diffusion (rus. диффузия) — (Latin diffusio “spreading, scattering”) a process of matter transfer by random motion of molecules, atoms or ions in a single- or multi-component system.


Diffusion is a nonequilibrium process, however, it leads to an equilibrium distribution of substance concentrations within the system. All types of diffusion are governed by the same laws. The diffusion rate is proportional to the area of the surface being perpendicular to the direction of transfer, to the difference in concentrations, temperatures, or the charge density (in case of relatively small values of these parameters). The amount of substance that diffuses over a given time, and distance traveled by the diffusing substance, are proportional to the square root of the time.

In most practical cases the flow of diffusing particles is proportional to the concentration gradient C with a coefficient of proportionality called the diffusion coefficient D [cm· sec1]. The diffusion coefficient D usually depends on the temperature.

Diffusion types observed in a solid body are bulk and surface diffusion, as well as diffusion along grain boundaries (grain-boundary diffusion), which differ greatly in velocity.


  • Zhuravleva Natalya G.
  • Saranin Alexander A.


  1. Molecular diffusion // Wikipedia, the free encyclopedia. — http://en.wikipedia.org/wiki/Molecular_diffusion (reference date 12.12.2011).
  2. Diffusion // Chemical encyclopedia (in Russian). V. 1. — Moscow.: Sovetskaja ehnciklopedija, 1990. 102 p.

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