Diffusion

· The jump rate, R, depends upon the joint probability of an interstitial having an energy, ΔG, and motion towards the barrier:
R = ν exp (- ΔG/kT)
Jumps will occur in both the +x and -x directions with equal probability.
· The net flux, J_x, of interstitials in the +x direction depends on the difference between these two fluxes, and the number of interstitials on the A and C planes:
{Net flux A-> C} = N_Α ν exp(- ΔG /kT) - N_C ν exp(- ΔG /kT)
i.e.: J_x = (N_Α - N_C) ν exp( - Δ G /kT) and J_x is zero if (N_Α - N_C ) is zero
· Interstitials still move in the lattice, but as many in one direction as the other.
· A symmetric diffusion barrier, therefore, requires a concentration gradient (chemical potential gradient) for a net flux of interstitials to occur