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

Abstract

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 .

Analysis of the microstructure of bulk MgB2 using TEM, EBSD and t‐EBSD

“…There were in some cases also relatively large areas of MgO seen in the EBSD mappings with dimensions of several tens of micrometres. As the size of these areas is much too large for being MgO grains (which would have been observed in X‐ray measurements), we ascribe them to surface contamination effects due to the oxygenation, possibly a formation of an Mg‐B‐O phase (Wenzel et al ., ; Prikhna et al ., ). From the map, another interesting result is obtained: within the scanned area of 6 × 6 μm 2 , we obtain a GB length of 417.6 μm, which can provide significant flux pinning at the GBs.…”

Higher borides and oxygen-enriched Mg–B–O inclusions as possible pinning centers in nanostructural magnesium diboride and the influence of additives on their formation

“…The HP-synthesized samples from mixtures with great quantity of boron (up to Mg:B=1:20) exhibited SC properties [1], Fig. 1h, k and the highest j c as well as T c about 37 K were demonstrated by samples with near MgB 12 composition of matrix (prepared from the Mg:B= 1:8 and 1:20 mixtures).…”

“…The most apparent advantages of high-pressure synthesis of MgB 2 are the possibility to suppress the magnesium evaporation and formation of near theoretically dense nanostructural material with good connectivity between grains and high critical current density, j c in a short time (1 hour) [1]. It is considered that due to the comparatively large coherent length pining centers in MgB 2 can be grain boundaries, nanosized grains of secondary phases, and inhomogeneities of the structure.…”

Nanostructural Superconducting Materials for Fault Current Limiters and Cryogenic Electrical Machines

“…The higher coherent length of MgB 2 (1.6-12 nm [12]) as compared to YBa 2 Cu 3 O 7−δ (or Y123), gives the possibility to attain high critical current densities, j c , and trapped magnetic fields in the polycrystalline MgB 2 -based material, because the grain boundaries are not the obstacles for superconductive current flow as in case of Y123 and can act as pinning centers, so the structure with nanosized grains is preferable for higher j c . Besides, the pinning centers in MgB 2 can be nanosized inclusions of the second phases [13][14][15]. One can increase the critical current density of the material by chemical alloying [16].…”