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.