Boron as Fuel for Ceramic Thermites

Boron submicrometer particles (22.3 m 2 /g) were used as fuel to prepare energetic compositions with bismuth(III) oxide (2.7 m 2 /g) and copper(II) oxide (10.7 m 2 /g), leading to unconventional submicrometer-thermites being made solely of ceramic compounds. The morphology of the mixtures was studied according to their boron content: (i) by comparing their calculated apparent density with their experimental one and (ii) by a photometric technique based on the analysis of the gray levels of the samples. The analysis of the early step of the reaction, corresponding to the preignition exotherm observed in DSC experiments, has shown that boron is first reduced by Bi 2 O 3 leading to a molten bismuth layer at the surface of boron particles. The dissolution of Bi 2 O 3 in this metallic coating favors the reaction. Conversely, the B 2 O 3 formed by boron oxidation further reacts with Bi 2 O 3, forming a glass like layer that limits the diffusion of Bi 2 O 3 . The reaction is decelerated by this phenomenon and requires a higher temperature to reach completion (second exotherm). This mechanism accounts for the incomplete oxidation of boron in Bi 2 O 3 /B compositions. The nature of the crystallized phases present in the combustion residues was identified by X-ray diffraction (XRD) and correlated to the evolution of combustion heat measured by calorimetry. For both compositions (Bi 2 O 3 /B and CuO/B), the combustion heat remains at a high level over a wide composition range. This result was explained by the formation of different boron oxides (B 2 O 3, B 7 O). Boron-based thermites are relatively insensitive to friction and impact but possess extremely low sensitivity levels to electrostatic discharge and ignite quite easily in contact with an open flame. The investigation of the ejection rate and the reactive power has shown that boron-based nanothermites are 2 orders of magnitude less reactive than their aluminum counterparts. Because of their moderate reactivity, boron-based submicrometer thermites are promising candidate materials for ignition devices and for specific propulsion applications.