Effect of higher borides and inhomogeneity of oxygen distribution on critical current density of undoped and doped magnesium diboride

The effect of doping with Ti, Ta, SiC in complex with synthesis temperature on the amount and distribution of structural inhomogeneities in MgB₂ matrix of high-pressure-synthesized-materials (2 GPa) which can influence pinning: higher borides (MgB₁₂) and oxygen-enriched Mg-B-O inclusions, was established and a mechanism of doping effect on j_c increase different from the generally accepted was proposed. Near theoretically dense SiC-doped material exhibited j_c= 10⁶ A/cm² in 1T field and H_irr =8.5 T at 20 K. The highest jc in fields above 9, 6, and 4 T at 10, 20, and 25 K, respectively, was demonstrated by materials synthesized at 2 GPa, 600 °C from Mg and B without additions (at 20 K j_c= 10² A/cm² in 10 T field). Materials synthesized from Mg and B taken up to 1:20 ratio were superconductive. The highest j_c (6×10⁴ A/cm² at 20 K in zero field, H_irr= 5 T) and the amount of SC phase (95.3% of shielding fraction), T_c being 37 K were demonstrated by materials having near MgB₁₂ composition of the matrix. The materials with MgB₁₂ matrix had a doubled microhardness of that with MgB₂ matrix (25±1.1 GPa and 13.08±1.07 GPa, at a load of 4.9 N, respectively).