K. Togano

Bi2Sr2CaCu2Ox/Ag multilayer tapes with Jc>500 000 A/cm2 at 4.2 K and 10 T by using pre-annealing and intermediate rolling process

Abstract The high transport critical current density ( J c )>500u2008000 A/cm 2 at 4.2 K, 10 T (critical current>700 A) is obtained in the Bi-2212/Ag multilayer tape fabricated by using the PAIR ( P re- A nnealing and I ntermediate R olling) and melt–solidification process. The combination of the improvement in geometric configuration of the tape and PAIR process make it possible to obtain the most preferable microstructure and enormous J c . Effect of processing and geometry of the tape on J c and Bi-2212 grain alignment has been studied. The enormous transport J c is attributed to highly textured grain alignment and uniformity in the tapes, which are achieved by PAIR process. After post annealing in flowing argon at 450°C for 4 h, transition temperature T c is increased from 83 K to 92 K and transport J c >25u2008000 A/cm 2 at 77 K is obtained. PAIR process is applicable for fabricating long Bi-2212 tapes with large transport J c for practical applications.

Effects of PAIR (pre-annealing and intermediate rolling) process on superconducting properties of Bi2Sr2CaCu2Ox/Ag multilayer tapes

Abstract PAIR process, which is the combination of pre-annealing (PA) and intermediate rolling (IR) processes and is performed prior to melt-solidification, enhances J c of the Bi-2212/Ag composite tapes. The effects of pre-annealing and intermediate rolling process on microstructure and J c of the Bi-2212/Ag multilayer tapes fabricated by the oxide-painting method have been studied. The results show that J c has been increased strongly by performing pre-annealing at 800–880°C and intermediate rolling with >25% deformation. Transport J c of 3.3×10 5 A/cm 2 in magnetic field (4.2 K, 10 T) is obtained. The enhancement of J c is assigned to the microstructure with high grain alignment and uniformity, as well as the improvement of grain coupling by the PAIR process.

The effect of Ag on the formation of Bi2Sr2Ca1Cu2Oy thick film

Abstract We studied the effects of Ag on the melting process, the solidification process and the microstructure of Bi 2 Sr 2 Ca 1 Cu 2 O y (2212). Samples were 2212 thick films prepared on Ag substrate and on MgO substrate prepared by the pyrolysis technique of organic acid salts. Addition of Ag-octylate or Ag powder to the 2212 layer was also tried for these samples. Ag enhanced the decomposition of the 2212 phase in the heating process, no matter how Ag was used. Ag substrate or Ag powder were more effective on the decomposition than Ag-octylate. The solidification temperature of 2212 was lowered by Ag substrate or by addition of Ag. The lower the solidification is, the faster the rate of solidification becomes. However, in a 2212 layer prepared on MgO substrate, many small swellings were observed and texturing was hindered at the swellings. A silver substrate was most effective to produce highly textured 2212 microstructures.

Formation and growth mechanisms of Bi2Sr2CaCu2Oy on an Ag substrate in the partial melting method

We investigated the phase changes of a Bi-Sr-Ca-Cu-O/Ag composite tape during cooling from a partially molten state using the high-temperature X-ray-diffraction and oil-quenching techniques. In the cooling process, (SrCa)CuO2 and Bi(SrCa)2Oy appeared at the areas where the (SrCa)2CuO3 phase disappeared. At around 860 degrees C, these solid phases melted into the liquid phase, and then Bi2Sr2CaCu2Oy (Bi-2212) grew rapidly at the surface. First, nucleation of Bi-2212 occurred at the surface of the liquid phase. With decreasing temperature the Bi-2212 crystal growth proceeded slowly from the surface to the inner area with the a-b plane almost parallel to the surface. During the growth of Bi-2212, excess Bi was ejected from Bi-2212 and a thin Bi-enriched layer was formed at the growth front of Bi-2212.

Optimization of melt-processing temperature and period to improve critical current density of Bi2Sr2CaCu2Ox/Ag multilayer tapes

Abstract We have studied the effect of melt-processing temperature and period on microstructure and critical current density ( J c ) of Bi-2212/Ag multilayer tapes. High J c values of 4.0×10 5 A/cm 2 with the dispersion J c values by using the melt process with a wider and lower temperature range for a longer period than the conventional melt process. Excellent reproducibility in J c value and wide range of optimal processing parameters are expected to lead to more efficient production of long tapes with high uniformity. It is applicable for fabricating large Bi-2212/Ag coils and/or magnets with high quality for practical applications.

