Dielectric investigation of the thermal decomposition of sodium azide

Abstract

The thermal decomposition of sodium azide has been investigated by monitoring changes in dielectric constant, dielectric loss, and pressure with time at a fixed frequency during decomposition at 262–305°C, and by monitoring changes in dielectric properties with frequency in the range 100 Hz to 300 kHz for samples cooled to 20°C after partial decomposition at 290°C. Curves of ε′ and ε″ against time both show maxima at the stage where the fractional decomposition α shows a marked increase. Three stages of decomposition (A, B and C) can be identified for α < ca. 0.02, and activation energies for the three stages and for two types of sodium azide vary between 28.0 and 73.4 kcal/mole. At 100 kHz, and during stage B, changes in ε′ and ε″ are both related to changes in α by simple power laws, the exponents for which show an approximate linear decrease with increasing temperature. At frequencies below ca. 1 kHz, d.c. conductance is mainly responsible for the dielectric loss, whereas at higher frequencies Debye or Maxwell-Wagner type relaxation processes predominate. The difference in static dielectric constant at 20°C between partially decomposed and undecomposed samples is directly proportional to α² for values of α corresponding to stage B. Changes in dielectric properties during decomposition are discussed in terms of “dipoles” involving the defect structure of the solid.