  Bulk MgB2 - based materials with the following critical current densities
(jc) in 1 T field: 570 kA/cm^2 at 10 K, 350 kA/cm^2 at 20 K and 40 kA/cm^2 at
30 K and in 10 T field: 650 A/cm^2 at 10 K have been high-pressure (HP)
synthesized from Mg and B with 2 - 10 wt.% addition of Ta. 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 Mg-B-O-matrix phase, as well as, the impurity nitrogen and
oxygen in Mg-B grains (black MgB2 single crystals distributed over the matrix).
Vickers microhardness of HP-synthesized material is Hv=12.54 (+/-)0.86 GPa (at
0.496-N load) 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).
