AU1650500A

AU1650500A - Method of producing superconducting tapes

Info

Publication number: AU1650500A
Application number: AU16505/00A
Authority: AU
Inventors: Wei Guo Wang
Original assignee: American Superconductor Corp
Current assignee: American Superconductor Corp
Priority date: 1998-12-22
Filing date: 1999-12-20
Publication date: 2000-07-12
Also published as: NO20013175L; WO2000038251A1; EP1142037A1; NO20013175D0; SK7992001A3; JP2002533874A

Description

WO 00/38251 PCT/DK99/00710 Title: Method of producing superconducting tapes Technical Field The present invention relates to a method for producing superconducting Ag-sheathed ceramic tapes. In particular, the invention relates to a mechanical deformation met 5 hod including a placing of oxide Ag sheathed composite bar into a metallic outer tube, deforming the resultant bar to a wire and rolling the wire to a tape. Background Art The powder-in-tube method offers a simple approach to produce superconducting wires and tapes having good and reproducible properties (K. Heine et al. "High-field critical 10 current densities in Bi 2 Sr 2 CaiCu 2 0 8 +/Ag wires " Appl. Phys. Lett. 55(23), 4 December 1989). A great potential market is expected in the Bi-based supercondutors produced by the powder-in-tube method. In particular, Bi-2223 and Bi-2212 tapes have been used to obtain high temperature superconducting prototypes, such as cables, magnets, motors, generators, fault current limiters, transformers, as well as superconducting magnetic energy 15 storage units. Brief Description of the Invention It has been indicated that the cost/performance must be reduced if the products should have a commercial interest. A high engineering critical current density J. (Ic/Ao) could reduce the cost/performance. J, can be enhanced by increasing J, (I, /A.xide) of the super 20 conducting core and reducing the silver matrix. However, by a strip cutting technique experiments demonstrate that there are very large local variations of J, in a single filament tape . Tapes with average Je values of 12-15000A/cm 2 (77"K, OT) had local J, values up to 76000 A/cm 2 which, however, depend on the local microstructure. (D.C. Labalestier, X.Y. Cai, Y. Feng, H. Edelman, A. Umezawa, G.N. Riley Jr. and W.L. Carter, "Position- WO 00/38251 PCT/DK99/00710 2 sensitive measurements of the local critical current density in Ag sheathed high-tempera ture superconductor (Bi,Pb)2Sr 2 Ca 2 Cu 3 0, tapes," Physica C, vol. 221, pp. 299-303, 1994.) Great differences in the microstructure of filaments were found in a 55 filaments tape and depend on the position in the cross section of the tapes. A variation of Je by a factor of 5 about 10 of the outer filaments compared to the filaments in the centre of the tape was observed. (Th. Schuster, et al., "Current capability offilaments depending on their position in (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 0l,+-multifilament tapes," Appl. Phys. Lett., vol. 69, pp. 1954-1956, 1996). This phenomenon was due to the inhomogeneous deformation of the tapes during the mechanical deformation which caused the irregular shape and inhomogeneous density 10 distribution of the filament. Measurements of Vickers hardness also demonstrated the variation of the density in the oxide core. After a heat treatment the density was highest in the centre region (Vickers hardness was 120 kg/mm 2 ) and lowest in the edge region (Vic kers hardness was 90 kg/mm 2 ), (Y. Yamada, M. Satou, T. Masegi, S. Nomura, S. Murase, T. Koizumi, and Y. Kamisada, microstructural effect on critical current density in 15 (Bi,Pb) 2 Sr 2 Ca 2 CuOAg sheathed tape, Advances in Superconductivity VI, T. Fujita and Y. Shiohara (Eds.), 1994, 609.) This density distribution resulted in a corresponding variation in the local current density in the direction of width. This is probably due to variable stress distribution induced during cold rolling. Two aspects in connection with powder-in-tube method are to be considered. The first one 20 is the low oxide density in a tape compared with the theoretical density. The second is the inhomogeneous distribution of the filaments in the direction of width. Densification effect on the microstructure and Jc in a Bi-2223 single filament tape has been reported by M. Satou, et al.. The highest relative density (true density/theoretical density) of a drawn wire was about 80 %. A green tape having a relative density of up to 90 % can be obtained by 25 rolling the highest density wire (M. Satou, Y. Yamada, S. Murase, T. Kitamura and Y. Kamisada, Densification effect on the microstructure and critical current density in (Bi,Pb) 2 Sr 2 Ca 2 CuOAg sheathed tape, Appl. Phys. Lett. 64, 1994, 640.) By increasing the density of a green tape, a final tape having a Jc value of more than 60000 A/cm 2 was obtained. A monotonously sharp increase of Jc with the relative density of the wire was 30 observed. By increasing the degree of cold working during drawing and rolling, the forma- WO 00/38251 PCT/DK99/00710 3 tion of a strong linked microstructure was promoted. It has been reported by Wang et al. that the stress-strain distribution of the superconduc ting oxide core can be controlled in response to curvature and hardness of a steel sheet in a "sandwich" rolling process. In this process, the tape was deformed between two steel 5 sheets and a high density and a smooth interface were obtained. (W.G. Wang, H.K. Liu, Y.C. Guo, P. Bain and S.X. Dou, Mechanism of deformation and sandwich rolling process in Ag-clad Bi-based composite tapes, Appl. Supercond., 3, 1995, 599.) A high compressive stress is essential for obtaining a high density in a green tape. A higher tensile sheath material than a pure Ag sheath would be effective to constrain the sheath in 10 order to obtain this condition. An Ag alloy is a possibility. However, Cu or a Cu alloy would be even better. According to 4-294008 (A) 1992-10-19 (JP), Appln. No. 3-58957 in the name of K. Matsumoto and concerning a method of manufacturing of a compound (A 3 B type) super conducting wire, a Cu-Ni alloy was used as an outer additional sheath to support a super 15 conducting wire in the drawing process. After an initial drawing, the outer Cu-Ni alloy sheath was removed by means of nitric acid. An outer Cu sheath has been used in pre paration of Bi-2223 single filament tapes which has been reported by I. Husek et al. (I. Husek et al., "Microhardness profiles in BSCCO/Ag composites made by various techno logical steps," Supercond. Sci. Technol. Vol. 8, 1995, pp. 617-625.) In one ofthese proces 20 ses, the Cu sheath was used as an additional outer sheath in the drawing process, then etched away before rolling. In another process, a Cu sheath was used as a supporting sheath in a hot extrusion process, whereafter a drawing and a rolling were performed to provide the final product. The highest densities were obtained for large deformation in one passage during extrusion. However, a hydrostatic extrusion does not allow a production 25 of superconductors in their final form as reported in this paper. This paper claimed that the highest density and homogeneity were obtained in the tapes by conventional wire drawing and rolling without an additional Cu sheath. In another paper reported by W. Pachla et al., the effect of hydrostatic pressure on precursor core densification, densifica- WO 00/38251 PCT/DK99/00710 4 tion of the billets for further processing, deformation at high strains and strain rates and core densities in composite wires and final tapes are demonstrated and discussed. (W. Pachla, et al., The effects of hydrostatic pressure on the BSCCO compound for the OPIT procedure," Supercond. Sci. Technol. Vol. 9, 1996, pp. 957-964.) Accordingto this article, 5 "application of harder sheath materials, such as alloyed silver or copper-clad silver makes the OPIT technology more difficult due to the strain-hardening phenomena and the decrea se in the conductor's fill factor" and according to "such texturisation ratios cannot be generated easily or at all in as-drawn or as-swaged wires. Before the further stages of deformation such as drawing and rolling, the external copper layer of bimetallic sleeve can 10 (but need not) be removed by chemical etching conf. Fig. 6, page 960. One of the figures (Fig. 6) illustrates a rolled single filament tape by using outer Cu sheath. There are no density measurements and Je results for this tape, no further information at all. This prior art only discloses that the single filament tape can be made with an outer Cu sheath, but no advantage has been indicated and no further results were obtained by this outer Cu 15 sheath. According to this article, no improvement of final tape performance was obtained by using an outer Cu sheath. It was demonstrated that a wire with a higher density resulted in a tape with a higher density after rolling to a certain degree. However, optimized rolling processes are needed to further increasing of the density of the tape. The prior art has not indicated the advanta 20 ge of using an additional outer metal strong sheath. How to achieve high density of the final tape that leads to ahigh performance oftheproduct has not been indicated too. There has been no report concerning multifilament wires and tapes that have been used for all types of applications by using an additional outer metal sheath. The object of the invention is to provide a method of improving superconducting 25 performance of oxide superconductors and superconducting composites by enhancing density of the oxide core, reducing secondary non-superconducting phases, reducing the inhomogenity of the filament distribution during processing of oxide superconductors and superconducting composites.

