(334c) Synthesis and Characterization of Aluminum Boride Particles Coated with Plasma Nanofilms for Energetic Applications

Aluminum (Al, 31 kJ/g) and boron (B, 58 kJ/g) based energetic materials oxidize exothermically to produce light, heat, and thrust, which find applications in solid fuels, propellants, and pyrotechnics. The primary inhibiting factor in extracting the stored chemical energy is the native oxide layer present on their surfaces, which occupies a significant mass fraction of the nano and submicron particles, acts as a diffusion barrier to the oxidizer, and does not contribute to the energy. In order to improve the reactivity and energy release, the oxide layer must be removed or chemically converted into materials that promote combustion and enhance the amount of energy released. In this study, we anneal a mechanical mixture of Al and B to synthesize submicron-sized aluminum boride. We optimize the parameters such as the composition of the mixture, time, and temperature through experiments. The advantages associated with Al boride are that the B oxide present in the particles can be reduced to B by metallic Al through an exothermic redox reaction during oxidation. This increases the B content of the material at the expense of less energetic Al, resulting in enhanced energy release. The boride is then treated by plasma-enhanced chemical vapor deposition (PECVD) of perfluorodecalin (C₁₀F₁₈) to produce a thin surface layer of CFx species. This layer provides passivation against air and humidity and contributes to combustion via the formation of metal fluorides which contribute to the energy content of the material. Effectively we are converting the native oxides into energetics. Several experiments are performed to find the ideal coating thickness according to its passivation ability and energy release during oxidation.

We characterize the material by a variety of experimental techniques. Its composition is studied by X-ray diffraction and X-ray photoelectron spectroscopy to probe the bulk and the surface, respectively. Particle size analysis using dynamic light scattering is used to measure the size of the synthesized particles. The shape, morphology, and elemental distribution are studied using high-resolution transmission electron microscopy and scanning transmission electron microscopy combined with energy dispersive spectroscopy. The extent of oxidation and corresponding heat release of the synthesized material is measured using thermogravimetric analysis and differential scanning calorimetry. The combustion heat is measured using bomb calorimetry. The core-shell structures formed show great promise as energetic materials with enhanced heat release and more extended storage stability.