Formation of Higher Borides During High-Pressure Synthesis and Sintering of Magnesium Diboride and Their Positive Effect on Pinning and Critical Current Density

Critical current density (j 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 j c by promoting the higher boride formation via impurity hydrogen absorption, thus preventing MgH 2 detrimental for j c being formed, which possibly increases the MgB 12 nucleation barrier. SiC (0.2-0.8 mum) addition increases j c of MgB 2 , allowing us to get j 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 1050degC, 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 j c (7middot10 4 - 2middot10 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.