Sensitizing micron-sized Al/KIO4 compositions for potential use in chemical delay detonators

Abstract

Fast-burning micron-sized compositions were investigated for potential use in chemical delay detonators. This investigation stems from the drive to replace heavy-metal-containing compositions and compounds within these detonators owing to environmental concerns. The Al/KIO4 composition was modified by adding Al/CuO and Al/PTFE thermites to it to assist with the ignitibility of the composition. The blend of Al/KIO4/CuO/PTFE had the following mass distribution: 23.56/64.98/7.79/3.68. The addition of copper(II) oxide and PTFE improved the ignitibility of the composition by reducing the ignition temperature of the base composition by approximately 150 °C. With the improved ignitibility came a drastic increase in enclosed combustion performance. The flame front for the sensitised composition approached 1 km·s−1. This is especially remarkable when considering the fact that all components used in the investigation were micron-sized. The increased ignitibility and combustion rate are thought to be due to a change in the ignition mechanism brought about by the addition of the two additional oxidisers. The Al/KIO4 would have a gas-solid mechanism, while the Al/KIO4/CuO/PTFE composition is thought to have a combination of gas-solid and condensed phase reactions caused by the pre-ignition reactions between the oxidisers and the native oxide layer on the fuel.

An attempt was made to utilise the composition as a direct replacement for the primary explosive lead azide (Pb(N3)2). It was thought that due to its high combustion rate, combustion temperature and the quantity of gas released during its combustion it might be able to initiate multiple reaction zones in a porous pentaerythritol tetranitrate (PETN) bed, thereby accelerating the deflagration to detonation transition. However, this direct replacement was not possible using the detonator shells used commercially since the shells burst before a transition to detonation could occur. The detonator failures were attributed to the comparatively slower transition to detonation that occurs when compared with a primary explosive. The timeframe involved allowed the detonator shell to fail by bursting before a transition could reliably occur. A simple flying plate detonator (FPD) was designed by utilising elements similar to those used for time delays with the addition of a steel flyer. The elements were made from steel as aluminium elements could not contain the pressure. In the trials, the two best-performing designs had muzzle velocities of 434 ±14 m·s-1 and 475 ± 17 m·s-1. The simple design used compares favourably with PETN initiation data from the literature, indicating that the composition may well find use in lead-azide-free chemical delay detonators. At this stage however the detonator shells remain the weak link in the design as shell failure occurs before reliable initiation of the PETN base charge used. Keywords: aluminium thermites, fast burning, sensitisation, burn rate modification.