The thermal decomposition of crystals of barium azide

Abstract

In solid reactions of the type A solld = B solld + C gas the reactions are sometimes autocatalytic, being accelerated by the solid product. The solid product may be disseminated throughout the crystal in a diffuse manner, as has been shown by the microscopical examination of partially decomposed crystals of mercury fulminate (Garner and Hailes 1933) or potassium azide (Garner and Marke 1936), or may be aggregated into visible nuclei as in the case of alkaline earth (Harvey 1933 ; Marke 1937; Maggs 1939) or lead azides (Garner and Gomm 1931). In previous investigations an induction period has been found during which the evolution of gas is negligible, which is followed by an accelerating rate of development of pressure. For a certain period of the reaction the relationship of pressure and time is that of an exponential type which in a number of cases takes the simple form, log p = kt + const. This expression is obeyed over the early part of the reaction for single crystals of mercury fulminate (Garner and Hailes 1933), barium azide (Harvey 1933), and lead styphnate (Hailes 1933). It also holds for silver oxalate (Macdonald 1936 a , b ) in a finely crystalline form and for ground mercury fulminate. Attempts have been made to account for the exponential relationship in terms of the spread of reaction chains throughout the crystal, but there are certain difficulties in the way of the application of the chain theory, especially when nuclei are formed. There are, besides, cases where this simple expression does not fit the experimental results, notably that of ground calcium (Marke 1937) and barium (Harvey 1933) azides.