High pressure synthesis and sintering of MgB/sub 2

High pressure (HP) synthesized MgB/sub 2/ (from Mg scobs and amorphous B with Ta addition in the form of powder of about 2-10 wt%) exhibits critical current density j/sub c//spl ges/10/sup 5/ A/cm/sup 2/ up to 4 T field at 10 K, 2.5 T at 20 K and 0.5 T at 30 K; j/sub c//spl ges/10/sup 4/ A/cm/sup 2/ up to 7.3 T at 10 K, 4.5 T at 20 K and 1.5 T at 30 K; j/sub c//spl ges/10/sup 3/ A/cm/sup 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 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 Ta/sub 2/H, TaH, TaN/sub 0.1/, etc., thus promotes the reduction of MgH/sub 2/ in samples, as well as, the impurity nitrogen and oxygen in Mg-B grains (black MgB/sub 2/ single crystals distributed over the matrix). In the materials with better superconductive characteristics, we observed some amount of unreacted Mg and higher density of MgB/sub 2/ single crystal inclusions. We have got some evidence that the amount of oxygen in the matrix Mg-B-O and the microdeformation of MgB/sub 2/ lattice influence the critical current density of the material. The HP-synthesized samples have exhibited a microhardness of H/sub v/=12.54 /spl plusmn/ 0.86 GPa at 0.496-N load and a fracture toughness of 4.24 MN /spl times/ m/sup -3/2/ at 4, 96-N load. The HP-sintered MgB/sub 2/ has a microhardness of H/sub v/=17.1/spl plusmn/1.11 GPa at 4.96 N-load, H/sub v/=10.12 /spl plusmn/ 0.2 GPa and fracture toughness of K/sub 1C/=7.6 /spl plusmn/ 2.0 MN /spl times/ m/sup -3/2/ at 147.2 N-load and the Young modulus of 213 /spl plusmn/ 18 GPa. The hardness (at 60-mN load) of MgB/sub 2/ single crystals located in a sample matrix is 35.6 /spl plusmn/ 0.9 GPa that is higher than the hardness of sapphire (31.1 /spl plusmn/ 2.0 GPa) and their Young modulus is 385 /spl plusmn/ 14 GPa (sapphire has 416 /spl plusmn/ 22 GPa) and that gives us grounds to assign MgB/sub 2/ to superhard materials as well.