MODIFIED BSCCO PRECURSORS FOR PRODUCING SUPERCONDUCTING ARTICLES

Abstract: The invention relates to a method of producing a superconducting article based on a bismuth(lead)-strontium-calcium-copper-oxide powder composite material (BSCCO material) using the powder-in-tube process to fabricate single-or multi-filamentary wires, whereby a modified BSCCO precursors is employed which contains a small amount of alkali halides or alkaline earth halides, preferably CaF2.

Description: MODIFIED BSCCO PRECURSORS FOR PRODUCING SUPERCONDUCTING ARTICLES The present invention relates to the production of superconducting articles, such as tapes and wires. The invention particularly relates to the production of high performance superconducting articles based on BSCCO powder materials. The invention also relates to superconducting articles produced by this method, modified BSCCO precursors for producing a superconducting article and superconducting articles produced from said modified BSCCO precursors. In particular the present invention relates to superconducting articles produced from a particular group of BSCCO precursor powder materials suitable for the production of superconducting articles with improved performance, and to the superconducting article with improved performance. Superconducting articles based on metal oxide composites and the production thereof are well known m the art. A superconducting article, such as a wire or a tape, is based on a superconducting material, which is defined as a material that has essentially zero resistance to the flow of electrical current at temperatures below a critical temperature. The superconducting material is normally based on copper oxide composite material which may be single phased or raultiphased. The current-carrying capacity of a superconducting oxide composite depends significantly upon the degree of alignment and the connection of the superconducting oxide grains. This degree of alignment is referred to as the degree of texture. While connection is to referred to degree of grain connectivity. A high degree of texture and a high degree of grain connectivity results m a high current- carrying capacity of the superconducting oxide composite. Superconducting articles may be smglefllamentary, but are most often multifllamentary articles, particularly when they are designated for use m AC applications and m applications where mechanical strength is needed. A filament comprises a textured superconducting oxide composite material extending along the length of the article and a constraining member substantially surrounding the filament. A multifllamentary superconducting article comprises a bundle of constrained filaments and a constraining member substantially surrounding the bundle of filaments. A superconducting article is normally produced by I) filling a precursor powder composite metal oxide material into a metal container, such as a tube (normally denoted at the "powder-m-tube method'*)/ or layering the precursor material with one or more grooved or billet sheet so that the precursor material is or is about to be surrounded by a constraining member such as a metal layer, II) deforming the article e.g. by stretching, pressing or rolling to obtain the desired form of the article, and III) subjecting the article to a sintering process by heating it up to a temperature m the range of from 600 °C to 875 °C . The individually constrained filaments may be bundled and constrained m a outer constraining member before or after the heat treatment, and the one or more deformations and/or heat treatments may be performed. The density and degree of texture of the oxide composites are developed by the one or more steps of deformation and the one or more sintering processes. The precursor powder composite metal oxide material is normally prepared by mixing a number of individual metal compounds, e.g. in the form of its nitrates or in the form of B1.O3, CaCO,, SrC0 3 , PbO and CuO powder, and calcinating the mixture (See e.g. the examples of WO 96/08045) . The use of a large number of different metal oxides m the production of superconducting article is well known m the art. US patent No. 5,661,114 mentions a number of these metal oxides, and describes particularly the use of a BSCCO metal oxide material for use m the production of superconducting articles. At present, the BSCCO metal oxide material is the most promising precursor material for the production of superconducting articles. The BSCCO material comprises a large number of phases intermixed with each other. The size, structure, number, and types of different internal phases m the material are very dependent on the stoichiometric composition of the metal- elements m the BSCCO material, on the calcinating treatment and on the further deformation and sintering process of the material. US patent No. 5,661,114 particularly relates to a method of preparing a BSCCO- 2223 superconducting article, wherein the final step of the production is an annealing treatment of the superconductor article at low 0 . pressure. A number of other patent publications refer to specific methods of deforming and heat treating the deformed textured article. WO 97/1772 e.g. relates to a particular two-step heat treatment of the deformed article. WO 96/39366 relates to a method of producing a superconducting article by using a one step deformation and sintering process (IDS process). It is well known m the art that a key requirement for improving the current densities (J-'s) of superconducting oxide composites is a high degree of crystallographic alignment or texture of the oxide grains. Since the texture of the oxide grains, as it is explained