Synthesis and Sintering of MgB2 under High Pressure

High-pressure (HP) synthesis and sintering are promising methods for manufacturing of the bulk MgB2 superconductive material. The available high-pressure apparatuses with 100 cm3 working volume can allow us to use the bulk MgB2 for practical applications such as electromotors, fly-wheels, bearings, etc. We have found that the Ta presence during HP synthesis (especially) or sintering process (in the form of a foil that covered the sample and as an addition of Ta powder of about 2-10 wt.% to the starting mixture of B and Mg or to MgB2 powder) increases the critical current density (jc) in the magnetic fields up to 10 T and the fields of irreversibility (Hirr) of MgB2-based bulk materials. We observed the strong evidences that Ta absorbs hydrogen and nitrogen during synthesis and sintering to form Ta2H, TaH and TaN0.1 and prevents or reduces the formation of MgH2 (both with orthorhombic and tetragonal structures). Ta also essentially reduces the amount of impurity nitrogen in black Mg-B (most likely, MgB2) crystals of the matrix phase. The presence of Ta during synthesis allowed us to obtain a bulk materials with the following critical current densities jc in 1 T field: 570 kA/cm2 at 10 K, 350 kA/cm2 at 20 K and 40 kA/cm2 at 30 K. Besides, the materials demonstrated high mechanical characteristics (microhardness, fracture toughness, Young modulus).