Atomic diffusion occurs in metallic solids mainly by (1) a vacancy or substitution mechanism and (2) an interstitial mechanism. In the vacancy mechanism atoms of about the same size jump from one position to another, using the vacant atomic sites. In the interstitial mechanism very small atoms move through the interstitial spaces between the larger atoms of the parent matrix. Fick’s first law of diffusion states that diffusion takes place because of a difference in concentration of a diffusing species from one place to another and is applicable for steady-state conditions (i.e., conditions that do not change with time). Fick’s second law of diffusion is applicable for non–steady-state conditions (i.e., conditions in which the concentrations of the diffusing species change with time). In this book the use of Fick’s second law has been restricted to the case of a gas diffusing into a solid. The rate of diffusion depends greatly on temperature, and this dependence is expressed by the diffusivity, a measure of the diffusion rate: Diffusivity D = Doe-Q/RT. Diffusion processes are used commonly in industry. In this chapter we have examined the diffusion-gas-carburizing process for surface-hardening steel and the diffusion of controlled amounts of impurities into silicon wafers for integrated
circuits.
