Experimental determination of the peritectic transition temperature of MgB2 in the Mg–B phase diagram

• We have created a high pressure, high temperature material synthesis apparatus.
• This apparatus is programmable and capable of various thermal analysis procedures.
• Using our apparatus we have established the peritectic transition temperature of MgB2.
• This temperature is 300 °C lower than established values for the same pressure.
• We have shown that this discrepancy is likely the result of impurities such as C.

We have studied thermodynamic phase stability in the Mg–B system through use of a high-pressure, high-temperature apparatus consisting of a large pressure vessel and an RF induction heater. The incongruent melting temperature for MgB2 was found to be ∼1450 °C at 10 MPa using thermal analysis data applied to both MgB2 powder samples and Mg/B mixtures. The experimental temperature is ∼300 °C lower than temperatures shown in calculated phase diagrams of the Mg–B system at the same pressure and the discrepancy demonstrates the need for further experimental investigations of phase stability in binary Mg–B and ternary Mg–B–X systems. In this study C (as an impurity in boron) was found to have a large effect on the peritectic temperature, with a relatively small (0.7 wt% C) impurity concentration resulting in a ∼40 °C elevation of the peritectic temperature. Along with morphological characterization, XRD and EPMA analyses were used to identify phases and confirm the peritectic transformation in the Mg–B phase diagram.