Reactive Composite Boron–Magnesium Powders Prepared by Mechanical Milling

Abstract

Composite reactive material powders combining boron with 40 mass % of magnesium are prepared by mechanical milling of elemental boron and magnesium powders. Commercial boron is washed with acetonitrile to remove surface boron oxide, which can react with magnesium during milling. Thermogravimetric measurements showed two oxidation steps. X-ray diffraction analysis of partially oxidized samples showed that selective oxidation of magnesium occurred during the first step. It was followed by oxidation of boron during the second step. Prepared composite powders were ignited using an electrically heated filament, by electric spark, and in constant-volume explosion tests. Ignition is caused by selective oxidation of magnesium. Ignition temperature of the composite powders obtained in heated filament tests was lower than that measured for the pure magnesium. A faster initiation by electric spark was observed for powders prepared using washed boron. This was attributed to an increased effect of heterogeneous oxidation for high heating rates and thus to an accelerated heterogeneous oxidation for powders achieved after removal of the natural boron oxide and hydroxide surface layers. Following the ignition, boron continued to react, contributing to the released combustion heat and pressure generated by the flame in both constant-volume explosion and spark ignition tests.