In order to investigate the MgO doping effects on critical current densities in oxide superconducting Bi-2212 crystals, crystals doped with and without sub-micron size MgO particles were prepared by the self-flux method using the large-temperature gradient technique. From scanning electron microscope (SEM) observation and energy dispersion spectroscopy (EDS) analysis, it was found that MgO particles in the crystal were condensed to large-size cubic particles with almost 1μm size and dispersed in the Bi-2212 matrix. With increasing MgO amount, cracks were observed. In the low-temperature region (i.e., from 5 to 10K) critical current density (Jc) for 1wt% MgO-doped crystal increased 3 times as much as that of MgO-free Bi-2212 single crystal. On the assumption that the cubic MgO particle works as a pinning center, the calculated Fp value based on the core interaction model was smaller than that of the observed value. This result suggests that the enhancement of Jc is due to defects or/and dislocation introduced by the MgO doping. Increase in the irreversibility field (Birr) due to MgO doping was observed over the entire temperature region measured (i.e., from 20 to 90K for 1wt% MgO-doped sample and from 20 to 29K for the 5wt% MgO-doped sample). The exponent n of the temperature dependence of Birr for all samples in the low-temperature region agreed with that of the calculated value based on the flux creep model. In the high-temperature region, the n values for the 1wt% MgO and 5wt% MgO-doped samples almost agreed with the calculated values assumed by the local model of a large normal precipitate as a pinning center. This result suggests that the doped MgO particles work as the pinning centers in the high-temperature region.
The design of electric insulation is important for designing High-Tc superconducting DC cable (HTS DC cable). In HTS AC cable, it is known that liquid-nitrogen-impregnated laminated-paper-insulation is a promising insulation for superconducting cables. We thought that this insulation would be effective for the insulation of HTS DC cable. Therefore, DC breakdown tests, impulse breakdown tests and DC polarity reversal tests were carried out to obtain the fundamental properties of laminated-paper-insulation. The insulation of a 500kV HTS DC cable was designed by using design stresses decided from test results. Furthermore, a 100km HTS DC submarine cable suitable for 500kV-4, 000MVA was designed.