Makoto Hiraoka

AC losses of Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox monofilamentary and multifilamentary tapes

Abstract The AC losses are investigated at 77 K on the Ag-sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x monofilamentary tape and multifilamentary tapes with filament numbers equal to 7, 19, 37 and 61, through an AC transport method under self-fields and an AC magnetic method under longitudinal fields. Although the main contribution to the AC losses around a commercial frequency of 50 Hz comes from the hysteresis loss independent of the measuring methods, the AC transport losses for the monofilamentary tape are larger than the AC magnetic losses by about one order of magnitude. Such a discrepancy is explained by a large aspect ratio of the superconductor core, which lowers only the magnetic losses. The superconductor subdivision in the multifilamentary tapes causes no notable influence on the AC transport losses ascribed to the hysteresis loss. This is compared with the result for the AC magnetic losses that the subdivision suppresses the contribution of the hysteresis loss while enhancing that of the eddy current loss. The density distribution of the AC transport losses obtained from numerical calculations based on a critical state model shows a resemblance between the monofilamentary tape and the multifilamentary tapes, which conforms to the experimental results for the AC transport losses.

Effect of Cu+/Cu2+ ratio on the thermal stability and crystallization in Bi2Sr2CaCu2Ox glass

Abstract The Cu+/Cu2+ ratio in B2Sr2CaCu2Oχ glass was controlled by adding glucose (0−2.5 wt%) during glass melting, and the thermal stability and crystallization behavior of glasses with different Cu+/Cu2+ ratios was examined. In a glass melted with 2.0 wt% glucose, more than 90% of Cu ions were present as Cu+ ions. It was found that thermal stability increased with increasing Cu+. At ∼ 780°C, a liquid phase was created in the presence of Cu+ ions in the interior of samples. After oxidation of Cu+ ions to Cu2+ ions due to the oxygen diffusion through a liquid phase, the superconducting B2Sr2CaCu2Oχ phase was formed.

Tensile strain/transverse compressive stress dependence of critical current in Ag-sheathed Bi(2212) 7-core superconducting wires☆

Abstract The mechanical properties and tensile strain/transverse compressive stress dependence of the critical current I c in Ag-sheathed Bi(2212) 7-core superconducting wires of different constructions were evaluated at a temperature of 4.2 K and a magnetic field of 14 T. The increase in the volume fraction of the superconductor shifted the stress-strain curves upwards. The tensile strain for I c degradation onset was in the range 0.14–0.27%. This strain depends upon the residual compressive strain and the thickness of the superconductor layer. On the other hand, the transverse compressive stress for I c degradation onset was in the range 9–20 MPa. Wires with a thin Ag-sheath layer showed a remarkably low stress for I c degradation onset. There was no welldefined correlation between the onset stress for degradation of I c caused by the tensile stress and that caused by the transverse compressive stress.

Stress/strain dependence of critical current in Cu-Ag externally reinforced Ag-Zr/Bi-2212 superconducting tapes

Abstract The effects of external reinforcement on the mechanical property and the axial tensile stress/strain dependence of the critical current I c in Bi(2212) superconducting tapes were evaluated at a temperature of 4.2 K and a magnetic field of 14 T. The monocore and 19-core superconducting tapes with three kinds of sheath—Ag, Ag-Zr and Ag-Zr/Ag double layer—were incorporated with a reinforcing Cu-Ag tape. As a result of this reinforcement, the 0.2% proof stress of superconducting tapes increased markedly without significant degradation of the I c . The tolerance limit in strain for I c degradation in the tapes was slightly improved and those of stress were markedly improved. The improvement in the tolerance limit in strain or stress for the degradation was more evident in the tapes with the Ag-Zr sheath, compared with those with a pure Ag sheath.

