Flashcards in Dental Materials Deck (293)
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
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)
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
Define ceramic coordination number
Number of anions around central cation
In crystalline ceramics what 3 areas can unit shapes share?
Corners: share 1 ion
Edges: share 2+ ion
Faces: share 3+ ion
What are the 7 crystal systems (unit cells) of crystalline ceramics?
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
What is a glass?
Inorganic product of fusion material that has cooled to rigid condition w/o crystallisation
Why can't glasses be defined by shape?
Have random, amorphous structure
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
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
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
What're the majority of glasses?
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
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
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
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
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
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
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
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
Describe the general properties of ceramics
High elastic modulus, brittle, hard
Some are bioactive and bioresorbable
Crystalline: less reactive, better mechanical properties
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
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
What are the 2 forms of silica and their subtypes?
a: quartz alpha and beta
b: cristobalite alpha and beta
c: tridymite alpha and beta1,2
a: vitreous (fused)
c: pyrolytic (fumed)
What are the 2 types of silica transformation?
quartz -> tridymite -> cristobalite
Involve breaking binds; difficult, rarely happen
alpha to/from beta
Alpha to beta: straighten bonds, causing expansion
Easy and rapid
Describe the properties of silica
Crystalline and virtuous relatively inert: only attacked by hydrofluoric acid
What are the uses of silica?
1. Filler: composite, investment materials, porcelains, cement, alginate
2. Component: GICs, porcelains
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