Thermomechanical Initiation and Propagation of Fast Decomposition

Historically, thermal and mechanical mechanisms for the initiation of fast reactions have been closely linked because, while it is common experience that explosions can be thermally initiated, it is also common to assume that the energy of any mechanical stimulus applied to an azide is first converted to heat before an explosion results (Chapter 8). Alternative mechanisms whereby the mechanical energy is absorbed as strain energy or in the fracture of crystals have not been given extensive consideration, although such possibilities have been proposed by Taylor and Weale [45], Ubbelohde [46,47], and others from the 1930s onward. The question of whether localized plastic flow or adiabatic shear can lead to the initiation of explosives under the action of low-intensity shocks has recently been given renewed attention [48]. Such considerations draw further attention to the lack of data on the mechanical properties of azides and explosives upon which to base quantitative assessments. Mechanical stimuli can also produce electric effects, and some of the possible consequences are discussed in the next section. 

The thermal growth of fast reactions in exothermic systems is described quantitatively by a three-dimensional heat-transfer equation which includes a factor for the rate of evolution of heat (taken for simplicity to have an Arrhenius dependence)  

In the equation the rate of rise of temperature dT/bt depends upon the balance between heat conduction (given by the first term on the right-hand side) and heat evolution (which involves the reaction rate parameter A and exothermicity parameter Q). C is the heat capacity per unit volume, k the thermal conductivity, E the activation energy, and k the Boltzmann constant. The equation cannot be solved analytically, but approximate solutions were first described for gas-phase reactions [49] and later for decomposing solids [50]. 

The problem of thermal initiation can be seen to be that of confining the heat produced in a sufficiently small volume so that the temperature rises enough to generate a fast reaction. In this section some mechanisms are considered in which hot spots are produced mechanically. But first, it will be instructive to establish a framework for the discussion by considering further the macroscopic features of thermal decomposition and the comparative reactivity of the azides. 

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