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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 Glass
Another 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.
Powder Processing
First step is milling the ceramic to form aparticular size range, followed by sizing.
Second stage may involve adding a lubricantand binders to permit particles to flow and bond together during later processing.
Third stage involves compacting the powdersin a mold which is the final shape of thepart. It is usually oversize.
A “green” compact is formed after these operations.
Particle Sintering
The 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.
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