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.