Ceramics and Glasses

Ceramics and Glasses

1. Ceramics

   • Raw Material: Clay. Fine particles of hydrous aluminum silicate. Kaolinite, Al2SI2O5(OH4)
     • When mixed with water, clay becomes formable and moldable. When heated to a sufficiently elevated temperature (firing), it fuses into a dense, strong material.
   • Products: Construction (bricks, clay pipe, building tile; Refractory ceramics (furnace walls, crucibles, molds); Concrete; Whiteware (pottery, stoneware, fine china, porcelain); Abrasives (Aluminum oxide, silicon carbide); Cutting tool materials (tungsten carbide, cubic boron nitride); Insulators; Bioceramics (artificial teeth, bones)
   • Properties
     • Hard, high melting temp, low conductivity of electricity and heat
     • Difficult to shear plastically (ionic nature of bonding that makes them brittle / non-ductile): Fracture once crack propagates
     • Compared to metallic materials, they are harder because covalent and ionic bonding are stronger than metallic bonding. However they cannot slip under stress and are hence more rigid.
   • Coordinated network structure of silica/ ordered crystalline structure

2. Glass: Amorphous "ceramics"

   • Uncoordinated random network structure of amorphous glass of silica that can be network-modified to affect glass-transition temperature Tg
   • Products: Window glass, containers, light bulbs, glassware, glass fibers (insulation, fiber optics), optical glasses (lenses)
   • Glassy state: Insufficient time allowed during cooling from the molten state for the crystalline structure to form. It cools to a rigid condition without crystallizing
   • Crystalline: crystallize at melting temp, Tm. Has abrupt change in specific volume at Tm.
   • Glasses: Do not crystallize, specific volume varies smoothly with T and there is a glass transition temp, Tg
   • Network Formers
     • Silica Glass (SiO2) BEST glass network former. Si-O strong covalent bonds. Each Si bonds to 4 O atoms --> 3D Structure
       • Easy to cool liquid SiO2 into an amorphous state as quartz forms over a geologic time span
       • Naturally transforms into a glassy state upon cooling from the liquid, while most ceramics crystallie upon solidifcation
     • B, Si, Ge, P, V, As
   • Network Modifiers: Modify the basic glass network to modify properties
     • Na, Ca, Mg, K, B
     • Promotes fusion during heating acting as a flux. Increases fluidity in molten glass for processing. Improve chemical resistance. Add colour. Alter index of refraction for optical applications
   • Intermediate oxides: Do not dramatically affect the glass structure
     • Ti, Zn, Pb, Al, Be, Zr, Cd

3. Glass-ceramic

   • conversion of glass into a polycrystalline structure through heat treatmet
   • Nucleation: Small amounts of nucleating agents such as TiO2, P2O5, ZrO2. Glass is cooled and reheated to cause dense network of crystal nuclei to form.
     • High density of nucleation sites inhibits graingrowth, leading to fine grain size
   • Crystallization
     • Heat treatment continued from nucleation at a high temperature to cause growth of crystalline phases
     • Grain size (0.1-1 micrometre) significantl smaller than conventional ceramics. Fine crystal structure makes glass-ceramics much stronger than the gasses from which they are derived. Proportion of crystalline phase ranges from 90-908%. Remainder is unconverted vitreous material.
       • Glass-ceramics are opaque usually grey/white rather than clear

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