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211

Describe the structure of ceramics

Have mostly ionic bonds, some covalent
Polymorphic: can exist as 1+ crystalline form OR as both crystalline and non-crystalline
Dependent on how subunits are structured

212

What 3 factors determine shape of ceramics

1. Max. electrostatic attraction between cation and anion (O2)
2. Min. electrostatic repulsion between anions
3. Anion to anion size ration (anion usually larger)

213

Describe how ceramic units are connected in crystalline and non-crystalline ceramics

Crystalline: regular repeat pattern
Non-crystalline: non-regular, random pattern, short-range molecular order

214

Define ceramic coordination number

Number of anions around central cation

215

In crystalline ceramics what 3 areas can unit shapes share?

Corners: share 1 ion
Edges: share 2+ ion
Faces: share 3+ ion

216

What are the 7 crystal systems (unit cells) of crystalline ceramics?

1. Cubic
2. Tetragonal
3. Orthorhombic
4. Rhombohedral
5. Monoclinic
6. Triclinic
7. Hexagonal

217

What are the 4 Bravais lattices of crystalline ceramics?

1. Primitive/simple: atoms at corner
2. Body: simple + atom in centre of shape
3. Face: simple + atom in centre of all faces
4. Base: simple + atom in centre of top and bottom face

218

What is a glass?

Inorganic product of fusion material that has cooled to rigid condition w/o crystallisation

219

Why can't glasses be defined by shape?

Non-crystalline
Have random, amorphous structure

220

Describe the formation of glasses

High viscosity melt cooled rapidly above critical cooling rate
Crystalline structure doesn't have time to form
As temp. dec., viscosity inc. until form rigid solids w/ random structure of liquids

Are in metastable state
Have no define MP

221

Describe glass transformation

Tg depends on cooling rate
Glasses formed when cooling rate > critical rate
At Tg, high viscosity restricts mobility of molecules, can't move quickly enough to get closer thus shrinkage rate lower

222

Explain correlation between cooling rate and glass density

Cooling rate >>> critical rate glass will have low density as less time to shrink

Cooling rate just > critical rate glass will have high density as more time to shrink

Higher density = greater physical properties

223

What're the majority of glasses?

Oxide glasses

224

What are the 4 rules of oxide glass formation?

1. O2 atom linked to = 2 glass forming atoms
2. Coordination number of glass forming atoms small
3. O2 polyhedral share corners w/ each other; not faces, edges
4. Polyhedral linked in 3D network

225

What are the 3 different types of oxide in glass oxide composition?

1. Glass former: forms 3D network, form glass alone
2. Intermediate: can't form glass alone, takes part in network, cation exchanges for glass forming cation
3. Modifier: disrupts network; breaks bonds, add O2, red. network connectivity

226

What is the network connectivity of oxide glasses?

Av. no. bonds linking each repeat unit in silicate network
Red. viscosity and fusing temp
Inc. coefficient of thermal expansion

227

Describe the processing of ceramics

Most formed from powder either dry or in solution
Formed into required shape by: slip casting, throwing, compaction of powders

After shape formed, article sintered (fired)
When particles packed still gaps between them (porosities)
Shrinkage occurs during sintering due to red. in porosity size
Porosity min. by control of particle size and packing density

228

Describe the effect of particle packing on porosity vol.

Vol. porosity depends on particle size, shape, distribution, packing

Single size spheres: porosity 40%
Can red. by introducing another size, further red. by introducing more

229

Describe sintering effect on porosity

Causes densification as particles merge together
Can occur in solid state (vitrification) or liquid phase
Driving force is red. in surface energy by red. porosity size

230

Explain brittle fracture

How all ceramics fail

Propagation and growth of micro-cracks, usually from surface
As cracks grows inc. stress conc., at critical crack length will run through material and cause failure

Fracture below elastic limit
Stress-strain almost linear
Generally fail @ low strain

231

Describe fatigue in ceramics

Failure: cyclic loading @ lower load than elastic limit; moist environment red. fatigue life

Static: in presence of water, stress enhanced chemical reaction @ tip of crack causes fracture to occur w/ no inc. load

232

Describe the general properties of ceramics

High elastic modulus, brittle, hard
Relatively inert
Some are bioactive and bioresorbable
Crystalline: less reactive, better mechanical properties

233

What are 5 crystalline ceramics used in dentistry?

1. Silica: filler in cements, investment materials
2. Alumina: high strength core of crown and bridge, filler in cements, reinforcing porcelains
3. Hydroxyapatite: artificial tooth root, RC filler
4. Gypsum: stone and plaster as model and die materials
5. Zinc oxide: power component for cements

234

What are 5 non-crystalline ceramics used in dentistry?

1. Fluoroluminosilicate glasses: GIC
2. Radiopaque strontium/barium glasses: filled in composite resin
3. Feldspathic glasses: porcelains
4. Fumed/colloidal silica: microfine filler in composite resin
5. Diatomaceous earth: 80-90% silica, filler in alginates impression materials

235

What are the 2 forms of silica and their subtypes?

1. Crystalline
a: quartz alpha and beta
b: cristobalite alpha and beta
c: tridymite alpha and beta1,2

2. Amorphous
a: vitreous (fused)
b: gel
c: pyrolytic (fumed)

236

What are the 2 types of silica transformation?

1. Reconstructive
quartz -> tridymite -> cristobalite
Involve breaking binds; difficult, rarely happen

2. Displacive
alpha to/from beta
Alpha to beta: straighten bonds, causing expansion
Easy and rapid

237

Describe the properties of silica

Crystalline and virtuous relatively inert: only attacked by hydrofluoric acid

238

What are the uses of silica?

1. Filler: composite, investment materials, porcelains, cement, alginate
2. Component: GICs, porcelains

239

What are the forms of alumina?

Most commonly crystalline forms as corundum, alpha-alumina
Also eta, chi, gamma, delta, theta
Can also be produced from bauxite

240

Describe the properties of alumina

Elastic modulus: GPa > zirconia
Flexural strength: MPa < zirconia
Fracture toughness: MPa.m1/2 < zirconia
Hardness 9 on Moh scale
Slightly soluble in strong acids and alkalis