A. Nagata

Formation and texture of Bi-2223 phase during sintering in high magnetic fields

Abstract The formation and texture of Bi-2223 phase during sintering in high magnetic fields were investigated. Samples contained in silver crucibles were sintered at different temperatures from 835 to 860 °C for 120 h in a vertical tube furnace installed in a solenoid-type superconducting magnet. A vertical magnetic field, H a of 10 T can be applied parallel to the long axis of the furnace. It has been found that the mixed structures of the Bi-2223 and the Bi-2212 grains with their c -axis parallel to the magnetic field are formed in samples sintered at 850–855 °C in 10 T magnetic field. Below 845 °C sintering in 10 T, a high proportion of Bi-2223 phase is obtained, however no preferred orientation is observed. It is suggested that a liquid phase is important for the formation of the textured structure in magnetic fields.

Texture and formation of (Bi,Pb)-2223 phase after partial-melting and solidification in high magnetic fields

We investigated the texture and formation of (Bi,Pb)-2223 phase after partial-melting and solidification in high magnetic fields. It has been found that the (Bi,Pb)-2212 grains with their c-axis parallel to the magnetic field were formed after the partial-melting and solidification in 8 T magnetic field, and transformed into the (Bi,Pb)-2223 grains with c-axis alignment during the further sintering process at 840/spl deg/C without magnetic field. The conversion (Bi,Pb)-2212 grains to (Bi,Pb)-2223 grains has the heredity in grain alignment.

Analysis of orientation distribution in YBa2Cu3O7−x polycrystals by electron channeling patterns

A unit standard map for YBa2Cu3O7−x superconductors was constructed and indexed from a series of electron channeling patterns (ECPs). (001) pole figures of YBa2Cu3O7−x tapes and bulk specimens with various values of critical current density (Jc) were drawn by using the map covering [001‐110‐110] stereographic quadrant and ECPs taken from individual grains of the specimens. It was found that preferred orientation of (001) formed in the tape with a decreasing thickness of the tape, which raised Jc significantly. It is concluded that the ECP technique is very useful for the analysis of orientation distribution in small‐sized YBa2Cu3O7−x specimens such as tapes or thin films.

Effect of MgO and Ag2O on the microstructure and superconducting properties of the (Bi,Pb)-2223 phase in the partial-melting and sintering process

Abstract The effect of MgO and Ag2O on the microstructure and superconducting properties of the (Bi,Pb)-2223 phase during sintering after partial melting has been investigated. In the Ag/oxide interface layer, a well-aligned Bi-2223 phase was preferentially formed. Both the degree of texturing and the volume fraction of the Bi-2223 phase decreased by increasing the distance from the Ag interface. Whereas, in the MgO/oxide interface layer, although well-aligned Bi-2223 grains existed at the MgO interface, a larger volume fraction of the Bi-2223 phase appeared in the center rather than at the MgO interface. The flux pinning was not clearly improved by the addition of MgO, Ag2O or PtO2 particles. However, MgO addition did not affect the formation rate of (Bi,Pb)-2223 phase, and could suppress the growth of Bi-free non-superconducting secondary phases. Ag2O or PtO2 addition resulted in coarse secondary phases, and slowed the formation rate of the Bi-2223 phase.

Crystal growth of Bi-2201 phase in high magnetic fields

Abstract The crystal growth of Bi-2201 phase during partial-melting and solidification in high magnetic fields was reported. Thin crystals (∼20 μm) with a size of 5–10 mm 2 exist only in the surface layers of samples treated in zero magnetic field. Whereas thick crystals (∼100 μm) exist not only on the surface, but also in the center of samples treated in 8 T magnetic fiel. T c of Bi-2201 phase treated in 0 T is higher than that treated in 8 T. Bi-2201 phase and Bi-2212 phase coexist in the sample treated in 0 T, whereas only Bi-2201 phase exists in the sample treated in 8 T. For the sample treated in 8 T magnetic field, a clear second peak structure was observed in the magnetic hysteresis loop in the direction H m ∥ H a at 5 K, and the width of the hysteresis loop ΔM at 5 K and 0 T in the direction H m ∥ H a is about 10 times as large as that in the direction H m ⊥ H a , indicating that for this sample an obvious anisotropy exists and the Bi-2201 grains align with their c -axes parallel to H a .

