L03 Types Of Dental Materials: Unit Cells And Metals

Question 1

What is a crystalline structure?

Answer 1

For any atom, arrangement of neighbouring atom is identical. Bonds that form are aiming to get to their lowest energy state.

Question 2

Which structures are crystalline?

Answer 2

Most metals are crystalline (unlike plastics)
Ceramics in the middle can be either

Question 3

Describe amorphous structures

Answer 3

No symmetry of atomic structures

Question 4

What materials are amorphous?

Answer 4

Amorphous ceramics are glasses

Question 5

What determines whether something is crystalline or amorphous?

Answer 5

A thermodynamic process. Time and temperature lead. Depends on how long it takes and what temperature it occurs at.

Question 6

What is a cubic structural cell?

Answer 6

Atoms only on corner of cubes.

Question 7

What is a BCC structural cell?

Answer 7

Body centred cubic - atoms on corners of cube and one in the middle of the body of the unit cell.

Question 8

What is an FCC structural cell?

Answer 8

Face centred cubic arrangement - atoms aligned on corners and the face of each side.

Question 9

What are factors relating to atoms occupying different volumes?

Answer 9

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

Question 10

What is phase transition?

Answer 10

A transition temperature. For crystalline materials, there will be a discrete change in volume when the material changes phase.

Question 11

What happens to nucleation as temperature decreases?

Answer 11

Crystals (grains) grow
Grains impinge (hit each other) giving grain boundaries
As temp decreases, extent of nucleation increases

Question 12

Are crystals and grains the same?

Answer 12

Yes, crystals = grains

Question 13

What is nucleation?

Answer 13

Nucleation = nuclei of crystallisation are imperfections e.g., bits of dust or grain refiners which cause crystals to grow within solution

Question 14

What do properties of metal crystallisation depend on?

Answer 14

Grain size

Question 15

How does grain size affect properties of metals?

Answer 15

Rapid cooling produces smaller grains, increased area of grain boundaries (increases material properties)

Question 16

How do grain boundaries affect properties of metals?

Answer 16

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.

Question 17

How can you change the crystal orientation of a metal after it has set and solidified by changing grains and grain boundaries?

Answer 17

Temperature or cold-working using force

Question 18

What is an alloy?

Answer 18

A mixture of 2 or more metals

Question 19

How are alloys formed?

Answer 19

By fusion of the 2 metals above their melting point. 2 metals soluble in each other. Forms a solid solution.

Question 20

How is melting point of a single element different to that of an alloy?

Answer 20

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.

Question 21

What is a substitutional solid solution? (Alloy)

Answer 21

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%)

Question 22

What is an interstitial solid solution?

Answer 22

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.

Question 23

Give examples of substitutional solid solutions?

Answer 23

Gold copper alloy, copper and zinc (brass), sterling silver (silver and copper)

Question 24

Give examples of interstitial solid solutions

Answer 24

Steel (carbon atoms are smaller and occupy interstitial spaces in iron)

Question 25

What are intermetallic compounds?

Answer 25

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.

Question 26

Applications of metals and alloys: what metals can be used in amalgams?

Answer 26

Ag/Sn/Cu alloy, Hg

Question 27

Applications of metals and alloys: what metals can be used in inlays/onlays?

Answer 27

Au alloys
Pd alloys

Question 28

Applications of metals and alloys: what metals can be used in crowns?

Answer 28

S/ steel (Fe, C, Ni, Cr)
Au alloys
Pd alloys
Ni/Cr alloys

Question 29

Applications of metals and alloys: what metals can be used in bridges?

Answer 29

Au alloys
Pd alloys
Ni/Cr alloys

Question 30

Applications of metals and alloys: what metals can be used in dentures?

Answer 30

Co/Cr alloys
S/ steel

Question 31

Applications of metals and alloys: what metals can be used in dental implants?

Answer 31

Ti