During detonation chemistry may precede heat

Thermal activation of exothermic chemical reactions produces heat which may activate more reaction thereby creating a self- sustained event. However, reactions can also be activated athermally. Photo activation is one example; and mechanical activation is another. Then heating follows the chemical event, rather than leading it. Which sequence prevails depends on the circumstances, particularly the time available for the process. The mechanical sequence, being electronic, is much faster than the thermal sequence. Thus deflagrations may be thermally activated, while detonations may be mechanically activated. To introduce mechanical activation, a brief review of mechanochemistry and of chemical reactivity is given. Examples of reactions that demonstrate the importance of purely shear strains (elastic) are: (1) 'hammer chemistry'; (2) oxidation of silicon at stressed crack tips; and, (3) increase of catalytic rates by surface acoustic waves. A computational example is the bending of TATB molecules. It is argued that the shear strains in the leading edges of detonation fronts initiate the detonation process. Because the process is confined, pressures in the megabar range are generated. The resulting hot plasma appears within several femtoseconds. Observable 'hot spots' follow this chemical process. An interpretation of the 'size effect' associated with initiation is given in terms of cage recombination. The behaviour at compressed cavities is also discussed.