Transitions in materials Flashcards
What are the 5 classes of materials?
Metals, ceramics, glasses, polymers, composites
Give examples of primary and secondary bonds
Primary: covalent (share), ionic (donate), metallic (weaker, sea of e-s)
Secondary: Van der Walls, H bonds
Which metals are used in dentistry?
Amalgam for fillings
Stainless steel (iron and chronium) for braces
Gold for fillings
Titanium for implants
What property helps implants to bond with bone?
Titanium oxide on surface
Properties of metals
Hard, opaque, lustrous
Good conductors of heat and electriity
Generally malleable
Can be cast and amalgamated
Define the metallic bond
Consists of a lattice of bonded metal cations with a sea of electrons
Why are metals lustrous?
Photons are absorbed by free e-s, which move up to a higher energy level. When they fall back down the energy is reemited as light
Why are metals ductile?
Cations are able to align while being shielded by their fluid like sea of e-s
Why do metals not shatter?
They just dent. The mobile e’s sheild cations from each other, preventing violent repulsion and allowing metal to change shape.
How are metal cations organised?
7 different crystal lattices
What is a transition in dental materials?
Reorganisation of distribution of atoms in a material. Occurs at transition temp
What are the common transitions for metals?
Solid > liquid > solid
Solid + liquid > solid
Solid > solid
Which ceramics and glasses are used in dentistry?
PerioGlas, zirconia, lithium disilicate, porcelain, HA
What are ceramics?
Inorganic, non-metallic compounds
Usually crystalline
Compounds of metal ions but not metallic themselves
Occasionally term ceramic is used to encompass glasses (inorganic amorphous materials)
Properties of ceramics
Brittle, hard
high melting points
weak in tension, high compression strength
ordered 3D structure of covalently and ionically bonded compounds of a metal with a non-metal e.g. Al2O3
How are ceramics produced?
- Powder synthesis
- Preconsolidation processing (milling/ grading/ additives)
- Compaction/ shape forming (produce ‘green body’
- Drying
- Densification (sintering)
MIX SIEVE SQUASH DRY HEAT
Methods for producing a ‘green body’ ready for drying
Dry pressing Isostatic pressing Slip casting Tape casting Extrusion Injection moulding
Common transitions in ceramics
Powder > sinter (shrinkage) > solid
Solid > hot press > solid
Slid + liquid > solid (setting reaction)
What are glasses?
Brittle, hard materials
Non-crystalline solids
No long range (atomic) order
Supercooled liquid where nucleation has been avoiding
How are glasses formed?
By melting, sol-gel techniques or vapour deposition
What does supercooled mean?
Cooled below melting point without solidification (usually very past), then chucked in water
Common transitions in glass
Normal cooling –> Crystallisation –> No glass formation
Supercooling –> no time to organise molecules –> amorphous –> forms a glass
What is a classic glass former?
Basic building block for silica (ceramic or glass) is silica tetrahedron with identical bond lengths and bond angles. Tetrahedra bonded to neighbouring tetrahedra via bridging oxygens
What changes the melting point of a glass?
Putting ions in the way, disrupting structure
What are glass-ceramics formed by?
Formed by controlled crystallisation of glass
2 stage heat treatment: nucleation & growth
What is the structure of glass-ceramics?
Typically pore free, little measurable decrease in volume on ceramming
Name 3 polymers used in dentistry?
Acrylic resin
Polyether
Alginate
What is the glass transition temperature?
The T at which the polymer chains begin to flow past each other. No immediate point of melting
Common transitions in polymers
Solid > liquid > solid
Monomer > solid
polymer liquid > polymer solid
-polymers melt slowly with heat and solidify when being cooled
What are composites?
2 materials mixed together
3 composites used in dentistry
GIC
IPS e.max lithium disilicate
Dental composite (-resin)
Common transitions in composites
Depend on exact constituents on composites
Cannot make generalisations
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