High Speed Photography of Electrical Breakdown and Explosion of Silver Azide Crystals

Abstract

FAST decomposition in solid explosives can be initiated by electric fields and electric discharges, but the precise mechanisms are not known. Most solid explosives are insulators or semiconductors, and it has been suggested that, first, the electrical resistance breaks down and then, as a consequence, an explosion may be initiated. Bowden and McLaren¹ applied a d.c. field of 4.5 × 10³ V m⁻¹ across a single crystal of silver azide 1.0 mm × 0.5 mm × 0.5 mm and measured the current flowing through the crystal at different times. They found that within a few minutes the current increased to 150 µA (for low fields the resistance of the crystal was 10¹² Ω, and it was ohmic) and then the crystal exploded. If the electrical field was removed within a few seconds of initiation of the electrical breakdown, the crystal resistance would recover to its original value. They believed that when the field is sufficiently high, the electrons from the impurity atoms in the crystal move towards the anode causing a build-up of space charge fields at the cathode, which cause injection of electrons into the crystal from the cathode; these electrons produce secondary free electrons and decomposition of the crystal. The exothermic decomposition leads to self-heating and initiation of fast decomposition if the rate of decomposition of silver azide by the injected electrons is high enough. Zakharov and Sukhushin² made experiments on compacts of Cu(N₃)2 and T1N₃. These explosives were in the form of disks 0.2 mm thick with density similar to that of the crystal. A high voltage pulse of microsecond rise time and a few microseconds duration was applied to the sample and the voltage and the current through the sample were monitored. The field was strong enough to cause the electrical breakdown and its explosion. By using electrodes of different work function they concluded that the breakdown of the resistance of Cu(N₃)₂ took place by the injection of holes from the anode, whereas for T1N₃ electrons from the cathode were responsible. The mechanism of initiation of fast decomposition in Cu(N₃)₂ they suggest is similar to that proposed by Bowden and McLaren, but for T1N₃ they suggested that the mechanical shock produced by the electrical breakdown was the important factor. Sukhushin et al.³ have suggested that the electrical breakdown of lead azide is due to electrons, but the mechanism of initiation of fast decomposition is similar to that of Bowden and McLaren. On the other hand, Mel'nikov et al.⁴ believe that following the impulse breakdown of pellets of T1N³ and AgN³ the initiation of fast decomposition is thermal. That is, the first step after the electrical breakdown is the heating of the reactive material around the discharge channels and then self-heating and initiation of fast decomposition. On the proposed model of Bowden and McLaren (and Zakharov and Sukhushin) the first step is decomposition by the impact of electrons and holes followed by self-heating and explosion.