Spontaneous explosion during crystal growth of lead azide

Abstract

Spontaneous explosion of lead azide during crystallisation is well known but little understood. Results of many experiments are analysed covering crystallisation by metathesis, including hydrazoic acid gas reacting with lead salt solutions, and by recrystallisation of lead azide from aqueous ammonium acetate. Vigorous agitation and hydrophilic colloids as additives suppress spontaneous explosions. There is no evidence connecting their occurrence with three polymorphic modifications. They can be predicted with reasonable accuracy by the authors' ammonium acetate method which has proved also that the explosions originate in the very early stages of crystal growth and are not related to larger crystals. Experiments using miniature detonators have shown that ammonium acetate solutions saturated with lead azide are normally incapable of propagating explosion. A finely divided form of lead azide has been prepared which is especially sensitive to ignition by electric spark. From these results a possible mechanism is proposed which assumes that a thin layer of nuclei forms at the surface of the supersaturated solution from which electrical energy is channelled toward a point of critical potential difference with respect to a successive layer. The resulting discharge, which is observable, is ineffective unless there is also sufficient continuity of nuclei of critical size to sustain propagation to explosion.