greek keramikos = burn stuff

solid compounds formed by heat (&/P)

applications followed by coolingat least 2 elements; 1 is a non-metal, the

other may be (a) metal(s) or (an)other non

Oxygen and silicon are abundance in nature:rocks, dust, clay, mud


4/26

Atom arrangement

In ceramics: long-range

order, short-range order or

both combination

Long-range order: repeatperiodicity >>bond length

cryatalline solid;ceramics

Short-range order:No

periodicity amorphous,

glassy or noncrystalline

solid; glass


5/26

1 unit cell: the smallest

region when repeated,

completely describes3D atoms of crystal

Crystalline solid: single

crytals & polycrystalline

solids

Single crystal: solid in

which the periodic &

repeated arrangementof atom is perfect &

extends specimen

entire, no interruption

Polycrystalline solid:

many single crystals

(grains), separatedeach other by disorder

area (grain boundaries)


6/26

General Characteristics of

Ceramics

Hard

Wear-resistant

Brittle

Prone to thermal

shock

Resistant to usual

treatment Oxidation resistant

Chemically stable

Nonmagnetic

Electrically &

thermally insulative


7/26

Ionic Vs Covalent Bonding

Ionic compound form between very active

metallic & nonmetallic elements

To form AX ionic bonding, A loses e easily,

X accepts e without too much energy input

Covalent bonding occurs if energies of

bonding electrons of A & X are comparable

If the electron energy of the atoms is

different transfer energy (ionic bonding)


8/26

Ceramic Structures

Atomic bonding: purely ionic to totally

covalent, many exhibit combination type

Degree of ionic character

electronegativities of atoms

Electronegativity (e greed): the power of

atom to attract electrons to itself

Two elements forming a bond have similar

electronegativity share e; covalent bond


9/26

Mutual attraction

lower potential energy If the electronegativity difference between

them (x)is large (indicating 1 element is

greedier than other), e attracted to the

more electronegative elemention attract

each other

x > 1.7ionic

x < 1.7covalent


10/26

Character of ionic compounds: have high

melting & boiling points (the bonds are strongand omnidirectonal), hard, brittle, poor electrical

and thermal conductors


11/26

Crystal Structure

Ionic bondingcomposed of electricallycharge ions instead of atoms

Metalic ions (cation, + charge) give

valence electron to the nonmetalic Characteristics of ions which affect crystal

structure:

1. magnitude of electrical charged of eachions

2. relative size of + and - ion


12/26

Crystalelectrically neutral; (+) charges

must be balanced by an equal number of

() related to the chemical formula

Each cation prefers as many neighbour

anions, anions also desire a maximum

number of cation. Coordination no. (number of anions

neighbors for a cation)related to rc/ra

Stable structures require that cations andanions touch


13/26

Table: Coordination

numbers and

geometries for

various rc/ra Blue cation

Redanion

Common coordinationnumbers for ceramic:

4, 6 and 8


14/26

The size of an ion depend on:

1. coordination number

Ionic radius increase as the number of opposite

charge neighbor ions increases2. number ofe of atom/ion

Removing e from atom/ion, the remainingvalence electrons become more tightly bound to

the nucleus decrease ionic radius. Ionic sizeincreases when electrons are added to an atomor ion.


15/26

AX-type crystal structures

Ceramic of metal & nonmetal element (AX)

with equal number of A (cation) & X (anion)

Ionic MgO; 2 e of A transferred to X, result

in Mg2+ & O2-

CovalentZnS; sharing elektron

3 structures: rock salt, CeCl and ZnS


16/26

Rock salt (NaCl) structure

The most common AX crystalstructure

Electrostatic attraction betweenNa+ & Cl- hold the crystaltogether

Max. electrostatic interactioneach Na+ has 6 Cl-, no Na+neighbours (vice versa)

Coordination number for both +

& - is 6 (octahedral) 1 unit cell generated from

FCC of anion with 1 cation incubic center & 1 at centered ofeach of 12 cube edge

NaCl, MgO, MnS, LiF and FeO


17/26

Cesium cloride (CsCl) stucture

Coordination number

for both ions is 8

(cubic)

The anions are ateach of the corners of

a cube

Single cation is at the

cube center


18/26

Zinc Blende (ZnS) structure

Coordinate number for

both ions is 4

(tetrahedral)

all corner and face

positions of the cubic cell

are occupied by S atoms

the Zn atoms fill interior

tetrahedral positions

Most often the atomic

bonding is highly covalent

ZnS, ZnTe, and SiC


19/26

SILICATE CERAMIC

Material composed primarily of silicon and oxygen

Each silicon atom bond strongly to 4 oxygen atom

Basic unit in all silicates tetrahedron (oxygen are

situated at the corners, oxygen is at the centre)


20/26

The most simple: silicondioxide/silica

Pure silica no metal ions,every oxygen becomes abridge between 2 silicon atoms

Every corner oxygen atom isshared by adjacent tetrahedra

The materials electricallyneutral, all atoms have stableelectronic structures

Ratio Si to O1:2 (indicatedby chemical formula)

If tetrahedras are arranged in aregular & order crystalline

3 polymorphic: quartz,cristobalite & tridymite


21/26

Tetrahedra link: directly or viametal ion (M)

Silicas combined with metaloxides (e.g. MgO, CaO,

Al2O3) MO/SiO2 2separated

SiO4 monomer linked by MOmolecules

MO/SiO2 < 2 silicadimers;1oxygens sharedbetween 2 tetrahedra(bridging oxygen)


22/26

Decrease amount of MO increase degree ofpolymerisation

Chain form

linked tetrahedra BackboneSi-O-Si-O-Si-;

2 oxygens of each tetrahedron are shared (2bridging oxygens)

The others forms ionic bonds between chains,joined by MO; weaker than backbonefibrous

E.g. asbeston


23/26

Sheet form3 oxygen of each tetrahedron are

shared; e.g. clay

M attached to one of side of sheet (side withspare oxygen on it)

The sheet is polarised a net + charge on one

surface, - charge on the other This interaction strongly with water, attract a

water layer between the sheets clay plastic


24/26

Silica Glasses

Commercial glasses are based on silica Silica also exist as noncrystalline solid/glassfused

silica/vitreous silica

Pure silica forms glass with high softening T (1200 C)

Great strength and stability, low thermal expansion but hard to

work with because high in viscosity


25/26

Commercial glasses silica glasses add with

other metal oxide toreduce viscosity

E.g. CaO, Na2O addpositive ion to the

structure &break up thenetworknetworkmodifiers

Add 1 Na2O molecules

introduces 2 Na+, eachattaches to 1 oxygen oftetrahedronnonbridging


26/26

TiO2, Al2O3 substitute silicon; become partand stabilize the network intermediates

Reduction in cross-linking/ substitute silicon

soften the glass, reduce Tm, viscosity of glass easier to form at low T

Common window glass: 70% SiO2, easily workat 700 C

Pyrex: 80% SiO2, less modifier, better thermalshock resistance, thermal expansions lower,harder work (800 C)

Ceramics and Glass

Documents

Transcript of Ceramics and Glass