Toshio Egi

High critical‐current density of Nd(Ba,Nd)2Cu3O7−δ single crystals

We have studied the superconducting properties of Nd(Ba1−xNdx)2Cu3O7−δ (Nd123, x≊0.1) single crystals grown by the traveling‐solvent floating‐zone method under 0.1% O2 in Ar atmosphere. An anomalous peak effect in the magnetization hysteresis (M–B) loop is observed in the Nd123 single crystals as well as in the Nd123 bulk crystals prepared by the oxygen‐controlled melt growth (OCMG) method. The critical current density (Jc) of the Nd123 single crystals is 70 600 A/cm2 in 1.0 T at 77 K for the applied field perpendicular to the a‐b plane. Uniform flux density distribution with the same shape as the sample is observed in the field perpendicular to the a‐b plane by the magneto‐optical flux‐density observation. Finely dispersed white regions in the dark‐field image due to Nd substitutions for Ba are observed in the Nd123 matrix by the transmission electron microscopy. The high Jc value of the Nd123 single crystals in the applied field is explained by the field‐induced pinning centers caused by the Nd–Ba subst...

Field-induced pinning effect of Nd(Ba1−xNdx)2Cu3O7−δ single crystals grown by the traveling-solvent floating-zone method

We have studied the superconducting properties of Nd(Ba1−xNdx)2Cu3O7−δ (Nd123, x ≈ 0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O2 in Ar atmosphere. The enhancement of the magnetization with increasing field is observed in the hysteresis (M-H) loop in fields both parallel and perpendicular to the c-axis of the Nd123 single crystals as well as in the bulk crystals prepared by the oxygen-controlled-melt-growth (OCMG) method. The composition variation of Ba/Nd is observed in the matrix of Nd123 crystals by an analytical TEM equipped with a cold field-emission gun. It turns out that the enhancement is due to the field-induced pinning effect ascribed to the weak superconducting NdBa substitution regions in the Nd123 matrix.

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>>

Segregation and dissolution reactions of the 2223 phase in the Bi, Pb-Sr-Ca-Cu-O system on annealing in air

Abstract Thermal behavior of the Pb-substituted Bi-2223 phase was examined by characterizing samples annealed at 500°C to 850°C in air and also those subsequently reheated at 850°C in air by XRD, SEM and magnetic measurements. Very interesting reactions occurred, which were segregation of precipitates on annealing below 830°C and dissolution of the precipitates on subsequent reheating at 850°C into the 2223 phase. The precipitates were identified as Sr 2.5 Bi 0.5 Pb 3 Ca 2 CuO 2 . The precipitation and the dissolution were clearly confirmed by morphological observations of a fixed portion of a sample before and after heat treatments. These results strongly suggest that the 2223 phase changes its composition reversibly between the relatively Pb-rich composition stabilized above 850°C and relatively Pb-poor ones stabilized in essentially the same structure below 830°C. However, T c remained almost constant at 113 K.

Investigation of Heat-Treating Conditions for Silver-Sheathed Bi2212 Superconducting Coils

Fabrication of silver-sheathed Bi-2212 superconducting coil has been attempted by means of the partial melt growth process. Silver-sheathed Bi-2212 tapes have been cooled down slowly from the temperature range of 875-890°C to 850°C at the rate of 6°C/h. The characteristics of Jc-H, AC magnetic susceptibility and X-ray diffraction pattern have been measured, and the SEM image of the cross section has been observed. The pancake-type test coil has been fabricated using an optimized heat-treating condition. This coil generates 0.22 T in the external magnetic field of 0 T and 0.07 T in the external magnetic field of 6.64 T at 4.2 K.

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.

A.c. losses in Bi-2212 superconducting magnet at 20 K

Abstract A Bi-2212 oxide superconducting magnet has been cooled down to temperatures around 20 K employing a Gifford-McMahon (G-M) cryocooler. An a.c. magnetic field with a peak value of 0.44T is stably generated at frequencies up to 0.25Hz (0T → 0.44T → 0T, 4s per cycle) at ≈ 20K. The a.c. loss characteristics arising in the magnet are presented. The expected potential of the magnet for a.c. operation with a higher peak magnetic field is also discussed in terms of a.c. loss behaviour.

Magneto‐optical and high resolution electron microscopy studies of flux pinning at grain boundaries in Bi2Sr2CaCu2Ox crystals

We have studied the flux pinning behavior of Bi2Sr2CaCu2Ox multicrystalline samples using a magneto‐optical flux observation method. When a magnetic field is applied parallel to the ab plane, we find low flux density regions at the intersections of small angle grain boundaries. The flux density distribution can be explained by considering both flux pinning at the small angle grain boundaries and the intrinsic pinning effect at the BiO layers. High resolution electron microscopy observations show that amorphous patches with columnar structure exist at the small angle grain boundaries (<20°), while other defects such as misfit dislocations and distorted microstructures appear at general grain boundaries. The fact that flux pinning only occurs at small angle grain boundaries (<20°) indicates that the columnar amorphous structures could act as flux pinning centers.

Characterization of a Twin-Free Orthorhombic YBa2Cu3Ox Crystal

Characterization of a twin-free orthorhombic YBa2Cu3Ox crystal is described in this paper. A tetragonal single crystal was grown by a crystal pulling method and the crystal was annealed in an oxygen atmosphere under a uni axial compression in order to prevent the twin formation. The resultant crystal had a twin-free structure. The onset T c of the crystal was about 91.8 K and the J c value was one fifth as small as that of the twinned crystal at 1 T.

Operation of Bi-2212 Superconducting Magnet with a R.T. Bore of 4 cm in Diameter

Bi-2212 superconducting magnet with a room temperature (R.T.) bore of 45 mm in diameter has been fabricated. The magnet has been cooled by a GM-cryocooler, and it has generated magnetic field up to 0.68 T. In order to prepare a oxide superconducting magnet with such a large R.T. bore, reproducibility in critical current density (Jc) of Bi-2212 silver-sheathed tape must be realized in long rage such as 1500 m. This problem has been solved by extending calcination time of Bi-2212 powder.