Rate processes in the solid state

Processes in which solids play a rate-determining role have as their principal kinetic factors the existence of chemical potential gradients, and diffusive mass and heat transfer in materials with rigid structures. The atomic structures of the phases involved in any process and their thermodynamic stabilities have important effects on these properties, since they result from the distribution of electrons and ions during the process. In metallic phases it is the diffusive and thermal capacities of the ion cores which are prevalent, the electrons determining the thermal conduction, whereas it is the ionic charge and the valencies of the species involved in non-metallic systems which are important in the diffusive and the electronic behaviour of these solids, especially in the case of variable valency ions, while the ions determine the rate of heat conduction. 

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The generally-accepted sequence of reactions that are responsible for the chemical changes in a polyolefin on oxidation are shown in Figure 1. It is possible to associate an elementary rate equation with each reaction, although it is recognised that the true rates are difficult to measure both because of the physical state of the polymer, possible diffusion-controlled rate processes in the solid state and the many competing reactions. In a homogeneous rate model, this sequence of rate equations may be solved and a total reaction rate then fitted to the measured oxidation rate (3). 

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