High-pressure synthesis of MgB2 with and without tantalum additions

The high-pressure (HP) synthesis seems to be very promising for manufacturing of the bulk MgB2 superconductive material. The positive influence of tantalum in the form of a foil that covered the sample and as an addition of Ta powder in the starting mixture of B and Mg on critical current density in the magnetic field and on the field of irreversibility of MgB2 has been observed during MgB2 synthesis. At 20 K HP-synthesized MgB2 has a critical current density of 68 kA/cm(2) in the 1 T field and of 160 kA/cm(2) in the zero field, its field of irreversibility at 20 K being 5.7 T. Vickers microhardness of the samples is 12.79 +/- 1.14 GPa at a 4.96-N load, fracture toughness is 4.1 MN/m(3/2) at the 147.2-N load (while at the 4.96-N load there are no cracks from the corners of the indent). The density of HP-synthesized MgB2 samples is 95-97% the theoretical one. (C) 2002 Elsevier Science B.V. All rights reserved.