Investigating the effect of NiO and NiF2 on boron carbide combustion

Boron-based fuels, recognized for their high energy density and potential in energetic applications, encounter challenges such as long ignition delays and incomplete combustion, which result in reduced combustion efficiency and limited performance in aerospace propulsion. In this study, boron carbide (B₄C) is investigated as an alternative fuel to pristine boron due to its favorable gas-phase combustion. Both metal oxide (nickel oxide (NiO)) and metal fluoride (nickel fluoride (NiF₂)) are selected as oxidizing modifiers to enhance the reactivity of B₄C. A method combining laser ignition with optical diagnostics is employed to investigate the enhancing effects of different oxidizers on the ignition and combustion characteristics of B₄C. Both NiO and NiF₂ can significantly increase the combustion radiation intensity and reduce the time to maximum intensity of B₄C. Differential scanning calorimetry, in-situ X-ray diffraction, and Fourier transform infrared spectroscopy were used for simultaneous thermal analysis of the B₄C composite powders. Combined thermal analysis showed that the effects of NiO and NiF₂ on promoting B₄C combustion is mainly achieved via the formation of Ni_mB_n and the release of a large number of gas products. It is reasonable to speculate that the phase separation at the B₂O₃/Ni_mB_n interface forms new pathways for oxygen diffusion and reaction with the B core. The difference in the combustion mechanism of B₄C with NiO and NiF₂ lies in the gas phase products, i.e., CO₂ and BF₃, respectively, thus leading to significant differences in their reaction processes.