Kompetenzzentrum für optische Spezialfasern

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 jc=1.2 - 1.0 E6 A/cm² (with 10% SiC doping) and jc=9.2 - 7.3E5A/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) fromB with high amount of impurityC(3.15%) andH(0.87%) has A/cm in 8 T field at 20 K, highest irreversibility fields (at 18.4 K T) and upper critical fields (at 22 K ? ? T) but 17% porosity. HP materials with stoichiometry near MgB ? can have ? K and ? A/cm² at 0 T and 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?A/cm². Dispersed inclusions of higher magnesium borides, which are usually present in MgB? structure and obviously create new pinning centers can be revealed by Raman spectroscopy (for the first time a spectrum of MgB 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.