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There are several well known explosives which can be mechanically triggered. I am curious about the mechanism of such a reaction, though.

I guess one can give an easy answer when the explosive is sensitive to everything, flames and mechanical activation, too. What puzzles me is that several of these compounds, e.g. nitroglycerin or ammonium-nitrate much more difficult to trigger by simple flame. How mechanical activation is different?


marked as duplicate by bon, M.A.R., ron, Community Sep 18 '15 at 8:08

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  • $\begingroup$ I've seen somewhere analysis of this, but dunno if I'll find it again $\endgroup$ – Mithoron Sep 17 '15 at 10:36

There are differing propagation mechanisms for an explosion:

  1. Thermal: a deflagrating explosive such as gunpowder has a comparatively slowly propagating wave-front, depending on heat propagating through the bulk material to set fire to the next layer. This is similar to a candle slowly burning down.

  2. Compression: some brissant (but opaque: see 3. below) explosives create a shock wave. "[High explosives] detonate to produce a defining supersonic over-pressurization shock wave."

  3. Light: fuel-air mixtures, glyceryl trinitrate and nitrogen triiodide explosion can be initiated by light (auto manufacturers are looking at laser "spark plugs" to improve internal combustion engine efficiency). In these types of explosives, light from the explosion itself might propagate a supersonic blast front.

So initiating an explosion through heat alone may initiate just a deflagration, without enough over-pressure to form a blast wave.


Mechanical initiation (hitting with hammer) causes shock wave running through the bulk of the material. This means phonon (vibrational) excitation. In some compounds, the vibronic coupling enables the transfer of vibrational energy to electrons, meaning the molecule reaches excited state. This in turn can lead to bond breaking and the energy rich molecules and with plenty of nitro groups have enough oxygen and heat to "burn". But, as noted, in the whole volume of sample in the same moment.

Compared to flame initiation, where the substance can burn only on the surface.

  • 2
    $\begingroup$ Thank you for the answer. My problem is phonon/vibrational energies are generally at least an order of magnitude lower than electronic excitation energies - however if there are low electronic excitation, high temperature (i.e. ignition) should trigger a similar reaction. It is not true that substances burn on the surface. Many explosive materials have a positive oxygen balance and don't need air/oxygen. $\endgroup$ – Greg Sep 17 '15 at 9:26
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    $\begingroup$ I am not an explosion scientist, just did some stuff on mechanical activation of chemical reactions. So I just quickly searched on this specific topic. To say, explosives are energy rich, but the activation process is heavily system-dependent. See e.g. pubs.acs.org/doi/abs/10.1021/jp300711m $\endgroup$ – ssavec Sep 17 '15 at 11:10

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