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Theory of solid-state thermal decomposition reactions

Scientific stagnation or chemical catastrophe? An alternative approach appraised and advocated

Author: Andrew K. Galwey

Abstract

A case is presented to recommend strongly that scientists interested in thermal chemistry should make comprehensive, conscientious, clinical and critical analyses of the strengths and weaknesses of The L'vov Thermochemical Theory (L'vov, Thermal decomposition of solids and melts—new thermochemical approach to the mechanism, kinetics and methodology, Springer, Berlin, 2007), used to interpret the kinetics and mechanisms of reactions that occur on heating. The shortcomings underlying the theory (some originally developed for solid decompositions) currently uncritically accepted in this field are reviewed, and these deficiencies are contrasted with the successes of L'vov's approach. To promote the use of this alternative theory, features that may have discouraged researchers unfamiliar with its assumptions, methodology and applications are discussed here. A new scientific theory cannot be ignored or discounted without adequate consideration and testing, particularly in a stagnant area of chemistry that lacks guiding principles and unifying concepts. Novel ideas in the literature (L'vov 2007) deserve recognition, critical appraisal and, if possible, exploitation to maintain the progress of scientific research.

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Melting and thermal decompositions of solids

An appraisal of mechanistic interpretations of thermal processes in crystals

Author: Andrew K. Galwey

Abstract

This analysis of interface phenomena considers the alternative processes that may result from heating a crystal, particularly including thermal decomposition, involving chemical reactions, and melting, involving loss of long-range structural order. Such comparisons are expected to provide insights into the factors that determine and control the different types of thermal changes of solids. The survey also critically reviews some theoretical concepts that are currently used to describe solid-state thermal reactions and which provides relevant background information to models used in a recently proposed theory of melting. Probable reasons for the current lack of progress in characterizing the factors that control chemical changes and mechanisms of thermal reactions in solids are also discussed.

It is concluded that some aspects of the macro properties of reaction interfaces in crystal reactions have been adequately described, including geometric representations of interface advance during nucleation and growth processes. In contrast, relatively very little is known about the detailed (micro) processes occurring within these active, advancing interfacial zones: reactant/product contacts during chemical reactions and crystal/melt contacts during fusion. From the patterns of behaviour distinguished, a correlation scheme, based on relative stabilities of crystal structures and components therein, is proposed, which accounts for the four principal types of thermal changes that occur on heating solids: sublimation, decomposition, crystallographic transformation or melting. Identifications of the reasons for these different consequences of heating are expected to contribute towards increasing our understanding of each of the individual processes mentioned and to advance theory of the thermal chemistry of solids, currently enjoying a prolonged quiescent phase.

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Authors: Boris V. L'vov and Andrew K. Galwey

Abstract

The thermochemical approach to analysis of thermal decomposition of solids, developed earlier by L'vov, is extended here, for the first time, to interpret the kinetics and mechanism of the reduction of an oxide (NiO) by a gas (H2). This approach is based on the mechanism of congruent dissociative vaporization of the reactant, Langmuir kinetics and determination of the Arrhenius E parameter by the third-law method. The calculated enthalpy of the reaction is in good agreement with the experimentally measured E value. Many other mechanistic and kinetic features of the reaction are explained within the framework of the given theoretical approach. These include: the formation of metal nuclei; the initial autocatalytic behavior; the formation of nanocrystalline structure of the reduced metal product; the equimolar and isobaric modes of reduction; the dependence of reduction rate on hydrogen pressure; the more than twofold decrease of the E parameter with the extent of reaction α, and the systematic increase of E with temperature.

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