Hiromi Takei

Development of in-plane aligned YBCO tapes fabricated by inclined substrate deposition

Abstract High temperature superconducting thin film tapes, which consist of a flexible metal substrate, buffer layer, and Y 1 Ba 2 Cu 3 O x (YBCO) layer have been developed for power application. Ni-alloy tape was used as the substrate. Both the YBCO layer and the buffer layer were formed by pulsed laser deposition (PLD) because of an advantageous of high deposition rate. Inclined substrate deposition (ISD) was developed to introduce in-plane alignment for the buffer layer on the non-textured metal substrate. YBCO films epitaxially deposited on the ISD buffer layer by normal PLD had high J c values over 10 5 A/cm 2 at 77 K. For verification of high rate productivity of the PLD/ISD method, we demonstrated high rate deposition of the yttria stabilized zirconia buffer layer and YBCO superconducting layer. We also performed continuous deposition of over 10 m YBCO thin film tapes with a reel-to-reel substrate transfer system combined with 200 W high power laser equipment.

Development of Ag-sheathed Bi-2223 superconducting wires and their applications

Silver-sheathed multifilamentary BiPbSrCaCuO 2223 superconducting wires with long length and high Jc of over 3/spl times/10/sup 4/ A/cm (77.3 K, 0 T) were developed by using the powder-in-tube method. For future industrial applications, high Jc with high Ic wires, high strength wires and low AC loss wires were also developed for large scale magnet applications and electric apparatus for AC use. In the case of AC conductor, twist effect was investigated in order to reduce the wire AC loss, PVF coating technology was applied to high strength wire. PVF coating technology made it possible to obtain amperage parallel conductor with low AC loss, because each strand is electrically insulated. As a progress of technology, we could fabricate many application prototypes. In the high amperage conductor application, current leads and cable conductors were developed. In the magnet application, pancake magnets cooled by GM refrigerator and operated at around 20 K were developed, We have also developed the transformer as an AC application operated in the liquid nitrogen.

Development of a 1 MJ cryocooler-cooled split magnet with Ag-sheathed Bi2223 tapes for Si single-crystal growth applications

A project to develop a high-temperature superconducting split magnet for Si single-crystal growth applications began in October 1997 and is scheduled to be completed for three years. The project is being executed on the basis of collaboration between Toshiba Corp., Sumitomo Electric Industries Ltd. and Shin-Etsu Handotai Co. Ltd., and is partially funded by Ministry of International Trade and Industry (MITI) of Japan. The purpose of this project is to confirm the energy-saving performance and high reliability of a large HTS split magnet (1 MJ) using Bi2223 tapes. This split coil system is composed of 2 coils, each consisting of 18 pancakes, and the total length of HTS tapes is approximately 80 km. The magnet is to be cooled to below 20 K by a highly efficient GM-type cryocooler in order to make overall current density of the magnet close to the density of metal superconducting magnets. In the first year of this project, a conceptual design was established and R&D of the fragile HTS tapes was carried out. In the second year, the design, fabrication, testing and evaluation of an experimental magnet, incorporating pancake coils of the same size as those of the actual magnet, has been accomplished. This work will contribute to the improvement of the design and fabrication of the full-scale magnet in the final year of this project.

Crystal Growth and Superconductivity of Tl-Ca-Ba-Cu-O System

Single crystals of oxides in the Tl-Ca-Ba-Cu-O system with the general formula Tl2Can-1Ba2CunOx (n=1, 2, 3, 4) have been successfully grown by the flux method. All crystals were of the platelet form ranging from 0.6 to 5 mm2. The transition temperatures of as-grown crystals, determined by DC SQUID magnetometer, were 84 K, 110 K, and 118 K for n=1, 2, and 3, respectively, but slightly lower for 115 K, where n=4. The results of the differential thermal analysis (DTA), X-ray diffraction measurement and energy dispersive X-ray microanalysis (EDX) will be also reported.

Development of Bi-2223 magnetic separation system

A prototype of a cryocooler cooled Bi-2223 magnet system for magnetic separation was constructed. The magnet system has 200 mm room temperature bore and generates fields higher than 1T in 11-liter room temperature space. The magnet was tested at various temperatures and excitation speeds. The magnet was cyclically excited up to 1.7 T at the speed of 1.7 T/min with various periodic times. The temperature gradually increased with the number of excitations, however, the temperature saturated at 28-38 K for the periodic times longer than 9 min, and stable operation of the magnet was obtained. A demonstration of magnetic separation of aquatic slurry containing fine /spl alpha/-hematite paramagnetic particle was performed by using ferromagnetic stainless steel fibers as a magnetic filter. Almost 100% of hematite particles were successfully separated from the slurry.

