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As follows from the consideration of the third-law method (Chapter 4), its use for determining the reaction enthalpy requires estimation of the equivalent pressure Peqp of the gaseous product under the conditions of free-surface vaporization of the reactant. This, in turn, involves determination of the absolute rate of decomposition J (kg m?2 s?1) and, hence, of the effective surface area of the decomposing sample. This problem, as applied to crystals, powders, and melts, is discussed below. [ABSTRACT FROM AUTHOR]

Confirmation of the CDV Mechanism The most important result following from the analysis of the material presented in Parts I and II of this book is the confirmation of the CDV mechanism, which provides the basis for the thermochemical approach. The major arguments confirming this mechanism are listed in Table 13.1. Some of the arguments result from the effects discovered experimentally, and some are predicted from theory. Among the latter are: the increase of the reaction enthalpy with temperature (for decompositions with formation of a solid product); the deceleration of the decomposition rate during reactant melting, and the peculiarities of the A and E parameters in the isobaric mode of decomposition. [ABSTRACT FROM AUTHOR]

Thermal decomposition of solids and melts is a process of basic significance involved in many natural phenomena and industrial technologies. Karst phenomenon and volcanic activity, rock weathering, dehydration of rock-salt deposits, which are actually manifestations of the evolution of the Earth and of the Universe as a whole, have been subjects of curiosity of mankind from the time immemorial. The history of our civilization is intimately connected with development of a variety of technologies based on heat treatment of natural materials with the purpose of obtaining new objects of practical importance. Among them are both the oldest earthenware and building crafts and pyrometallurgy, and many modern technologies closely associated with industrial chemistry, production of catalysts, ceramics and optoelectronic materials. Table 1.1 presents examples illustrating the use of thermal decomposition in science and technology. [ABSTRACT FROM AUTHOR]

As can be seen from Tables 9.2 and 9.3, for many reactants the composition of the primary gaseous products (oxygen and nitrogen) differs from the equilibrium composition. These products are released, completely or partially, in the form of free atoms. This phenomenon is of remarkable theoretical and practical interest. Some general features of this phenomenon, related to the decompositions of oxides, nitrides and, to some extent, of phosphorus, arsenic, and antimony, are described below. [ABSTRACT FROM AUTHOR]

Thermogravimetric analysis (TGA or TG) is one of the most widely used methods of TA. With this method, changes in the sample mass, maintained in a specified atmosphere, can be monitored during heating, cooling, or being kept at a constant temperature. For most reactants, the sample mass decreases as a result of evaporation of the adsorbed or chemically bound water, pyrolysis of organic substances, and decomposition or evaporation of the material. For some samples in some atmospheres, the sample mass increases owing to, e.g., oxidation of metals to oxides or carbonation of oxides. [ABSTRACT FROM AUTHOR]

An abnormal change of the dehydration rate, J, of crystalline hydrates with an increase of water vapour pressure, Pw, was discovered by Topley and Smith (T-S) in 1931 [1] in their studies of the dehydration rate of MnC2O4 2H2O. In contrast to the expected monotonous decrease of the rate with increasing Pw, the dehydration rate, on reaching a certain critical pressure (about 0.1 Torr), begins to increase, passes through a maximum (about 1 Torr), and then decreases (Fig. 7.1). [ABSTRACT FROM AUTHOR]

The vaporization coefficient, αv, is usually defined as the ratio of the actual flow of gaseous decomposition product J to the flow Jmax coming from an effusion cell, in which, it is assumed, decomposition products are in an ideal equilibrium with the reactant. For many substances, as found from comparative Knudsen-Langmuir TG measurements, ?v << 1, i.e., their free-surface decomposition proceeds much more slowly than would be expected from effusion observations. It is a common practice to explain this discrepancy by a multistage character of the evaporation process, by surface relief peculiarities or by impurities and defects (imperfections) in the reactant lattice. [ABSTRACT FROM AUTHOR]

Starting in 1981 [1], L'vov and his colleagues tried to interpret the experimental parameter E in the Arrhenius equation as the molar enthalpy, ΔrH◦T /ν, of the desired decomposition reaction. However, it did not affect the traditional interpretation of the parameter E. One of the potential reasons for mistrust of this approach could be due to the unreliability of E values measured by the traditional Arrhenius plot method. As an illustration, a comment by Vyazovkin [2] can be quoted: "The comparison of theoretical values of the activation energy with the experimental ones may itself present a considerable challenge as the reported values tend to be widely different." [ABSTRACT FROM AUTHOR]

The goal of Part II is to attempt to interpret, in terms of the thermochemical approach developed in this work, some unusual effects and phenomena accompanying the decompositions of solids and melts and to use these results to confirm the correctness of the approach itself. The analysis starts from the most common relationships relating the molar enthalpy to temperature and proceeds further to the reaction mode and stoichiometry. [ABSTRACT FROM AUTHOR]

The role of self-cooling in endothermic decompositions has been discussed in many works. However, only in a few studies [1-3] performed during the period 1930-1950 has this effect been taken into account in measurements of dehydration rates and of the corresponding Arrhenius parameters (E and A). Most of the other researchers assume (often implicitly) that the magnitude of self-cooling is insignificant and may be neglected. Much greater attention has been given to the problem of self-heating during the processes of pyrolysis, carbon gasification, and decomposition of high-energy materials. [ABSTRACT FROM AUTHOR]

Books and reviews dealing with the kinetics of solid-state reactions (e.g., [1-3]) usually pay little attention to the analysis and comparison of the metrological characteristics of the methods employed in TA measurements although a correct choice of the method to be used in measurement and calculation of the quantity of interest determines the reliability of the results obtained. This chapter addresses these points. [ABSTRACT FROM AUTHOR]

By the kinetic description of decomposition reactions of solids one usually understands analysis of isothermal ??t curves which characterize the evolution of the degree of decomposition of the reactant (or of the product yield) ? with time t. Such an analysis reduces essentially to choosing the equation that fits best the real kinetic curves. The set of equations derived from different models describing the mechanisms of separate stages (induction, acceleration, and deceleration), or of their combination, is well known and provides a basis for what is presently called formal kinetics [1-8]. The information obtained in this way on the contribution of these stages to the observed kinetics is used to develop mechanisms and schemes for the evolution of the decomposition process with time. This approach is not capable, however, of yielding any data on the thermochemical characteristics of a reaction (including the composition and stoichiometry of the products, the enthalpy and the entropy of the process) and on how the decomposition rate is affected by experimental conditions, such as the temperature and the presence of gaseous products in the reaction system. [ABSTRACT FROM AUTHOR]

It appears reasonable to define the concept "decomposition mechanism", to be discussed below, before we cross over to its consideration. (As René Descartes said: "Define the meaning of the words, and you will free mankind of half of its misconceptions.") One may find in the literature different approaches to interpretation of this concept. This should be largely assigned to the methods employed by researchers to unravel it, and the goals pursued in its subsequent application. [ABSTRACT FROM AUTHOR]