Chapter 12 Flashcards
What is a Frenkel defect?
A defect consisting of a cation-vacancy and cation-interstitial pair.
What is a Schottky defect?
A defect consisting of a cation-vacancy and anion-vacancy pair.
What is an anion?
A negatively charged ion.
What is a cation?
A positively charged ion.
What is a defect structure?
Relating to the kinds and concentrations of vacancies and interstitials in a ceramic compound.
What is electroneutrality?
The state of having exactly the same numbers of positive and negative electrical charges (ionic and electronic)—that is, of being electrically neutral.
What is flexural strength?
Stress at fracture from a bend (or flexure) test.
What is an octahedral position?
Where there are 6 nearest-neighbor atoms.
What is stoichiometry?
For ionic compounds, the state of having exactly the ratio of cations to anions specified by the chemical formula.
What is a tetrahedral position?
Where there are 4 nearest-neighbor atoms.
What is viscosity?
The ratio of the magnitude of an applied shear stress to the velocity gradient that it produces—that is, a measure of a noncrystalline material’s resistance to permanent deformation.
What is the most common type of mixed bonding in ceramics?
Covalent-Ionic bonding.
What are oxide structures?
- The oxygen anions are larger than metal cations
- Close-packed oxygen in a lattice (usually FCC)
- Cations fit into interstitial sites among oxygen ions
What are the factors that determine crystal structure?
- Relative sizes of ions (maximize the number of opposite charged neighbors for stability)
- Maintenance of charge neutrality (net charge = 0)
What is the relationship between coordination number (nearest-neighbor) and ionic radii?
The coordination number increases as rcation/ranion increases.
What are the five types of stable atomic structures?
- Linear (2 nn) [0 - 0.155]
- Triangular (3 nn) [0.155 - 0.225]
- Tetrahedtral (4 nn) [0.225 - 0.414]
- Octahedral (6 nn) [0.414 - 0.732]
- Cubic (8 nn) [0.732 - 1]
What are 5 common ceramic crystal structures?
- Rock salt (sodium chloride)
- Cesium chloride
- Zinc blende (sphalerite)
- Fluorite
- Perovskite
What are the 3 AX structure types?
- Rock salt (sodium chloride)
- Cesium chloride
- Zinc blende (sphalerite)
What is the AX2 structure type?
- Fluorite
What is the ABX3 structure type?
- Perovskite
What are the characteristics of rock salt (sodium chloride)?
- AX
- FCC
- 6 cations 6 anions
What are the characteristics of cesium chloride?
- AX
- Simple cubic
- 8 cations 8 anions
What are the characteristics of zinc blende (sphalerite)?
- AX
- FCC
- 4 cations 4 anions
What are the characteristics of fluorite?
- AX2
- Simple cubic
- 8 cations 4 anions
What are the characteristics of perovskite?
- ABX3
- FCC
- 12/6 cations 6 anions
How is density calculated for ceramics?
(number of formula units(ΣAc + ΣAa))/(unit cell volumeavogadro’s)
What are silicate ceramics?
- Have tetrahedral shapes
- SiO2 polymorphic forms are quartz, cristobalite, and tridymite
- Strong Si-O bonds result in high melting temps
What are silicates?
- Bonding of adjacent SiO44- by sharing of common corners, edges, or faces
- Cations within maintain charge neutrality and ionically bond SiO44- to each other
What is the structure of glass?
Glass is noncrystalline (amorphous) that has impurity cations
What are layered silicates?
- SiO4 tetrahedra connected together to form a 2-D plane
- Net negative charge associated with each unit
- Negative charge is balanced by an adjacent plane rich in cations
- Adjacent sheets bonded by van der Waal’s
Ex: clays, mica, talc
What are the 3 polymorphic forms of carbon?
- Diamond
- Graphite
- Fullerenes or carbon nanotubes
What are the characteristics of diamond?
- Tetrahedral
- Hardest material with high thermal conductivity
- Large single crystals are gemstones
- Small crystals are used for cutting/grinding
- Thin films are used as surface coatings
What are the characteristics of graphite?
- Layered structure of hexagonal arrays of carbon
- van der Waa’s forces between layers
- Good lubricant since planes slide easily
What are the characteristics of fullerenes/nanotubes?
- Single layers of graphene
- Fullerenes are soccerball shaped made up of hexagons and pentagons
- Nanotubes are carbon tubes
What are the types of point defects in ceramics?
- Vacancies (small holes) for both cations and anions
- Intersitials (inserts) for just cations
How is electroneutrality maintained when impurities are present?
If there is a substitutional cation impurity, there will be a cation vacancy to balance the charge. The same goes for anions
Why are ceramics more brittle than metals?
Dislocation motion is difficult in ionic solids
- Few slip systems
- Resistance to the motion of ions of like charge
What do flexural tests measure?
Room temperature mechanical behavior, 3-point bending test is often used
What is the room temperature behavior of ceramics?
Elastic with brittle failure
What are the Hume-Rothery rules?
a) similar atomic sizes (within <15%)
b) the same crystal structures
c) similar electronegativity
d) similar valence systems
