The monograph deals with a physicochemical approach to the problem of solid-state growth of chemical compound layers and reaction diffusion in binary heterogeneous systems formed by two solids as well as a solid with a liquid or a gas. It is expla ined why the number of compound layers growing at the interface between initial phases is usually much less than the number of chemical compounds on the phase diagram of a given binary system. For example, of eight intermetallic compounds existing in the Al-Zr binary system, only the ZrAl3 phase was found to grow as a separate layer at the Al-Zr interface under isothermal conditions. The physicochemical approach predicts that in most cases the number of compound layers should not exceed two, with the main factor resulting in the occurrence of additional layers being crack formation due to thermal expansion and volume effect.
It is shown that many experimentally observed kinetic dependences of the layer thickness (or mass) upon time (examples presented include linear growth of Cu6Sn5 between Cu and Sn, parabolic growth of NiBi3 between Ni a nd Bi, linear-parabolic growth of SiO2 between Si and oxygen, asymptotic and paralinear growth of two oxides, simultaneous parabolic growth of the Al3Mg2 and Al12Mg17 layers between Al and Mg, etc .) can readily be derived from a single theoretical viewpoint based on two almost obvious basic postulates: (i) about the summation of the time of diffusion of reacting species and the time of subsequent chemical transformations with their par ticipation and (ii) about the independency of elementary acts of solid-state chemical reactions.
The reasons for the kinetic instability of the layers of chemical compounds, which may result in their gradual degradation (disappearance) with passing time, are discussed. An example is the Ti-Ti3Al-TiAl-TiAl2-TiAl3-Al reaction system, with the layers Ti3Al, TiAl and TiAl2 disappearing during isothermal annealing at temperatures below the melting point of aluminium. The instability of this kind is by no means connected with the thermodynamic stability of the compounds under given temperature-pressure conditions.
A comparative analysis of growth kinetics of the same compound layer in various reaction couples consisting of elements A and B and their other chemical compounds of a multiphase binary system is presented. Mathematical expressions relating the growth rate of the layer in one of the couples to that in the others are proposed. The formation of duplex structures in these couples is discussed, with the emphasis on the determination of the ratio of the sublayers of a layer of the same compound, which look like the layers of two quite different chemical compounds. The FeSn layer between Fe and FeSn2 consists of two sublayers of equal thickness, differing by the shape of grains. The same applies to the Co3O4 layer between CoO and oxygen and to many other compound layers.
The dissolution in the solid-liquid systems as well as the evaporation in the solid-gas systems was shown to play a significant role in determining the layer-growth rate. These effects were taken into account in equations describing the rate of growth of the layer under conditions of its simultaneous dissolution in the liquid phase or evaporation into the gas phase. Calculations carried out for the Fe2Al5 layer growing between solid iron and molten aluminium showed that theoretical expressions fit the experimental data fairly well.
The reasons for the great difference in values of reaction- and self-diffusion coefficients of the components of a chemical compound are analysed. For example, in the case of Fe3-< d O4 the reaction-diffusion coefficient is two orders of magnitude greater than the self-diffusion coefficient of iron ions. For other compounds (Al2O3, Fe2Al5, Pd2Si, AlSb, etc.) this difference varies from five to ten orders of magnitude. After the normalisation to the same vacancy concentration the values of reaction- and self-diffusion coefficients of the same component become close, if not identical, as it should be from a physical viewpoint.
The difference in diffusivities of the components in a growing chemical compound layer is often connected, especially in the literature on physics and metallurgy and especially in relation to intermetallics, with the Kirkendall effect. From historical and scientific viewpoints, in many cases this does not seem to be sufficiently substantiated. In particular, this is so in the case of formation of chemical compound layers at the interface of initial substances. A brief consideration was presented to s how that different diffusional contributions of the components to the growth process of a chemical compound layer can hardly be regarded as a manifestation or result of the Kirkendall effect.
Comparison of the consequences following from the physicochemical and purely diffusional approaches is given to show that the latter is one of the limiting cases of the former. Theoretical conclusions are illustrated by the available experimental data on the formation of intermetallics, silicides, oxides, salts and other chemical compounds.
The monograph only contains the material which cannot be found in any other book, with the exception of two previous books "Kinetics of Solid State Chemical Reactions" (Naukova Dumka, Kiev, 1992, in Russian) and "Growth Kinetics of Chemical Compound Layers" (Cambridge International Science Publishing, Cambridge, 1998). Compared to those, it gives a more detailed, revised and better illustrated consideration of the peculiarities of reaction diffusion and solid-state heterogeneous kinetics, though the basic principles, on which this consideration rests, remained unchanged.
The book is addressed to scientific workers, engineers, students and post-graduates (physical, solid-state and inorganic chemists, metal and solid-state physicists, materials and corrosion scientists, metallurgists, etc.) involved into the stud y of solid-state processes and their practical applications including solid-state synthesis of inorganic substances, protective coating, corrosion, all-in-one joining of dissimilar metals, welding, brazing and thin-film technology. It may equally be used by theoreticians, experimentalists and technologists to satisfy, to a greater or a lesser extent, their specific needs.