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Critical current density ( c ) of high-pressure (2 GPa) manufactured MgB 2 -based superconductors depends on the amount and distribution of higher borides (MgB 12 ) in MgB 2 matrix, which in turn are determined by the nature of the initial components first of all B or MgB 2 and the temperature of sintering or synthesis. Ti and Ta additions can improve c by promoting the higher boride formation via impurity hydrogen absorption, thus preventing MgH 2 detrimental for c being formed, which possibly increases the MgB 12 nucleation barrier. SiC (0.2-0.8 m) addition increases c of MgB 2 , allowing us to get c = 10 6 A cm 2 at 20 K in the 1 T field: pinning is increased by SiC and higher boride grains and there is no notable interaction between SiC and MgB 2 . As the synthesis temperature increases from 800 to 1050 C, Ti and SiC additions may affect the oxygen segregation and formation of Mg-B-O inclusions enriched with oxygen as compared to the amount of oxygen in the MgB 2 matrix, which can also promote an increase in pinning. Materials high-pressure synthesized from Mg and B taken in 1:4, 1:6, 1:7, 1:8, 1:10, 1:12, 1:20 ratios were superconductive with T c of about 37 K. High c (7 10 4 2 10 4 A cm 2 in zero field at 10-30 K, respectively) showed materials with the matrix composition near MgB 12 stoichiometry, they have doubled microhardness of MgB 2 .

Superconducting (SC) and mechanical properties of spark plasma (or SPS) produced MgB2 –based materials allow their efficient applications in fault current limiters, superconducting electromotors, pumps, generators, magnetic bearings, etc. The synthesized from Mg and B at 50 MPa, 1050 °C for 30 min material has a density of 2.52 g/cm3, critical current density, jc = 7.1•105 A/cm2 at 10 K , 5.4 •105 A/cm2 at 20 K, and 9•104 A/cm2 at 35 K in zero magnetic field; at 20 K its field of irreversibility Birr(20)=7 T and upper critical field Bc2(20)=11 T; microhardness HV=10.5 GPa and fracture toughness K1C =1.7 MPa•m1/2 at 4.9 N-load. SPS-manufactured in- situ MgB2-based materials usually have somewhat higher jc than sintered ex-situ. The pressure variations from 16 to 96 MPa during the SPS-process did not affect material SC characteristics significantly; the jc at 10-20 K was slightly higher and the material density was higher by 11%, when pressures of 50-96 MPa were used. The structure of SPS-produced MgB2 material contains Mg-B-O inclusions and inclusions of higher borides (of compositions near MgB4, MgB7, MgB12, MgB17, MgB20), which can be pinning centers. The presence of higher borides in the MgB2 structure can be revealed by the SEM and Raman spectroscopy.

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The study of high pressure (2 GPa) synthesized MgB 2 -based materials allows us to conclude that higher borides (with near MgB 12 stoichiometry) and oxygen-enriched Mg-B-O inclusions can be pinning centers in nanostructural magnesium diboride matrix (with average grain sizes of 15-37 nm). It has been established that additions of Ti or SiC as well as manufacturing temperature can affect the size, amount and distribution of these inclusions in the material structure and thus, influence critical current density. The superconducting behavior of materials with near MgB 12 stoichiometry of matrix is discussed.

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