On the Pressure Generated by Thermite Reactions Using Stress‐Altered Aluminum Particles

Abstract

This study examines pressure build-up and decay in thermites upon impact ignition and interprets reactivity based on the holistic pressure history. The thermite is a mixture of aluminum (Al) combined with bismuth trioxide (Bi 2 O 3) powder. Four different Al particles sizes were examined that ranged from 100 nm to 18.5 μm mean diameter and for each size, two different Al powder treatments were examined: stress-altered compared to untreated, as-received Al powder. Stress-altered Al powders have been shown to be more reactive, such that the stress-altered Al powder thermites offer a metric for analyzing thermite reactivity in terms of pressure development compared to untreated Al powder. In a binary thermite system, multiple phase changes and interface chemistry influence the transient pressure response during reaction. Results reveal three key pressure metrics that need consideration specifically for thermite combustion: (1) delay time to peak pressure, (2) peak pressure, and (3) decay after peak pressure. Our experiments show that a lower peak pressure corresponds with higher thermite reactivity because aluminum consumption of oxygen generated by decomposing solid oxidizer reduces the peak pressure. Faster rates of reaction consume oxygen at higher rates such that pressure development becomes more limited than less reactive thermites and the result is a lower peak pressure. This conclusion is opposite of traditional studies using metal fuels with a gaseous environment that typically show higher peak pressures correspond with greater reactivity.

Pioneering insights into the superior performance of titanium as a fuel in energetic materials

“…As for some applications, is desirable to lower the ignition temperature, much research effort is dedicated to the increase of Al ignitability and decrease of its ignition temperature. For example, Williams and Pantoya [14] modified the Al particles reactivity by modifying the Al-Al2O3 core-shell microstructure through annealing and quenching of the powder, to vary interfacial strain and associated stress. Correlation between altering the Al microstructures by stress and lowering ignition point was also observed in [15,16].…”

Comprehending Metal Particle Combustion: a Path Forward

“…The coexistence of two regimes of Al particle combustion, i. e., the existence of two different characteristic burn times for the same particle diameter, explains results on the performance enhancement upon various metal particle treatments for the same particle size [19,20,21]. Successful treatment leads to preferential combustion of particles at shorter times, i. e., an increase in the fraction of particles burning in the fast regime.…”

“…The thermal processing of Al powder, specifically its annealing followed by fast quenching had been developed by Hill et al [19]. A number of subsequent studies further demonstrated distinctly different performance of as-received (i. e., called UN, untreated) and thermally processed (i. e., called SQ, super-quenched) powders in different energetic formulations [20,21,23]. Thermal processing the powder resulted in high internal particle stress [19,23], but a link to the mechanism for performance variation remained unclear.…”

Ignition and Combustion Characteristics of Al/TiB2-Based Nanothermites: Effect of Bifuel Distribution

“…This has led to the pursuit of various physical or chemical modifications of Al–Al 2 O 3 core–shell microstructure, aiming to lower aluminum’s ignition temperature and consequently enhance the thermite ignition and combustion. In addition, thermal and plasma pretreatments − in Al microstructures have shown to lower the initiation onset and the burn time. Because mass transport is a key factor in setting the reaction pathway and hence regulating the ignition point, several other studies have been populated by approaches modifying the native Al 2 O 3 using a polymeric material , or different oxides. , While all of these strategies prove to be categorically effective, it is undeniable that they necessitate time-consuming and cost-ineffective engineering efforts.…”