Phase transition detonation induced

Gas-phase results provide insight into the reaction pathways for isolated HE molecules however, the absence of the condensed-phase environment is believed to affect reaction pathways strongly. Some key questions related to condensed-phase decomposition are as follows (1) How do the temperature and pressure affect the reaction pathways (2) Are there temperature or pressure-induced phase-transitions that play a role in the reaction pathways that may occur (3) What happens to the reaction profiles in a shock-induced detonation These questions can be answered with condensed-phase simulations, but such simulations would require large-scale reactive chemical systems consisting of thousands of atoms. Here we present results of condensed-phase atomistic simulations, which are pushing the envelope toward reaching the required simulation goal. 

The explosive phenomena produced by contact of liquefied gases with water were studied. Chlorodifluoromethane produced explosions when the liquid-water temperature differential exceeded 92°C, and propene did so at differentials of 96-109°C. Liquid propane did, but ethylene did not, produce explosions under the conditions studied [1], The previous literature on superheated vapour explosions has been critically reviewed, and new experimental work shows the phenomenon to be more widespread than had been thought previously. The explosions may be quite violent, and mixtures of liquefied gases may produce overpressures above 7 bar [2], Alternative explanations involve detonation driven by phase changes [3,4] and do not involve chemical reactions. Explosive phase transitions from superheated liquid to vapour have also been induced in chlorodifluoromethane by 1.0 J pulsed ruby laser irradiation. Metastable superheated states (of 25°C) achieved lasted some 50 ms, the expected detonation pressure being 4-5 bar [5], See LIQUEFIED NATURAL GAS, SUPERHEATED LIQUIDS, VAPOUR EXPLOSIONS... 

Immediately ahead of the detonation firont the explosive rests quietly in its metastable state, while to the rear the shocked and reacted material flows at several kilometers per second with a pressure of several hundred thousand atmospheres and temperature of several thousand Kelvins. The rapid compression and heating of matter to these extreme conditions and the associated high velocity flow are properties of detonations that can be shared by strong shockwaves. However, with detonations the heated and compressed flow is selfsustaining. Typically, detonations are maintained by the exothermic chemistry they induce. Detonations driven by first order phase transitions have been envisioned, but have not yet been observed. 

The possibility of a dissociative phase transition induced by a detonation is fascinating because the behavior could result in more efficient conversion of chemical energy to useful work. Although detonations accompanied by phase transitions are perfectly compatible with continuum theory, they have yet to be reported in detonating solids. In any event, the results presented in this section, when compared to those fox either... 

Boron nitride is an important superliard material. Preparation of superhard w-BN and c-BN by phase transition of g-BN induced by shock waves has been a hot research topic in the last decade. In China, early work in this field was done by Yun and colleagues [lO]. Using an annular detonation wave generator to produce sliding detonation, about 20% of g-BN in a steel tube could be converted to w-BN. 

Therefore, successful realization of detonation-induced /t-BN -> c-BN phase transition requires that the hydrolysis of BN is suppressed, but detonation gases usually contain water vapour (see above) at high temperature. However, using the hydrogen-free explosive benzotrifuroxan, CeNeOe (and thus excluding water in the detonation cloud) we have achieved the detonation synthesis of c-BN with a 80 % yield [266]. 

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