Pressure-temperature stability diagram of MgB2 – Consequences for sintering of dense MgB2 ceramics

Abstract

The MgB₂ compound has been largely studied due to its high superconducting properties. Such compound is well known to be difficult to densify. Utilization of Spark Plasma Sintering (SPS) to achieve high density ceramics has been proposed by many authors. However, the phase analysis of the samples from these papers systematically presents secondary phases such as MgB₄, MgB₇ or MgO. Thermodynamic studies seem to indicate that high pressures should increase the temperature stability domain of MgB₂. Therefore, in order to achieve high density pure MgB₂ ceramics, spark plasma sintering at low temperatures and high pressures has been investigated. Experiments demonstrated that increasing the pressure can reduce and even eliminate the presence of MgB₄. However, they also show that increasing the dwell time to achieve higher density leads to the reappearance of the MgB₄ compound. Therefore, high pressures rather reduce the kinetic of MgB₂ decomposition than increasing the MgB₂ temperature stability domain. The highest density achieved, without any composition change compared to the precursor powder, is 95 % relative density for a ceramic sintered at 700 °C under 1000 MPa during a dwell time of 2 h. These promising results make it possible to consider the investigation of the superconducting properties on a pure MgB₂ ceramic and to determine the influence of the MgB₄ phase on these properties.