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Optimization of a High Energy Ti-Al-B Nanopowder Fuel Albert Epshteyn, Michael Raymond Weismiller, Zachary John Huba, Emily L. Maling, and Adam S. Chaimowitz Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02321 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on January 2, 2017
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Energy & Fuels
Optimization of a High Energy Ti-Al-B Nanopowder Fuel Albert Epshteyn1,* Michael R. Weismiller2,‡, Zachary J. Huba2,‡, Emily L. Maling3, and Adam S. Chaimowitz3 1
Staff, Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5342 Former NRC Postdoctoral Associate, Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5342 3 Former NREIP Student Intern, Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5342 ‡ These authors contributed equally to this work 2
ABSTRACT Sonochemically-generated reactive metal nanopowders containing Ti, Al, and B represent a new class of highenergy-density nanopowder fuels with superior energy content and air stability as compared to nano-Al. In this work, we optimize the energy density of a Ti-Al-B reactive metal nanopowder fuel by varying the Ti:Al:B ratios using a sonochemically-mediated complex metal-hydride decomposition. After heating the recovered solids under vacuum to temperatures in the range between 150 °C and 300 °C, the powder’s air stability is significantly improved so that it can be handled in air. Variable temperature vacuum heat treatment was used to produce fuels tuned to be stable with a gravimetric energy density exceeding that of pure bulk Al (> 31 kJ/g). The density of the powder was found to be 2.62 g/cc by helium pycnometery, which translates to an impressive volumetric energy 3 content of 89 kJ/cm . In PMMA protected bomb calorimetry tests commercial nano-Al (SkySpring Nanomaterials, 20% oxide) produced only 25 kJ/g, whereas the sonochemically generated Ti-Al-B nanopowders released 24% more energy per unit mass and 19% more energy per unit volume in identical experiments.
INTRODUCTION Hydrocarbons are currently the mobility fuels of choice because they are liquids at ambient conditions and are easy to contain, store, and transfer; also, combustion energy from burning hydrocarbons is easy to turn into useful work due to the copious production of gaseous combustion products. Another important characteristic of hydrocarbons is that as a class of energy carrier compounds they have a high gravimetric energy content of ~ 40 kJ/g when completely oxidized with O2. Metal powders, which traditionally have been used as a fuel in energetic formulations such as explosives, propellants, and pyrotechnics, can also contain significant amounts of energy. Recently, Bergthorson and coworkers proposed that metal powders could even be used as mobility fuels, and the metal oxide combustion products could be trapped, collected and regenerated, which would make the metal powders essentially an energy storage medium.1 Regardless of the considerations for implementing such a proposal, the comparison between hydrocarbons and metal powder fuels is a useful one to make in order to explore the possibilities for improvement of the fuel characteristics of metal powders. Only a select group of light elements can equal or exceed hydrocarbons in gravimetric energy content (Figure 1), the elemental forms of H, Li, Be, B, Mg, Ti, and Al, are quite competitive. H, 1 ACS Paragon Plus Environment
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Be, and B are particularly noteworthy, being the only elements that exceed hydrocarbons’ gravimetric energy density. Unfortunately, H, Be, and B, each have significant flaws as fuels in their elemental forms. Hydrogen’s Achilles’ heel is that while gravimetric energy content is important for efficiency, volumetric energy content is also a major factor for fuel storage considerations, and in its elemental form H is the least dense element, making it inconvenient for most applications unless a denser form of hydrogen storage were developed.2 The use of Be in fuel applications is impractical due to the acute toxicity of Be and its oxide.3 And finally, although B and its oxide are relatively innocuous in terms of toxicity,4 and both the gravimetric and volumetric energy contents of B are superlative, significant problems with B combustion due to the physical properties of B and its oxide have plagued researchers for decades.5 Due to interest from the energetic materials community in improving energetics technology’s capabilities by increasing rates of energetic reactions, in the past 20 years there has been significant research activity focused on the synthesis and passivation of reactive metal particles and powders of various elements, reaching well into the nanometer size regime.6 Research has primarily focused on the study of the combustion properties of either pure elemental powders that are commercially available, or the use of mechanical methods to produce composites in an attempt to improve their combustion properties.6-7 A significant challenge for the usefulness of nanometer scale reactive metal powder fuels is their generally highly oxophilicity; meaning that smaller particle size also produces a greater surface area available to react with air. This in turn means particles of decreasing size are less air stable and require surface passivation. The most common approaches include producing a thinner layer of oxide via a controlled atmospheric exposure, or employing surface coatings such as polymers or coordinating molecular ligands.8
Figure 1 The energy released on a gravimetric and volumetric basis for several light elements (O2 as oxidant) which produce attractive energy release. Automotive gasoline is included as a point of reference.
