two types of diffusion
- interdiffusion - liquid or solid diffusion with 2 materials mixing
- self-diffusion where an atom moves within its material
challenge with diffusion
diffusion is very slow, especially in perfect crystals, defects help with diffusion
when is fick’s first law used
steady state - rate of diffusion into a system is the same as the rate of diffusion out of a system
ficks first law
J(x) = -D * dc/dx
J(x) = flow rate/flux of diffusion of a species in x direction
D = diffusion coefficient or diffusivity
dc/dx = concentration gradient
when is fick’s second law used
for transient or non steady state conditions
fick’s second law for semi-infinite solid
(c(x)-c(o)) / (c(s)-c(o)) = 1 - erf(x / 2*sqrt(Dt))
c(s) = surface concentration
c(o) = initial bulk concentration
x = position of interest
t = time
diffusivity depends on _____
temperature - D = D(o)exp(-q/kT)
when is diffusion fast
on the surface
when is diffusion medium pace
along grain boundary
when is diffusion slowest
bulk (volume)
