CHE 333 Class 24

Ceramics, Glass and Carbon.

Glass Silica Tetrahedra

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.

Ceramic Phase Diagrams

Applications of “White ware” as a function of composition.

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.

Pore Density V Sintering Time

Ceramic Properties

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

Ceramic Slip Casting

Glass Residual Stress

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”.Rope to moon possible, filters, fuel cell electrodes