Takemi Muroga

Enhancement of critical current density for Bi-2212/Ag tape conductors through microstructure control

Abstract We investigated the microstructures of Bi2Sr2CaCu2Oy (Bi-2212) at the free-surface and at the interface with silver substrate. The alignment of Bi-2212 grains near the silver substrate was better than that near the free-surface. Based on this result, we fabricated tape conductors having various cross-sectional configurations. Jc increased with increasing oxide/Ag interface area and with decreasing oxide layer thickness. However, Jc was almost independent of the area of the free-surface. These results suggest that most of the superconducting current flows locally near the silver substrate. Furthermore, we found that the alignment of Bi-2212 grains, and hence, Jc can be improved by decreasing cooling speed during the heat treatment. The application of cold-rolling to the tape before the heat treatment is also effective in improving the microstructure and Jc of Bi-2212/Ag tape. About 2 times larger Ic and Jc were obtained by ∼50% reduction of the oxide layer. The highest Ic and Jc obtained in this experiment were Ic=450 A and Jc=3.5×105 A/cm2 (in 10 T, at 4.2 K), respectively.

Study of a bi-system superconducting wire with an oxide barrier using the strand and formed method

We present the strand and formed method of manufacturing Ag-sheathed Bi-system superconducting round wire. The method strands three round multifilamentary wires into a single round wire. For the strand and formed Bi2Sr2Ca1Cu2Ox (Bi-2212) wire, the critical current Ic exhibited almost no change until the bending strain exceeded 0.6%, while for the conventional round Bi-2212 wire Ic deteriorated when the bending strain exceeded 0.2%. Also, we have measured the alternating-current transport loss of both a strand formed round Bi-Pb2Sr2Ca2Cu3Ox (Bi-2223) wire with a barrier and a conventional tape Bi-2223 wire without a barrier. The measured loss normalized by the Ic of the former was more reduced than that of the latter.

Effect of Ag substrate on the microstructure and Jc of Bi-2212/Ag tapes prepared by a dip-coating method

We investigated the effect of silver substrate on the microstructure and Jc of Bi-2212/Ag tapes prepared by applying the melt-solidification. X-ray diffraction analysis indicated that the grain alignment of Bi-2212 near the silver substrate was better than that at the free-surface. We fabricated tape conductors having various oxide/silver interface areas and free-surface areas. Jc increased with increasing oxide/silver interface area and with decreasing the Bi-2212 layer thickness. However, Jc was almost independent of the Bi-2212 free-surface area. These results suggest that the most of the superconducting current flows at the local Bi-2212 area near the silver substrate. Cold-rolling before the heat treatment is also effective to improve the microstructure of Bi-2212. In this experiment, the highest Jc of 4.0 × 105A/cm2(in 10T, at 4.2K) was obtained.

Superconducting Joints of Bi-2212/Ag Tapes Prepared by Dip-Coating Method

We found that the alignment of Bi-2212 grains near the Ag substrate was better than that at the free-surface of Bi-2212 layer for dip-coated Bi-2212/Ag tapes. Therefore, we fabricated tapes without free-surface of Bi2212 layer by folding the dip-coated tapes along the long direction. The Bi-2212 layers sandwiched with Ag tapes had excellent critical current Ic with small Ic variation between the samples. Then, we made superconducting joints using the tapes. We developed a technique to obtain good grain alignment at the joint. As a result, we attained superconducting joints having Ic almost equal to Ic of the tapes with small Ic variation from joint to joint.