Solid-state reactions fundamentals

The fundamental processes involved in a solid state reaction are twofold. First, there is the reaction itself - the breaking and forming of bonds. Second, there is the transport of matter to the reaction zone. A number of models aiming to describe solid state reactions exist. They are generally based on sigmoidal kinetic curves. The general form of the kinetic equation is as follows ... 

In 1937, dost presented in his book on diffusion and chemical reactions in solids [W. lost (1937)] the first overview and quantitative discussion of solid state reaction kinetics based on the Frenkel-Wagner-Sehottky point defect thermodynamics and linear transport theory. Although metallic systems were included in the discussion, the main body of this monograph was concerned with ionic crystals. There was good reason for this preferential elaboration on kinetic concepts with ionic crystals. Firstly, one can exert, forces on the structure elements of ionic crystals by the application of an electrical field. Secondly, a current of 1 mA over a duration of 1 s (= 1 mC, easy to measure, at that time) corresponds to only 1(K8 moles of transported matter in the form of ions. Seen in retrospect, it is amazing how fast the understanding of diffusion and of chemical reactions in the solid state took place after the fundamental and appropriate concepts were established at about 1930, especially in metallurgy, ceramics, and related areas. 

In the solid-state reaction, nucleation and growth have a fundamental role, because, in essence, the solid-state reaction is a phase transformation. In this type of reactions, nucleation and growth follow similar principles as those previously analyzed in Section 3.1 the principal difference being the increased role of diffusion in solid-state reactions [30],... 

We shall summarize here fundamental results which point to newly discovered mechanisms which permit a control of ageing processes in catalysts. These mechanisms involve the acdon of surface mobile species, so-called spillover. The spillover species can stabilize catalysts against harmful solid-state reactions, in particular prevent reduction to less selective phases. Such reactions occur very frequently in selective oxidation catalysts, and constitute a major cause of deactivation. A typical example is constituted by vanadium phosphate catalysts used in the selective oxidation of butane to maleic ahydride. A few years ago, for example, many such catalysts lost a large part of their selectivity in a few months this selectivity dropped from the modest initial molar value of 55-60% to 45% or less. 

When multicomponent solid systems are used to prepare a catalyst, homogenization of the precursors (mixing at the molecular level) is extremely important The activity of the finished catalyst should not differ in the different parts of a catalyst charge, or from batch to batch of it. Two fundamental aspects of solid-state reactions involved in the preparation of catalysts are nuclcation and the growth from solution of the nuclei or elementary particles into distinct solid phases in the... 

Using differential scanning calorimetry (DSC) (or, less directly, differential thermal analysis (DTA)) (see Section 2.8.5., above) it is possible to measure several of the thermodynamic properties of solids and of solid state reactions. The DSC response is directly proportional to the heat capacity, Cp, of the sample, so that by use of a calibrant it is possible to obtain values of this fundamental thermodynamic property, at a particular temperature, or as an average over a specified temperature range. Other thermodynamic properties are readily derived from such measurements ... 

In solid-state reactions, the reactants are initially in contact and combine chemically to form the reaction product. The kinetics of the initial stage of such a reaction depend on the parameters of the interface (the crystallography of the contacting surfaces, etc.). The fundamental point is that we start with one interface and immediately create two new interfaces. We will consider the example of Ni0/Al203. Similar systems include Mg0/Al203 and Fe0/Fe203. 

The ideal crystal is an abstract concept that is used in crystallographic descriptions. The lattice of a real crystal always contains imperfections. Chemical reactions in the solid state are fundamentally dependent upon imperfections, so that an exact characterization of all possible crystalline defects is essential to an understanding of the reactivity of solids. 

Corrosion is the attack of a gaseous or liquid phase upon a metal or other solid with a resultant destruction of the structure. The fundamental aspects of the problem of gaseous corrosion were discussed in the previous section. Since we are concerned here with solid state reactions, we shall discuss only those corrosion processes in liquid media (i. e. particularly in aqueous... 

In this chapter we shall discuss some solid state reactions which are of technological importance. We shall not be too concerned with providing a quantitative treatment. Technological processes are often much too complex to permit us to give a complete quantitative description of the overall physico-chemical course of the reaction. Instead, we shall apply the previously discussed concepts in two ways. Firstly, we shall attempt to separate out the elementary reaction steps and to describe these steps in terms of fundamental atomic parameters. Secondly, we shall try to describe the processes on the basis of simple limiting cases which can be tested in practice. The examples to be discussed have been chosen somewhat arbitrarily. However, many different areas of solid state chemistry are covered by these examples, and the wide range of applicability of the basic concepts of solid state reactions is well illustrated. 

The fundamental goal of nanoparticle research is to assemble atoms in a controllable way and design nanostructured materials with the desired physical and chemical properties. A major part of the research in the field of nanoscience is dedicated to the development of synthesis routes to nanoparticles and nanostructures. Conventionally, solid-state reactions between powders have successfully been employed for the low-cost production of bulk metal oxides. However, to obtain metal oxide nanoparticles with well-defined shape, size, and composition, these solid-state routes are unsuitable. In contrast to these high-temperature processes, liquid-phase synthesis routes, and in particular sol-gel routes, offer better possibility to control the variation of structural, compositional, and morphological features of the final nanomaterials [1,2]. 

The kinetics of reactions in ionic solids is of profound importance not only in the understanding of the fundamental principles involved but also in many aspects of industrial research and development. These relationships, which are necessarily complex, cover a very broad spectrum, from the initial interaction through all stages of reaction to nucleation and growth of each individual phase. Their academic and applied implications touch on every varied aspect of solid-state reactions and merit even more attention than they have yet been accorded. 

The aim of the series is to present the latest fundamental material for research chemists, lecturers and students across the breadth of the subject, reaching into the various applications of theoretical techniques and modelling. The series concentrates on teaching the fundamentals of chemical structure, symmetry, bonding, reactivity, reaction mechanism, solid-state chemistry and applications in molecular modelling. It will emphasize the transfer of theoretical ideas and results to practical situations so as to demonstrate the role of theory in the solution of chemical problems in the laboratory and in industry. 

Palladium hydride is a unique model system for fundamental studies of electrochemical intercalation. It is precisely in work on cold fusion that a balanced materials science approach based on the concepts of crystal chemistry, crystallography, and solid-state chemistry was developed in order to characterize the intercalation products. Very striking examples were obtained in attempts to understand the nature of the sporadic manifestations of nuclear reactions, true or imaginary. In the case of palladium, the elfects of intercalation on the state of grain boundaries, the orientation of the crystals, reversible and irreversible deformations of the lattice, and the like have been demonstrated. 

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