Defects, Driving Forces and Definitions of Diffusion Coefficients in Solids

Defects assist the diffusion process, whereas driving forces influence the rate of diffusion depending on the thermodynamics of alloying, diffusion mechanisms, diffusion paths, and defect production. Different types of diffusion are defined based on defects assisting the diffusion process and whether atomic transport occurs in the absence or presence of an additional driving force different from purely entropic one. In this chapter, different types of zero-dimensional (point defects), one-dimensional (edge and screw dislocations), two-dimensional (grain and phase boundaries, surfaces), and three-dimensional (voids) defects are introduced. The origins of internal (chemical) and external (electric current) driving forces are discussed. Based on defect types and driving forces, different types of diffusion coefficients are introduced.