Ceramics Flashcards
What is the bonding in a ceramic?
Mostly ionic, some covalent
Why does %ionic character vary in ceramics
% ionic character increases with difference in electronegativity
Describe ceramic oxide structures (3)
- The oxygen anions are much bigger than the metal cation
- The oxygen atom are close-packed in an FCC lattice
- The cations are in the holes of the oxygen lattice
Why is the size of the site important when the cation is undergoing site selection? (2)
- The size of the site
- Does the cation fit?
- Stable ceramic compounds form when the anions surrounding a cation are all in contact with said cation
What is there to consider when a cation selects a site? (3)
- Size of the site
- Stoichiometry
- Bond hybridization
What is important to maintain when judging whether a cation can occupy a space when regarding size?
Charge neutrality
Net charge in the structure should be 0, so only a certain amount of possibilities is possible, even id more cations can fit.
How can you determine exactly how many cations can fit around an anion?
Find the coordination number
What is a coordination number?
The ratio of the radius of the cation to the radius of the anion
Coordination # = rC / rA
List the coordination numbers and then the corresponding range of cation-anion ratios and the geometries
Show that the minimum cation-to-anion radius ratio for the coordination number 3 is 0.155
How does the stoichiometry factor into site selection for ceramics?
Stoichiometry dictates that if all of one type of site is full, the remainder have to go into other types of site
e.g. FCC crystal structures have 4 OH and 8 TD sites
If for a specific cell each unit cell has 6 cations, and the cation prefer OH sites
4 in OH
2 in TD
What type of anion packing do most AX structure types have? Eg: NaCl, MgO, FeO, Zincblende like ZnS and SiC
What is the exception?
What anion packing does this structure have?
FCC
CsCl (Cesium chloride)
Simple cubic
How does bond hybridization effect which site an anion occupies?
If there is significant covalent bonding present in a compound, the hybrid orbitals can have a significant impact
Covalent bonds are directional, hybridization locks the direction, lattice distortion is made harder.
How do you work out exactly how covalent/ionic a compound is?
Work out %ionic character
E.g. SiC
%ionic character = 100e-1/4(XSi-XC)^2
Where XSi and XC are electronegativity values.
How would you predict the structure for a compound?
- rcation/ranion
Match to the corresponding coordination number
Match structure
What is the anomaly to the method for predicting the structure of a compound?
The zinc blende structure
The ratio implies that they should prefer OH sites
But bonding hybridization of zinc favours TD
What type of anion packing do most AX2 molecules use?
Simple cubic
These have a fluorite structure (FCC)
What is UO2 used as?
nuclear fuel
What is ZrO2 used as?
Structural ceramic
What type of anion packing do most ABX3 have?
FCC
The structure name is perovskite
Why are ceramics more brittle than metals?
Slippage along planes is much harder due to the ionic bonding and sp3 hybridization
What are the three silica (SiO2) structures?
Quartz, crystobalite, tridymite
Why is silica such a strong material with a high melting point?
The Si-O bond is very strong
How many of the different types of site does an FCC lattice structure have?
4 OH
2 TD
Why is silica amorphous?
H+ ions can attach to it to neutralise it and stop it crystalising
Why are some materials transparent?
They are either single crystal or have blockers
Why do clear polymers stop being clear when you pull them?
The molecules are forced to align and so the material crystallises.
What is silica glass?
A dense form of amorphous silica with the addition of Na+
