Aluminum added to propellants, explosives, and pyrotechnics, boosts their energy density. Different approaches were investigated that could shorten aluminum ignition delays, increase combustion rates, and decrease the tendency of aluminum droplets to agglomerate. Here, Al-based reactive, mechanically milled materials are prepared and characterized.
For powders with Fe, Ni, or Zn additives, the particles consist of an aluminum matrix and inclusions of Fe, Ni, or Zn comprising 10 at % of the bulk composition. For additives of Ni and Zn, only short milling times can be used to prepare composites; intermetallic phases form at longer milling times. Thermogravimetric analysis shows selective oxidation of Zn and Ni at low temperatures, prior to a characteristic first step of Al oxidation. At higher temperatures, the powders oxidize following, qualitatively, the stepwise process reported earlier for the pure Al. The magnitude and kinetics of the low-temperature aluminum oxidation steps are affected by the presence of metal additive inclusions. Heated-filament ignition experiments show that all three prepared composite powders ignite at lower temperatures than pure Al powder. Comparison of the Al-metal composites with Al·Al2O3 reference composites prepared with similar milling conditions suggests that the altered Al morphology, such as developed grain boundary network produced in the milled powders, is primarily responsible for their accelerated low-temperature oxidation. It is concluded that the improved ignition dynamics for the prepared Al-metal composites is due to a combination of the accelerated low-temperature oxidation with reaction mechanisms altered by the presence of metal inclusions.
For Al·Mg alloys, both internal structures and particle size distributions are adjusted for powders with 50-90 at. % Al. Previous work showed that particles of mechanically alloyed (MA) Al·Mg powders burn faster than similarly sized particles of pure aluminum. However, preparation of MA powders with particle sizes matching those of fine aluminum used in energetic formulations was not achieved. Milling protocol is optimized to prepare equiaxial, μm-scale particles. Ignition temperatures are much lower than those of pure Al powders and are close to those of Mg. For aerosolized powders ignited in air, maximum pressures are higher, rates of pressure rise are greater, and ignition delays are shorter for the MA powders than for pure Al. Single particle laser combustion experiments show that the MA-particles burn in two stages, while the first stage is gradually disappearing at higher Al concentrations.
Finally, it has been shown that a range of MA-Al·Mg powders can be prepared with different compositions and particle sizes. Conventionally, such alloys are prepared by melt processing. Here, the oxidation, ignition, and combustion characteristics are compared for two powders of Al·Mg with similar bulk compositions and particle sizes: one produced via MA, and another, produced via grinding of a cast alloy. Low-temperature exothermic features are observed for the MA powder but not for the cast-alloyed powder in thermo-analytical experiments. MA powders have slightly lower ignition temperatures than cast-alloyed powders. Aerosol combustion experiments show a substantial increase in both the maximum pressure and rate of pressure rise for the MA powders as compared to the cast alloyed powders. In single particle laser ignition experiments, MA particles ignite more readily than cast alloyed particles. MA powders burn in a staged sequence, with the first stage dominated by combustion of Mg and the second stage representing primarily combustion of Al. No similar staged combustion behavior is observed for the cast-alloyed powders, which generates very short emission pulses with a relatively low brightness, and thus may not have burned completely. It is proposed that the difference in the structure and morphology between the MA and cast alloyed particles results in different ignition and combustion scenarios.
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