Thermodynamic and kinetic aspects of decomposition of MgB2 in vacuum: Implications for optimization of synthesis conditions

Investigation of the high-temperature decomposition of MgB2 under thermodynamic and kinetic control provides fundamental knowledge required for optimizing the synthesis conditions of this promising high-temperature superconducting material. In this paper, a new thermodynamic p–T phase stability diagram for the Mg–B binary system is presented and an investigation of some of the kinetic aspects of the high-temperature vaporization of magnesium diboride is reported. By comparing experimental results for the decomposition of MgB2 under kinetic and thermodynamic control, vaporization coefficient of magnesium diboride and the activation enthalpy and entropy of the reaction 2MgB2(s) ¼ MgB4(s)+Mg(g) have been derived. MgB2 shows a small and slightly temperaturedependent vaporization coefficient av;1000 K ¼ ð5:9 1:0Þ 102; the activation enthalpy of the high-temperature decomposition of magnesium diboride is DactH 1010 K ¼ 266:3 5:6 kJ mol1 to be compared to the measured equilibrium decomposition enthalpy of DrHo 992 K ¼ 239:1 5:6 kJ mol1. Implications of the kinetic limited MgB2 high-temperature decomposition for synthesis condition optimization of this material are discussed.