Evaluation and design of metal-based gas-generating energetic materials

Metal-oxidizer nanocomposites have recently attracted attention as potential gas-generating energetic materials (EMs) that can replace or augment common CHNO compounds in selected applications. Experimentalists use enclosed chambers to ignite charges of metal-based EM and report pressures, P. Unfortunately, the reported pressures cannot be compared directly between studies because of differences in the chamber volume, V, and mass, m, of the EM used in different experiments. A parameter P·V/m, proportional to the released energy available for mechanical work is proposed to harmonize the reported experimental data. This parameter is both predicted using thermodynamic equilibrium analysis (here, employing a well-known and readily available NASA CEA code) and recovered from published experimental data. Calculations also determine the adiabatic flame temperature, T, and molar amounts of the generated gas products, n, which are not easily obtained from experiments. Comparisons of calculations with experimental data suggest that some Al-based nano-thermites with CuO and MoO₃ as oxidizers have achieved their theoretical performance levels, at least in small scale tests. Similarly, in some Al-ammonium nitrate EMs, theoretically predicted pressures have been observed experimentally. Available experimental data for B-based EMs suggest that all such EMs can be improved to achieve energetic performance closer to that predicted by thermodynamic equilibrium calculations. Calculating P·V/m and n/m for EMs with varied fuel/oxidizer ratios and different charge mass normalized per volume, m/V, offers an approach for designing thermodynamically optimized EM compositions with performance maximized at the temperature ranges of interest and achievable in practical scenarios. Finally, calculations also show that for both Al and B as fuels, ammonium nitrate is superior as an oxidizer in terms of both gas generation and ability to perform work. These benefits are despite somewhat lower adiabatic flame temperatures, e.g., compared to the analogous EMs using ammonium perchlorate as an oxidizer.