Diffusion Flashcards
Define diffustion
Mass transport by atomic motion
What is the relation between diffusion and concentration of atoms?
Atoms diffuse from [high] to [low]
What is it called when atoms of one metal diffuse into another?
Interdiffusion or impurity diffusion.
What is it called when atoms diffuse within a single metal?
Selfdiffusion
What must be true for diffusion to occur
Atoms must have vacancies to jump to, and enough energy to do so.
Therefore, diffusion usually occurs upon application of heat
What are the common mechanisms of diffusion?
Vacancy diffusion and interstitial diffusion
What is required for interstitial diffusion that is not required for vacancy diffusion?
Interstitial diffusing atoms must be very small in order to fit in the interstitial positions.
How fast is vacancy diffusion compared to interstitial diffusion?
Interstitial diffusion is faster, because there are more spots.
Define diffusion flux.
The mass of atoms moving through a perpendicular cross sectional area per unit of time.
J=M/At
Define steady state diffusion? What are it’s characteristics?
Steady state occurs when the flux is constant.
- flux is constant
- gradient is constant
- profile is linear
What is the concentration profile?
A plot of [] over distance.
What is the concentration gradient?
The slope of the [] profile.
dC/dx
What is Fick’s First Law?
For a steady state diffusion
J = -D(dC/dx)
Where D is the diffusion coefficient.
What is Fick’s Second Law?
For a non-steady state diffusion:
J = D(d^2C/dx^2)
What is the Qd in
D = Do e^(-Qd/RT)
Activation energy to diffuse one mole of atoms.
