Dielectric Breakdown of Silver Azide

Change log
Tang, Tong, Bor 

The breakdown of single crystals of heavy metal azides has been investigated. In silver azide, the low-field conduction is by mobile interstitial cations and their enthalpies ,of formation and hopping have been determined from the Arrhenius plot of the conductivity. Based on AC conductivity · measurements and the observed time dependence of the current, it is shown that ionic polarisations occur at the electrode interfaces. With fields higher than 15 kV.m- l (for silver or carbon electrodes) bipolar field injection i s though t to take place. These fields can lead to dielectric breakdown, although the incubation period may l as t up to several days - but less than a second if the field is 0.4 MV.m- l or higher. Evolution of nitrogen has been measured by mass spectrometry under this condition. Based on a comparison with thermal decomposition of AgN 3 , the following mechanism is proposed. for the 'electrical decomposition'. Holes injected from the anode combine bi-molecularly via traps and produce N2. Some of the electrons from the cathode arc localised at impurity centres or defects . The discharge of mobile Ag+ at the electron traps results in silver atoms which, if formed on the surface , may migrate as well as aggregate because of their high diffusivity even at room temperature. By optical and electron microscopy silver nuclei have been detected on the surface of a crystal to which a strong field is applied through conducting contacts. It has also been observed that post-breakdown disruption in the form of an explosion occurs. The breakdown is explained as due to the formation on the crystal surface of filamentlike silver films, and the initiation of deflagration and subsequent detonation, to the appearance along the metallic conduction path of 'hot spots' arising from Joule heating and leading to thermal decomposition and self-heating. This explanation is supported by evidence such as initiation locality.

In the comprehensive study on the thermal decomposition of AgN 3, new methods have been proposed for the analysis of kinetic data, both isothermal and dynamic, from solid state reactions.

Parallel observations, in less detail, suggest that dielectric breakdown and explosion initiation by similar mechanisms occur in thallous and probably in lead azides. It is a possibility that breakdown of long incubation period can be brought about by analogous 'electrical decomposition' processes in certain metal halides, oxides, hydrides , alkali azides and other ionic metal compounds, although in substances which are not highly exothermic no explosion will follow the breakdown.

azides, conductivity, decomposition
Awarding Institution
University of Cambridge