Jun-ichi Shimoyama

Effect of high‐energy ion irradiation and electron irradiation on textured Bi2Sr2CaCu2Ox—180‐MeV Cu11+ and Br11+ irradiations and 3‐MeV electron irradiation

180‐MeV Cu11+ and Br11+ irradiations and 3‐MeV electron irradiation were carried out on textured Bi‐2212 tapes and the effects of irradiation on the microstructure and superconducting properties were investigated. Ion irradiation, which creates linear tracks, is very effective in increasing both intragrain Jc and irreversibility field at temperatures below 60 K when field is parallel to the tracks. At 60 K, an order of magnitude higher Birr is obtained for both Cu11+ and Br11+ irradiations. Electron irradiation which creates point defects, on the other hand, is not as effective as ion irradiation except for low temperatures of ∼4.2 K. Apparent pinning potential energy U0 was increased by ion irradiation from ∼15 to ∼40 meV at 41.8 K, while U0 was unchanged by the electron irradiation. However, even U0 of the ion‐irradiated sample is still small. This small U0 increases flux creep at temperatures above 60 K and makes ion irradiation less effective. A small increase of transport Jc was obtained by both ion ...

Processing and fabrication of Bi2Sr2CaCu2Ov/Ag tapes and small scale coils

Abstract Optimipation of heat treatment conditions for a Bi2Sr2CaCu2Op(Bi-2212)/Ag melt-solidified tape was carried out. Effects of the maximum temperature for partial melting, cooling rate in solidifying process, cooling rate after the melt-solidification and partial pressure of oxygen on Jc were examined. A tape prepared under optimized conditions showed a high Jc value of 2.4 × 105 A/cm2 at 4.2 K in 12 T. Small pancake coils were fabricated using Bi-2212/Ag long tapes prepared by a continuous dip-coating method. The generated field of a coil was higher than 1 T at 4.2 K in bias fields less than 4 T and its critical current remained greater than 200 A even at 20 K under zero bias field. This means that the Bi-2212/Ag coil has sufficient potential for practical use as a superconductng magnet at 20 K.

Fabrication of Flexible Ribbons of High-Tc Bi(Pb)-Sr-Ca-Cu-O Superconductors

Thin ribbons (30 µm thick) of high-Tc Bi(Pb)-Sr-Ca-Cu-O superconductor have been successfully prepared by the combined process of doctor blade casting, cold rolling and sintering. A zero-resistance state has been achieved at 107 K. The highest Jc obtained so far is about 1850 A/cm2 at 77 K and in zero magnetic field. The ribbon has sufficient flexibility to be bent to about a 30 mm diameter even after the final sintering.

Oxygen nonstoichiometry and phase instability of Bi2Sr2CaCu2O8+δ

Abstract Oxygen nonstoichiometry, δ , of Bi 2 Sr 2 CaCu 2 O 8+δ was determined by thermogravimetric measurement as functions of temperature and partial pressure of oxygen (PO 2 ). Dependencies of thermodynamic quantities, lattice constant and critical temperature on δ show anomalous behaviors at same oxygen content of δ =0.235. A quite sluggish and a large amount of weight increase observed at about 600°C under PO 2 =1 atm was found to correspond to the decomposition of Bi 2 Sr 2 CaCu 2 O 8+δ phase to BiSrCaO, Bi 2 Sr 2 CuO y and CuO. This suggests that the Bi 2 Sr 2 CaCu 2 O 8+δ phase is unstable at low temperature under high PO 2 .

Preparation of Ag-I Intercalated Bi2Sr2CaCu2Oy Superconductor

We have found that silver and iodine can be intercalated simultaneously into a Bi2Sr2CaCu2Oy superconductor. By this intercalation, the lattice parameter along the c-axis was significantly expanded from 30.9 to 45.6(=22.8×2) A. From X-ray diffraction analysis, it was assumed that silver and iodine atoms are inserted between double Bi-O layers in the stacking sequence of Bi-I-Ag-I-Bi, and that the stoichiometric composition is AgI2Bi2Sr2CaCu2Oy. Tc(onset) of this intercalated new material is 60.7 K, which is comparable to that of the stage-1 IBi2Sr2CaCu2Oy obtained through the same heat treatment.

