Ceramics and glasses

What bonding do ceramics and glasses use and what properties do they have

covalent and ionic; bonds difficult to rearrange; stiff hard and brittle materials (difficult to machine and shape); low thermal conductivities and high heat capacities

What are non-technical ceramics

often clay based; polycrystalline: usually more than one crystalline phase, often contain some glassy materials, open porosity (connect to surface)

What are technical ceramics

high grade materials (expensive); polycrystalline; may be oxides, carbides, nitride; possibly contain glassy grain boundary phase; usually only closed porosity

Clay based ceramics

structural applications: bricks, roof tiles, cement and concrete; interior: floor and wall tiles, sanitaryware; pottery: earthenware, porcelain; relatively cheap

What are refractories

materials resistant to heat decomp or chemical attack

What are some clay based refractories

fire clay; high alumina

What are some non-clay refractories

silica; periclase; extra high alumina; zircon; silicon carbide based materials

Give some key families of technical ceramics

alumina; zirconia; silicon carbide; silicon nitride; boron carbide; tungsten carbide; barium titanate

What are some uses of technical ceramics

piezoelectric devices; oxygen sensors; solid oxide fuel cells (ceramic electrolyte); radomes; dental implants; wear resistant parts for hip replacements; thermal barrier coatings

How are ceramics processed

wet processing or powder pressing then fired to form product; sintering and densification cause shrinkage and chemical reactions; some pores remain open after firing

Describe wet processing

suspend ceramic in liquid; slip casting; hydroplastic forming; tape casting

What is Powder pressing

powder pressed into green body: shape of final product (density ~60%) with enough strength to hold together; uniaxial, isostatic (in fluid) or hot pressing

What is slip casting

easy method; precursor slurry of ceramic crystallites; shape into final form; air dry; fire; result contains porosity and has mechanical weakness

What factors are important in ceramics processing

powder processing: milling and mixing; green body formation: pressing; densification: sintering; uniformity: composition, particle size, porosity, particle orientation

What is sintering

high temperature firing; removal of binders; bonding of individual particles; grain growth; pore removal

What are the types of sintering

solid state; liquid phase (solution precipitation); viscous composite sintering (vitrification); viscous glass sintering

Describe the stages in sintering and desification

initial stage: neck formation; intermediate stage: pore structure remains interconnected, later rapid grain growth can occur and pores become isolated; final stage: density >92%, isolated pores, slow densification, significant grain growth (large grains reduce mechanical properties)

What are the stages of liquid phase sintering

melting: additive melts and permeates solid particles; rearrangement: liquid acts as lubricant and capillary action allowing solid particles to move past each other and densify; precipitation: some solid dissolves in liquid and precipitates in the pores; pore removal: solid skeletal growth and pores are removed by solid state diffusion

What is the microstructure of glass

amorphous; often continuous 3D network

What is a glass

non crystalline solid that retains the atomic structure of the liquid; changes continuously from solid to liquid (and vice versa)

Glass transition region

T«Tg = elastic solid; T~Tg = viscoelastic solid/liquid; T>Tg = liquid

Composition of bulk glass

typically a soda-lime-silica composition Na2O CaO SiO2 plus some other stuff

Glass fibre reinforcements

typical E-glass comp CaO Al2O3 SiO2

Raw materials for making glass

high quality sand (low iron SiO2); carbonates: sodium, calcium, dolomite (Ca Mg carbonate)

What are the components of soda-lime-silica glass

silica SiO2 melts at 1710; soda ash Na2CO3 melts at 852; limestone CaCO3 600-1000 decomposes to form refractory CaO

Describe the formation of a carbonate rich liquid in glass melting

Na2CO3 and CaCO3 form a liquid carbonate phase at lower temp; Si reacts with this liquid creating CO2 bubbles; SiO2 content of the liquid increases slowly and solution rate falls (liquid has different comp to final melt); need 1550-1600C and a few hours for process to complete; melt then forms a glass when it cools down

Describe what happens in glass melting

heat to produce initial melt (not same comp as product); further heating and sand dissolves into melt; Batch free = all raw materials gone; heat longer to remove bubbles (viscous liquid); homogenise: allow time (for diffusion) to form uniform melt; forms glass as it cools

Once double carbonates form …

silica starts to dissolve

batch factor =

batch component FW / oxide FW

Weight of batch component for 100g of glass =

batch factor x oxide wt %

How does the viscosity change while cooling

by many orders of magnitude

How is glass cooled

in an annealing lehr; rapidly above annealing point (where internal stresses are relieved); slow between anneal and strain point to avoid residual stresses; rapidly after strain point (cheaper)

Why might glass composition deviate from intended (batched) comp

losses during melting; impurities in raw material; contamination from refractories