(616b) Aluminum-Nickel Fluoride Reactive Materials

Aluminum powders are most common metal fuel additives in diverse energetic formulations. Aluminum as a fuel is attractive due to its high energy density and accessibility. Sustained efforts have focused on modifying aluminum with the goal of accelerating its ignition and combustion and minimizing agglomeration of aluminum particles prior to ignition. One approach is to combine aluminum with a modifier, such as fluorinated oxidizer. The thermite-like reaction yielding aluminum fluoride is expected to boost ignition. Furthermore, in oxygenated environments, metal-rich aluminum-metal fluoride composites are expected to generate gas-phase combustion products, desirable for many applications. Recently, it was observed that aluminum composites with cobalt and bismuth fluorides prepared by arrested reactive milling (ARM) are readily ignited by heating but are insensitive to ignition by electrostatic discharge. This work is exploring nickel fluoride, NiF₂, as a potential additive to improve and tailor combustion of aluminum. Nickel fluoride is one the most stable and easy to handle metal fluorides. Here, it is explored as an oxidizer for a thermite-like composition for the first time. Several Al·NiF₂ composite powders were prepared by ARM with the weight fraction of aluminum varied from 50 to 90 % with respective equivalence ratios for the Al-NiF₂ thermite varied from 5.3 to 47. For each composition, milling conditions were identified leading to preparation of homogeneously mixed, fully dense composite particles. Prepared materials are examined by electron microscopy and x-ray diffraction. Samples of the prepared powders are ignited using an electrically heated filament. It is observed that the ignition temperatures increase significantly, e.g., from 400 to 1200 °C when the concentration of NiF₂ decreases from 50 to 10 %. This suggests an ability to tune the ignition temperature of the Al·NiF₂ composite powders. The effect of heating rate on the ignition temperature for all prepared materials is also quantified. Experiments are presently in progress correlating the ignition of the prepared composites with the exothermic reactions observed in thermo-analytical measurements. Combustion experiments with the prepared composite powders are also in progress. Results of these experiments will be presented and discussed in this talk.