Energy release, explosive output, and critical diameter

Energy release, explosive output, and critical diameter 

For most explosives, where a small volume of a sohd is converted into a large volume of gas, a good approximation of the energy release AG is dominated by the enthalpy change AH AG = AH—TAS, where AH is given by the heat of formation of the products minus the heat of formation of the reactants [see Eqs. (1) and (2)]. Hence, it is desirable that chemical explosives have as positive a heat of formation as possible. 

It is somewhat confusing that the term critical diameter is also used by those interested in the potential of an energetic material to undergo thermal runaway. Because, by definition, the energetic material releases heat when it decomposes, it has the potential to increase its local environmental temperature. Depending on the decomposition kinetics of the material, at some critical dimension the charge can self-heat to catastrophic reaction. This can be referred to in terms of the critical diameter or, more often, in terms of the initial environmental temperature that allows this scenario, the critical temperature . 

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