Mass Transport Properties of Materials

We bring the analogies of momentum, heat, and mass transfer in materials to a close with a description of mass transport, often called mass diffusion,orsimply diffusion. Many of the phenomena we have already studied depend upon diffusion. For example, most phase transformations (excluding “diffusion-less” transformations such as martensitic) rely on the movement of atoms relative to one another. Corrosion can- not occur unless there is a movement of atoms or ions to a surface that allows the appropriate chemical reactions to take place. And, as we will see in Chapter 7, some processing methods such as chemical vapor deposition are wholly dependent upon the transport of chemical reactants to the location where reaction conditions allow products to form. This is why the term “diffusion-controlled” is often used to describe the kinetics of a process—the reaction is rapid relative to the time it takes to transport reactants to and/or products from the reaction site. For these reasons, it is important to study diffusion in materials, even though the driving forces are complex and the mass transport properties of a material are highly dependent upon the diffusing species of interest. Unlike momentum and heat transport, the entity being transported is not the same in all diffusion situations. We must describe not only the material in which the transport is taking place, but also what is being transported (see Appendix 6, for example). The movement of hydrogen gas through metals, for example, is much different from the movement of liquid water through metals. As a result, the systems we will address will be highly specific and not at all general. Nonetheless, we can begin as in the previous sections of this chapter, with a description of mass transport properties from a fundamental viewpoint, and extend this to liquids and solids, with the recognition that as systems become more complex, theoretical predictions must give way to empiricisms and correlations.