Effect of Gaseous Atmosphere Composition on the Thermal Decomposition of Calcium Hypophosphite

The thermal decomposition of calcium hypophosphite is studied using simultaneous differential thermal analysis and thermogravimetric analysis (DTA/TGA) in the temperature range from 25 to 600°C, with a heating rate of 10°C/min, under both a nitrogen atmosphere and an air atmosphere. In the nitrogen atmosphere, the decomposition consists of a broad exothermic effect, comprising four consecutive stages with peaks at 368, 386, 400, and 434°C, and a total heat release of 1.51 kJ/g. In the air atmosphere, the decomposition shows an exothermic effect with a heat release of 9.01 kJ/g. During the decomposition in air, researchers experimentally observe an increase in the sample weight by 6 wt % in the temperature range of 370 to 400°C, which may be explained by the oxidation of phosphorus with the formation of orthophosphoric acid. The condensed products formed during the thermal decomposition appear as a porous mass of reddish-brown color, which may be explained by the formation of an intermediate decomposition product—calcium phosphide. It is reasonable to assume that the formation of phosphide as an intermediate product is due to the nonequilibrium nature of the process, kinetic factors, and diffusion limitations during the thermal analysis experiment, given that calcium phosphide is thermodynamically stable up to 1000°C. Previously, these factors were considered in relation to the processes of self-propagating high-temperature synthesis of phosphides. Furthermore, the formation of new phases during the thermal decomposition of calcium hypophosphite, according to the Hedvall effect, may explain the rapid formation of calcium phosphide and its preservation.