Diffusion Iin Solid-Lec.2

INTRODUCTION
Numerous chemical reactions or micro-structural changes in solids take place through solid
state diffusion, i.e. the movement and transport of atoms in solid phases. The diffusion takes
place because of the presence of defects in solids. Point defects, e.g. vacancies and interstitial
ions, are responsible for lattice diffusion. Diffusion also takes place along line and surface
defects which include grain boundaries, dislocations, inner and outer surfaces, etc. As
diffusion along linear, planar and surface defects is generally faster than in the lattice, they
are also termed high diffusivity or easy diffusion paths. Another frequently used term is short
circuit diffusion.
The relative contribution of the different types of diffusion in oxides and other inorganic
compounds are functions of the temperature, partial pressures or activities of the constituents
of the compounds, the microstructure, grain size, porosity etc. Grain boundary and dislocation
diffusion generally have smaller activation energies than lattice diffusion and as a result
become increasingly important the lower the temperature in solids with a given
microstructure.