above, is a result of deformation and heat treatment of the material, much effort has been placed m developing improved methods of deforming (texturing) and heat treating (sintering) the superconducting article. In addition to the above mentioned patent publications reference should also be made to WO 97/28557 which relates to a process of producing biaxially aligned superconducting articles, and to WO 96/08045 which relates to a method of texturing a multifllamentary superconducting article. In the production of superconducting articles it has for long oeen a major problem to obtain superconductors exhibiting high quality and high current densities m repeated productions. Furthermore, it is quite normal that superconducting articles produced from a specific oxide precursor material having a specific stoichiometric formula and obtained from the same supplier, and which have been deformed and sintered by similar methods m repeated productions may result m superconducting articles exhibiting large differences m current densities . Using a BSCCO powder type as superconducting material, it is well known that the superconducting phase is constituted by the(Bι,Pb) Sr Ca . Cu-0 referred to as the (Bι,Pb)-2223 phase. This phase should mainly be produced during the sintering of the superconducting article, since it also is important that the (Bι,Pb)-2223 grains are aligned along the article. If the (Bι,Pb)-2223 phase are produced prior to the texturing, the (Bι,Pb)-2223 grains will not be well-aligned. The object of the present invention is to provide a method of producing superconducting articles, by use of which method it is possible to obtain superconducting articles exhibiting improved current densities. Furthermore, it is an object to provide a method of producing superconducting articles, by use of which method it is possible to obtain superconducting articles exhibiting equally high current densities m repeated productions. Another object of the present invention is to provide a method of producing superconducting articles to obtain superconducting articles having improved properties with respect to low resistance to the flow of electrical current at operable temperatures. These and other objects are obtained by the invention as defined m the claims. The method of producing a superconducting article comprises the steps of: a) providing a BSCCO precursor, b) mixing the BSCCO precursor with from 0.1 to 5 ° (w/w) of one or more alkali-halides or alkaline earth-halides or a mixture of them to obtain a modified BSCCO precursor c) preparing at least one precursor filament by surrounding the modified BSCCO precursor with at least one constraining member to obtain at least one green smglefllamentary article, d) deforming and texturing at least one green smglefllamentary article, preferably by rolling and/or drawing and/or pressing, m one or more steps to obtain at least one deformed smglefllamentary article; e) if there is more than one smglefllamentary article, bundling the smglefllamentary articles by surrounding the BSCCO precursor with at least one constraining member to obtain a multifllamentary article, and optionally deforming and texturing the multifllamentary article, f) sintering the deformed multi- or smglefllamentary article m one or more steps by heat treatment, g) optionally subjecting the article to further steps of deforming and/or sintering. In order to allow the electric current to flow from one ceramic gram into the adjacent ones, the contact surface between adjacent grains must be as clean (no impurities left) and well crystallised as possible. By using the method according to the invention it has surprisingly been found that the gram connectivity of the (Bι,Pb)-2223 grains is highly improved relative to the gram connectivity of the (Bι,Pb)-2223 grains m the prior art superconducting articles. Furthermore the superconducting articles produced according to the method of the invention contains a major BSCCO 2223 phase, and m most situations the BSCCO 2223 phase is relatively larger than m the known superconducting articles. Another object of the invention is to provide a method of producing superconducting articles by improved transformation of BSCCO 2212 into BSCCO 2223, preferably at a processing temperature lower than the processing temperature normally used for converting BSCCO 2212 into BSCCO 2223. Furthermore, it has been found that by using the method according to the invention the thermal expansion of the material normally occurring during heat treatment is reduced, which result m a superconducting articles having relatively high densities compared to known superconducting articles. Also the superconducting articles produced according to the method according to the invention exhibit improved gram connectivity. All together the improvement described above result superconducting articles which is highly improved with respect to high current density. Furthermore it has been found that the transformation of the BSCCO precursor into BSCCO 2223 by using the method according to the present invention can be conducted at a processing temperature lower than the processing temperature normally used for converting BSCCO precursors into BSCCO 2223. Probably the processing speed will be faster at the same temperature. The alkali-halide may be selected from a group consisting of LiF, LiCl, NaF, NaCl, KF, KC1, RbF, RbCl, CsF, CsCl, MgF_, MgCl_, CaF„, CaCl., SrF_, SrCl_, BaF ^ , and BaCl . In a preferred embodiment the alkali-halide used is CaF_ . The amount of added alkali-halide should be from 0.1-5 % (w/w), preferably in the range of 0.1-4.0 % (w/w), more preferably 0.2-3.0 % (w/w) and even more