As was observed in the last topic, the known types of diffusion are volume (vol), grain boundary (gb) and surface (sur). The quantities of the activation energy Q needed for each of these types is: \[Q _ {vol} > q _ {gb} > Q _ {sur}\]
In systems where the three Q activation enthalpies has already been determined,the proportion between \[Q _ {vol} : q _ {gb} : Q _ {sur} \approx 4 : 3 : 2 \: \: or \:\: 4 : 2 : 1\]
Typical values of the diffusion coefficients D0 vary between 0.1 and 1.0 cm2/s. The comparison of for each of the diffusion types in the figure above is the following:
\[D_{0vol.}D_{0gb.}>D_{0sur.}\]
For the solid state reaction temperatures that count, the three diffusion coefficients can be shown as:
\[D _ {sur} > D _ {gb} > D _ {vol}\]
Figure 2.9 (below) presents typical data for diffusion of silver (Ag) as a function of temperature. This data indicates that only in temperatures much lower than the melting point of silver, when the diffusion coefficient of the volume falls much below the others, the surfaces and grain boundaries become important as mass transport paths, as the volume diffusion is really slow at low temperatures. The grain boundary diffusion values only approximate the diffusion values in volumes of fine grained materials, depending on the relationship between Dgb and Dvol .
