Volatilization of boric acid formed on the surface of compacted samples of the boron–titanium SHS-system

Abstract

Self-propagating high-temperature synthesis (SHS) is based on solid-flaming combustion of a compacted mixture (boron and titanium) in the solid phase. Сompacted mixture combustion is generally initiated by local pulsed heat supply to the object (glowing solid contact, electric arc, radiant energy flow, etc.), further combustion propagates and is supported by exothermic chemical reaction heat. In some cases the absence of ignition was observed in the initiation of the boron–titanium SHS-system (deterioration of sensitivity to ignition). This phenomenon is caused by the presence of boric acid crystals on the compacted sample surface. The boric acid crystals significantly increase the ignition delay time. The aim of the present research is the investigation of boric acid volatilization process on the boron–titanium compacted SHS sample surface. Boron-titanium compacted SHS sample surface has been analyzed by optical and electron microscopy and by infrared spectrometry. The analysis results show that some amount of meta boric acid (НВО2) is formed on the boron–titanium compacted SHS sample surface under normal climatic storage conditions. Microcrystals of meta boric acid (НВО2) present in the surface layer of boron–titanium compacted SHS samples, probably are nuclei (growth centers) of relatively large boric acid (НВО3) crystals. We have developed the volatilization technology of boric acid crystals on the boron-titanium compacted SHS sample surface. This technology has allowed recovering sensitivity of boron–titanium SHS-system to ignition by means of local pulsed heat supply.