Solid-state reactions homogeneous

In both 4.1.1. and Table 4-1, S, L G refer to solid, liquid and gas, respectively. Note that we have also classified these heterogeneous mechanisms in terms of the same PHYSICAL CHANGES given above for homogeneous transformations. For the most part, the initial material will be a solid while the nature of the final product will vary according to the type of material undergoing solid state reaction. 

As stated above, we can clcissify solid state reactions as being either homogeneous or heterogeneous. The former involves reactions by a single compound whereas the latter involves reactions between two different compounds. As stated above in Table 4-1, there are four (4) t es of solid state reactions. 

Although we have covered mechanisms relating to solid state reactions, the formation and growth of nuclei and the rate of their growth in both heterogeneous and homogeneous solids, and the diffusion processes thereby associated, there exist still other processes zifter the particles have formed. These include sequences in particle growth, once the particles have formed. Such sequences include ... 

If iron metal would oxidize to form a homogenous oxide layer without flaking off, draw a diagram showing the reaction conditions, the phase boundary formed and the diffusion conditions likely to prevail in the solid state reaction. 

Because observed rate enhancements are usually small, or zero, nonthermal effects do not seem to be important in MW heated reactions in homogeneous media, except possibly in some reactions of polymers and reactions in nonpolar solvents. Relatively few studies have been conducted on MW-assisted reactions of polar reactants in nonpolar solvents. Also, since there is some disagreement as to whether or not these reactions are accelerated significantly by MW, in comparison with conventionally heated reactions at the same temperature, more research on the effect of MW irradiation on the rates of these reactions is required. Nonthermal effects may, however, explain the more substantial MW rate enhancements in solvent-free reactions on solid supports [44] (see Chapt. 5) and solid state reactions [68, 69]. 

The solid state reactions are extremely complex due to intervention of many physical parameters and it becomes often necessary to make some generalizations in the complex reactions. The rate in solid state reactions cannot be defined in the same way as that for a homogeneous reactions because the concept of concentration in solid state reactions has no significance. The energy of activation in a solid state reaction has also no significance, except in some rare cases. 

When mixtures of substances are investigated, e.g. in solid state reactions, the mixture should be completely homogeneous and of uniform particle size. Smaller particle size, i.e. higher ratio surface area/volume, are important for such types of reaction ... 

Many solid-state reactions may be pictured as proceeding in two steps. First a homogeneous process leads to product molecules dissolved in residual parent matrix. Curtin and Paul, in a review on thermal solid-state reactions (6), divide this step into a number of stages First, there is a loosening of the molecules at the reaction site to be, then molecular change (the true reaction), and finally solid-solution formation. When the concentration of the accumulated product exceeds the solubility limit the second step, the decomposition of this solid solution into separate reactant and product phases, occurs. However, in some cases the solubility limit is very low, so that the overall process appears to become simpler ... 

The ideal condition for carrying out a solid state reaction in order to obtain a homogeneous product in the shortest time at the lowest possible temperature is to ensure homogeneous mixing of the reactants on an atomic scale. This, however, cannot be achieved in the ceramic method or its modifications. The only way to achieve this is to prepare a single phase (a chemical compound) in which the reactants are present in... 

Another solid state reaction problem to be mentioned here is the stability of boundaries and boundary conditions. Except for the case of homogeneous reactions in infinite systems, the course of a reaction will also be determined by the state of the boundaries (surfaces, solid-solid interfaces, and other phase boundaries). In reacting systems, these boundaries are normally moving in space and their geometrical form is often morphologically unstable. This instability (which determines the boundary conditions of the kinetic differential equations) adds appreciably to the complexity of many solid state processes and will be discussed later in a chapter of its own. 

For elemental solids and stoichiometric compound crystals, the primary influence of irradiation on their kinetic behavior is due to the increase in Acv(s Ac,). We would expect the enhancement in the component diffusion to be in proportion to the increase in the (average) defect concentrations, thus influencing all homogeneous, inhomogeneous, and heterogeneous solid state reactions. 

---