Thermodynamics of MgB/sub 2/-by calorimetry and Knudsen thermogravimetry

Abstract

The vapor pressure of MgB/sub 2/, corresponding to the reaction, 2MgB/sub 2/(cr)/spl rarr/MgB/sub 4/(cr)+Mg(g), has been measured by Knudsen effusion vacuum thermogravimetry. The vapor pressure over the range 600/spl deg/C-850/spl deg/C can be expressed as: logp/sub mg/(Pa)=-1.129141(10000/T)+7.328161, where T is the temperature in Kelvin. By extrapolation, the 0.1 MPa decomposition temperature of MgB/sub 2/ is estimated to be 1268/spl deg/C. In a separate calorimetric investigation on the same material, using the isoperibol solution method, we measured the enthalpy of formation of MgB/sub 2/, /spl Delta//sub f/H/sub MgB2//spl deg/(298.15), to be -(155.9/spl plusmn/14.2)kJ/spl middot/mol/sup -1/. By combining the vapor pressure data and the enthalpy data, the Mg-rich part of the Mg-B phase diagram can be calculated. The measured vapor pressures are approximately an order of magnitude higher than those derived from published thermodynamic data. The lower limit of Mg pressures for the deposition of high-purity MgB/sub 2/ thin films at any given temperature may therefore be at a higher value than previously thought.