As a result of improvement on melt/solidification processing, the critical current density (Jc) at 77K of YBCO bulk superconductors has been increased even in a magnetic field. Especially, QMG and MPMG method has become to give a Jc value high enough for practical use. Zonemelting method has been utilized to fabricate superconducting wires. Recently, a single crystal Y 123 has been also continuously fabricated by using a crystal pulling method. The growth mechanism of 123 crystal from the partial molten state is gradually clarified and the special peritectic reaction mechanism has been proposed. Applicable process (e.g. superconductiog wires, magnetic bearing) are expected to be developed.
Specific heats of polyvinylformal, which is insulator for superconducting wires, and carbon for thermometers are measured in the temperature range from 14K to 248K for the former and ranges from 14K to 201K and from 243K to 274K for the latter by an adiabatic calorimeter. Experimental results are represented by 6th- or 7th-polynomial equations with respect to temperature. Specific heats of polyvinylformal above 20K and ones of carbon for thermometers above 50K are precise enough for numerical analysis of the thermal stability on a superconducting coils.
AC loss characteristics of sub-micron filament Nb-Ti superconducting wires with Cu-Ni-Mn alloy matrix for AC use are described. To investigate the relationship between the proximity effect and hysteresis loss, the Nb-Ti superconducting wires are prepared, 1) changing Ni (10 and 30wt%) and Mn (0-1.78wt%) contents in Cu-Ni-Mn alloy matrix, 2) the variation of ratio of filament spacing (dn) to filament diameter (df). The addition of Mn to Cu-Ni matrix reduces the proximity effect by magnetic scattering and the allowed efficient Mn content to reduce hysteresis losses of Nb-Ti filaments is in the range of 0.9wt% to 1.78wt%. The relationships between dn and hysteresis losses were investigated and the optimum dn to reduce the proximity effect were dn=0.2μm (Cu-10%Ni matrix), dn=0.08μm (Cu-10%Ni-0.9%Mn), dn=0.052μm (Cu-30%Ni-0.9%Mn) and dn=0.048μm (Cu-30%Ni-1.34%Mn). Weak coupling due to the proximity effect has no influence on increasing hysteresis loss at an amplitude field of more than Bm=0.6-0.8T.
Critical current density and stability of sub-micron filament Nb-Ti superconducting wires with Cu-Ni-Mn alloy matrix are described. Characteristics of critical current density Jc depend on filament diameter df, kinds of matrix and ratio of filament spacing dn to df. In the case of large dn/df(=0.65-1) wires, increasing Jc with decreasing df is slight due to low pinning force. Residual Resistance Ratio (RRR) of the wires which were heat treated (100-250°C×1hour) at final size increased to 70-80. These values are three times those of the as drawn wires. The final heat treatment at more than 300°C degraded RRR and Jc, because of Ni diffusion. AC (50Hz) quench current at high field (>2T) of the secondary twisted cable with 36 strands (copper ratio 1.9) is higher than DC critical current of the secondary twisted cable.
Superconducting properties in an artificially designed pin structure composed of Nb/NbTi multilayers in a NbTi superconducting wire are analyzed theoretically with the experimental results and it is found an aspect ratio of plate-like Nb artificial pins in the pin structure gives effect to the critical current density in the wire. This is useful parameter for obtaining more high critical current densities, since both the elementary pinning force and the pin density can be changed without reducing the pinning efficiency in the pin structure.