Phase transformations, heterogeneous

The ideas developed in this chapter are descriptive of shock waves in fluids. Solids have many significant features that distinguish them from liquids and gases, such as shear strength, polymorphic phase transformations, heterogeneous structure, anisotropy, and viscoplastic behavior. The influences of these special properties of solids on shock compression are the topics of several of the other chapters, and for the most part are ignored in this introduction to the basic principles of shock compression. 

It was found that in the catalytic transformation of 2- and 4-t-butylphenol in the liquid phase on heterogeneous KSF and K10 montmorillonite catalysts under micro-wave and conventional conditions the microwaves affected both the rate and the selectivity of the reaction. 

The scope of kinetics includes (i) the rates and mechanisms of homogeneous chemical reactions (reactions that occur in one single phase, such as ionic and molecular reactions in aqueous solutions, radioactive decay, many reactions in silicate melts, and cation distribution reactions in minerals), (ii) diffusion (owing to random motion of particles) and convection (both are parts of mass transport diffusion is often referred to as kinetics and convection and other motions are often referred to as dynamics), and (iii) the kinetics of phase transformations and heterogeneous reactions (including nucleation, crystal growth, crystal dissolution, and bubble growth). 

The second type is simple phase transitions in which one phase transforms into another of identical composition, e.g., diamond graphite, quartz coe-site, and water ice. This type sounds simple, but it involves most steps of heterogeneous reactions, including nucleation, interface reaction, and coarsening. 

Name some kinetic problems that you have already encountered or you may encounter in the future in your research. Then decide whether they belong to (i) kinetics of homogeneous reactions (ii) mass transfer, or (iii) kinetics of heterogeneous reactions (including phase transformation). 

These brief remarks on Ostwald ripening conclude the discussion of nucleation and early growth stages of heterogeneous reactions at this point. Some of the concepts are deepened in Chapter 12 on phase transformations [see also R. Wagner, R. Kampmann (1991)]. 

In Chapter 3 we described the structure of interfaces and in the previous section we described their thermodynamic properties. In the following, we will discuss the kinetics of interfaces. However, kinetic effects due to interface energies (eg., Ostwald ripening) are treated in Chapter 12 on phase transformations, whereas Chapter 14 is devoted to the influence of elasticity on the kinetics. As such, we will concentrate here on the basic kinetics of interface reactions. Stationary, immobile phase boundaries in solids (e.g., A/B, A/AX, AX/AY, etc.) may be compared to two-phase heterogeneous systems of which one phase is a liquid. Their kinetics have been extensively studied in electrochemistry and we shall make use of the concepts developed in that subject. For electrodes in dynamic equilibrium, we know that charged atomic particles are continuously crossing the boundary in both directions. This transfer is thermally activated. At the stationary equilibrium boundary, the opposite fluxes of both electrons and ions are necessarily equal. Figure 10-7 shows this situation schematically for two different crystals bounded by the (b) interface. This was already presented in Section 4.5 and we continue that preliminary discussion now in more detail. 

The foregoing classification is not without ambiguity. For example, it is common practice to call the reaction A - B +C° (see Fig. 6-1) induced by decreasing the temperature a phase transformation. The similar (peritectoid) reaction C = a+fi (Fig. 12-2) induced by a temperature increase, however, is named a decomposition reaction. In addition, the isothermal reaction AO = A+j02, which occurs if the intensive variable fio2 is decreased so that AO decomposes, is called a metal oxide reduction. It is thus categorized as a genuine heterogeneous solid state reaction (the... 

Phase transformations in heterogeneous catalysis have been described recently by topochemical kinetic models [111-115]. These models were taken from solid chemistry, where they had been developed for "gas-solid reactions. The products of such reactions are solids. When gas is in contact with the initial solid, the reaction rate is negligible. But as nucleates of the phase... 

Interfacial Aspects of Phase Transformations, Nato Advanced Studies, Institute Series. Series C, Vol. 87 (1982), B. Mutaftchiev, Ed., D. Reidel Publ. Cy. (Dordrecht, the Netherlands). (Surface structure, adsorption and heterogeneous nucleation.)... 

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