Ignition and heterogeneous combustion of aluminum boride and boron–aluminum blend

Abstract

Adding other metals is an efficient method of breaking through the energy release efficiency limitation of boron-based fuels. A laser ignition testing system was used for investigating the ignition and heterogeneous combustion characteristics of aluminum boride and boron–aluminum blend. The emission spectra and combustion flames of the samples during combustion were recorded by a fiber optic spectrometer and a high-speed camera, respectively. The component, morphology, and distribution features of the condensed combustion products (CCPs) were then analyzed using an X-ray diffractometer, a scanning electron microscope, and an X-ray energy dispersive spectrometer. The emission spectral intensity curves show that the boron–aluminum blend has better ignition feature than aluminum boride (the ignition delay time was 45% shorter). However, the combustion intensity and self-sustaining combustion time of boron–aluminum blend are both inferior to those of aluminum boride. In full wave emission spectra of the samples, the characteristic peaks of combustion intermediate products like BO2, AlO, and AlO2 were found. The combustion flames of the two samples were both yellow green and tapered. The variation trend of the flame size corresponded to that of the emission spectral intensity curves. During the first half of the combustion, large amounts of sparks were ejected from the combustion flame of boron–aluminum blend, which caused the acute fluctuation of combustion intensity. B, Al, B2O3, Al2O3, AlN, and Al5O6N were detected in the CCPs of both samples. In addition, the CCPs of aluminum boride contained unburned AlB2, and those of boron–aluminum blend contained Al4B2O9. Webbed AlN fibers were observed only in the CCPs of boron–aluminum blend, while AlN and Al5O6N whiskers were found in those of both samples. The observed bulks were partially oxidized AlB2 in the CCPs of aluminum boride, or partially oxidized B in those of boron–aluminum blend. As both diffusion and melt-dispersion reactions of aluminum were proceeded during the combustion of boron–aluminum blend, oxidation products of aluminum with different morphology were found in the CCPs of boron–aluminum blend as follows: smooth aluminum spheres with low oxidation degree, rough aluminum oxide spheres with high oxidation degree, empty aluminum oxide shells with hollows or cracks, and fine aluminum oxide particles adhered on the surface of boron bulks.