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the special case of diffusion in a pure substance or a solution of constant composition where the substance’s own particles undergo diffusion. In self-diffusion, the atoms that participate in the diffusion movement have the same chemical properties but can differ in their physical characteristics—in the composition of their nuclei (see ISOTOPES). In the case of a difference in the isotopic composition of the substance, self-diffusion can be observed through the use of radioactive isotopes (see ISOTOPE TRACERS) or through analysis of the isotopic composition by means of mass spectrometers.
The change over time in the concentration of a given isotope in the volume of the substance under consideration is described by the usual equations for diffusion, and the rate of the process is characterized by the coefficient of self-diffusion (see DIFFUSION). When such forces as surface tension, gravity, elastic forces, or electrical forces act on a solid specimen for a long time, the diffusion displacements of the solid’s particles can result in a change in its shape or in other effects. These diffusion processes are evidenced in the joining together of two polished specimens of the same substance placed next to each other, in the sintering of powders, and in the stretching of a body under the action of a load suspended from it (diffusion creep). The study of the kinetics of these processes permits determination of the coefficient of self-diffusion of a substance.