Effect of Impurities on Thermal Decomposition

Various impurities significantly affect the thermal decomposition kinetics of metal azides the yet unfulfilled challenge is to determine by what mechanism these impurities operate. 

Types of inorganic impurities can be conveniently classified in two ways extrinsic or foreign ion impurities and intrinsic or ion vacancy and interstitial impurities. 

The structure and chemical formula of the surface contaminants on lead azide have yet to be determined their effect on the main decomposition reaction is also unknown. However, Fox [10] found that freshly prepared crystals contained only about 0.1 monolayers of contaminant, and reproducible kinetics were obtained using aged samples which were stored in vacuo in the dark [47]. 

The atmosphere in which azides decompose significantly influences decomposition kinetics. Reitzner [88] investigated the effect of water vapor on the decomposition of lead azide and concluded that the induction period was the result of the reaction of water vapor with lead nuclei. Not until all the water was consumed by the reaction did the azide decomposition accelerate. The length of the induction period was, thus, proportional to the vapor pressure of water in the system. However, in the presence of large amounts of water the azide reacted to produce a basic material which decomposed in a complex manner. Slow decomposition of lead azide in air also produced various oxides, hydroxides, and carbonates of lead [2,4]. 

In an attempt to test the nudeation-autocatalysis theory of azide decomposition, Reitzner et al. [89] decomposed samples of lead azide powders coated with silver and found a substantial decrease in the induction period prior to the main acceleratory stage and a slight decrease in rate of decomposition. However, the mechanism of these effects is not known, although the silver may have acted as nuclei for decomposition. 

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