Thermal decomposition of sodium azide: Crystal size effects, topochemistry and gas analyses

Abstract

The kinetics of thermal decomposition of sodium azide are shown to be dependent on the specific surface area of crystals. Decomposition commences at the edges of crystal platelets and advances inward in directions parallel to the alternating planes of sodium and azide ions. The acceleration and decay of the decomposition after an initial first stage is dependent on the development of a closed two-dimensional envelope at the crystal edges and a subsequent contraction of the envelope. The boundary between decomposed and undecomposed material is indicated by three colored zones of the sodium decomposition product in different states of aggregation. With crystals grown slowly from aqueous solutions a residue remains when crystals are decomposed at temperatures up to 370°C. Residual gas analyses during decomposition suggest that the residue results from a back reaction between impurity gases such as H 2O 2, CO and CO 2 and the sodium, and the deposition of the more stable compounds on islands of undecomposed azide. At higher temperatures than 370°C, where NaOH is appreciably volatile, little residue remains.