The physical approach to the interpretation of the kinetics and mechanisms of thermal decomposition of solids: the state of the art

The first part of this review is devoted to a substantiation of the physical approach to the interpretation of the exponential dependence of the rate of heterogeneous reactions on temperature proposed by Hertz and Langmuir and developed by the author as an alternative to the traditional chemical approach based on the Arrhenius concept of the activation effect. The second and third parts of this work are devoted to the application of this approach to the quantitative interpretation of some important features of crystolysis reactions and identification of decomposition mechanisms fur several classes of solids. The features of considered crystolysis reactions are the following: the mechanism of nucleation and autocatalytic development of decomposition reactions, the retardation of decomposition in the presence of gaseous products, the low vaporization coefficients alpha (v) for many substances, the effect of self-cooling on the measurement of kinetic parameters. the Topley-Smith effect and the kinetic compensation effect. The physical approach has been successfully applied to the interpretation of kinetics of sublimation/dissociative evaporation of more than 110 substances from 20 different classes of solids: metals, nonmetals, oxides, hydroxides, sulfides, selenides, tellurides, nitrides, azides, carbides, borides, fluorides, chlorides, bromides. iodides, carbonates. nitrates, sulfates, oxalates and hydrates. As examples of unusual results obtained. two types of reaction mechanisms should be noted: the mechanism of low-temperature (< 1000 degreesC) 'carbothermal reduction' of Fe, Co, Ni and Cu oxides and the desorption mechanism of release of sub-nanogram masses of analytes from graphite and metal surfaces. In the course of substantiation and application of the physical approach, the results obtained by different techniques (quadrupole mass spectroscopy, electrothermal atomic absorption spectrometry (ET AAS), Knudsen effusion and Langmuir free-evaporation experiments and the traditional thermal analysis) were used. (C) 2001 Elsevier Science B.V. All rights reserved.