Dynamics of phase formation during the synthesis of magnesium diboride from elements in thermal explosion mode

Abstract

The influence of the heating rate of the Mg + 2B mixture on the dynamics of the phase formation during the thermal explosion in the helium medium is investigated by the time-resolved X-ray diffraction method. It is shown that the MgB₂ phase appears without the formation of intermediate compounds. The presence of impurity oxygen is a substantial factor affecting the formation kinetics of MgB₂. The oxide film on the surface of magnesium particles has no time to form with the heating rate of the charge mixture of 150–200°C/min. A result of this circumstance is the reaction diffusion mechanism of the Mg + 2B = MgB₂ reaction immediately after the melting of magnesium. Synthesis products mainly consist of MgB₂ and MgO traces at a level of 5%. The thermal explosion temperature is 1100°C. A comparatively thick oxide film which retards melt spreading and shifts the onset of the formation reaction of MgB₂ by 8–9 s grows on the magnesium surface at a heating rate of 30–50°C/min. Synthesis products contain MgB₂ and up to 15% MgO. The thermal explosion temperature is 1020°C in this case.