ceramics

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Structure &
Properties
of Ceramics

Lecture 3
EBB113

Why study ceramic materials?
– Very “traditional” (crude civil engineering material) – BUT also new high-tech ceramics and applications.
• Optical (transparency) opto-electronic
• Electronic (piezoelectrics, sensors, superconductors)
• Thermo-mechanical (engine materials)
• Cutting tools
Today, the
U.S. market is
estimated to be over
$35 billion.

2

Introduction
keramikos - burnt stuff in Greek → desirable properties of ceramics are normally achieved through a high-temperature heat treatment process (firing). Usually a compound between metallic and non-metallic elements. Always composed of more than one element (e.g.,Al2O3, NaCl, SiC, SiO2) Interaction bond either totally ionic or having some covalent character Properties :

• Generally hard and brittle
• Generally electrical and thermal insulators (exceptions: graphite, diamond, AlN… and others)
• Can be optically opaque, semi-transparent, or transparent High chemical stability and high melting temperature.
3

Class of ceramic
Traditional Ceramics: primary raw materials is clay
Example: porcelain, bricks, tiles, glasses.

4

Class of ceramic

Oxygen sensor

Now new generation of this materials
have evolved
Engineering Ceramics
High-temperature ceramic
Advance ceramic
Electroceramic

Aerospace product-Ni
Ti, Stainless steel
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Ceramics Structure

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Ceramic Crystal Structures
Ceramic which predominately ionic-composed of cation (+ve charge) and anion (-ve charge)
Atomic Bonding: Mostly ionic, some covalent.
% ionic character increases with difference in electronegativity. CaF2: large
Ionic SiC: small
12%

Ionic
89%

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Crystal Structure
Two characteristic which influence the crystal structure
1.
Magnitude of electrical charge of each component ions
2.
The relative sizes of the cation and anions

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1. Magnitude of electrical charge of each component ions
Electrically must be neutral-Net charge in structure should be zero

CaF 2 :

Ca 2+ +
cation

Fanions
F-

A m Xp

m, p determined by charge neutrality

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2. The relative sizes of the cation (rC) and anions (rA)

Stable structures will be formed when anions surrounding a cation are all in contact with that cation

-

+

-

-

+

-

-

+

-

Stable
crystal
structure

unstable

stable
stable
Each cation prefer to have as many nearest-neighbor anions
The anions desire a maximum no of cation nearest-neighbor

Anion
surrounding
cations are
all in contact
with cations
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Coordination number
Coordination no (i.e no of anions nearest-neighbors for a cation) is related to the
r cation

r anion
Coordination no increases with

r cation
r anion

There is a critical or minimum rC/rA ratio for which this cation-anion contact is established and this ratio can be determined from geometrical consideration (see following example)

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Example: Determine minimum rcation/ranion for octahedron site (C.N. = 6)

2ranion + 2rcation = 2a

a = 2ranion
2ranion + 2rcation = 2 2ranion
ranion + rcation = 2ranion

rcation = ( 2 −1)ranion

rcation
= 0.414
ranion
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The coordination no and geometries for various rC/rA
ratios.
Important table!
r cation

ZnS
(zincblende)

r anion

Coordination
no

< 0.155

2

linear

0.155 - 0.225

3

triangular

0.225 - 0.414

4

tetrahedron

0.414 - 0.732

6

octahedron

0.732 - 1.0

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cubic

NaCl
(sodium
chloride)
CsCl
(cesium
chloride)

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AX-type Crystal structure
Rock Salt Structure
Example: NaCl (rock salt) structure
The coordination no for both cation and anion is 6.
A unit cell is generated from FCC arrangment of anions with one cation in centre and one at center of 12 cube edges
rNa = 0.102 nm
rCl = 0.181 nm
rNa/rCl = 0.564
∴ cations prefer octahedron sites
Example: NaCl, MgO, MnS, LiF, FeO

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AX-type Crystal structure...
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