Chemical diffusion in non-stoichiometric metal sulphides

Abstract

Transport properties of transition metal sulphides have been discussed in terms of chemical and self-diffusion coefficients. It has been shown that in the case of highly non-stoichiometric sulphides (Co1−yS) the chemical diffusion coefficient may easily be obtained from thermogravimetric measurements of re-equilibration kinetics. If the non-stoichiometry and thereby defect concentration is low (Mn1−yS), the re-equilibration kinetics is difficult or impossible to follow thermogravimetrically, and the electrical conductivity method can be applied. If the non-stoichiometry of a given metal sulphide is known as a function of temperature and sulphur activity, chemical diffusion data may successfully be utilized for calculation of parabolic rate constants of metal sulphidation and to obtain better insight into the growth mechanism of the sulphide scale. Using this procedure it has been shown that the sulphide scales on cobalt and manganese grow by the outward volume diffusion of cations. The chemical diffusion coefficient may also be used in the calculation of the self-diffusion coefficient of cations (or anions) if the non-stoichiometry data of a given sulphide are available. It has been shown that the self-diffusion coefficients of manganese in Mn1−yS obtained in such a way are in full agreement with those determined experimentally.