Diffusion

Question 1

What are the three main types of diffusion?

Answer 1

1. Vacancy diffusion: self-diffusion and substitutional solute diffusion by flow of atoms and vacancies
2. Interstitial diffusion: movment of interstitial atoms from one interstitial site to another
3. Other mechanisms involving atom exchange

Question 2

Describe Vacancy diffusion

Answer 2

Vacancy diffusion: using substitutional sites, can be self-diffusion (same atoms as lattice) or substitutional solute diffusion, depends on the number of vacancies.
* Often found in imperfect crystal structures

Question 3

Describe Interstitial diffusion

Answer 3

• Interstitial diffusion: small solute atoms using interstitial sites (always lots of interstitial sites, so always faster than substitutional).

Question 4

Describe atom exchange

Answer 4

• Atom exchange: atoms swap locations. Requires high energy so often slower than substitutional and interstitial.

Question 5

What is interdiffusion or impurity diffusion?

Answer 5

• Inter or impurity diffusion: diffusion between two solids
• Occurs in a solid that countains more than one element (an alloy)
• Atoms migrate from high concentration regions to low concentration regions.

Question 6

What is Flux, J, and its equation (Not Fick’s first law)?

Assuming stead state diffusion

Answer 6

• Flux, J, is the rate of diffusion, this is linear if steady state diffusion is occuring.
  * J = Flux = moles (or mass) diffussing / (surface area) (time) = mol/cm^2 x s = kg / m^2 x s
• J = 1/A * dM/dt
• Where M is mass diffused, A is surface area and t is time.

Question 7

True or False:

The concentration gradient isn’t proportional to Flux

assuming steady state diffusion

Answer 7

False
* dC/dx = concentration gradient
* At steady state diffusion this is linear
* We can use this proportional relationship to derive Fick’s first law

Question 8

State Fick’s First Law

Answer 8

• The Flux (rate of mass transfer) is proportional to the concentration gradient
• Atoms flow down the concentration gradient
  * J = -D x dC/dx
• Where J is flux, D is diffusion coefficient (-ve D as direction of diffusion is down the gradient) and dC/dx is the concentration gradient

Question 9

What are the effects of the materials strucutre on diffusion?

Answer 9

• Volume diffusion - occurs through bulk of material. Activation energy for diffusion is high and rate of diffusion is low - limited by number of vacancies.
• Grain Boundary diffusion: activation energy is lower and atom movement is faster in this disordered region - lots more vacancies
• Interstitial diffsusion - much faster than vacancy diffusion (lots of interstices, but they’re small)
• Diffusion promoted by: open crystal structures (i.e. low density), low melting points and small diffusing atoms.

Question 10

What are some factors that also impact diffusion?

Answer 10

• Type of diffusion - such as vacancy vs interstitial
• Temperature of the material - faster at higher temps
• Unit cell packing - BCC unit cell is not closed packed, as apposed to FCC, so it diffuses faster.

Question 11

State Arrhenius equation and what it does?

Answer 11

• Arrhenius equation shows how Diffusion coefficient, D varies with temperature (which of course relates back to Fick’s equation with D).
• D = D0 e^(-Q/RT)
• where D0 is diffusion const, Q is activation energy, R is gas const, and T is absolute temperature (K).