(546d) Fuel-Rich Metallic Energetics with Metal Fluoride Oxidizers

The importance of thermites over the years has risen due to the continuous improvement in its performance and tunability. The improvements in combustion and ignition behaviors were achieved through the particle size reduction, extending the contact area between fuel and oxidizer. Nanocomposite powders were prepared, in which particles remained in the micron-sized range, while containing nano-sized inclusions of individual metal and/or metal oxide phases. In the current work, the focus is to alter the properties of energetic thermite-like materials by changing the oxidizer to a metal fluoride, instead of the common metal oxides. The materials are prepared mechanochemically. The oxidizer; a meta-stable metal fluoride is ball-milled with a reactive metal fuel. Four composites, with fuel/oxidizer ratios fixed at 50-50 wt. % have been prepared and analyzed for the preliminary study; Al/BiF₃ (Equivalence ratio Ï• =9.8), Al/CoF₂ (Ï• =5.4), B/BiF₃ (Ï• =24.6), and B/CoF₂ (Ï• =13.4). The aluminum compositions so obtained are micron-sized composites with ca. 20-nm inclusions of the metal fluorides in the aluminum matrix. Experiments using an electrically heated filament showed that all prepared materials have very low ignition temperatures, with aluminum based ones around 480 °C while for the boron based ones, the temperatures are around 350 °C. Interestingly, all the prepared compositions are insensitive to electrostatic discharge except Al/CoF₂, which may be ignited by high-energy sparks. Thermal analysis of these materials indicates low-temperature reactions in both inert and oxidative environments. Aluminum samples have lower temperature exothermic reactions and no substantial mass loss in inert atmospheres. The Al/CoF₂ sample is more reactive of the two aluminum-based compositions. Boron samples have a mass loss of 25% when heated to 450°C in inert atmosphere suggestive of the formation of boron trifluoride gas. The ignition behavior of the prepared samples shall be studied further using laser and other ignition stimuli. Thermo-analytical kinetic studies of the materials in oxidative and inert atmospheres will be performed to explain and understand ignition behavior as well. Both as-prepared materials and their reaction products will be characterized by x-ray diffraction; their particle sizes will be measured to interpret the experimental observations.