(676d) Plasma Processing for the Enhancement of Energetic Potential of Boron Nanoparticles

The oxidation of metals and metalloids (B, Al, Mg) is a highly exothermic reaction, a property that can be used to enhance the energy content of liquid fuels through the addition of a small volume fraction of nanoparticles. The strong tendency of these materials to oxidize leads to the formation of few nanometers thick surface oxide layer, which for particles in the nanometer size range may represent a quarter of the total particle volume or more. A requirement for the efficient use of energetic nanoparticles is to remove this oxide layer and passivate the surface against re-oxidation. Here we demonstrate the enhancement in the gravimetric energy density of boron nanoparticles (70 nm) using non-thermal plasma processing. Hydrogen plasma was used to reduce the native oxide layer from the surface of boron nanoparticles as a result of surface reactions between excited species of hydrogen and boron oxide. The surface reduction of oxide minimizes the diffusion barrier for the oxidizer during the oxidation and combustion process of the boron nanoparticles. This was followed by in-situ passivation of the boron particles to prevent re-oxidation of nanoparticles. Passivation was done by plasma-enhanced chemical vapor deposition (PECVD) of perfluorodecalin. After performing the series of experiments for different time durations, we found that 120 minutes of hydrogen plasma treatment with 15 minutes of PECVD treatment resulted in a 19% increase in the gravimetric energy density of boron nanoparticles. Analytical techniques like HRTEM, STEM-EDS in high angle annular dark field, and XPS were employed to characterize the reduction in the surface oxide. The reduction in the oxide leads to increase in gravimetric energy density which is shown by characterizing the untreated and plasma- treated particles using thermogravimetric analysis and differential scanning calorimetry. PECVD improved the storage life of the nanoparticles in ambient conditions by providing excellent passivation against air and humidity for 60 days. Thus, the application of in-situ hydrogen plasma and PECVD leads to the amelioration of energetic performance and storage life of boron nanoparticles, making them conducive for nanoenergetic applications.