R. Ogawa

Fabrication of superconducting joints for Bi-2212 pancake coils

To evaluate the possibility of persistent current operation for NMR/MRI using a Bi/sub 2/Sr/sub 2/CaCuO/sub 8/ (Bi-2212) superconductor at 4.2 K, the temperature dependence of magnetic relaxation of the single crystal has been measured. It was found that the operating current must be one-fourth of J/sub c/ at 4.2 K to satisfy the stability condition for NMR/MRI operation ( Delta J/J less than 0.005 ppm/h). A superconducting coil made of Bi-2212 silver-sheathed wire was fabricated by the partial melt growth process. The authors were successful in generating 1.60 T in a zero external magnetic field at 4.2 K with a coil made of silver-sheathed Bi-2212 tape which was 100- mu m thick, 10-mm wide and 150-m long. Superconducting jointed pancake coils with critical current reaching 80 A in an external magnetic field of 1.0 T at 4.2 K were also fabricated.<<ETX>>

Production of YBa2Cu3O7-x Thick Films on Ag Metal Substrate Controlled by Oxygen

YBa2Cu3O7-x superconducting thick films were formed on Ag substrate using the sol prepared from pertinent metal alkoxides. The firing conditions which cause incongruent melting and a peritectic reaction in the film were investigated. We call this the IMPERCO (incongruent melting and peritectic reaction controlled by oxygen) method. It has been found that films which undergo both reactions have excellent superconducting properties. Accordingly, the critical current density (Jc) of 1.18×104 A/cm2 at 77 K and zero magnetic field has been obtained. Roles of the boundary of the film and Ag substrate were also discussed.

Critical fluctuations in the magnetization of Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8] near the [ital H][sub [ital c]2]([ital T]) line

The magnetization measurements on Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8] single crystals with [ital H] parallel to the [ital c] axis are presented to show that the critical fluctuations in the magnetization near the [ital H][sub [ital c]2]([ital T]) line can be well studied within the two-dimensional nonperturbative scaling theory developed by Te[hacek s]anovic [ital et] [ital al]. By using this scaling theory and basic thermodynamic arguments, we obtained the Ginzburg-Landau parameter [kappa] ([congruent]115) and [ital dH][sub [ital c]2]/[ital dT] ([congruent][minus]2.7 T/K) near [ital T][sub [ital c]] for this material. In addition, we found that these derived parameters are in good agreement with those determined from the study of reversible magnetization in an intermediate field at low temperature.

Structural and superconducting properties of oxygenated La2−χMχCuO4+y (M=Sr, Ba; O≦χ<0.1)

Abstract Oxygen stoichiometry, superconductivity and structural properties of La 2−χ M χ CuO 4+ y (M=Sr, Ba; 0≦χ P O 2 =360 atm at 600°C were studied by TG-DTA, electrical resistivity, magnetic susceptibility and X-ray diffraction measurements. All oxygenated samples showed superconducivity. The phase separation into two phases with different orthorhombic strain was observed in the samples with 0≦χ≦0.03 under ambient atmosphere, and the phase with the smaller orthorhombic strain was the superconducting one. The superconductivity and the phase separation in this system appear to be related to the average valence of Cu ions and the structural properties, respectively, which are affected by both oxygen nonstoichiometry and substitution.

Comparative study of Jc‐H characteristics for silver‐sheathed superconducting Bi(2:2:1:2) and Bi(2:2:2:3) tapes

Critical current densities Jc of silver sheathed Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10 composite tapes fabricated by a partial melt and a powder‐in‐tube process, respectively, were measured at 4.2, 27, and 64 K as a function of applied magnetic field and the angle between the tape face and the direction of applied magnetic field. These measurements indicate that (1) the fraction of the grain boundaries, which are strongly coupled, are greater in the Bi(2:2:1:2)/Ag tapes than in the Bi(2:2:2:3)/Ag tapes; (2) the alignment of the Bi cuprate platelets with the c‐axis perpendicular to the tape face is significantly greater for the Bi(2:2:1:2) than for the Bi(2:2:2:3)/Ag tapes; and (3) the E‐J curves for both kinds of the tapes are described well by an expression, E∼exp[−(J0/J)μ], where μ and J0 are constants below their transition magnetic fields H*g.

