Solid state Flashcards
What is an amorphous solid
Locally ordered, but no long range order
What is a crystalline solid
Periodicity in structure both locally and long range
What is allotropy
Give examples
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. πΌ-Fe (BCC) , πΎ-Fe (FCC) , πΏ-Fe (BCC)
What is polymorphism
When a compound has more than 1 crystal form
Braggs Law
2dsinπ = nπ
Why do defects form?
Increase in internal energy compared to perfect crystal but Gibbs free energy decreases due to increased entropy as the number of defects increases
Gibbs Free energy Equation
G = H - TS
Number of defects equation
nd = Ne^-πd/KT
Plot graph of Gibbs Free energy against number of defects
dy/dx = 0
Minimum of graph is most likely number of defects
Natural diamonds vs Synthetic
Natural diamonds have defects which give characteristic optical properties
Synthetic diamonds are almost defect free
Uses of materials with minimal defects
Electronic devices require perfect LC alignment
High strength polymers require few defects for optimal strength
Elastic deformation
Bonds are stretched under load but structure is not permanently deformed and will return back to original when load is removed
Plastic deformation
When load is applied bonds are broken and reformed (crystal planes shift) and the structure is permanently deformed
Ductile
Ability to draw wires
Low stress and withstand high strain before fracturing
Brittle
Strong but fractures
Can take high stress with little strain/ extension
Large youngs modulus
Stiffness
Ability to resist deformation
Strength
Ability to withstand a load
Stress
π = force / area
Strain
π = extension/ starting length
Force
mass x gravity
Tension
The act of stretching or straining
Compression
Decrease in volume as a result of applied stress
Steel 550 vs Steel 650
Steel 550 has smaller grain and has a high youngβs modulus so cant extend much before fracturing
Steel 650 has larger grain and low youngβs modulus so can extend much more for a given stress
Poissons ratio, π
Ratio of transverse vs longitudinal strain
π = -πt / πl
π = -πt / πl
What is an auxetic material?
Has a negative poissons ratio, if stretched in one direction it expands in all others
Bulk Modulus
K = -V dP/dV
P = Pressure
V = initial volume
Shear Modulus
G = π / πΎ
π = shear stress
πΎ = shear strain
Example of a high tensile strength polymer and why?
e.g. Kevlar
Has a high youngβs modulus due to hydrogen bonding between amide groups in the polymer chain. This means the layers cannot slide over each other. = high tensile strength
What is specific strength?
Specific strength is a materials strength (force per unit area) at failure divided by the density
Units Nmkg-1
When do deformations via slip occur?
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.
What does the ease of dislocation glide depend on?
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
Why does slip occur in certain directions only?
Only see defects in close packed layers directions
How can you stop slipping?
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
