Peculiarities of high-pressure and hot-pressing manufacture of MgB2-based blocks with high critical currents for electrical machines

Structure and properties of MgB2-based materials in the form of cylinders, rings, quadratic and rectangular blocks manufactured using high-pressure (2 GPa), hot-pressing (30 MPa), hot isostatic pressing (0.1 GPa), and atmospheric pressure (with predensification by broaching) from different types of B and MgB2 are considered. The blocks have been synthesized from Mg and B or sintered from MgB2 at 800 - 1100 °C (with and without additions of Ti or Ta). The inclusions of higher borides (with stoichiometry near MgB12 in the high pressure-manufactured magnesium diboride or near MgB7 in the hot-pressing-synthesized material) can effect critical current density: higher amount and finer dispersion of the above inclusions are observed in the materials with higher critical currents. Samples synthesized (at 4 GPa) from the MgB12 stoichiometric mixture of Mg and B that, according to the SEM microprobe analysis, contained MgB12 (more than 50 %) and MgO phases and, according to X-ray analysis, along with the above phases some amount of MgB2 demonstrated superconducting behavior. From high-pressure synthesized MgB2 blocks the first reluctance-type superconductive electromotor (1,3 kW) has been constructed and successfully tested.