Grain alignment and critical current densities of (Bi,Pb)-2223 phase in the partial-melting and sintering process

The grain alignment of the partially melted samples exists only in their surface layers. The Bi-2201 phase with the c-axis alignment, (Sr,Ca)CuO2 and (Sr,Ca)2CuO3 coexist in the surface layers of the air-cooled sample, whereas in the surface layers of the sample sintered at 840 °C for 18 h after being partially melted, the Bi-2212 phase with the c-axis alignment exists as a major phase. After prolonged sintering to 240 h, the c-axis-aligned Bi-2212 phase transforms to the c-axis-aligned Bi-2223 phase. The Bi-2201 phase in the air-cooled sample shows the sharpest and narrowest grain alignment, whereas the Bi-2223 phase in the sample sintered at 840 °C for 240 h exhibits the bluntest and widest alignment. Jc values of tapes which were heat treated by partial melting before wrapping with Ag sheets are larger than those of tapes without partial melting, and the tape which was sintered at 840 °C for 240 h after being partially melted at 875 °C prior to wrapping exhibits the highest Jc value. These results indicate that the partial-melting process is of real importance to the improvement of Jc in the (Bi,Pb)-2223 oxide superconductor.

Microstructure and Critical Current Density of YBa2Cu3O7-x-Ag Composites Compressed at High Temperature

Microstructures of YBa2Cu3O7-x-Ag composites compressed at high temperatures have been investigated, and relationship between the microstructures and superconducting properties is discussed. YBa2Cu3O7-x specimens doped with 20 and 30 w/o Ag2O were well compressed to 55% plastic strain under a low stress level compared to the Ag2O-undoped sample. No microcracks were observed in the central zone of the composites reoxidized at 773 K for 18 h after compression at 1203 K and at 1.3×10-5/s. From this result, it is suggested that uniformly distributed Ag particles play an important role not only in ductile deformation at high temperatures but also in relieving the anisotropic stresses induced by phase transition and reoxidizing heat treatment. The maximum Jc(trans) value of 1400 A/cm2 at 77 K and 0 T was obtained in 20 w/o Ag2O-doped composites. The agglomerations of Ag particles, which were formed during compression of the 30 w/o Ag2O-doped sample, reduced the path of the current flows and eventually decreased Jc(trans).

Effect of post annealing on superconducting properties and microstructures of Bi(2212)/Ag tapes

Bi(2212)/Ag tapes were fabricated by the melt-growth method at various cooling rates in air and Bi atmosphere followed by furnace cooling. Post-annealing at 810/spl deg/C was carried out for improving T/sub c/ and J/sub c/. T/sub c/ of as-solidified tapes was /spl sim/80 K, which increased to /spl sim/90 K after post-annealing. J/sub c/ of as-solidified tapes was scattered even at the same fabrication condition. Post-annealing reduced scattering of J/sub c/ and improved J/sub c/ of the tape with lower J/sub c/ values. However, samples with high J/sub c/ (4.2 K, 0 T) of 6/spl times/10/sup 4/ A/cm/sup 2/ after solidification was little improved by post-annealing.<<ETX>>

Transport critical current in silver sheathed BiPbSrCaCuO tapes

The temperature and magnetic field dependences of transport critical current density, J/sub c/, in silver-sheathed Bi/sub 2.1/Pb/sub 0.4/Sr/sub 2.2/Ca/sub 3.4/0/sub 4.2/ tapes have been studied. J/sub c/ was weakly dependent on magnetic fields at temperatures below 30 K. The effect of field-variation history on J/sub c/ was observed in the same temperature region. The magnetic field dependence of J/sub c/ and its anisotropy with respect to the field direction increased rapidly with increasing temperature. The irreversibility field and its anisotropy were also studied. Flux pinning and weak coupling properties are discussed.<<ETX>>

Microstructure and superconducting properties of Bi-2223/Ag tapes fabricated in high magnetic fields

The microstructure and superconducting properties of Bi-2223/Ag tapes fabricated in high magnetic fields were investigated. The results show that the partialmelting temperature has influence on the formation of Bi-2223 phase and the Jc. The optimum partial-melting temperature is 855°C. The magnetic field during sintering at 830°C has no influence on the formation of Bi-2223 phase and the Jc. However, the magnetic field during partial-melting process has obvious influence on the formation of Bi-2223 phase and the Jc. The tape partially melted with 10 T magnetic field shows stronger c-axis alignment of Bi-2223 phase and higher Jcvalue than that partially melted without magnetic field.