The reason for the two types of diffusion stems from the relationship between their relative atomic sizes. Vacancy diffusion occurs primarily when the diffusing atoms are of a similar size, or substitutional atoms. The movement of a substitutional atom requires a vacancy in the lattice for it to move into. Interstitial diffusion occurs when the diffusing atom is small enough to move between the atoms in the lattice. This type of diffusion requires no vacancy defects in order to operate.
Diffusion depends on five main variables.
Diffusivity is defined as the rate at which the solute or impurity penetrates into the the solid, base, material. The time and temperature are fairly self-explanatory, though it should be noted that the temperature increases the rate of diffusion with increasing temperature. Senior Project , Sanjiv D. Parikh, Virginia Tech, 1995. The Diffusion section of the Class Notebook was a production of the Class of 1997 .