CHE 333 Class 23.

Ceramics

Ceramic Structures

Two Criteria for ceramic structures

1. Electrical Neutrality using ionic charge

cation (positive charge) must equal anion

( negative charge) amounts.

Eg NaCl Na one positive, Cl one negative

2. Cations – usually smaller, must contact

Anions in structure. Determines the

coordination number of the structure for

ceramics. Within certain ranges only

particular coordination numbers will

accommodate the different sized ions

contacting each other.

AX Ionic Structures

NaCl structure CN 6 CsCl structure CN 8 ZnS structure CN 4

All these have a 1 to 1 cation to anion ratio, but different coordination numbers.

AnXm and AnBmXp Structure

Calcium Flouride CaF2 Barium Titanate Piezoelectric material

Ceramic Structures

Glass Silica Tetrehedra

Tetrahedral Structure SiO44-

Si – O ionic ratio is 0.29 tetra CN

Unit cell of silicates. High Crystobalite 1470 to 1710C

Soda GlassAnother structure using silica tetrahedron

as a basic building block. However there is

no three dimensional regularity to the

structure, so it is regarded as “Amorphous”

It is the structure associated with glass.

Silicates

(Si2O5)2- silicateKaolinite clay structure. Secondaryvan der Waals bonds hold sheetstogether unless water is present.

Particle SinteringThe green compact has little structural strength

and is “Sintered” to increase the density of the

part and increase its strength. Sintering is

conducted at temperatures above 0.66Tm, as

diffusion is a major part of the process.

A “neck” is formed between adjacent

particles by surface diffusion. This is a new

chemical bond. The porosity in the part

decreases, so the external dimensions of

the part decrease. The higher the sintering

temperature and the longer the time, the

denser the part and the smaller the voids.

Pore Density V Sintering Time

Ceramic Properties

Strength depends upon pore volume Refractoreis because of low thermalconductivities and high melting points

Carbon

Diamond structuredrills,Rare, expensive

Graphite structureLubricants, pencils, copiers, tyres.Common, cheap, good conductor

“New” Carbon Structures

Fullerenes consist of hexagonal and pentagonalarrangements of carbon atoms.

Carbon nanotubes can be produced more expensivethan diamond, very strong “ Nano Nano”.Rope to moon possible, filters, fuel cell electrodes

Last Slide

Have a good vacation.