Difference between self diffusion and interdiffusion
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Diffusion is a result of the kinetic properties of particles of matter. The particles tend to mix until they are evenly distributed throughout a substance. A brief and greatly oversimplfied explanation of the difference between self diffusion and interdiffusion:
Self diffusion takes place within a mass of a pure substance when the thermal energy of atomic vibrations causes some atoms to shift their positions into intersticial voids, which could be thought of as holes in a lattice-like 3-dimensional structure.
Interdiffusion takes place when atoms shift their position within an inhomogeous substance, the process of diffusing causing the atoms to reposition throughout the mixture. The mechanisms of interdiffusion are primarily random (Brownian) motion in gases and liquids, and vacancy diffusion or interstitial diffusion in solids such as metal alloys.
Answer:
The direction of flow of atoms is opposite the vacancy flow direction. Interstitial diffusion is generally faster than vacancy diffusion because bonding of interstitials to the surrounding atoms is normally weaker and there are many more interstitial sites than vacancy sites to jump to.