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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.