High pressure synthesis and sintering of MgB2

High pressure (HP) synthesized MgB2 (from Mg scobs and amorphous B with Ta addition in the form of powder of about 2-10 wt%) exhibits critical current density j(c) greater than or equal to 10(5) A/cm(2) up to 4 T field at 10 K, 2.5 T at 20 K and 0.5 T at 30 K; greater than or equal to 104 A/cm(2) up to 7.3 T at 10 K, 4.5 T at 20 K and 1.5 T at 30 K; j(c) greater than or equal to 10(3) A/cm(2) up to 10 T at 10 K and 6 T at 20 K. In our high-pressure apparatuses we can produce samples about 60 nun in diameter and 20-30 mm in height that might be used for superconductive devices such as electromotors, generators, flying wheels, current limiters, noncontact bearings, etc. In synthesis and sintering process Ta plays the role of an absorbent of impurity gases (hydrogen, nitrogen, etc.) and forms Ta2H, TaH, TaN0.1, etc., thus promotes the reduction of MgH2 in samples, as well as, the impurity nitrogen and oxygen in Mg-B grains (black MgB2 single crystals distributed over the matrix). In the materials with better superconductive characteristics, we observed some amount of unreacted Mg and higher density of MgB2 single crystal inclusions.. We have got some evidence that the amount of oxygen in the matrix Mg-B-O and the microdeformation of MgB2 lattice influence the critical, current density of the material. The HP-synthesized samples have exhibited a microhardness of H-v = 12.54 +/- 0.86 GPa at 0.496-N load and a fracture toughness of 4.24 MN X m(-3/2) at 4, 96-N load. The HP-sintered MgB2 has a microhardness of, 11, = 17.1 +/- 1.11 GPa at 4.96 N-load, H-v = 10.12 +/- 0.2 GPa and fracture toughness of K-1C = 7.6 +/- 2.0 MN X m(-3/2) at 147.2 N-load and the Young modulus of 213 18 GPa. The hardness (at 60-mN load) of MgB2 single crystals located in a sample matrix is 35.6 +/- 0.9 GPa that is higher than the hardness of sapphire (31.1 +/- 2.0 GPa) and their Young modulus is 385 +/- 14 GPa (sapphire has 416 +/- 22 GPa) and that gives us grounds to assign MgB2 to superhard materials as well.