Paul E. Anderson

The reaction kinetics of aluminum and other metals in detonations has long been studied with the goal to obtain full enthalpy energy of oxidation at early timeframes. This requires oxidation kinetics to occur at the same rate as explosive CHNO reactions. While the literature claims some success with model formulations, few fielded formulations obtain such performance due to competing carbon oxidation of the inert formulation binder ingredients. Moreover, from gas analysis data in detonation calorimetry, it is hypothesized that high pressure/high temperature gas equilibrium concentrations are one of some factors that play a role in obtaining early reactivity of metals in the detonation. The ability to ascertain the extent and rate of aluminum oxidation, as well as other inorganic additives, in early timeframes of a detonation are compared between cylinder expansion, detonation calorimetry, and emission spectroscopy.

Developmental testing of high explosives for military applications involves small-scale formulation, safety testing, and finally detonation performance tests to verify theoretical calculations. For newly developed formulations, the process begins with small-scale mixes, thermal testing, and impact and friction sensitivity. Only then do subsequent larger scale formulations proceed to detonation testing, which will be covered in this paper. Recent advances in characterization techniques have led to unparalleled precision in the characterization of early-time evolution of detonations. The new technique of Photonic Doppler Velocimetry (PDV) for the measurement of detonation pressure will be shared and compared with traditional fiber-optic detonation velocity and plate-dent calculation of detonation pressure. In particular, the role of aluminum in explosive formulations will be discussed. Recent developments led to the development of explosive formulations that result in reaction of aluminum very early in the detonation product expansion. This enhanced reaction leads to changes in the detonation velocity and pressure due to reaction of the aluminum with oxygen in the expanding gas products.

Hydrogen fluoride (HF) is a known product from the combustion or detonation of explosives formulated with fluoropolymer binder systems. This presents the user with elevated risk levels during unintended combustion events or detonations in confined situations. In an effort to remediate the production of HF, calcium disilicide (CaSi₂) was added to explosive formulations and the amount of HF formed was monitored. Viton A/calcium disilicide mixtures were made and the thermal decomposition characteristics studied using thermal gravimetric/differential thermal analysis (TG/DTA). The activation energy ranged from 145–190 kJ mol⁻¹, indicative of CF scission in the Viton A binder prior to calcium fluoride formation. An energetic formulation was prepared which consisted of approximately a 5/3 mass ratio of Viton A/CaSi₂. Combustion calorimetry in oxygen and air, and subsequent analysis of the residues using X‐ray diffraction (XRD) showed evidence of calcium fluoride (CaF₂) formation. The decrease in HF formation was determined by trapping off gases and subsequent analysis in anion exchange chromatography of combustion and detonation products. Upon introduction of calcium disilicide into the formulation, a small decrease in HF formation was observed along with appearance of calcium fluoride and free Si in the residue. Such products are consistent with the mechanism following a general decomposition path of 2F⁻+CaSi₂→CaF₂+2Si. Under detonation conditions, the decomposition path followed nearly the same route with a net ca. 30 % decrease in HF formation, but with a portion of the silicon oxidizing slightly further to SiO₂.

In an effort to improve the insensitive munition (IM) response but maintain performance of aluminized formulations, silicon was investigated as a possible replacement for aluminum. An RDX‐based silicon explosive was developed in which nearly 90 % reaction of silicon to silicon dioxide was realized by 7 volume expansions as measured by the 2.54 cm diameter copper cylinder expansion test. In spite of the low nitramine loading in the formulation (79 wt.‐%), the corresponding Gurney constant for the explosive was 2.81±0.02 km s⁻¹, which is superior to Composition A‐3 under the same experimental conditions (91 % RDX, 2.69±0.02 km s⁻¹). Energy calculations from detonation calorimetry also indicate reaction of the silicon, which was further confirmed by both silicon metal and silicon dioxide in the analyzed residue. The energy release, despite it being equivalent to a highly loaded explosive, was found to lag behind the rate of A‐3. This indicates silicon oxidation may occur sometime after lighter gas reactions in the reaction front, but is fast enough to impart work in the copper cylinder test.

The objective of this study was to determine compositional variables that result in early reaction of aluminum in detonations of pressed high explosive compositions, defined as reaction by 7 volume expansions as measured by 2.54 cm diameter copper cylinder expansion tests. In order to accomplish this in an economical fashion, statistical mixture design of experiments (DOE) was used in conjunction with anaerobic detonation calorimetry. The effect of binder type (e.g. energetic vs. inert), binder content, HMX content, aluminum content, and aluminum particle size was investigated. It was determined an energetic binder must be used to obtain significant aluminum reaction at volume expansions less than 7 V/V₀. Aluminum particle size was only a minor factor. Furthermore, the compositional oxygen balance only provides a general indication of which compositions exhibit more aluminum reaction than others.