Solid-state reactions spinel formation

Reactions between two solids are analogous to the oxidation of a metal, because the product of the reaction separates the two reactants. Further reaction is dependent on the transport of material across this barrier. As with oxidation, cracking, porosity and volume mismatch can all help in this. In this section, the case when a coherent layer forms between the two reactants will be considered. The mechanism of the reaction may depend on whether electron transport is possible in the intermediate phase, and the rate of reaction will be controlled by the rate of diffusion of the slowest species. To illustrate the problems encountered a typical solid-state reaction, the formation of oxide spinels, is described. 

Of all solid state reactions, the formation of oxide spinels is at present the most thoroughly investigated [4, 5, 33]. The first reason for this is the relatively simple crystallographic structure of the spinel lattice. Essentially, this consists of a nearly close-packed face-centred-cubic sublattice of oxygen ions. The tetrahedral and octrahedral interstices of this sublattice are filled in a certain way by the cations. The second reason is that spinels are technically very interesting substances, and one would like to be able to find optimal methods for their preparation. For instance, ferrites are used as control or circuit elements in the electronics industry, and chromite brick is used as cladding in ovens which are used for the production of steel. Therefore, the formation of spinels will now be discussed in detail as a model of a classical solid state reaction. 

Let us now turn to diffusion in the general case, without worrying about the exact mechanism or the rates of diffusion of the various species. As an example to illustrate how we would analyze a diffusion-limited solid state reaction, we use the general equation describing formation of a compound with spinel (cubic) structure and stoichiometry ... 

Figure 70. Change from reaction to diffusion control in a spinel formation reaction4 indicated by the mass change. (Reprinted from H. Schmalz-ried, Solid State Reactions, Verlag Chemie, Weinheim. Copyright 1981 with permission from WILEY-VCH Verlag GmbH.)...

One of the difficulties with the classical solid-state reaction is that mechanical mixing methods are relatively ineffective in bringing the solid reactants in contact with one another. Diffusion lengths, on an atomic scale, are still enormous and the temperatures required may preclude the formation of phases that might be stable at intermediate temperatures. One method, called a precursor method, involves the formation of a mixed-metal salt of a volatile organic oxyanion such as oxalate by wet chemical methods, which result in mixing essentially on the atomic level. The salt is then ignited at relatively low temperatures to form the mixed-metal oxide. The method has been applied successffilly to the preparation of a number of ternary transition metal oxides with the spinel structure. ... 

As kaolinite is heated beyond 980°C, the small fraction of mullite crystals that formed at 980°C continue to grow, albeit at a slow rate. Mullite growth is accompanied by the disappearance of the spinel phase, although the amount of mullite formed is lower than expected based on the spinel loss [33], Mullite formation does not approach completion until a second exothermic event occurs at approximately 1200°C, as recorded by differential thermal analysis [33], When formed by solid-state reaction, mullite has... 

There are many solid state reactions with interesting morphologies in which other modes of transport besides ionic transport in the reaction product can be very important. As a first example, let us consider a reaction which normally occurs by the counter-diffusion of cations, as illustrated in Fig. 2-1 for the formation of spinels in the reaction couple AO/AB2 O4/B2 O3. It was assumed in this case that the mobility of the oxygen ions is negligible. However, if the reactant oxides AO and B2 O3 are very porous, then the reaction product AB2 O4 will also be porous. Furthermore, if the reaction product is an electronic conductor 1), as is generally... 

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. 

The formation of NiCr204 spinel as a continuous layer in between NiO and Cr203 is a result of the solid-state reaction between these pure oxide... 

Two mechanisms have been suggested for the formation of spinel Direct oxidation of the alloy melt through cracks in the MgO film [48,49] or an interfacial displacement reaction between aluminum and MgO, followed by solid state diffusion of magnesium through the oxide to repeat the oxidation to MgO at the surface [47,51],... 

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