Combustion behavior of discrete magnesium diboride particles in the state of motion.

Magnesium diboride (MgB 2) is regarded as a potential substitute for the amorphous boron (B) due to its excellent energy-release characteristics. In this work, the motion track, flame, and reaction process of discrete MgB 2 particles were investigated in the state of motion to reveal their combustion mechanism. The effect of different gaseous mediums on the sustained combustion time and the ingredients and microstructure characteristics of combustion residues were also analyzed. Furthermore, the size distribution of grains in the initial MgB 2 particles, MgO cluster particles, and burnt grains in the combustion residues was obtained using graphic analysis. Based on the different motion tracks and flame features obtained, three different combustion modes can be identified; single-stage combustion, two-stage combustion, and micro-explosion combustion. Among them, the single-stage combustion and the two-stage combustion exhibit similar sustained combustion times (12.1 ms and 12.3 ms, respectively), whereas the micro-explosion combustion exhibits a significantly shorter sustained combustion time (5.6 ms) due to the disintegration of particles. Besides, the average sustained combustion time of MgB 2 particles was prolonged in the oxygen but shortened in the water vapor. The obtained X-ray diffractometer patterns showed that the combustion residues were composed of MgB 2 , MgO, Mg, B 2 O 3 , and MgB x (indicating an incomplete oxidation during the combustion), and that the burn-off rate of discrete MgB 2 particles could still be improved. The scanning electron microscopy images showed a coating layer of condensed B 2 O 3 and clusters of MgO particles on the surface of the combustion residues. The average diameter of burnt MgB x grains and MgO particles were calculated as 1.04 μm and 0.18 μm, respectively. The burnt MgB x grains were products obtained after the coagulation of liquid B 2 O 3 layer and initial MgB 2 grains and were larger than the initial MgB 2 grains (diameter of 0.24 μm). The MgO cluster particles were condensed from gas-phase reaction and agglomerated to produce a porous structure. • Single-stage, two-stage, and micro-explosion modes exist during MgB 2 combustion. • Micro-explosion mode exhibits shorter sustained combustion time due to disintegration. • Oxygen benefits the heat release but water vapor slows down the oxidation reaction. • Stepwise decomposition and incomplete combustion of the particles are confirmed. • B 2 O 3 condensation and MgB x recrystallization cause size increase of product grains. [ABSTRACT FROM AUTHOR]