Improvement of J/sub c/ in Bi-2212-Ag composite superconductor by controlling Po/sub 2/ on the partial melt process

In a partial melt process for Bi-2212-Ag composite tapes, round and rectangular shape superconductors, the effects of oxygen partial pressure (Po/sub 2/) on the critical current density (J/sub c/) have been studied. It was found that the J/sub c/ of Bi-2212-Ag composite tapes increased from 4.5/spl times/10/sup 4/ A/cm/sup 2/ to 2.2/spl times/10/sup 5/ A/cm/sup 2/ at 4.2 K in zero magnetic field with increasing Po/sub 2/ from 0.1 bar to 1.0 bar. SEM observation for the cross sectional image and J/sub c/ measurement revealed that the partial melt process under higher Po/sub 2/ was effective in realizing well-aligned structure along the silver wall. These results were explained by the idea that crystallization of the Bi-2212 system starts at the interface between the liquid phase and the oxygen rich gas phase. In order to prove the potentiality of this process for the rectangular shape conductors, fabrication of a solenoid magnet has been attempted using rectangular shape conductors under high oxygen partial pressure. The typical results of this solenoid magnet, 2.7/spl times/10/sup 4/ A/cm/sup 2/ in J/sub c/ with generating magnetic field 0.70 T, and 4.3/spl times/10/sup 4/ A/cm/sup 2/ in quenching current density with generating magnetic field 1.13 T have been observed at 4.2 K in the self magnetic field.<<ETX>>

Preparation of YBa2Cu3O7−x (YBCO) superconductive film using sol-gel method

Using alkoxides of Y and Ba and Cu-acetate as raw materials, Y Ba2Cu3O7-x (YBCO) superconductive films were produced through the sol-gel method. When the nonaqueous mixed solution was hydrolyzed under an appropriate condition, a pure and homogeneous YBCO powder could be prepared. Controlling the metal concentration and the temperature of the sol, precursor(gel) films were formed on YSZ substrate by dip-coating. And heating the films in the range of 200°-950°C, changes of the films morphologies, the generated phases and the film reactions with the substrates were studied using scanning electron microscopy and X-ray diffraction. It has been known that to improve the superconductivity of the film it is necessary to sinter it at as high as 950°C and increase its thickness over about 3 µm after sintering. A superconductive film with Tc.off=75 K was obtained by adopting these conditions.

Fabrication of silver-sheathed Bi(2:2:1:2) superconducting magnet by means of partial melt and slow cooling process

Abstract Fabrication of silver-sheathed Bi(2:2:1:2) superconducting coil has been attempted by means of the partial melt and slow cooling process. In order to establish the fabrication process of long length Bi (2:2:1:2) Ag tape with high Jc, the nominal compositions and heat-treating conditions have been optimized. d.c. and a.c. current operation around 20 K has been carried out using GM-cryo-cooler, a.c. loss characteristics have been discussed. The large scale magnet (81/185 mm in inner/outer diameter and 218 mm long) succeeded in generating magnetic field of 0.68 T at the center part of room temperature bore of 45 mm in diameter at 20 K. In order to obtain the guide for material selection between these materials for the cryogen-free application, a comparative study of Jc-B characteristics for Bi(2:2:1:2) and Bi(2:2:3) tapes has been done at 4.2 and 27 K. Detailed measurements of V-I characteristics have been performed at 27 and 64 K as a function of applied magnetic field for Bi (2:2:1:2) Ag and Bi (2:2:2:3) Ag . The E−J curves are well described by an expression, E ∼ exp [ − ( J 0 J ) μ ] below their transition magnetic fields Hg, rather than the commonly used power law, i.e. E ∼ Jn, where μ, J0 and n are constants. For realizing the cryogen-free medium field magnet (e.g. H ∼ 10 T), Bi (2:2:1:2) Ag will be useful only below 20 K and Bi (2:2:1:2) Ag will be feasible at temperature range from 20 to 30 K.

Operation of superconductively jointed Bi-2212 solenoidal coil in persistent current mode

Abstract A Bi 2 Sr 2 Ca 1 Cu 2 O x superconducting solenoidal coil with a persistent current switch (PCS) has been newly designed and fabricated, for the first time as a superconductively jointed oxide superconducting coil. The operation behaviour of the solenoidal coil at 4.2 K has been evaluated when the PCS is off or on. When the PCS was off, the solenoidal coil worked well, generating magnetic fields of over 95% of the designed value. After the PCS was turned on, a magnetic field of about 100 gauss was continuously generated for 100 h and then the PCS was turned off. The residual resistance of the entire circuit was estimated to be below 1.6 × 10 −11 Ω from the last 1900 min (= 31.7 h) of operation. This resistance value reaches the practical level of commercial metallic superconducting magnets used for magnetic resonance spectroscopy, being operated in the persistent current mode. This work has proved that the Bi-2212 solenoidal coil has great potential for practical use with persistent current mode operation.

Preparation of Bi based high- T c superconductors containing Pb and Sb by the sol-gel method

Preparation of Bi based high- T c superconductors having Pb and Sb elements has been attempted using the sol-gel method. Production of the high- T c phase was promoted by doping a little Sb (0.05 mol) together with Pb. The nominal composition of the system is Bi 0.96 Pb 0.19 Sb 0.05 Sr 1.0 Ca 1.1 Cu 1.6 O y . It has been found that the reaction scheme of Sb doped samples is different from that of Sb nondoped samples.