Session Y2: Energetic and Reactive Materials XI: Metals

Reactivity and Fragmentation of Aluminum-based Structural Energetic Materials under Explosive Loading

Aluminum-cased warheads have been observed to generate enhanced blast and target damage due to reactivity of the aluminum fragments with ambient air. This effect can more than double the output of a conventional warhead. The mechanism by which the aluminum reacts under these conditions remains poorly understood. We undertake a highly controlled experimental study to investigate the phenomenon of aluminum reaction under explosive loading. Experiments are conducted with Al 6061 casings and PBX-N9 explosive with a fixed charge to case mass ratio of 1:2. Results are compared to inert casings (steel), as well as to tests performed in nitrogen environments to isolate aerobic and anaerobic effects. Padded walls are used in some tests to isolate the effects of impact-induced reactions, which are found to be non-negligible. Finally, blast wave measurements and quasi-static pressure measurements are used to isolate the fraction of case reaction that is fast enough to drive the primary blast wave from the later time reaction that generates temperature and overpressure only in the late-time fireball. Fragment size distributions, including those in the micron-scale range, are collected and quantified.   

Combustion Characteristics of Printed Biocidal Formulations

Iodate based biocidal formulations are traditionally plagued by aging and degradation of the iodate and/or the aluminum, which in turn reduces their efficacy. It would be ideal if we could apply these formulations to surfaces, devices, or the like, rather than working with loose reactive powders. In this effort, fluoropolymers were selected as i) the protective agent for both the iodates and the aluminum within the formulations, and ii) as the polymer for development of a printable biocidal. This study examines the effectiveness of the fluoropolymer in terms of protecting the respective iodates under accelerated aging conditions (70 $^{\circ}$C, 30{\%} RH) and the combustion characteristics of printed traces. Simultaneous differential scanning calorimetry (DSC) and thermo-gravimetric (TG) analyses were performed to elucidate the complex interactions between the flouropolymers, iodates, and aluminum. Printable formulations were made with varying polymer content and printed using a pen-type deposition system. The combustion characteristics are presented as a function of polymer loading and print dimension. The necessary rheological characteristics and the associated safety characteristics of the printed formulations will also be detailed.   

Probing Aluminum Reactions in Combustion and Explosion Via the Kinetic Isotope Effect

The mechanism that controls the reaction speed of aluminum in explosion and combustion is poorly understood, and experimentally difficult to measure. Recently, work in our laboratory has demonstrated that during the combustion of nanoparticulate aluminum with H2O or D2O, different reaction rates due to the kinetic isotope effect are observed. This result is the first-ever observed kinetic isotope effect in a metal combustion reaction and verifies that chemical reaction kinetics play a major role in determining the global burning rate. During or shortly after a detonation, however, the reaction rates are dramatically faster and the physical mechanism controlling Al reaction is likely different than during combustion events. To utilize the kinetic isotope effect to probe Al reactions in detonation, formulations were produced that contain powdered Al in deuterated high explosives and high-fidelity detonation velocity were determined along with PDV measurements to observe early wall velocity expansion measurements. The JWL equation of state was solved to determine temperature, pressure and energies at specific time periods, in addition of Gurney energies, which enables the elucidation of Al reaction extent. By comparison of the Al oxidation with LiF, data indicate that Al oxidation occurs on an extremely fast time scale and isotope effects in both the HE detonation and post-detonation Al reactions are discussed.   

Ignition Behavior of an aluminum-bonded explosive (ABX)

We report the results of a study on the ignition behavior of a novel concept and design of heterogeneous energetic material system called ABX, or aluminum-bonded explosives. The idea is to replace the polymeric binder in polymer-bonded explosives (PBX) with aluminum. The motivation of this study is that a new design may have several desirable attributes, including, among others, electrical conductivity, higher mechanical strength, enhanced integrity, higher energy content, and enhanced thermal stability at elevated temperatures. The analysis carried out concern the replacement of the Estane binder in a HMX/Estane PBX by aluminum. The HMX volume fraction in the PBX and HMX is on the order of 81{\%}. 2D mesoscale simulations are carried out, accounting for elasticity, viscoelasticity, elasto-viscoplasticity, fracture, internal friction, and thermal conduction. Results show that, relative to the PBX, the aluminum bonded explosives (ABX) show significantly less heating and lower ignition sensitivity under the same loading condition. The findings appear to confirm the expected promise of ABX as a next-generation heterogeneous energetic material system with more desirable attributes.   

Fine fragmentation distribution from structural reactive material casings under explosive loading

Structural reactive material (SRM) can be used for explosive casings to provide additional blast energy. SRM fragments can react either promptly or after impact with nearby structure. Better understanding of fine fragment distributions from SRM casings is important for optimization of initiation and reaction of the SRM fragments. Key to this is knowledge of the initial fragmentation character before it has been altered by early reaction or by subsequent impact with surrounding structure. The study must be conducted beyond critical charge diameter to minimize effects of the expansion wave on fragment sizes. The collection and analysis of fragment distribution down to 40 micron size from thick SRM casings are therefore investigated in a 1.18 m diameter, 2.1 m$^{3}$ closed cylindrical chamber filled with artificially-made pure snow packed to density 0.35 g/cm$^{3}$. The snow quenches early reaction of SRM fragments and soft-catches the fragments before impact with the chamber walls. A 100 g cylindrical C-4 explosive charge is used, packed in a 3.3 cm inner diameter SRM casing, with length-to-diameter ratio of L/d $=$ 2, and casing-to-explosive mass ratio of M/C $=$ 1.75. Three types of SRM are investigated, including a baseline of Aluminum 6061 for comparison. The cased charge is suspended in an argon filled cavity, 20 cm in diameter and 40 cm long, within the snow filed chamber.