Air-Stable Nanopowders of Mixed Reactive Metals as Fuel Additives
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Air-Stable Nanopowders of Mixed Reactive Metals as Fuel Additives Michael R. Weismiller1*, Zachary J. Huba1, Emily L. Maling2, Albert Epshteyn3, and Bradley A. Williams3* 1
NRC Postdoctoral Associate, Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5342 NREIP Student Intern, Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5342 3 Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5342 2
ABSTRACT Nano-sized metallic powders have advantages as fuels including faster, more complete combustion than micron-sized metal powder particles; however, the unpassivated nanoparticles of some metals of interest, such as Al, are pyrophoric and highly reactive, making them difficult to handle. Additionally, metal-hydrides are of great potential interest for a significant gravimetric energy increase without a penalty in the volumetric energy content, as the inclusion of hydrogen in the metallic matrix does not significantly decrease overall density of the material. Reactive metal amorphous powders produced by the sonochemical decomposition and dehydrogenation of an in situ-produced mixed borohydride-tetrahydroaluminate of titanium and containing hydrogen are examined in the current work and show exceptional air-stability and higher energy content than nano-Al. An aerosolized powder burner is used to investigate the combustion behavior of these powders mixed with gaseous fuels to quantify the energy density and reaction rate as compared to commercial aluminum powders in an effort to benchmark the performance of the sonochemically generated amorphous Ti-Al-B powder fuels. INTRODUCTION One aspect that is particularly attractive in using reactive metals as fuels is their high volumetric heats of combustion with oxygen, the most outstanding being boron (137.7 kJ/cc). Energy densities on both a gravimetric and volumetric basis are shown in comparison to liquid hydrogen and a high density hydrocarbon fuel (i.e. JP-10; exo-tetrahydrodicyclopentadiene) in Figure 1. The use of solid fuels is limited for many common applications in part because they can have slower rates of energy release, with boron being especially problematic.1, 2 Additionally, the products generated from the oxidation of metals are typically condensed phase oxides and less useful for generating higher specific impulse (Isp, impulse per unit weight of propellant). Low atomic mass reactive metal hydrides also have high energy densities and generate more gas phase and low molecular weight products when combusted. For example, AlH3 has an energy density of 61.8 kJ/cc, and is 10 % hydrogen by mass. As a result, metal hydrides have significantly higher theoretical Isp values. A major hurdle in adapting metal hydrides for use as practical fuels is their inconvenient reactivity with oxygen and moisture, and therefore a lack of stability presenting challenges with safety, storage, and compatibility when incorporating them into practical formulations. We have previously reported the sonochemically-mediated synthesis of bimetal
