Effect of electric fields on oxygen exchange reactions during sodium perchlorate thermolysis

Abstract

Alkali metal perchlorates are known as oxidizers widely used in propellants, pyrotechnics, catalysts, and other fields [1]. These compounds contain significant proportions of oxygen, so they can also be successfully employed as solid oxidizers in the self-propagating high-temperature synthesis (SHS) of functional oxide materials (ferrites, ferroelectrics, high- T c superconductors, and others) [2]. In the combustion synthesis of complex oxides, the quality of the ultimate product is significantly affected by the degree of coincidence of the peak of perchlorate decomposition with oxygen gas evolution and the peak of metal fuel oxidation [3]. Usually, numerous test-and-trial experiments are needed to solve this problem and to find an optimal batch composition. In this context, it is pertinent to search for means of controlling such processes, especially for distant control means. It was discovered that, for example, strong electric fields generated by a condenser under SHS conditions substantially change the combustion process parameters [4]. The magnetic parameters of some ferrites are substantially modified during SHS in electric fields. To explain the effect of external physical actions on combustion and phase formation during the synthesis of complex oxide materials, it was hypothesized that electromagnetic fields applied to the zone of intense chemical reactions affect the kinetics of the decomposition of solid intrareaction oxidizers [5]. In addition, it cannot be ruled out that such actions affect oxygen exchange during the formation of complex oxides. In order to elucidate possible effects of electric fields, we believed it important to recognize the major SHS stages and to analyze the field effects on each stage. In this context, by way of example we studied the decomposition kinetics in electric fields for pure sodium perchlorate. The experimental setup used to study sodium perchlorate thermolysis (Fig. 1) was a vertical furnace equipped with a kanthal wire heater, with two tubular chambers 20 mm in inner diameter. The furnace was mounted between two copper plates, which were isolated with Teflon liners 1 mm thick and to which voltage up to 25 kV could be applied. Crucibles 18 × 24 mm in size, one containing powdered sodium perchlorate and the other containing a reference (NaCl), were simultaneously inserted into the chambers preheated to the required temperature ( 750°C ). Chromel‐ Alumel thermocouples were embedded beforehand into the powder to a depth of about 3 mm. Thermocouple signals were recorded with a potentiometer equipped with an xy plotter. Sodium perchlorate (99+%) from Acros Organics pounded in an agate mortar with a pestle was used in the experiments.