Mechanism of Diffusion

Abstract

Diffusion mechanisms in material including ceramic materials are either via vacancy, direct interstitial or interstitialcy mechanisms. The most dominant of these is the vacancy mechanism. For the frequency of a vacancy-atom exchange two conditions are required, jumping energy and the amount of time that the vacancy resides in the vicinity of the energetic atom. Without sufficient energy for the jump, the atom oscillates about its equilibrium site and no atom-vacancy exchange can occur. Similarly, the fraction of time that the vacancy resides in the atom’s vicinity determines the frequency of jumps. In the absence of any of these factors, an atom will not be able to change places with the vacancy. Direct interstitial mechanism occurs when an atom is of small radius. Smaller atoms cause less distortion of the lattice during migration and diffuse more readily than ones having larger atomic radii; being small allows them to fit into interstitial sites and to jump from one interstitial site to another without greatly displacing the solvent atoms from their normal lattice sites. The interstitialcy mechanism is characteristic of diffusion when the size of the diffusing atoms is large, perhaps equal to or greater than dimensions of the host atoms. Diffusion by this mechanism is likely when an atom (perhaps a tracer) is located either on a lattice in some substitutional solution or in an interstitial site forming an interstitial solid solution. These mechanism are the subject of this chapter.