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

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 degrees 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.