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 2MgB(2)(s) = MgB4(s) + Mg(g) have been derived. MgB2 shows a small and slightly temperature-dependent vaporization coefficient alpha(v),(1000) (K) = (5.9 +/- 1.0) x 10(-2) ; the activation enthalpy of the high-temperature decomposition of magnesium diboride is Delta H-act*(1010K) = 266.3 +/- 5.6 kJ mol(-1) to be compared to the measured equilibrium decomposition enthalpy of 1(110 K Delta H-r degrees(992) (K) = 239.1 +/- 5.6 kJ mol(-1). Implications of the kinetic limited MgB2 high-ternperature decomposition for synthesis condition optimization of this material are discussed. (c) 2006 Elsevier B.V. All rights reserved.