Diffusion, Diffusion Coefficient, D(T)
Diffusion in solids is an atomic level transport process driven by chemical potential gradients. The diffusing material may be an atom of the host lattice (self diffusion) or an atom of another type (impurity diffusion). The diffusing species move through the solid using either normal lattice sites or interstitial sites. When normal lattice sites are used the transport process is mediated by lattice vacancies. The interstitial process may involve either thermally produced interstitials of lattice species (self interstitials) or interstitial impurities.

The macroscopic transport behavior is described by Fick's Laws. In steady state diffusion, Fick's first law describes the flux, J, of material. In one dimension this has the form:      J = -D(T) dc/dx        where D(T) is the temperature dependent diffusion coefficient and (dc/dx) is the concentration gradient of the diffusing species. Fick's second law describes the change in concentration of the diffusing species as a function of time in an element of the sample: 
                                    (dc/dt) = D(T) (d2c/dx2)
The temperature dependent diffusion coefficient can be written as: 
             D(T) = D0exp(-Q/kT)      where D0 is a material dependent, temperature independent parameter and Q is the activation energy of the transport process in electron volts. For metals: 0.5 < Q < 3.0 eV; 10-7 < D0 < 10-4m2/s