Formation of Higher Borides During High-Pressure Synthesis and Sintering of Magnesium Diboride and Their Positive Effect on Pinning and Critical Current Density
in: IEEE Transactions on Applied Superconductivity (2009)
Critical current density (jc) of high-pressure (2 GPa) manufactured MgB2-based superconductors depends on the amount and distribution of higher borides (MgB12) in MgB2 matrix, which in turn are determined by the nature of the initial components first of all B or MgB2 and the temperature of sintering or synthesis. Ti and Ta additions can improve jc by promoting the higher boride formation via impurity hydrogen absorption, thus preventing MgH2 detrimental for jc being formed, which possibly increases the MgB12 nucleation barrier. SiC (0.2-0.8 mum) addition increases jc of MgB2, allowing us to get jc = 106 A/cm2 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 MgB2 . 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 MgB2 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 Tc of about 37 K. High jc (7middot104 - 2middot104 A/cm2 in zero field at 10-30 K, respectively) showed materials with the matrix composition near MgB12 stoichiometry, they have doubled microhardness of MgB2.