CHE 333 Class 23. Ceramics
Ceramic Structures Two Criteria for ceramic structures • Electrical Neutrality using ionic charge cation (negative charge) must equal anion (positive charge) amounts. Eg NaCl Na one positive, Cl one negative 2. Anions – usually smaller, must contact Cations 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
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 van der Waals bonds hold sheets together unless water is present. (Si2O5)2- silicate
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 thermal conductivities and high melting points
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 Nano”. Rope to moon possible, filters, fuel cell electrodes
Last Slide Have a good vacation.