Differential thermal analysis of potassium perchlorate

Abstract

A study has been made of the differential thermal analysis of (i) potassium per~ chlorate in powdered form, lii) potassium perchlorate in pelletized form, (iii) potassium perchlorate recrystallized from liquid NH3, and (iv) potassium perchlorate preheated for 24 hours at 375 ~ Pretreatment of potassium perchlorate leads to a desensitization of both endothermic and exothermic processes. Additionally, the pretreatment tends to convert the symmetric exotherm into an asymmetric exotherm due to merging of the two exotherms. An analysis of the factors causing asymmetry in the exotherm has thrown fresh light on the mechanism of thermal decomposition of potassium perchlorate.Potassium perchlorate (KC10~) has gained importance as an oxidizer in solid composite propellants since 1950. Although the thermal decomposition of KC104 has been studied extensively with a view to understanding the mechanism of its decomposition [1-i0], relatively little attention has been paid to clarifying the existing controversy on the differential thermal analysis (DTA) curve of KC104. The DTA of KC104 shows two endotherms, one at 300 ~ corresponding to the phase transition from orthorhombic to cubic, and another around 600 ~ corresponding to melting of the eutectic mixture. The two endotherms are followed by an exothermic reaction, around which is centred a controversy. Anderson and Freeman [8] studied the DTA of KC104 in air by taking 0.086 g of doubly recrystallized material. They located the thermocouples in the samples and reference standard. Their observations reveal that KC10~ has two endotherms and two exotherms. The first exotherm appears after the "melting endotherm at 606 ~ Its exothermicity decreases as it reaches 636 ~ At 636 ~ a sharp rise in exothermicity occurs, with a peak at 641 ~ followed by the rapid decay of the exotherm. Markowitz and Boryta [9], on the other hand, observed a single exotherm, pointing to a reaction occurring in a single stage. The latter authors used shielded thermocouples and varied the sample size from 3 to 0.1 g, and observed only one exotherm. Further, the latter authors attributed the "resolved exotherm" of Anderson and Freeman [8] to the "possible catalytic effect of the chromel-alumel thermocouple". Later work [10] in this laboratory has confirmed the results of Markowitz and Boryta [9]. More recently, while studying the effect of precompressi0n on the