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Modeling Metal Fuels Simulations show the promise of nanostructured solid-state boron as a fuel.
Gasoline, ethanol, hydrogen and other fluid fuels burn well in air. But in airless environments, solid fuels may work better. Specially designed metals may have the right stuff for fueling tomorrow's submersible vehicles and high-altitude, low-speed aircraft, as well as providing electric and propulsive power for hypersonic spacecraft. Because metals are high-density energy carriers, their combustion can produce tremendous heat. Combustion of metal particles a few billionths of a meter across requires such low activation energy that even a nine-volt battery can provide enough heat for ignition. Because nanoparticles have considerable surface area, they may collect sufficient oxygen to combust. Thus, nanoparticles require a thin layer of surface rust to prevent self-ignition, a process akin to dry hay catching fire on a hot day. Researchers at Oak Ridge National Laboratory have studied nanoparticles of iron and aluminum. Compared with gasoline, iron produces twice the energy per unit weight and aluminum yields three times as much. But boron spectacularly provides five times the energy. Unlike hydrocarbon fuels, which produce nitrogen oxides, carbon dioxide and soot upon combustion, ORNL-engineered metallic nano-fuels emit no pollutants.
Supported by ORNL's Laboratory Directed Research and Development (LDRD) program, computational chemist Bobby Sumpter helped investigate solid-state metals as fuels. He and theorists Vincent Meunier, Bill Shelton and Mike Drummond used computers to simulate nanoscale combustion. Simultaneously, experimentalists Parans Paranthaman, Solomon Labinov, Louis Qualls and David Beach carried out numerous laboratory studies that put the fundamental theory, initially proposed by Labinov, to the test. Previous ORNL experiments explored how iron nanoparticles burn, how efficient particle combustion is and how likely nano-fuel combustion by-products can be recycled by chemical reduction. That work spawned the world's first paper about metal fuels, published in a 2007 issue of Journal of Energy Resources Technology. The computational work employed the Cray X1E Phoenix and SGI Altix Ram supercomputers at the National Center for Computational Sciences, ORNL's leadership computing facility, to simulate combustion of boron (B) and boron oxide. Just as stop-action photography reveals events frame by frame, simulation shows every step of combustion at the atomic level. Using models based on quantum mechanics, the center's supercomputers simulated a process that in the real world lasts five trillionths of a second. The supercomputers required only two days to simulate the process with B12 oxidation and two weeks to mirror it with B80. Boron normally clusters in 12-atom icosahedral structures or flat sheets, each of which oxidizes differently. The researchers sought a material that could resist melting, or loss of structure. They modeled B12 and B24, as well as the particularly stable B80, discovered in 2007 through predictions by theorists at Rice University.
Adsorption and subsequent chemical reaction of just one oxygen molecule was enough to destroy the original structure of B12, the simulations showed. Although B80 tolerated several oxygen molecules, it too ultimately lost its structure. Drummond, Meunier and Sumpter published their initial theoretical studies in spring of 2007 in the Journal of Physical Chemistry A. Computation has confirmed that combustion of solid-state boron nanoparticles will produce considerable heat, reaching temperatures around 1,300°F. The calculations indicate that a boron particle could be generated with enough adsorbed oxygen for complete combustion. Thus, in theory, boron could work as either an aerobic or anaerobic solid-state fuel. Armed with that knowledge, the ORNL experimentalists succeeded in burning boron in a combustion chamber but could not sustain the flame. They are working to optimize their test setup. But don't expect to fill up your next car's fuel tank with boron nanoparticles. Boron will probably be used in more specialized applications. "A major problem with boron is that the nanostructure is lost during the oxidation process," Sumpter says. "Thus, we can't easily recycle it by using chemical reduction, as is the case with iron." Currently, the LDRD team is computationally exploring the encapsulation of boron nanoparticles in an iron or aluminum shell in an attempt to preserve the original nanostructure.—Dawn Levy Contact: Bobby G. Sumpter
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