In this work, we have developed a method to include the high energy density elements H and B, together with Ti and Al, simultaneously achieving greater energy content, higher air and formulation stability, as well as superior combustion characteristics, simultaneously addressing multiple problems associated with metal powder combustion. This was achieved via the use of intermetallic inorganic complex molecular-precursors in a sonochemically mediated liquid 2 ACS Paragon Plus Environment
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Energy & Fuels
slurry synthesis to produce an atomically mixed multi-metal powder fuel. We previously reported the development of a sonochemically-agitated slurry reaction process for producing RMNPs in hydrocarbon or ethereal solvents using transition metal halides and metal hydrides or alkali metals as starting materials.9 In the previously reported synthesis of group 4 metal RMNPs with aluminum, where the binary group 4 aluminide materials were annealed to temperatures above 700 °C, of the three group 4 aluminide materials originally explored, the TiAl materials showed the greatest energy content of 27 kJ/g.9c Recognizing an opportunity to push the fuel energy content further via the addition of B, we use lithium borohydride (LiBH4) in combination with lithium aluminum hydride (LiAlH4) in the reaction with titanium(IV) chloride (TiCl4), providing a direct synthesis of ternary powders containing Ti, Al, and B (Equation 1). We envisioned that including H and B in the previously studied Ti-Al intermetallic powder system could resolve or ameliorate any B-related combustion problems, as well as create an air-stable reactive metal nanopowder (RMNP) fuel that would be useable in a variety of applications.9c The optimization of the intermetallic RMNP fuel carried the requirements of a high energy density, good kinetic stability for ease of handling and storage, which is predicated on the ability to release the stored energy from the material on a useful timescale to be harnessed for useful work.
Eq. 1
Herein we describe the empirical optimization of the process to produce the fuel powders with the highest possible energy content, nanoscopic size, and acceptable air stability, by utilizing sequential steps, as follows: 1) the sonochemical synthesis of a Ti-Al-B stock raw powder, followed by 2) a thermal treatment for improved stability, then 3) a THF wash to remove byproduct LiCl, and finally 4) cryogenic ball milling to reduce the average powder particle size and homogenize the particle size distribution. The main technical challenge in the optimization of the ternary intermetallic RMNPs is in optimizing the balance between the fuel energy content and stability for ease of handling and formulating. Herein, we show that the new ternary Ti-Al-B RMNPs exhibit requisite characteristics of a practical and stable high energy density nanopowder fuel, with the best samples yielding a gravimetric energy content in the range of 31-34 kJ/g, at a mass density of 2.62 g/cc, as measured by helium pycnometry. This means the highest energy content samples have a volumetric energy density of 89 kJ/CC, which is significantly greater than other useful fuels. In this work we also report a cryogenic ball milling procedure, which yields relatively monodisperse powders in the ~380 ± 125 nm size range without significant loss of energy content or increase of crystallinity, as determined by powder X-ray diffraction (PXRD). This is particularly exciting, because this milled nanoscopic reactive metal fuel possesses greater gravimetric and volumetric energy density than commercially sourced nano-aluminum (nAl) and is also more kinetically stable than nAl, as
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confirmed by the activation energy of oxidation via Kissinger analyses from TGA/DSC data, making it safer and easier to handle.
EXPERIMENTAL General Considerations – For the purposes of this work, the RMNP samples will be referred to in accordance with the ratio of metals during initial synthesis of a given sample, and the subsequent anneal temperature – i.e. Ti:Al:3B-200 would be the sample that was synthesized by taking 1 equivalent of TiCl4 and adding 1 equivalent of LiAlH4 and 3 equivalents of LiBH4, which was then heated to 200 °C under dynamic vacuum before washing out the byproduct LiCl with THF. Manipulations of air sensitive reagents and materials when charging reaction vessels was performed in a Vacuum Atmospheres glovebox under an atmosphere of Ar (O2 and H2O concentrations of
