(143a) Boron Composite Nanoparticles for Enhancement of Biofuel Combustion

Boron (in small quantities) is a candidate fuel additive for biofuels of low energy density. Its own energy density is very high on both a volumetric and gravimetric basis. However, burning boron can be difficult, because the B₂O₃ oxide shell melts at 722 K, far lower than the boron core (2350 K), and it often adheres to the particles. This liquid B₂O₃ shell can significantly delay boron combustion. The addition of either a catalytic (e.g., CeO₂) or easily ignitable metal (e.g., Fe) as a shell to boron nanoparticles is hypothesized to solve this problem.

       In this study, we have synthesized such core (boron)-shell nanboparticles.  A simple mechanical ball milling method was sufficient to coat CeO₂ or rare earth oxide (REO) mixtures to the boron core. We examined three synthesis methods for coating with Fe: (a) cold milling in dry ice; (b) borohydride reduction of Fe salts; (c) H₂ reduction of precipitated Fe(OH)₂ . Simple mechanical ball milling of Fe and B nanoparticles failed because it resulted in some intermetallic (Fe₉B)_0.2, which is relatively inert to  combustion. Using emission measurements at various wavelengths, it was shown that the addition of these rare earth oxides and Fe to boron did enhance its burning behavior, reducing the ignition delay and providing greater overall heat release. The boron was completely oxidized in the ethanol-fueled, swirl-type combustor.