Tuning the Chemistry of Metal Powders to Optimize Ignition and Combustion*

Elemental Al or Mg powders are commonly used in high explosives, propellants, and pyrotechnics to enhance performance, yet in many cases combustion efficiencies are well below 100%. Some are estimated to be as low as 50%. Recent closed-chamber detonation studies suggest that multi-element or alloyed metal powders ignite more readily and reach much higher combustion efficiencies than elemental Al powders. However, our community is just beginning to explore the many alloys that can be formed by mixing elements with high heats of combustion such as Al, Mg, Li, B, Si, Ti, and Zr. Given small variations in the atomic fraction of four elements in an alloyed powder generate over a million potential chemistries, we are leveraging a combinatorial process to rapidly screen the ignition and combustion properties of alloyed metal powders. We do so by sputter depositing 30 micron thick coatings onto polymer meshes and then debonding the coatings to create elongated powders. In our first two combinatorial studies 1) we systematically vary the at% of Zr in Al-Zr and Al-Mg-Zr powders and 2) we vary the relative percentage of Al and B in Al-B-Ti powders. In this presentation we report on trends in ignition temperatures of the powders as a function of chemistry using hot wire experiments, and we describe their combustion temperatures, durations, and gaseous species using a novel, hyperspectral imaging system. The system allows us to track the motion, temperature, and gaseous species of many individual particles in one field of view, providing temperature over time and identifying microexplosion events on a per particle basis. Trends in the measured properties will be detailed for the alloyed powders as a function of chemistry, and they will be compared with those for elemental powders of Al, Mg, Ti, and B. As superior metallic alloy chemistries are identified, we will fabricate spherical, microscale powders in larger quantities using ultrasonic atomization for additional testing.