Generation of High Temperatures and Chemical Reaction

Mechanical Generation of High Temperatures and Chemical Reaction 

There appears in Chapter 8 a discussion of the generation of high temperatures by the adiabatic compression of gas pockets. In this section the direct generation of heat in solids by mechanical means is discussed. 

The earliest systematic studies of mechanically induced chemical reactions in solids were those of Cary-Lea [61] and Parker [62], who showed that the shearing action of grinding was able to bring about a number of doubledecomposition reactions between simple inorganic salts. Moreover, Parker concluded that this was due to local heating at points of contact between crystals, resulting in fusion and subsequent chemical reaction. This point of view was later taken up by Bernal [63], who proposed that energy could be concentrated into localized hot spots by friction. Detailed experimental studies on the initiation of explosion by impact and friction were subsequently carried out by Bowden and Yoffe [64] and Ubbelohde [65,66] and their coworkers. 

The theory of heat generation by mobile dislocations was given by Eshelby and Pratt [73] in terms of the temperature rise AT produced by the movement of n dislocations at a velocity F in a medium of thermal diffusivity a. Two cases were considered, one in which the mean spacing of the dislocations X is much smaller than A = 2a/F, and that for which X A. The final expressions have the form 

The metal azides are, by common experience, brittle when subjected to mechanical stress, they shatter before appreciable plastic deformation takes place. This arises because, as with most inorganic materials, dislocation densities are low, grown-in dislocations are usually immobile, and slip can take place only on a limited number of planes. However, with the possible exception of diamond and certain borides and nitrides, few materials are ideally brittle, and some plastic deformation is possible, the amount depending upon the temperature and the rate of strain low temperatures and high rates of strain both favor brittle behavior. 

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