Chemical and Ambipolar Diffusion

Of course, it is possible to write other and more complex defect reactions, which may be equally compatible with the results. In all of these examples, therefore, direct evidence is desirable before the mechanisms can be regarded as proven. 

The above sections have focused upon homogeneous systems with a constant composition in which tracer diffusion coefficients give a close approximation to selfdiffusion coefficients. However, a diffusion coefficient can be defined for any transport of material across a solid, whether or not such limitations hold. For example, the diffusion processes taking place when a metal A is in contact with a metal B is usually characterized by the interdiffusion coefficient, which provides a measure of the total mixing that has taken place. The mixing that occurs when two chemical compounds, say oxide AO is in contact with oxide BO, is characterized by the chemical diffusion coefficient (see the Further Reading section for more information). 

Ambipolar diffusion involves the transport of charged species, and in such cases overall electric charge neutrality must be maintained during diffusion. Moreover, during ambipolar diffusion the difference in the mobilities of the diffusing species sets up a field, the Nernst field, that influences the rates of motion of the particles. 

Corrosion reactions of a metal with gaseous species such as oxygen, chlorine, sulfur containing molecules or water vapor to produce a thin layer of product phase are typical of ambipolar diffusion reactions. For example, metal oxidation 

After initial attack, further reaction can only proceed if ions can diffuse across the product, either from the outside gaseous phase into the inner metal layer or else from the metal out to meet the gas. 

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