The kinetic analysis of formation behavior for the MgB₂ phase

By using differential scanning calorimetry (DSC) and X-ray diffraction (XRD), we have investigated the phase formation for the Mg-B system, where two kinds of alumina crucible, with a cover (denoted as sample A) and without cover (denoted as sample B), were used during measurements. Heating rates of 5, 10, 15 and 20 °C/min were employed in the DSC measurement for the mixed powder of Mg:B = 1:2. The DSC curves of sample A show that the Mg-B system undergoes three kinds of process; solid-solid, Mg melting, and liquid-gas-solid reaction with increasing temperature. But the DSC results of sample B present three changes of Mg melting, liquid-gas-solid reaction, and a possible oxidation process of MgB₂. The difference between samples A and B was further investigated in a sintering experiment similar to the DSC process. The XRD analysis for the sintered samples indicates that the reaction between Mg and B to form MgB₂ occurs easily in the Mg atmosphere. The activation energy of the solid-solid reaction for the Mg-B system was calculated by using the Ozawa-Flynn-Wall and Kissinger methods, and the calculated values are 58.18 and 72.75 kJ/mol, respectively. The calculated value of the pre-exponential factor is 2.0 × 10¹⁵ S^-1. The relatively small value of E indicates that the reaction between Mg and B to form MgB₂ occurs easily.