A unified theory of the oxidation of materials is formulated, according to which the differences between these processes are related to the degree of deviation from equilibrium of the processes of the generation and recombination of point defects at the boundary of the oxide film, which are responsible for mass transfer. Anodic oxidation occurs under nonequilibrium conditions and is controlled by the rate of generation of two types of basic point defects. The generation of both defects is characterized by the identity of parameters, and occurs as a result of a jump of the same particle (oxygen ion or metal) at the boundary of the oxide film, and this explains the phenomenon of simultaneous mobility of anions and cations. The mechanisms of isoparametric generation of a pair of point defects are proposed: the initiating jump mechanism and the displacement mechanism. Their implementation in oxides with excess or deficiency of oxygen is shown. Criteria are proposed for determining the mechanism of generation of defects during the oxidation of various substances. The conditions of thermal oxidation are close to equilibrium, and point defects have time to recombine at the oxide boundary; therefore, defects with a higher recombination activation energy predominate.
It is known that deviation in the stoichiometry of the composition of chemical compounds is associated with nonstoichiometric point defects, namely vacancies and interstitial ions. Until recently, only the behavior of point defects in the bulk of a solid was considered, while the question of the appearance of point defects in chemical compounds, and where and how they appear, remained uninvestigated. Previously, we showed that nonstoichiometric point defects arise exclusively on the surface of a solid, and the mechanisms of their occurrence were given. However, phase boundaries can also appear inside a solid, and this is due to the presence of pores inside them. Here we propose that a reactive-vacancy mechanism for pore formation and growth in chemical compounds is based on a quasi-chemical reaction that occurs on the pore surface between nonstoichiometric point defects; vacancies in one sublattice and interstitial ions in the other. This reaction leads to the formation of vacancies in the second sublattice and, together with vacancies from the first sublattice, to the growth of pores.
The principal operating characteristics of a fuel cell with a capillary membrane are considered. Mechanisms for the self-regulation of water elimination are discussed.
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