(701b) Aluminum-Metal Reactive Nanocomposite Materials

Multiple Al-based nanocomposite reactive materials have been explored recently, including thermite compositions, coated powders, and multilayered foils. In most cases, a highly exothermic reaction is expected to occur between aluminum and one of the additional components resulting in an accelerated ignition. However, adding solid oxidizers inevitably reduces the overall energy density of the metal fuel additives. Aluminum combustion was also shown to be enhanced when it is alloyed or mechanically alloyed with titanium and magnesium. Exothermic formation of an Al-Ti intermetallic phases and selective oxidation of Mg were proposed to increase the overall reactivity of the respective alloys. A new type of Al-based reactive nanocomposite materials is explored in this project. In these materials, aluminum is mixed on the nanoscale, but not alloyed, with a metal, rather than oxide. The materials prepared and characterized are micron-sized powders in which each particle has aluminum matrix and nano-sized inclusions of the metal additive. The bulk compositions are aluminum-rich to take advantage of the high aluminum combustion enthalpy. Such materials have higher energy density than nanocomposite thermites with similar morphology and are expected to be more reactive than alloyed powders with the same bulk compositions. It is expected that the reactivity of such composite materials will be improved compared to both pure aluminum and to its homogeneous alloys because of several reasons. Early selective oxidation of the added metal can offer an energy boost during the heating of respective particles. An exothermic intermetallic reaction can occur; this reaction would be particularly important for such systems as Al-Ni and others with relatively high reaction enthalpy. The kinetics of aluminum oxidation, controlled by the growth kinetics for various aluminum oxide polymorphs are also expected to change as a result of the combined oxidation processes of different metals and because of formation of ternary oxides, less protective, than pure alumina. Finally, an increase in the surface area of the reactive interface, including multiple grain boundaries in the prepared nanocomposite materials is expected to increase the rate of their heterogeneous oxidation. Different mechanisms are expected to improve reactivity of Al-metal nanocomposite materials for different metal additives. In this project, nanocomposite materials in the systems of Al-Zn, Al-Fe, and Al-Ni were prepared by mechanical milling of aluminum powders mixed with powders of the respective metal additives. For each material, structure and composition were characterized using electron microscopy and x-ray diffraction. Reactivity of the prepared powders was studied using thermal analysis and heated filament ignition experiments. It was observed that oxidation kinetics of the prepared aluminum-metal reactive nanocomposite materials differ substantially from the kinetics of aluminum oxidation. It was also observed that reduced ignition temperatures are achieved for different Al-metal nanocomposite materials. Detailed studies of the reaction processes are carried out to establish mechanisms of accelerated oxidation kinetics and reduced ignition temperatures for different materials.