L03 Types Of Dental Materials: Unit Cells And Metals Flashcards
What is a crystalline structure?
For any atom, arrangement of neighbouring atom is identical. Bonds that form are aiming to get to their lowest energy state.
Which structures are crystalline?
Most metals are crystalline (unlike plastics)
Ceramics in the middle can be either
Describe amorphous structures
No symmetry of atomic structures
What materials are amorphous?
Amorphous ceramics are glasses
What determines whether something is crystalline or amorphous?
A thermodynamic process. Time and temperature lead. Depends on how long it takes and what temperature it occurs at.
What is a cubic structural cell?
Atoms only on corner of cubes.
What is a BCC structural cell?
Body centred cubic - atoms on corners of cube and one in the middle of the body of the unit cell.
What is an FCC structural cell?
Face centred cubic arrangement - atoms aligned on corners and the face of each side.
What are factors relating to atoms occupying different volumes?
Packing factor - e.g., more packed in an FCC to a cubic
Possible for smaller atoms to occupy free space
These factors change the properties of the crystal and material
What is phase transition?
A transition temperature. For crystalline materials, there will be a discrete change in volume when the material changes phase.
What happens to nucleation as temperature decreases?
Crystals (grains) grow
Grains impinge (hit each other) giving grain boundaries
As temp decreases, extent of nucleation increases
Are crystals and grains the same?
Yes, crystals = grains
What is nucleation?
Nucleation = nuclei of crystallisation are imperfections e.g., bits of dust or grain refiners which cause crystals to grow within solution
What do properties of metal crystallisation depend on?
Grain size
How does grain size affect properties of metals?
Rapid cooling produces smaller grains, increased area of grain boundaries (increases material properties)
How do grain boundaries affect properties of metals?
Grain boundaries help to prevent cracks/defects from growing in the material. The more the grain boundaries, the more likely the crack/defect will stop growing. It will become arrested.
How can you change the crystal orientation of a metal after it has set and solidified by changing grains and grain boundaries?
Temperature or cold-working using force
What is an alloy?
A mixture of 2 or more metals
How are alloys formed?
By fusion of the 2 metals above their melting point. 2 metals soluble in each other. Forms a solid solution.
How is melting point of a single element different to that of an alloy?
Single element has a discrete melting point. Mixed alloys mean that at a higher temperature, everything is liquid = liquidus. At a temperature below a certain point everything becomes solid = solidus. Gradual change in the melting temperature not discrete.
What is a substitutional solid solution? (Alloy)
Substituting atoms that would otherwise be a single element, with alternative atoms that are of similar morphology (= same valency, same crystal structure, similar atomic size within 15%)
What is an interstitial solid solution?
Smaller atoms are able to occupy spaces between the larger atoms, creating some distortion in the unit cell being created. Distorts the purely crystalline structure to change properties. Solute atoms must be much smaller than solvent atom. Some distortion of the lattice will occur to accommodate the extra atoms.
Give examples of substitutional solid solutions?
Gold copper alloy, copper and zinc (brass), sterling silver (silver and copper)
Give examples of interstitial solid solutions
Steel (carbon atoms are smaller and occupy interstitial spaces in iron)
What are intermetallic compounds?
2 or more metals combine with a discrete composition, e.g., dental amalgam. They provide unique properties for metals, e.g., can become stiff or break in a brittle fashion.
Applications of metals and alloys: what metals can be used in amalgams?
Ag/Sn/Cu alloy, Hg
Applications of metals and alloys: what metals can be used in inlays/onlays?
Au alloys
Pd alloys
Applications of metals and alloys: what metals can be used in crowns?
S/ steel (Fe, C, Ni, Cr)
Au alloys
Pd alloys
Ni/Cr alloys
Applications of metals and alloys: what metals can be used in bridges?
Au alloys
Pd alloys
Ni/Cr alloys
Applications of metals and alloys: what metals can be used in dentures?
Co/Cr alloys
S/ steel
Applications of metals and alloys: what metals can be used in dental implants?
Ti
