Combustion performance of self-assembly ternary pyrotechnic time-delay compositions

Abstract

Pyrotechnic compositions are usually employed in time delay detonators to facilitate controlled initiation of explosive charges in mining and quarrying operations. Traditional fast and slow-burning formulations are problematic, as they contain heavy metals which are bio-accumulative and toxic to the environment. Thermite and intermetallic reactions offer the possibility of greener alternatives. Unfortunately, the high reaction temperatures associated with conventional thermites cause operational failures due to melting of the metal housings. Similarly, it is difficult to achieve sustained burning with intermetallic systems alone. This study investigated the use of a combination thermite and intermetallic-type reactions, produced using environmentally benign materials to achieve both slow and fast-burning effects while overcoming the aforementioned limitations.

In order to obtain the fast-burning effect, a ternary composition based on powder mixtures of aluminium, nickel and nickel oxide was investigated. The maximum adiabatic temperature of the standard Al-NiO binary standard thermite was ca. 3100 K at a ca.19.8 wt-% Al content. The highest combustion rate (191 ± 31 mm·s-1) was achieved for a binary composition with 35 wt-% Al content that was compacted in glass tubes with an internal diameter of 4.0 mm. Progressive dilution of the primary thermite reaction, , with the intermetallic reaction, , reduced the adiabatic reaction temperature by as much as 800 K while simultaneously maintaining consistent burn behaviour inside both glass and lead tubes. In the process, the burning rate gradually decreased from ca. 190 mm·s-1 to as low as 23 mm·s- 1. Analysis of the burn product residue revealed the presence of various Al-Ni intermetallic compounds.

For the slow-burning effect, a ternary composition comprising manganese and tin as fuels mixed with bismuth oxide as an oxidiser was explored. It was hypothesised that the high thermite reaction temperature would ensure a sustained intermetallic reaction, resulting in the desired slow-burning effect. Burn rates ranging from 2.9 to 10.9 mms−1 were obtained and were dependent on the relative proportions of the reagents. Burn rate predictions, using a Padè mixture model, were in close agreement with the measured data. The reaction products consisted of mixtures of metal oxides, manganese stannate, and Mn3Sn, in which the latter was the only intermetallic formed. The slowest burning compositions were those associated with the formation of this intermetallic compound.