Solid state

Question 1

What is an amorphous solid

Answer 1

Locally ordered, but no long range order

Question 2

What is a crystalline solid

Answer 2

Periodicity in structure both locally and long range

Question 3

What is allotropy
Give examples

Answer 3

The ability of an element to form more than 1 structure in a particular state.
E.g. graphite and diamond
E.g. O3 and O2
E.g. 𝛼-Fe (BCC) , 𝛾-Fe (FCC) , 𝛿-Fe (BCC)

Question 4

What is polymorphism

Answer 4

When a compound has more than 1 crystal form

Question 5

Braggs Law

Answer 5

2dsin𝜃 = n𝛌

Question 6

Why do defects form?

Answer 6

Increase in internal energy compared to perfect crystal but Gibbs free energy decreases due to increased entropy as the number of defects increases

Question 7

Gibbs Free energy Equation

Answer 7

G = H - TS

Question 8

Number of defects equation

Answer 8

nd = Ne^-𝜀d/KT

Question 9

Plot graph of Gibbs Free energy against number of defects

Answer 9

dy/dx = 0
Minimum of graph is most likely number of defects

Question 10

Natural diamonds vs Synthetic

Answer 10

Natural diamonds have defects which give characteristic optical properties
Synthetic diamonds are almost defect free

Question 11

Uses of materials with minimal defects

Answer 11

Electronic devices require perfect LC alignment
High strength polymers require few defects for optimal strength

Question 12

Elastic deformation

Answer 12

Bonds are stretched under load but structure is not permanently deformed and will return back to original when load is removed

Question 13

Plastic deformation

Answer 13

When load is applied bonds are broken and reformed (crystal planes shift) and the structure is permanently deformed

Question 14

Ductile

Answer 14

Ability to draw wires
Low stress and withstand high strain before fracturing

Question 15

Brittle

Answer 15

Strong but fractures
Can take high stress with little strain/ extension
Large youngs modulus

Question 16

Stiffness

Answer 16

Ability to resist deformation

Question 17

Strength

Answer 17

Ability to withstand a load

Question 18

Stress

Answer 18

𝜎 = force / area

Question 19

Strain

Answer 19

𝜀 = extension/ starting length

Question 20

Force

Answer 20

mass x gravity

Question 21

Tension

Answer 21

The act of stretching or straining

Question 22

Compression

Answer 22

Decrease in volume as a result of applied stress

Question 23

Steel 550 vs Steel 650

Answer 23

Steel 550 has smaller grain and has a high young’s modulus so cant extend much before fracturing
Steel 650 has larger grain and low young’s modulus so can extend much more for a given stress

Question 24

Poissons ratio, 𝜈

Answer 24

Ratio of transverse vs longitudinal strain
𝜈 = -𝜀t / 𝜀l
𝜈 = -𝜎t / 𝜎l

Question 25

What is an auxetic material?

Answer 25

Has a negative poissons ratio, if stretched in one direction it expands in all others

Question 26

Bulk Modulus

Answer 26

K = -V dP/dV
P = Pressure
V = initial volume

Question 27

Shear Modulus

Answer 27

G = 𝜏 / 𝛾
𝜏 = shear stress
𝛾 = shear strain

Question 28

Example of a high tensile strength polymer and why?

Answer 28

e.g. Kevlar
Has a high young’s modulus due to hydrogen bonding between amide groups in the polymer chain. This means the layers cannot slide over each other. = high tensile strength

Question 29

What is specific strength?

Answer 29

Specific strength is a materials strength (force per unit area) at failure divided by the density
Units Nmkg-1

Question 30

When do deformations via slip occur?

Answer 30

Slip occurs via dislocation motion once the shear stress acting in slip direction reaches critical value, causing rearrangement of atoms in crystal. Permanent macroscopic deformation.

Question 31

What does the ease of dislocation glide depend on?

Answer 31

The degree of distortion around the dislocation core
If distortion is spread over larger region, the dislocation is easier to move. Common in ductile materials

Question 32

Why does slip occur in certain directions only?

Answer 32

Only see defects in close packed layers directions

Question 33

How can you stop slipping?

Answer 33

Alloys - have different sized atoms which disrupt the regular structure and stop the planes sliding over one another = stronger material e.g. steel (iron + carbon) more carbon = stronger