WO 00/38251 PCT/DK99/00710 5 It is a further object ofthe inventionto prepare oxide superconducting tapes having higher Jc, Je and Ic than conventionally-processed tapes. A feature of the invention is an improved mechanical deformation which involves an outer additional strong metal sheath during wire drawing and tape rolling processes. 5 A method according to the invention of preparing an Ag alloy/ceramic superconducting tape includes a putting of a precursor powder into an Ag or Ag alloyed tube, drawing the bar into a wire of a predetermined diameter, cutting the wire to form a multifilament bar with an outer Ag or Ag alloyed tube, putting the resultant bar into a metal tube of e.g. Cu, Cu alloy, Al or steel. Deforming the multifilament bar by drawing, groove rolling, extru 10 sion, to form a wire and rolling the wire into a tape. The outer metal sheath will be remo ved either chemically or mechanically before heat treatment. Advantages of the invention are as follows: 1. Achievement of a higher density of the rolled tape. 2. Preventing contamination during deformation steps. 15 3. Achievement of a high superconducting fill factor. 4. By removing the Cu, especially at the edge part of the tape, the low density part is reduced. 5. Achievement of higher Jc, Jc and Ic. The term "ceramic" refers to oxide superconductor, e.g. Bi(Pb)SrCaCuO, TlBaCaCuO, 20 HgBaCaCuO, Y(Nd)BaCuO superconductors. Brief Description of the Drawings Fig. I is a flow diagram illustrating the processing steps of the method according to the invention.

