Evaluation of Bi2O3 and Sb6O13 as oxidants for silicon fuel in time delay detonators

Abstract

This study considered bismuth (III) oxide (Bi2O3) and antimony hexitatridecoxide (Sb6O13) as potential substitutes for the red lead (Pb3O4) and barium sulphate (BaSO4) oxidants currently used in time delay compositions for detonator assemblies. Fine silicon powders with a specific surface area of 2 - 10 m2/g were used as fuels. Some experiments were also done with a coarse manganese powder as fuel. Bi2O3 was synthesised by the thermal decomposition of (BiO) 2CO3 by heating at 460°C for 15 hours. The yield was near quantitative, ie. close to the 91,4% expected based on the complete conversion of the carbonate to the oxide. Sb6O13 was obtained by heating colloidal antimony pentoxide (Sb2O5) for 8 hours at 315°C. This resulted in a ca. 20 % mass loss and yielded a reactive black powder. In the Si-Bi2O3 system, compositions in the range 5 - 40% by mass Si were ignitable with shock tubing. Burn rates measured in lead tubes varied between 15 and 155 mm/s. This highest burn rate was obtained with 20% silicon. Addition of additives such as KMnO4 and boric oxide had little effect on the burn rate. The fast burning Si-Bi2O3 system is a potential replacement for the commercial Si - red lead system. The burning rate decreased with increasing compaction of the samples. Burn rate also decreased when the aluminium instead of lead tubes were used. This is attributed to a greater heat loss with the former. The combustion products were characterised using DTA, FT-IR, XRD and SEM. The results show that the combustion reactions led to reduction of the oxidant to the corresponding metal form. The Sb6O13-Si system requires an initiating composition such as Bi2O3-50%Si (Type 4). It is slow burning and thus a possible replacement for the commercial BaS04-Si system. The lowest sustainable and reproducible burn rate, in the absence of additives, was 4,8 mm/s. It was achieved using 10% silicon Type 4. Adding small amounts of fumed silica <2%) increased the burn rate. This is attributed to better mixing and compaction. However, lower burn rates (~2 mm/s) are possible if more fumed silica is added as inert diluent. Replacing the silicon fuel with manganese powder gave more exothermic and even slower burning compositions.