Spontaneous Explosions During Crystal Growth

Spontaneous explosions of LA occur without apparent reason during the course of its crystallization. This phenomenon is known but not very well tmderstood. These explosions are not exclusive to azides of lead but have been observed also in the case of cadmium, cupric, and mercuric [15]. In the case of lead azide, it has been frequently attributed to the formation of p-lead azide or the growth of large crystals to a point where internal stresses become important [12, 18, 34]. Both of these theories have httle or no experimental support [18]. 

The experimental results show conclusively that the suppression of spontaneous explosion during crystallization by metathesis from solutions of soluble lead salt (usually lead acetate or nitrate) and soluble azide (usually sodium azide) is obtained by (a) rapid and thorough mixing of the solutions and (b) the use of certain additives especially hydrophilic colloids (PVA, dextrin, eosin). Amounts as small as 0.02 % of PVA completely suppressed spontaneous explosions [18]. 

It has been further shown that explosions still occur when lead nitrate is replaced by lead acetate and sodium azide with calcium azide [12]. The explosimis are reported to occur at room temperature as well as at 0 C and 60 °C [12]. 

With the reduction of mixing efficiency during crystallization by metathesis the probabihty of explosion increases. When mixing was reduced until the solutimis merely diffused as is usual in the method for preparation of the p-form ( ), the incidence of explosions increased greatly— up to a 50 % probability [18]. 

The most reliable way to produce explosions of LA is reciystallization from ammonium acetate. A reasonable explosion control time is possible by carefully controlling the concentration, temperature, and conditions of cooling. Table 4.5 gives results of a series of experiments with times to explosion from concentrations from 0.6 to 1.0 % LA. The frequency of explosions decreases below 0.6% and falls to nil at 0.4 %. An interesting observatimi was made that intact crystals of LA could be found after the vessel had been shattered by an explosion [18]. 

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Taylor, A.C., Thomas, A.T. Spontaneous explosions during crystal growth of lead azide. J. Cryst Growth 3, 391-394 (1968)... 

Initiation, Spontaneous. This is really a misnomer. What it refers to is the self-heating of an explosive as a result of autocatalytic decomposition eventually leading to a thermal explosion. Spontaneous initiation may also occur during crystal growth eg in Lead Azide see Detonation (and Explosion), Spontaneous in Vol 4, pp D561-563... 

As stated above, Wohler and Krupko [80] noticed that lead azide may explode during crystallization. Miles [75] confirmed this and stated that spontaneous explosion could occur during the growth period of crystals of /Head azide, i.e. when the two solutions forming the crystals diffused slowly. When isolated, however, the crystals (some of which had reached a length of 4 cm) were not found to be particularly sensitive. 

According to Kaufman [91] spontaneous explosion can also take place during the growth of a-lead azide crystals, e.g. when a supersaturated solution of lead azide in ammonium acetate is seeded with crystals of the a-form. Spontaneous explosions have also been observed with mercuric azide and in some cases with cadmium azide. 

War Department Field Manual FM 9-40, Ordnance Unexploded Bombs Organization and Operation for Disposal (1943) states for lead azide, Sensitivity. . . varies with crystal size. .. Large crystals detonate spontaneously. Sensitivity concerns over crystal size is compatible with the manufacturing process for lead azide in which dextrin or polyvinyl alcohol is added to inhibit crystal growth during the precipitation stage. (See Explosives by Rudolph Meyer.) FM 9-40 also states for TNT, Impurities may cause exudation. This is undesirable because of probabihty of low order detonation and increased sensitivity to shock of the explosive. ... 

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