Development of Bi Based Superconducting Wires

Long length high-strength alloy sheathed Bi2223 tapes were developed. The tolerance for tensile stress improved to twice as high as that of the silver sheathed tape. Length and Jc at 77K of the tape achieve 800m and 30kA/cm2 respectively. The long length tape was coated with Poly-vinyl formval, and successfully insulated along the overall length without any Jc degradation. Special samples to study AC losses were fabricated and investigated. The samples consist of successfully separated filaments without any bridging. And twisting gives an obvious loss reduction especially in the external magnetic field parallel to the tape plane.

Preparation of Ag-Sheathed (Tl, Bi)–Ca–Sr–Cu–O Superconducting Wire

In the Tl-Ca-Ba-Cu-O system, Tl-O single layered and double layered structures with general formulas Tl1Can-1Ba2CunO2n+3(n=1~6) and Tl2Can-1Ba2 CunO2n+4 (n=1 ~4) have been obtained. We fabricated Ag-sheathed Tl-Ca-Ba- Cu-O superconducting wires using powders of Pb-added Tl1Ca 3Ba2Cu4+O11 and of Tl2Ca2Ba2Cu3O10 phases whose critical temperatures are the highest among the respective structures. The production process of the wires was cold- working and heat treatment. The measurement of transport critical current was carried out in the range of magnetic fields from zero to 2.5T. The critical current density Jc at 77K for the wire of (Tl,Pb)1Ca 3Ba2Cu4O11 is 10800A/cm2, and that value for Tl2Ca2Ba2Cu3O10 wire is 5500A/cm2. There is a tendency for the wire specimens with homogeneous structure of superconducting phases to have high Jc values. The Jc of the (Tl,Pb)1Ca3 Ba2Cu4O11 wire decreases rapidly by 0.1T, while the decrease of Jc above 1T is not so remarkable. The magnetic field dependency of Jc of the Tl2Ca2Ba2Cu3O10 wire shows a two-step decrease in Jc versus B curves; the first step is from zero to 0.1T, and the second step is beyond 1T. The direction of the magnetic field against the plane of the wire does not considerably influence the Jc values of either of the wires, because the anisotropic property of the wire is small.

Testing of a cryocooler-cooled HTS magnet with silver-sheathed Bi2223 tapes for silicon single-crystal growth applications

A project to develop a high-temperature superconducting split magnet for 8 inch silicon single-crystal growth applications began in October 1997. This split coil system is composed of 2 coils, each consisting of 18 pancakes. Each coil has an outer diameter of 1.2 m and a thickness of 0.1 m. Moreover, we developed a highly efficient GM-type cryocooler to cool a HTS coil to below 20 K. This paper presents the test results for a full-scale magnet. Each coil was cooled by the GM-cryocooler with a 3.3 kW compressor. The cool-down of the coil was completed within 480 hours with temperature difference among pancakes maintained at less than 18 K. In April 2001 this split coil was successfully energized up to rated current of 210 A at 20 K without a quench. The stored energy of this split coil reached 1.1 MJ at 216 A, 20 K, and the fastest charging time in our experiments was 1 minute.

Testing of the world's largest HTS experimental magnet with Ag-sheathed Bi-2223 tapes for Si single crystal growth applications

Funded by the Ministry of International Trade and Industry (MITI) of Japan, a project to develop a cryocooler-cooled high-temperature superconducting (HTS) magnet for Si single crystal growth applications began in October 1997 on the basis of collaboration between Toshiba Corp., Sumitomo Electric Industries Ltd, and Shin-Etsu Handotai Co., Ltd. So far, we have finished a design of the HTS magnet and basic tests using a model coil. The target magnet is composed of 36 single pancake coils using Ag-sheathed Bi2223 tapes and total length of the HTS tapes is approximately 80 km. Stored electromagnetic energy of the magnet will amount to 1 MJ at rated current operations. The magnet is to be cooled to below 20 K by a GM (Gifford-McMahon) cryocooler. Before fabricating the target magnet, we tested a model coil composed of 4 single pancake coils which were the same size as those of the target magnet, in order to investigate basic characteristics of the HTS coils. We performed a 290 A continuous current flowing operation using the model coil and reached 50 kJ of electromagnetic energy storage.

Superconductivity in the Tl-Bi-Ca-Ba-Cu-O and Tl-Bi-Ca-Sr-Cu-O Systems

The influence of the substitution of Bi for Tl in forming the superconducting phases in Tl-Bi-Ca-Ba-Cu-O and Tl-Bi-Ca-Sr-Cu-O systems was investigated. In the Tl-Bi-Ca-Ba-Cu-O system, the Tl Ca2Ba2Cu3O9 phase forms from a 10% to 40% substitution of Bi for Tl in the nominal composition. With a further increaseing Bi, the superconducting phase disappears because of the formation of BaBiO3. On the other hand, the superconducting phase of (Tl, Bi)CaSr2Cu2O7 forms from a 10% to 90% substitution of Bi for Tl in the Tl-Bi-Ca-Sr-Cu-O system. It is recognized that the substitution of Bi for Tl stabilizes and accelerates the formation of the (Tl, Bi)CaSr2Cu2Ox phase.