Basics of the Congruent Dissociative Vaporization CDV Mechanism

Foundation Thermal decomposition of a solid reactant, R, is usually described by two types of reaction by congruent dissociative vaporization of a reactant R into gaseous products A(g) and B(g), i.e., according to the scheme  

It appears much more probable to assume that solid products form as a result of collisions and condensation of low-volatility particles (atoms and molecules) that became free in the gaseous phase through decomposition by a common mechanism of CDV, i.e., by the scheme 

This reasoning leads to the main assumption (I) that the decomposition of all compounds most likely proceeds hy CDV and, in the case of formation of low-volatility products, includes an additional stage of condensation of these products. The reader may find this reasoning not convincing enough. Therefore, other arguments, both direct and indirect, will be presented below for the validity of this assumption. 

Other Basic Assumptions In addition to this main assumption (I), which forms the foundation for the CDV mechanism, there are two other, more obvious assumptions, namely, (II) that primary products of decomposition may differ from their equilibrium forms, and (III) that the energy released in condensation of a low-volatility product in the reaction interface decreases partially the total enthalpy of the decomposition reaction (including the condensation stage) compared to that of the primary stage of dissociative vaporization. 

Consider now the essence and validity of these assumptions. The decompositions of oxides of some metals (CuO, HgO, Pt02, and ZnO) at comparatively low temperatures ( 1,000K) were shown long ago [8] to proceed with liberation of oxygen in the form of free atoms. There are other similar examples [6] that support assumption II. The appearance of low-volatility products in the gaseous phase in congruent vaporization (in other than their equilibrium aggregate state) is a part of this assumption. 

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