Characteristics of Solid-State Reactions

There are several ways to induce reactions in solids. The application of heat, electromagnetic radiation, pressure, ultrasound or some other form of energy may induce a transformation in a solid. For centuries, it has been a common practice to subject solid materials to heat in order to determine their thermal stability, to study their physical properties, or to convert one material into another. One important commercial reaction, that producing lime, 

Reactions in solids are often vastly different from those that take place in solutions. Because many of the reactions in the solid state involve inorganic materials, an introduction to this important topic is presented in this chapter to show some of the principles that are applicable to this area of inorganic chemistry. The emphasis is on showing several types of reactions, but no attempt is made to present comprehensive coverage of the hundreds of reactions that take place in the solid state. Although some 255 

In order to test rate laws, a must be determined as a function of time using an appropriate experimental technique. If the reaction involves the loss of a volatile product as shown in Eq. (8.1), the extent of reaction can be followed by determining the mass loss either continuously or from sample weight at specific times. Other techniques are applicable to different types of reactions. After a has been determined at several reaction times, it is often instructive to make a graph of a versus time before the data are analyzed according to the rate laws. As will be shown later, one can often eliminate some rate laws from consideration because of the general shape of the a versus t curve. 

In addition to the complications described, other factors are important in specific reactions. If a reaction takes place on the surface of a solid, reducing the particle size (by grinding, milling, or vibration) leads to an increase in surface area. A sample of a solid treated in this way may react faster than an untreated sample, but in some cases changing the particle size does not alter the rate. This has been found to be true for the dehydration of CaC204-H20, which is independent of the particle size over a wide range of a values. 

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Rate equations not characteristic of solid state reactions... 

The stability of suspensions, emulsions, creams, and ointments is dealt with in other chapters. The unique characteristics of solid-state decomposition processes have been described in reviews by D. C. Monkhouse [79,80] and in the monograph on drug stability by J. T. Carstensen [81]. Baitalow et al. have applied an unconventional approach to the kinetic analysis of solid-state reactions [82], The recently published monograph on solid-state chemistry of drugs also treats this topic in great detail [83],... 

Heterogeneous solid state reactions occur when two phases, A and B, contact and react to form a different product phase C. A and B may be either chemical elements or compounds. We have already introduced this type of solid state reaction in Section 1.3.4. The rate law is parabolic if the reacting system is in local equilibrium and the growth geometry is linear. The characteristic feature of this type of reaction is the fact that the product C separates the reactants A and B and that growth of the product proceeds by transport of A and/or B through the product layer. 

Nuclei provide a large number of spectroscopic probes for the investigation of solid state reaction kinetics. At the same time these probes allow us to look into the atomic dynamics under in-situ conditions. However, the experimental and theoretical methods needed to obtain relevant results in chemical kinetics, and particularly in atomic dynamics, are rather laborious. Due to characteristic hyperfine interactions, nuclear spectroscopies can, in principle, identify atomic particles and furthermore distinguish between different SE s of the same chemical component on different lattice sites. In addition to the analytical aspect of these techniques, nuclear spectroscopy informs about the microscopic motion of the nuclear probes. In Table 16-2 the time windows for the different methods are outlined. 

This reaction has been studied in considerable detail and is mentioned here as providing a link between the nucleation and growth mechanisms characteristic of solid state decompositions and surface processes of the type discussed in heterogeneous catalysis. 

Rate equations based on concentration dependence (reaction order) Under some conditions, the rate characteristics of solid-state processes can be expressed through a concentration-type dependence. For example, if the decomposition of a large number of small crystallites is controlled by an equally probable nucleation step at each particle, then this is a first-order process (Al, FI). These rate equations are also used in nonisothermal kinetic analyses of rate data in the form g(a) = kt and are therefore included in Table 5.1. 

The solid state reactions at low-heating temperatures have their own characteristics. There are four steps in a typical solid state reaction diffusion, reaction, nucleation, and growth. At low heating temperatures, any step can be the rate determining step of solid state reaction. By means of this method, many new compounds have been synthesized. In recent years, much progress has been made in the preparation of nanomaterials by solid state reactions at ambient temperatures. 

There are several well known examples of photochemical reactions which exhibit a discontinuous change in rate constants and quantum yields when they are carried out in solid matrices and not in solution (see Table 2.5). The deviation from first-order kinetics for a reaction carried out in a matrix below its glass transition temperature is a characteristic feature of solid-state reactions. 

Books and reviews dealing with the kinetics of solid-state reactions (e.g., [1-3]) usually pay little attention to the analysis and comparison of the metrological characteristics of the methods employed in TA measurements although a correct choice of the method to be used in measurement and calculation of the quantity of interest determines the reliability of the results obtained. This chapter addresses these points. 

In the earliest studies of solid-state reactions between ceramic oxides, the width of the reaction product produced by bulk diffusion couples was determined by VLM. Using an SEM with a field-emission gun a much more precise EDS profile analysis can be performed providing chemical analysis at a spatial resolution of 2nm. Figure 10.33 shows a typical XEDS spectrum a plot of counts versus X-ray energy. The X-rays are produced as a result of electron transitions within the atoms in the sample. The transitions and, hence, the peaks are characteristic of specific atoms. A doped silicon crystal is used to detect the X-rays, where they cause the formation of electron-hole pairs. New methods for detecting the X-rays are being developed that use the change in temperature caused by the X-ray and are known as colorimeters. 

A review is given of studies of reactions in ionic solid systems and of the implications of these studies for industrial applications. Work on the kinetics of solid-state reaction systems is discussed, as are studies of reaction mechanisms and of the effects of process variables on product characteristics. As examples of the significance of these studies for industry the formation of ferrites and of other spinels by reaction in the solid state, the use of catalytic processes employing such solid catalysts as zeolites, and the development of batteries and fuel cells using solid-state electrolytes are described. 

A discussion is presented of problems associated with the corrosion of nonmetallic materials and of aspects of solid-state reactions related to corrosion processes. The effects of the various characteristics of surfaces and of attacking agents are considered, and the kinetics and some possible mechanisms of solid-state reactions are briefly reviewed. The effects of transition states, through their influence on reactivity, and of extreme environmental stresses are noted, as are epitaxial effects and effects produced by adsorbed gases. Emphasis is placed on the need for further research on problems of the corrosion of such materials as glasses, ceramics, plastics, and natural and synthetic stones, as well as on the need for interdisciplinary cooperation to help combat these problems. 

Characteristic Features of Solid-State Redox Reactions in Li1 xNi02... 

Solid state reactions are also very common in producing oxide materials and are based on thermal treatment of solid oxides, hydroxides and metal salts (carbonates, oxalates, nitrates, sulphates, acetates, etc.) which decompose and react forming target products and evolving gaseous products. Solid-state chemistry states that, like in the case of precipitation, powder characteristics depend on the speed of the nucleation of particles and their growth however, these processes in solids are much slower than in liquids. 

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