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7.2 Decomposition of solids: thermodynamic and kinetic aspects

Topic

explanation and teaching suggestions: Thermal decomposition of inorganic solids are

reactions where a solid reactant yields a new solid phase with molar volume lower than that of the reactant, and a gaseous product. In dealing with the kinetics analysis of this

complex sintering

phenomenon many features need to of the solid products, vaporization from

be the

considered. Nucleation, growth, interface reaction and diffusion in

to the capillary of the gaseous one are only a examples of role in determining the rate limiting steps. Although the kinetics of decomposition reactions of inorganic solids have in a large number of studies, there is at present a renewed

processes that can have a HT thermodynamics and been dealt with in the past interest in the mechanism

and thermal decomposition of different types of inorganic solids due to industrial applications. Indeed, the decomposition of e.g. carbonates,

their various sulfates, and

hydroxides remains a common process in the of decomposition conditions and mechanism

production of oxide ceramics; knowledge of formation of certain semiconductors is

important for film growth and processing at high temperature in vacuo and in environments. Illustration to students of the basic mechanistic aspects decomposition reactions of solids should be accompanied by discussion of examples chosen, e.g., among carbonates, sulfates etc.

reactive of the relevant

Experimental and theoretical studies aimed at interpretation of the kinetics and mechanism of thermal decomposition of solids have an history that dates back to many decades. Before the 1970’s the role of the solid state point of view was extensively explored (see books in the appended bibliography), but the implication of the vaporization theory of the gaseous products was quite neglected. At the end of 1970’s and subsequently, Alan W. Searcy (University of California, Berkeley, USA) and D. Beruto (University of Genoa, Italy) developed this approach in a series of experimental and theoretical papers, mainly on the decomposition of metal carbonates (and specifically calcite), that clarified the nature of the surface step due to the vaporization of the gaseous product, the subsequent diffusion and effusion processes into the solids porous matrix and the microstructure changes of the formed oxides due to the high temperature chemical adsorption of the gaseous product onto the oxide surface and to the catalytic effect that the gaseous phase may have in the sintering of the oxides nanocrystallites. From these informations a clear picture of the rate determining steps of the thermal decomposition kinetics can be formulated in terms of modified Langmuir-Hertz equation and of decomposition coefficients. The nature of this coefficient was clarified recently by the same authors as a function of surface chemical step of the gaseous products and of thermodynamic activity of the formed solid oxide. More recently a russian group (Boris V.L’vov, University of St. Petersburg, Russia, and coworkers), tried to extend further the implication of the vaporization theory in the thermodynamic and kinetic analysis of the decomposition reactions by proposing a physical approach (PA), in contrast to the traditional Arrhenius plot and second-law method, through

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