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≥10⁵ A/cm² up to 4 T field at 10 K, 2.5 T at 20 K and 0.5 T at 30 K; j_c≥10⁴ A/cm² up to 7.3 T at 10 K, 4.5 T at 20 K and 1.5 T at 30 K; j_c≥10³ A/cm² up to 10 T at 10 K and 6 T at 20 K. In our high-pressure apparatuses we can produce samples about 60 mm 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, TaN_0.1, 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 × m^-32/ at 4, 96-N load. The HP-sintered MgB₂ has a microhardness of H_v=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 × m^-32/ 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.