Electromagnetic Properties and Detailed Preparation Methods of Ag-sheathed Bi-Pb-Sr-Ca-Cu-O Superconducting Tapes

We conducted researches on Bi-Pb-Sr-Ca-Cu-O powders, for preparing Ag-sheathed superconducting tapes with high critical current density (Jc) at 77.3 K. Three different powder compositions (Bi8Pb2Sr10Ca10Cu15Ox, Bi9Pb2Sr10Ca11Cu15Ox, and Bi10Pb2Sr10Ca10Cu15Ox) were investigated. With these powders, superconducting tapes about 0.2 mm in thickness were prepared by powder-in-tube method and the process combining sintering and intermediate uniaxial pressing. The highest Jc was 19,500 A/cm2(77.3 K,0 T) on the specimen using Bi9Pb2Sr10Ca11Cu15Ox. Influence of powder compositions on electromagnetic properties such as Jc and A.C. susceptibility in various magnetic fields are also reported.

Mechanical and AC loss properties in mono- and multi-filamentary Bi-2223 Ag-sheathed tapes

The evaluation of the bending strain tolerance and AC loss properties for monoand multi-filamentary Bi-2223 Ag-sheathed tape were carried out at liquid nitrogen temperature. For tapes with a filament number of over 19, the critical current (Ic) was maintained at the same values up to the bending strain of 0.3%, although the Ic of the mono-filamentary tape at the condition of 0.2% strain degraded to 90% of the value for the no-strain condition. The AC loss of the monofilamentary tape was the hysteresis type. On the contrary, the AC loss of the multi-filamentary tape was substantially dominated by the eddy current loss in the Ag matrix.

A.C. susceptibility and critical current density of Bi-based oxide superconductors

Abstract Polycrystalline oxide superconductors exhibit a complex magnetic susceptibility which is dependent on temperature. For a given applied magnetic field, the imaginary component of the observed susceptibility can be maximized at T ci , the peak temperature. We have investigated the relationship between the dependence of J c on applied D.C. magnetic field H dc and the dependence of T ci on the magnitude of applied A.C. field H ac for samples of silver-sheathed Bi 1−x Pb 0.2 Sr 1 Ca 1+y Cu 1.5 O z superconducting tapes. From the investigation, it was found that T ci was, in all cases, inversely related to the applied A.C. field. However, the degree to which T ci was sensitive to H ac correlated with the degree to which J c decreased with increasing H dc .

Preparation of Superconducting Oxide Films by Sol-Gel Method

For preparing superconducting wires and tapes, the sol-gel method is one of the most useful techniques. We prepared highly-oriented Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O superconducting films using a solution of acetate and propionic acid. The Y-Ba-Cu-O films on Ag substrate were dense and homogeneous and achieved Tc of 88K and J c of 7,500A/cm2 (77K, 0T). The Bi-Sr-Ca-Cu-O films were prepared on MgO substrate. The film including some high-Tc phase and Tc=87K was obtained with the starting composition of 2223.

AC Transport Losses of Ag-Sheathed Bi2223 Monofilamentary and Multifilamentary Tapes

The AC transport losses are investigated at 77 K on the Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox monofilamentary tape and multifilamentary tapes. Despite the main contribution of the hysteresis loss to the AC transport losses, the superconductor subdivision in the multifilamentary tapes causes no notable influence on the AC transport losses. In addition, the absolute values of the losses for all tapes are equivalent in the theoretical results for a round wire with the same critical current value. The loss-density distribution obtained from numerical calculations shows a resemblance between the monofilamentary tape and the multifilamentary tapes, which conforms to the experimental result for the AC transport losses.

AC Losses of Bi2223 Superconducting Tape and Rod

The AC losses of the Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox superconducting wires in the tape or rod forms are evaluated under AC transport current or AC magnetic field at 77K. The losses for the rod measured by two different methods agree with each other and the magnetic field dependence is explained by the Bean model. However, in case of the tape, the losses under AC transport current are larger than those under AC magnetic field by about one order of magnitude. This discrepancy is explained by the difference in the dependence of the losses on aspect ratio between both cases.