Fully dense nano-composite energetic powders prepared by arrested reactive milling

Highly reactive metastable nano-scale composites of aluminum and metal oxides have been produced by arrested reactive milling (ARM), and their combustion performance has been preliminarily evaluated. Aluminum powder has been milled with powders of MoO3 and Fe2O3. The prepared composites are powders with particle sizes in the 1–100 μm range. Each individual particle comprises a fully dense, nano-scale mixture of the chemical reagents. These composites belong to a novel class of energetic materials characterized by an intimate mixing of reactive components on nanometer to atomic scale. Reactive components can be metal/metal oxide pairs or combinations of other materials capable of highly exothermic reactions such as B–Ti or B–Zr. High-energy milling of these components leads to mechanical initiation of the reaction. Highly reactive composites are obtained by arresting this process immediately before the initiation would occur if milling were allowed to proceed. An experimental parametric study of reactive milling in the Al–MoO3 and Al–Fe2O3 systems was conducted to establish at which milling times the reaction is spontaneously initiated under various conditions. Samples of nano-composite powders were synthesized by arresting the milling process, and characterized using electron microscopy, X-ray diffraction, and particle size analysis. Ignition temperatures of the materials were determined at heating rates in the range of 300–3000 K/s using an electrically heated filament. Activation energies of ignition were determined to be 152 ± 19 and 170 ± 25 kJ/mol for the Al-MoO3 and Al-Fe2O3 nano-composites, respectively. The activation energy obtained for the Al-Fe2O3 nano-composite is consistent with a previously reported value for the Al-Fe2O3 thermite reaction. Combustion tests were conducted in a constant volume pressure vessel in argon for both Al-Fe2O3 and Al-MoO3 and compared to respective blends of initial powders and to partially milled powders. The nano-composites showed higher respective reaction rates. Linear burning rates measured in an open channel of 2.5 × 2.5 mm cross-section were also higher for the ARM-prepared powders compared to partially milled materials.