The thermal decomposition of unirradiated and irradiated lithium azide
The thermal decomposition in vacuo of lithium azide, in powder and pellet form, has been investigated. The temperature range was 160° - 220° C. The results obtained were highly reproducible, and a study of the reaction kinetics was made for both the pelleted and powdered forms of the azide. Mathematical analysis of the results obtained indicated that discrete nuclei are formed over the induction period which then grow two-dimensionally over the acceleratory period. The nuclei increase in number linearly with time and overlap and ingestion of the nuclei occurs during the acceleratory period. This was shown by the applicability of the Avrami-Erofeyev equation with the exponent n assuming the value 3, for the analysis of the acceleratory period p/t plot. The nuclei are formed mainly over the external surfaces of the decomposing particles. The effects produced by pre-irradiation with γ-, X- and U.V.- radiation on the subsequent thermal decomposition have been studied. These studies were largely on the powdered lithium azide, but some attention was also given to pre-irradiating the pelleted material. Pre-irradiation of the powdered material resulted in a marked shortening of the length of the induction period followed by an increased acceleratory rate, for the types of radiation employed. X- and U.V.-radiation did not have any significant effect on the decay period reaction rater The p/t plots were sigmoid, as found for the unirradiated azide. Applicability of the Avrami-Enofeyev equation in the analysis of the acceleratory period for the irradiated azide indicated that two-dimensional nuclei formed and grew on the surfaces of the particles, except in the case of γ-irradiated lithium azide when the reaction was largely confined to internal grain boundaries. Similar mechanisms operate over the induction periods for the unirradiated and irradiated azide. A change in the value of the activation energy for the acceleratory periods, indicated that the rate determining step for this stage of the reaction is altered after pre-irradiation. The same mechanism was applicable for the decay stages of the unirradiated and irradiated azide. The effects of pre-irradiation with all three types of radiation, resulted in a double-sigmoid p/t plot in the case of pelleted lithium azide. The initial reaction was confined to the surface of the pellet pre-irradiated with U.V., and occured throughout the bulk of the pellets pre-irradiated with γ- and X-rays.
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