CVD-COMPOSITES AND SALINE SOLUTIONS-MELTS: SIMILARITY AND DIFFERENCES

Abstract

The general features and specific peculiarities of the thermodynamics of the processes occurring during the evaporation of CVD (Chemical Vapor Deposition) – composites based on germanium – metal chalcogenide systems and dissolution of poorly soluble compounds in salt melts are considered. The essence of both processes consists in the occurrence of exchange reactions between the initial components with the formation of highly volatile substances in the first case and highly soluble compounds in the second. Both processes are endothermic in their thermochemical essence, and their course is facilitated by the entropy component, the role of which increases with increasing temperature, deepening of the vacuum in the case of CVD composites, and dilution of the saline solution – melt. The peculiarities due to the difference between vacuum and salt melt in nature have also been established. If the interaction of molecules in a rarefied molecular vapor of evaporation products can be practically neglected, then in a salt solution-melt, as in a highly concentrated system, secondary reactions of complexation of ions and molecules of the dissolved compound and the main ions of the melt are characteristic. The latter factor significantly affects the solubility in the salt melt due to the shift in equilibrium. Kinetic factors, which differ significantly in both processes, are essential in the reverse reactions of condensation of a CVD‑composite vapor on a substrate and in the deposition of crystals during the crystallization of a solute from a salt melt. Due to the much higher rate of vapor condensation on the substrate, more significant vapor saturation is achieved and, accordingly, the nucleation rate than during crystallization of the salt melt. Therefore, the probability of nanostructuring or complete amorphization of a coating obtained from a CVD‑composite is much higher than for a salt solution-melt, in which the ability to form nanostructures is more limited.