Hg doping effects on synthesis, microstructure and superconducting properties of the Bi-2212 phase

Abstract We studied the effect of Hg addition to a Bi-2212 superconducting bulk materials. The mixture of Bi–Sr–Ca–Cu–O precursor and HgO was heat treated in an evacuated quartz tube. Hg was not incorporated into the Bi-2212 phase and no Hg compound was formed. The high superconducting transition temperature Tc of 97 K was observed for the starting composition of Bi(1.4)Hg(0.6)Sr(2)Ca(1)Cu(2) sample sintered at 840°C for 24 h. Melting point of Bi-2212 phase was lowered by the deviation from stoichiometric composition, and the grain growth was promoted. At low temperature (5 K), magnetic hysteresis (ΔM) of Hg-doped sample is larger than that of the un-doped sample. This can be explained by the small pinning centers such as point defects associated with nonstoichiometric composition.

BiSrCaCuO/Ag superconducting composite tapes

Bi2Sr2Ca1Cu2Ox/Ag composite tapes were prepared by the combined process of doctor blade casting and crystal growth from the partially molten state. A highly textured microstructure was obtained for oxide layer thicknesses of 15 mu m or less by solidifying from 890 degrees C The textured tapes have excellent transport Jc values greater than 105 A/cm2 at 4.2 K in magnetic fields above 25 T. The results indicate that textured Bi-based tapes are promising candidates for high field superconducting magnets.

Development of multilayer Bi2Sr2CaCu2Ox superconducting tapes

High critical density (Jc ) over 500 kA cm-2 (at 4.2 K and 10 T) was obtained in Bi2 Sr2 CaCu2 Ox multilayer superconducting tapes processed by the pre-annealing and intermediate rolling (PAIR) technique. The PAIR process was effective in creating a dense core with lower carbon content, resulting in homogeneous chemical reactions, small amount of impurity phases and good alignment of superconducting grains. In production of long tapes, reducing the carbon content was the most serious challenge. A Jc value of 7.1 ×105 A cm-2 at 4.2 K in self-field was obtained in a 100 m long tape when the carbon content could be controlled to be 0.002 wt%. We fabricated a double-pancake coil using a 100 m long tape and tested it at 4.2 K in backup fields. The coil generated 2 T in a backup field of 4 T.

Relationship between Bi2Sr2CaCu2Ox layer thickness and Jc enhancement by pre-annealing and intermediate rolling process

Abstract Pre-annealing and intermediate rolling (PAIR) process enables one to fabricate Bi 2 Sr 2 CaCu 2 O x (Bi-2212)/Ag tapes with high transport J c >5.0×10 5 A/cm 2 at 4.2 K, 10 T. We study the relationship between thickness of Bi-2212 layer and J c enhancement by PAIR process for monolayer Bi-2212/Ag tapes. J c enhancement by PAIR process is more significant for thinner Bi-2212 layer. For Bi-2212 layer less than 10 μm thick, oxide J c (4.2 K, 10 T) around 4.0 and 1.6×10 5 A/cm 2 are obtained for PAIR and no-PAIR samples, respectively, however, a large variation of J c is seen for both series of samples in this thickness range. Good reproducibility of J c is seen for the samples with the Bi-2212 layer around 20 μm with oxide J c (4.2 K, 10 T) around 3.2×10 5 A/cm 2 . PAIR samples show smaller J c variation than no-PAIR samples. J c enhancement by PAIR process diminishes with increasing Bi-2212 layer thickness and disappears at the thickness around 50 μm. PAIR process produces Bi-2212 layer with small variation of thickness (2–8% along the tape length and 2–19% across it), however, large variation of thickness (4–24% along the tape length and 30–80% across it) remains in no-PAIR samples. PAIR process has a large effect to improve the macroscopic homogeneity of Bi-2212 layer. This is supposed to be one of the reasons for the large J c enhancement achieved by PAIR process.

Fabrication of Bi2Sr2Ca1Cu2Ox superconductor using metallic substrate tape clad with Ag

Abstract With the intentions of the reduction of material cost and the improvement of mechanical properties, Ag/Ni and Ag/Stainless Steel (SUS) clad tapes were prepared as substrate materials for the fabrication of Bi2Sr2Ca1Cu2Ox (Bi-2212) superconducting tape. Bi-2212 formed on the Ag/Ni clad tape with a 10 μm Ag layer thickness showed good superconducting properties of Tc=88 K and Jc (4.2 K, 10 T)=1.7×105 A/cm2, comparable to those of Bi-2212 tape fabricated using pure Ag tape. However, Bi-2212 formed on the Ag/Ni clad tape with a 5 μm Ag layer thickness showed deteriorated properties due to the transfer of Cu and Ni across the Ag layer. The Ag/SUS clad tape did not work as a substrate material, because the Ag layer was eroded, forming many holes, and hence could not prevent a severe reaction between Bi-2212 and SUS. The Jc of the Bi-2212/Ag/Ni tape started to degrade at e=0.2–0.3% in the bending test. A tensile test indicated that the Ag/Ni clad tape had about three times larger yield strength than pure Ag tape. From these results, it is expected that the Bi-2212/Ag/Ni tape has a much improved stress tolerance over the conventional Bi-2212/Ag tape.