Air-stable, unoxidized, hydrocarbon-dispersible boron nanoparticles
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Air-stable, unoxidized, hydrocarbon-dispersible boron nanoparticles Brian Van Devener, Jesus Paulo L. Perez, and Scott L. Andersona) Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received 24 June 2009; accepted 7 August 2009)
Here we describe a simple method to produce boron nanoparticles with control over surface chemistry and dispersiblity in different solvents, with potential applications ranging from high energy density fuels to neutron capture therapy. The methodology should be adaptable to many hard materials; indeed, we have produced hydrocarbon-dispersible silicon nanoparticles using a procedure similar to that described below. The method, based on high-energy milling, with subsequent sedimentation to separate aggregates, produces gram quantities of nanoparticles in a narrow distribution of particle sizes centered around 50 nm, and should be readily scalable to industrial scale production.
Boron has a volumetric heat of combustion (136 MJ/L) around twice that of aluminum (81 MJ/L) and three to four times that for practical hydrocarbon fuels (e.g., jet fuels have 34–39 MJ/L). Boron’s potential as a fuel has not, to date, been realized, partly due to the difficulty in igniting and burning it efficiently.1 One problem is that boron is refractory (Tvap = 2800 K), and thus combustion depends on heterogeneous reactions, which tend to be slow and diffusion limited. This limitation can be mitigated, at least in principle, by using nanoparticulate boron, leading to large surface-area-to-volume ratios, as suggested by a number of previous researchers.2–4 A drawback to this approach is that boron exposed to air forms a passivating native oxide layer. Not only does the oxide inhibit combustion, but it limits the strategy of reducing particle size, by consuming an increasing fraction of the particle mass, and thus energy density. The method described here allows production of boron nanoparticles that are completely free of oxide. The particles, if desired, can also be made dispersible in liquids such as organic solvents or fuels. Practical preparation methods for propulsion fuels must generate materials on large scales, and at low cost. The method we have developed is based on high-energy ball milling, and is simple, inexpensive, and easily scalable to large batches. In one step, micron scale boron power is crushed to a nano-scale powder, rendered air stable by an organic ligand coating, and rendered dispersible in hydrocarbons. If desired, the particles can also be partially coated with catalysts or other surface chemistry modifiers, although this letter focuses only on air-stability. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0412
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 11, Nov 2009 Downloaded: 31 Jan 2015
Passivated, dispersible, boron-rich nanoparticles are also interesting for boron neutron capture therapy, as recently demonstrated.5,6 The method described below should be easily adaptable to make part
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