EP1142037A1

EP1142037A1 - Method of producing superconducting tapes

Info

Publication number: EP1142037A1
Application number: EP99959261A
Authority: EP
Inventors: Wei Guo Wang
Original assignee: Nordic Superconductor Technologies AS
Current assignee: Nordic Superconductor Technologies AS
Priority date: 1998-12-22
Filing date: 1999-12-20
Publication date: 2001-10-10
Also published as: NO20013175D0; NO20013175L; AU1650500A; JP2002533874A; SK7992001A3; WO2000038251A1

Abstract

A method of preparing an Ag alloy/ceramic superconducting tape comprising a putting of a precursor powder into Ag or Ag alloyed tube to form a bar, drawing the bar into a wire of a predetermined diameter, cutting and bundling the wire to form a multifilament bar with an outer Ag or Ag alloyed tube (3) and putting the resultant bar into an additional tube (4) of e.g. Cu, Cu alloy, Al or steel. The multifilament bar is deformed by drawing, groove rolling or extrusion to form a wire, and the wire is thereafter rolled into a tape. The outer metal sheath will be removed either chemically or mechanically before heat treatment.

Description

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- hod including a placing of oxide Ag sheathed composite bar into a metallic outer tube, defoπning 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 current densities in Bi₂Sr₂Ca₁Cu₂O_8+x/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 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_e (I_c/A,_otll|) could reduce the cost/performance. J. can be enhanced by increasing J_c (I_c /A_oxide) of the super- conducting core and reducing the silver matrix. However, by a strip cutting technique experiments demonstrate that there are very large local variations of J_c in a single filament tape . Tapes with average J_c values of 12-15000A/cm² (77°K, 0T) had local J_c values up to 76000 A/cm² 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- sensitive measurements of the local critical current density in Ag sheathed high-temperature superconductor (Bi,Pb)₂Sr₂Ca₂Cu₃O, 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 J_c by a factor of about 10 of the outer filaments compared to the filaments in the centre of the tape was observed. (Th. Schuster, etal., "Current capability offilaments depending on their position in i,Pb)₂Sr₂Ca₂Cu₃O_{U δ}-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 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/mm²) and lowest in the edge region (Vickers hardness was 90 kg/mm²), (Y. Yamada, M. Satou, T. Masegi, S. Nomura, S. Murase, T. Koizumi, and Y. Kamisada, microstructural effect on critical current density in (Bi,Pb)₂Sr₂Ca₂CuO_xAg 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 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 J_c 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 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)₂Sr₂Ca₂CuO_xAg sheathed tape, Appl. Phys. Lett. 64, 1994, 640.) By increasing the density of a green tape, a final tape having a J_c value of more than 60000 A/cm² was obtained. A monotonously sharp increase of J_c with the relative density of the wire was observed. By increasing the degree of cold working during drawing and rolling, the forma- tion of a strong linked microstructure was promoted.

It has been reported by Wang et al. that the stress-strain distribution of the superconducting 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 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 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 (A₃B type) superconducting wire, a Cu-Ni alloy was used as an outer additional sheath to support a super- 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 preparation 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 technological steps," Supercond. Sci. Technol. Vol. 8, 1995, pp. 617-625.) In one of these proces- 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 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- 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, "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 decrease 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 (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 J_c 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 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- ge of using an additional outer metal strong sheath. How to achieve high density of the final tape that leads to ahigh performance ofthe product has notbeen 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 performance of oxide superconductors and superconducting composites by enhancing density ofthe oxide core, reducing secondary non-superconducting phases, reducing the inhomogenity ofthe filament distribution during processing of oxide superconductors and superconducting composites. It is a further object ofthe invention to prepare oxide superconducting tapes having higher J_c, J_e and I_c 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.

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, puttingthe resultantbar into ametal tube of e.g. Cu, Cu alloy, Al or steel. Deforming the multifilament bar by drawing, groove rolling, extru- sion, to form a wire and rolling the wire into a tape. The outer metal sheath will be removed either chemically or mechanically before heat treatment.

Advantages ofthe invention are as follows:

1. Achievement of a higher density ofthe rolled tape.

2. Preventing contamination during deformation steps. 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 J_c, J_c and I_c.

The term "ceramic" refers to oxide superconductor, e.g. Bi(Pb)SrCaCuO, TlBaCaCuO, HgBaCaCuO, Y(Nd)BaCuO superconductors.

Brief Description of the Drawings

Fig. 1 is a flow diagram illustrating the processing steps of the method according to the invention. 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 Carrying out the Invention

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 additional 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 ofthe grains, as well as increased oxide/ Ag ratio were 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 that working tools densify the oxide into a highly constrained condition. The fast transformation and diffusion are obtained by the highly dense oxide core 1. This results in a phase with a high purity and good texture.

The outer Cu sheath 4 also gives a strong support for deforming high ratio superconducting oxide wires and tapes without breaking. During mechanical deformation, a contamination ofthe 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 loaded into a pure Ag or Ag alloyed tube 2. In case of a BiPbSrCaCuO superconductor, a nominal composition (Bi,Pb)₂Sr₂Ca₂Cu₃O_xisused. The precursor powder consists ofBi- 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- 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 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 conventionally processed samples (control sheath)

Precursor powder was prepared by spray pyrolysis with a nominal stoichiometry of Bij _κPb_{0 33}Sr, _x7Ca₂Cu₃θ_^ 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 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 mm² multifilament tape approximately about 250 meters 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 ofthe samp] es were measured using a criterion of 1 μ V/cm, 77 K and self-field. The results are indicated in Table 1 and show that samples processed according to the invention exhibited an improvement of almost a factor two in critical current properties

Table 1. A comparative study ofthe method ofthe invention with a conventional process

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, 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:

bl) the wire is cutted and bundled to provide a multifilament preform.

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

4. A method according to one ofthe 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 thereof.

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

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