High-Pressure Synthesized Nanostructural ${\hbox {MgB}}_{2}$ Materials With High Performance of Superconductivity, Suitable for Fault Current Limitation and Other Application

A variety of samples made via different routes were investigated. Samples are nanostructured (average grain sizes are about 20 nm). The advantage of high-pressure (HP)-manufactured (2 GPa, 800-1050°C, 1 h) MgB₂ bulk is the possibility to get almost theoretically dense (1-2% porosity) material with very high critical current densities reaching at 20 K, in 0-1 T j_c = 1.2 1.0 · 10⁶ A/cm² (with 10% SiC doping) and j_c = 9.2 - 7.3 10⁵ A/cm² (without doping). Mechanical properties are also very high: fracture toughness up to 4.4 ± 0.04 MPa · m^0.5 and 7.6 ± 2.0 MPa · m^0.5 at 148.8 N load for MgB₂ undoped and doped with 10% Ta, respectively. The HP-synthesized material at moderate temperature (2 GPa, 600°C, 1 h) from B with high amount of impurity C (3.15%) and H (0.87%) has j_c = 103 A/cm² in 8 T fleld at 20 K, highest irreversibility fields (at 18.4 K H_irr = 15 T) and upper critical fields (at 22 K H_C2 = 15 T) but 17% porosity. HP materials with stoichiometry near MgB₁₂ can have T_c = 37 K and j_c = 6 · 10⁴ A/cm² at 0 T and H_irr = 5 T at 20 K. The spark plasma synthesized (SPS) material (50 MPa, 600-1050°C 1.3 h, without additions), demonstrated at 20 K, in 0-1 T j_c = 4.5-4 10⁵ A/cm². Dispersed inclusions of higher magnesium borides, which are usually present in MgB₂ structure and obviously create new pinning centers can be revealed by Raman spectroscopy (for the first time a spectrum of MgB₂ was obtained). Tests of quench behavior, losses on MgB₂ rings and material thermal conductivity show promising properties for fault current limiters. Due to high critical fields, the material can be used for magnets.