Ceramics, Glass and Carbon. CHE 333 Class 24 Ceramics, Glass and Carbon.

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 Kaolinite clay structure. Secondary (Si2O5)2- silicate van der Waals bonds hold sheets together unless water is present. (Si2O5)2- silicate

Ceramic Phase Diagrams Applications of “White ware” as a function of composition.

Powder Processing First step is milling the ceramic to form a particular size range, followed by sizing. Second stage may involve adding a lubricant and binders to permit particles to flow and bond together during later processing. Third stage involves compacting the powders in a mold which is the final shape of the part. 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 thermal conductivities and high melting points

Ceramic Slip Casting

Glass Residual Stress

Carbon Diamond structure drills, Rare, expensive Graphite structure Lubricants, pencils, copiers, tyres. Common, cheap, good conductor

“New” Carbon Structures Fullerenes consist of hexagonal and pentagonal arrangements of carbon atoms. Carbon nanotubes can be produced more expensive than diamond, very strong “ Nano”. Rope to moon possible, filters, fuel cell electrodes