The tendency toward diffusion is very strong even at room temperature because of the high molecular velocities associated with the thermal energy of the particles. The diffusion coefficient is temperature dependent, and is can be described by the Arrhenius expression:
D = D 0 exp(- E a /RT)
k is Boltzmann's constant. D0 (is the maximum diffusion coefficient (at infinite temperature) and Ea (the energy of activation) is constants describing the diffusion. All kinds of diffusion require a certain minimum energy to occur and it can be called the activation energy. The higher the activation energy, the harder it is for diffusion to occur. The activation energy and maximum diffusion constant of different elements is given in Table 8.
Table 8 : The activation energy and maximum diffusion constant of different elements
|
S i |
B |
In |
As |
Sb |
P |
Units |
D 0 |
5 6 0 |
1 . 0 |
1 . 2 |
9.17 |
4.58 |
4.70 |
cm2 s -1 |
E A |
4.76 |
3 . 5 |
3 . 5 |
3.99 |
3.88 |
3.68 |
e V |