Ceramics

Question 1

What is the bonding in a ceramic?

Answer 1

Mostly ionic, some covalent

Question 2

Why does %ionic character vary in ceramics

Answer 2

% ionic character increases with difference in electronegativity

Question 3

Describe ceramic oxide structures (3)

Answer 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

Question 4

Why is the size of the site important when the cation is undergoing site selection? (2)

Answer 4

• 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

Question 5

What is there to consider when a cation selects a site? (3)

Answer 5

• Size of the site
• Stoichiometry
• Bond hybridization

Question 6

What is important to maintain when judging whether a cation can occupy a space when regarding size?

Answer 6

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.

Question 7

How can you determine exactly how many cations can fit around an anion?

Answer 7

Find the coordination number

Question 8

What is a coordination number?

Answer 8

The ratio of the radius of the cation to the radius of the anion

Coordination # = r_C / r_A

Question 9

List the coordination numbers and then the corresponding range of cation-anion ratios and the geometries

Answer 9

Question 10

Show that the minimum cation-to-anion radius ratio for the coordination number 3 is 0.155

Answer 10

Question 11

How does the stoichiometry factor into site selection for ceramics?

Answer 11

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 O_H and 8 T_D sites

If for a specific cell each unit cell has 6 cations, and the cation prefer O_H sites

4 in O_H

2 in T_D

Question 12

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?

Answer 12

FCC

CsCl (Cesium chloride)

Simple cubic

Question 13

How does bond hybridization effect which site an anion occupies?

Answer 13

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.

Question 14

How do you work out exactly how covalent/ionic a compound is?

Answer 14

Work out %ionic character

E.g. SiC

%ionic character = 100e^{-1/4(X_Si-X_C)^2}

Where X_Si and X_C are electronegativity values.

Question 15

How would you predict the structure for a compound?

Answer 15

• r_cation/r_anion

Match to the corresponding coordination number

Match structure

Question 16

What is the anomaly to the method for predicting the structure of a compound?

Answer 16

The zinc blende structure

The ratio implies that they should prefer O_H sites

But bonding hybridization of zinc favours T_D

Question 17

What type of anion packing do most AX₂ molecules use?

Answer 17

Simple cubic

These have a fluorite structure (FCC)

Question 18

What is UO₂ used as?

Answer 18

nuclear fuel

Question 19

What is ZrO₂ used as?

Answer 19

Structural ceramic

Question 20

What type of anion packing do most ABX₃ have?

Answer 20

FCC

The structure name is perovskite

Question 21

Why are ceramics more brittle than metals?

Answer 21

Slippage along planes is much harder due to the ionic bonding and sp³ hybridization

Question 22

What are the three silica (SiO₂) structures?

Answer 22

Quartz, crystobalite, tridymite

Question 23

Why is silica such a strong material with a high melting point?

Answer 23

The Si-O bond is very strong

Question 24

How many of the different types of site does an FCC lattice structure have?

Answer 24

4 OH

2 T_D

Question 25

Why is silica amorphous?

Answer 25

H⁺ ions can attach to it to neutralise it and stop it crystalising

Question 26

Why are some materials transparent?

Answer 26

They are either single crystal or have blockers

Question 27

Why do clear polymers stop being clear when you pull them?

Answer 27

The molecules are forced to align and so the material crystallises.

Question 28

What is silica glass?

Answer 28

A dense form of amorphous silica with the addition of Na⁺