Fabrication of Bi2Sr2CaCu2O8 Tapes and Coils

Textured Bi2Sr2CaCu2O8/Ag composite tapes were fabricated by both doctor blade process and dip-coating process applying partial melting and subsequent slow cooling. Texturing significantly improved current transfer across the grain boundaries and increased transport Jc. At 4.2K, Jc values well above 105A/cm2 were obtained up to 25T for the doctor blade processed tapes. Small coils with 64 turns were fabricated using the tapes prepared by the continuous dip-coating process. 6000Gauss was generated at 12T and 4.2K by this coil, which is one of the highest magnetic fields achieved by the coils made by high-Tc oxide superconductors.

Dependence of critical temperature and critical current of oriented Bi2Sr2CaCu2O8+ delta thick films on the oxygen content

Thermogravimetric measurement of Bi2Sr2CaCu2O8+ delta was carried out as functions of temperature and partial pressure of oxygen (PO2). Dependencies of thermodynamic quantities, lattice constant and critical temperature (Tc) on oxygen nonstoichiometry, delta , show anomalous behaviors at the same delta of 0.235. Furthermore, Bi2Sr2CaCu2O8+ delta phase is found to decompose to Bi-Sr-Ca-O, Bi2Sr2CuOy and other phases at low temperature under high PO2. Effects of oxygen nonstoichiometry and the phase decomposition on critical current density (Jc) of Bi-2212/Ag melt-solidified thick films were also studied.

Flux Pinning Characteristics in 180 MeV Cu11+-Irradiated Bi2Sr2CaCu2Ox

We investigated the effect of Cu11+ irradiation on the flux pinning in c-axis grain-oriented Bi2Sr2CaCu2Ox by means of the d.c. magnetization method. Irradiation significantly enhanced both hysteresis of d.c. magnetization and the irreversibility field at temperatures up to 60 K. Flux creep was suppressed by the irradiation. However, the apparent pinning potential energy U0 of irradiated samples was still small. The U0 values of irradiated and nonirradiated samples were 14 meV and 8 meV in 4 T at 4.2 K and 40 meV and 15 meV in 0.2 T at 41.8 K, respectively.

Properties of Bi2Sr2CaCu2Oy/Ag Composite Tapes and Coils Melt-Solidified with Bi2Al4O9

We have fabricated Bi2Sr2CaCu2Oy(Bi-2212)/Ag composite tapes and small scale pancake coils by applying melt-solidifying method. The Bi2Al4O9 compound was found to be an effective vapor source of bismuth oxide for suppressing its vaporization from Bi-2212 layer during the high-temperature heat treatment resulting in the improvement of the J c homogeneity of tapes and coils. Short Bi-2212/Ag tapes showed high J c values above 2×105A/cm2 at 4.2K in 10T. A double stacked pancake coil using Bi-2212/Ag long tapes generated 1.64T and 1.25T under bias fields of 0T and 6T, respectively, at 4.2K.

Bi-System Oxide Superconducting Tapes Based on Silver-Alloy for Power Current Leads

To investigate stabilized materials in contact with oxide superconductors for current lead, the electrical resistivity and thermal conductivity of Ag-alloy samples were measured. Measured values obtained for the Ag-Au alloys showed a strong influence by Au content at low temperature, while those for Ag-Cu alloys showed a weakly depends on Cu content. Superconducting tapes of Bi-2212 and Bi-2223 stabilized with the alloys mentioned above were prepared. The critical current density Jc of the tapes were measured by the four-probe method. A strong dependence of Jc degradation on Cu content was recognized, but, no influences by Au could be observed. Values q(Heat input per critical current at low temperature end 4.2K) were calculated to evaluate the materials as for current lead use. The Ag-Au alloy sheathed superconducting tapes can impede heat flow with a q values smaller by 2 digits than the copper leads even though a magnetic field is present around 77K.