WO 00/38251 PCT/DK99/00710 6 Fig. 2 is a cross section of a superconducting composite bar and Fig. 3 is a cross section of a superconducting tape. Best Mode for Carryin out the Invention 5 The present invention is a method of improving the critical current density of oxide superconductor wires and tapes by a novel mechanical deformation process with an addi tional outer metal sheath. By applying an outer strong metal sheath in additional to the Ag alloy sheath 3, an enhanced density of the oxide core 1, reduced non-superconducting secondary phases, improved texture of the grains, as well as increased oxide/Ag ratio were 10 obtained in the tapes resulting in a higher critical current density. The high compressive stress can be induced in the oxide core 1 by an additional outer strong sheath 4. The high tensile strength and toughness materials such as Cu can carry high deformation stress and strain without breaking so thatworking tools densify the oxide into a highly constrained condition. The fast transformation and diffusion are obtained by 15 the highly dense oxide core 1. This results in a phase with a high purity and good texture. Theouter Cu sheath4 also gives strong supportfor deforming high ratio superconducting oxide wires and tapes without breaking. During mechanical deformation, a contamination of the surface can be omitted by means of an outer protecting sheath. Fig. 1 shows the flow diagram of the processsing steps. The powder or powder bar was 20 loaded into a pure Ag or Ag alloyed tube 2. In case of a BiPbSrCaCuO superconductor, a nominal composition (B i,Pb) 2 Sr 2 Ca 2 Cu 3 0, is used. The precursor powder consists of Bi 2212 and secondary phases. In the first stage of making single filament wire, the outer Cu tube may be used to prepare very high oxide ratio single filament. If so, the outer Cu sheath 4 have to be removed before bundling the single filament wires to form a multifila 25 ment bar. Bundled wires will be put into an Ag alloy tube 3 and then fit into a Cu tube 4 as shown in Fig. 2. The resultant bar will be deformed by swaging, drawing, extrusion, or WO 00/38251 PCT/DK99/00710 7 groove rolling to a wire. The wires are rolled into a tape shown in Fig. 3. Examples The following example compares the critical current characteristics of a sample prepared by the process combined with an outer Cu sheath according to the invention to those of 5 conventionally processed samples (control sheath). Precursor powder was prepared by spray pyrolysis with a nominal stoichiometry of Bij.xPbo.

33 Sri. 87 Ca 2 Cu 3 0,. The powder was pressed to a round bar with diameter of 16 mm and a length of 40 cm. The billet was put into an Ag tube with an inner diameter of 18 mm and an outer diameter of 20 mm. Closing end and drawing the single filament wire down 10 to 2 mm were performed. 55 wires were bundled together and put into an Ag alloy tube with an inner diameter of 18mm and an outer diameter of 20 mm. This multifilament bar is thereafter put into a Cu tube with an inner diameter of 21 mm and an outer diameter of 23 mm. The rod was drawn through a series of dies down to a 1.4 mm wire. The round wire was rolled to form a 0.2x3.5 mm 2 multifilament tape approximately about 250 meters. 15 The Cu sheath was stripped away before heat treatment. A control tape to be compared with a tape according to the invention was made in same way but without an additional outer Cu sheath. Short tapes were cut from the long tape and heat treated. The critical current of the samples were measured using a criterion of 1 p V/cm, 77 K and self-field. The results are indicated 20 in Table 1 and show that samples processed according to the invention exhibited an impro vement of almost a factor two in critical current properties. Table 1. A comparative study of the method of the invention with a conventional process.

WO 00/38251 PCT/DK99/00710 8 Sample No. Tape size (mm 2 ) Ic (A) J, (A/cm 2 ) Example 1 0.51 52.5 10000 Example 2 0.51 52.7 10330 Example 3 0.51 53 10400 5 Control 1 0.51 27 5300 Control 2 0.51 30 5900 Control 3 0.51 32 6300

Claims

1. A method of preparing an Ag or an alloy/ceramic superconducting tape comprising the following steps: a) putting a precursor powder into an Ag or in an Ag alloy tube to form a preform, 5 b) drawing the preform into a wire and c) surrounding the preform by a metal tube of e.g. Cu.

2. A method according to claim 1, wherein before step c: bI) the wire is cutted and bundled to provide a multifilament preform.

3. A method according to claim 2,wherein that after step bI) the multifilament bar is 10 surrounded by a metal tube of Ag or Ag alloy.

4. A method according to one of the claims 1-3, wherein after step c) the wire is treated by drawing or grove rolling or by rolling, e.g. flat rolling or extrusion or a combination there of.

5. A method according to one of the claims 1-4, wherein the Cu is removed by a chemical 15 etching or by a mechanical stripping.

6. A method according to one of the claims 1-5, c h a r a c t e r i s e d in that in the manu facturing of the superconducting tape, BiPbSrCaCuO is used as superconducting material.