Defects and the Reactivity of Solids

Considerable empirical evidence for the role of defects in solid state reactivity has been accumulated over the years, although the systematic study of the simpler processes is a comparatively recent field of work. Hedvall, as early as 1906, concluded that the mobility 

Hiittig (32) has demonstrated that solid-solid reactions involving bulk phase diffusion take place at tempera-tures 0-52 ( X) only, those involving surfirce processes (i.e., sintering) occur at temperatures 0-52 but 0-25 r . The relation to volume and surface defect mobifey has been demonstrated by Bevan, Shelton and Anderson (33) by electronic conductivity measurements. 

Structure-sensitive reactions of solids, due to non-equilibrium defect content, are probably of greater industrial importance than those in which the reacting phases have equilibrium lattice defect concentrations and are free of other types of lattice disturbance. However, the structure-insensitive systems give some hope of establishing the general principles of the reaction mechanism in solids there is no essential difference in principle between the two types of reaction, only that the structure-insensitive systems offer some possibility of quantitative study. Moreover, the systematic study of simple reaction systems could not be pursued using powdered materials, as too many additional factors cloud the fundamental processes. Wagner s classical experiment on the reaction between silver metal and sulphur demonstrated quantitatively the reaction mechanism. 

Wagner found, for instance, that after heating at 220 C. for one hour the Ag block had lost 108 mg. in weight and the slabs of a-AgsS had both increased in weight, 

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