L4 - Diffusion

Question 1

What are the mechanisms for diffusion?

Answer 1

3
- vacancy diffusion
- interstitial diffusion
- mechanisms involving atom exchange

Question 2

How might vacancies be formed within a crystallographic structure?

Answer 2

• When crystals are cooling down, atoms are being packed closer to each other
• there may come an instance where atoms do not fill a gap
• leaving a vacancy

Question 3

What is vacancy diffusion?

Answer 3

• is driven by concentration gradients
• occurs at substitutional sites or vacancies within a structure
• incoming atoms can be same as lattice or different

Question 4

What determines the rate of vacancy diffusion?

Answer 4

• general diffusion factors
• number of vacancies within a crystal

Question 5

What is interstitial diffusion?

Answer 5

• smaller solute atoms diffuse through gaps within crystal lattice structure
• is always faster than vacancy as there are a large number of interstitial sites in any crystal

Question 6

What is atom exchange?

Answer 6

• atom is swapped into lattice

Question 7

What is interdiffusion?

Answer 7

• AKA impurity diffusion
• diffusion between two solids within an alloy
• atoms migrate from high concentration regions to low concentration regions

Question 8

What is flux?

Answer 8

• the rate of transfer of matter
• can be in terms of diffusing atoms or vacancies

Question 9

What is steady state diffusion?

Answer 9

• diffusion rate (flux) is time independent
• flux is proportional to concentration gradient

Question 10

What are the effects of structure on diffusion?

Answer 10

• Volume diffusion (occurs through bulk of material)
  
  • is vacancy diffusion or atom exchange
  • activation energy for diffusion is high
  • rate of diffusion is low, limited by vacancies
• Grain boundary diffusion
• is disordered region, more vacancies
• activation energy is lower
• atom movement is faster
• Interstitial diffusion
• faster than vacancy diffusion
• many interstices but very small

Question 11

What are the material properties of materials which have high rates of diffusion?

Answer 11

• open crystal structures (low density)
• low melting points
  
  • higher amplitudes of vibration are reached earlier, resulting in more space for atoms to diffuse through

Question 12

What factors influence diffusion?

Answer 12

• Vacancy vs Interstitial
• Temperature
• Unit cell packing structure