Transient flameout process of boron-magnesium agglomerates during combustion in oxygen-rich atmospheres

Abstract

In this study, boron–magnesium agglomerates with varying mass ratios were prepared by drying a micron-sized boron–magnesium mixed suspension, and the combustion process of these agglomerates under different oxygen-rich concentrations were investigated using a laser ignition system. The test results showed that when the mass fraction of magnesium powder in boron-magnesium agglomerates exceeded a certain threshold (between 2% and 5%), flame extinction and reignition occurred after a significant reduction in the agglomerate volume during combustion. This process is referred to as the transient flameout process, which is affected by the magnesium content of the agglomerate and the oxygen concentration in the ambient atmosphere. An increase in the magnesium content or oxygen concentration makes this phenomenon more pronounced. During weakening of the flame intensity, a dark film gradually covered the particle surfaces. X-ray diffraction and elemental analyses of the cross-section and outer surface of the condensed combustion product suggested that the dark film is primarily composed of Mg-B-O ternary oxides. This film prevents direct contact between boron and oxygen, thereby inhibiting surface and gas-phase reactions and leading to the occurrence of the transient flameout phenomenon.