(764b) Nano-Structured Aluminum Powders with Modified Protective Surface Layers

Aluminum powder is widely used as a fuel additive in propellants, explosives, and pyrotechnics due to its high volumetric combustion enthalpy and relatively low cost.  However, the thermodynamically predicted benefits of aluminum combustion are rarely achieved because of extended ignition delays associated with heterogeneous reactions involving protective alumina layer, which is always present on the aluminum surface.  In order to fully exploit aluminum’s high reaction energy, this effort focuses on adjusting its combustion dynamics by modifying its surface and structure.  The modification is achieved by cryo-milling aluminum with hydrocarbon compounds, which are liquid at room temperature.  At the temperature of liquid nitrogen, the hydrocarbons, such as cyclooctane, become solid and form composite with ball-milled aluminum.  The natural alumina layers are removed from the particle surfaces; freshly formed surfaces are coated with the softer hydrocarbon.  Once the milling is completed, most of the hydrocarbon becomes liquid and is easily separated from the prepared powder.  Thus, the prepared powder is nearly pure aluminum.  However, its surface remains modified and coated with a protective layer with properties significantly different from those of regular alumina.  The particle morphology is typical of mechanically milled metals and includes multiple grain boundaries, which are also expected to be coated with similar, hydrocarbon-modified layers.  Prepared powders ignite at temperatures substantially reduced compared to that of pure aluminum.  Their oxidation kinetics, as observed from thermo-analytical measurements, is also different from that of pure aluminum.  In addition to ignition and oxidation studies, experiments on combustion of the prepared powders using constant volume explosion setup and laser ignition setup will be discussed.   Effect of specific hydrocarbon on the properties of the prepared material will be investigated.