The thermal decomposition of silver azide

Abstract

The isothermal pyrolysis of single crystals ( ca . 5 x 0.1 x 0.1 mm 3 ) of silver azide has been investigated in the temperature range T ═ 513–558 K by using a thermobalance. The kinetic equation is found to be 1 – (1 – α ) ½ ═ t {2(1 – ∊) / B } u 0 exp (– E/kT) , where α is the molar fraction of decomposition, t the time, B the width of the crystal, ∊( ≪ 1) is an aspect ratio, u 0 ═ 10 2.5 –10 3.4 m s –1 , E ═ 1.23 ± 0.20 eV, and k is the Boltzmann constant. The reaction is topochemical, i. e. it is controlled by the movement of the phase boundary. The solid product (silver) has been examined with X-rays and with scanning and high voltage transmission electron microscopes. It is shown that silver aggregates in the form of ‘pebbles’ 7–14 μm in diameter, which protrude out of the decomposing surface, are randomly orientated but are probably monocrystalline. A mechanism based on Mott’s (1939) model is proposed. The rate-limiting step is the emission of valence electrons from silver azide into silver, and the decomposition is autocatalytic. The theoretical values of the activation energy and the pre-exponential factor are in general agreement with the experimental results. The mechanism has also been used to explain the effects of additives as observed by other workers. The topo­chemical and the autocatalytic characteristics of the reaction suggest that during pyrolysis a silver film covers the crystal surface.