preferably 0.4- 2.0 t (w/w) . The initial precursor powder may be any BSCCO material, but preferably the material comprises a major BSCCO 2212 phase, preferably more than 60 % (w/w) . This BSCCO material may have various nominal compositions, but a preferred nominal composition may be 1.8±0.5 : 0.3±0.2 : 2±0.4 : 2±0.4 : 3±0.4 (Bi : Pb : Sr : Ca: Cu) . The superconducting article according to the invention may have any geometry, e.g. a superconducting wire or a superconducting tape. The article may be singlefilamentary as well as multifilamenatary. Singlefilamentary articles according to the invention may have an average transverse thickness for the numbers mentioned less than about 500 μm, and preferably an average variation in thickness along its length of less than about 20%. Multifilamentary articles according to the invention may comprise from 2-1000 filaments, preferably from 7-200, wherein each of said filaments has an average transverse thickness less than 200 μm, and preferably an average variation m thickness along its length of less than about 201. In order to ensure that the constraining member or members are substantially non-reactive with respect to oxide superconductors and precursors and to oxygen under the expected conditions (temperature, pressure, atmosphere) of manufacture and use, the constraining member or constraining members are preferably of a noble metal selected among Ag, Au, Pd, and Pt, or alloys based on said elements. In a preferred embodiment according to the invention the constraining member or constraining members are of Ag or of an Ag-alloy. The deformation and texturing of the green singlefllamentary may by carried out m any conventional way, however m a preferred embodiment according to the invention it is performed, e.g. by a rolling process. Sintering of the deformed single- or multifllamentary article may be performed by one or more heat treatments at a temperature the range of 400-950 °C, preferably at a temperature m the range of 600-875 °C and more preferably at a temperature m the range of 750-840 °C. The oxygen pressure during the process may be m the range of 0.01-0.25 at, preferably 0.07-0.21 at. In the following the invention is described m more detail with reference to the drawings and examples below. Figure 1 shows XRD patterns recorded on powdered pellets after various heat treatments . Figure 2 shows HTD traces recorded during 72 h at reaction temperature. Figure 3 shows Ic values measured on reference tapes. Figure 4 shows Ic values measured on tapes containing KC1. Figure 5 shows Ic values measured on tapes containing CaF_. Example 1 Three portions of commercial precursor powder having a nominal composition of Bii „ 4 Pb 4 Srι ιCa_ 3 Cu 3 .. was mixed with 0.2, 0.5 and 1.0 % (w/w) of CaF_, respectively, under dry conditions m polyethylene jars with ZrO_ balls. For reference purposes one portion of the same powder free of CaF_ was handled under the same conditions . All 4 portions of precursor was transformed into powder pellets with initial specific gravity of about 4.4 g/ciM The pellets were then subjected to heat treatment at 836 °C . A further portion of precursor containing no CaF . was subjected to a heat treatment at 853 °C, and XRD patterns were recorded using Cu K α radiation in a STOE diffractometer . The results are shown m fig. 1. The weιght-5; of CaF and the sintering temperature are indicated on the respective patterns. • = Bι,Pb(2223) O = Bi (2212) ♦ = Ca_Pb0 4 Before the heat treatment the precursor powders consist mostly of the Bι,Pb(2212) and Ca_Pb0 4 phases as well as trace amounts of various alkaline earth cuprates. Fig. 1 shows that after 150 h of sintering at 836 °C the CaF_-free pellet still contained a significant amount of unconverted Bι,Pb(2212), whereas the sample initially containing 1.0 % (w/w) of CaF was almost completely converted to Bi,Pb(2223) under the same conditions. The optimum sintering temperature for the CaF-free sample was found to be around 851 °C, whereas the samples containing various amounts of CaF showed an enhanced conversion of Bi,Pb(2212) into Bi,Pb(2223) at temperatures from 825-840 °C . Highest conversion level was achieved for the sample initially containing 1.0 % (w/w) of CaF : . Example 2 Pellets produced in example 1 were subjected to differential dilatometric (HTD) investigations in a SETARAM instrument under air flow. 30 μm Au foils were placed between the sample and the pressing Al : 0 3 rods in order to avoid spurious reactions. The results are shown in fig. 2. a ) 0 . 0 (w/w) CaF_, dwell at 853 °C b ) 1 . 0 (w/w) CaF : , dwell at 823 °C c ) 1 . 0 (w/w) CaF, dwell at 834 °C For all samples, an initial expansion occurs from room temperature up to about 710 °C . However, it appears that the contraction during the first stage of the sintering of the precursor powders is much more significant in the CaF-containing samples. At the end of the heat treatment the CaF : -containg samples therefore have a lower overall expansion, which results in a much higher final density. Example 3 Reference samples of tapes as well as tapes containing KCl and CaF2 were subjected to standard thermo-mechanical treatment. The heat treatment schedule were as follows: Reference tape: - 10 h at 817 °C to 835 °C followed by pressing - 5 h at 817 °C to 835 °C followed by pressing - 817 °C to 835 °C - 787 °C to 805 °C with 0.3 °C/h cooling rate, then furnace cooled. Tapes containing KCl or CaF . : - 10 h at 801 °C to 819 °C followed by pressing - 5 h at 801 °C to 819 °C followed by pressing - 801 °C to 819 °C - 771 °C to 789 °C with 0.3 °C/h cooling rate, then furnace cooled. The results are shown m figs. 3, 4, and 5. o = l " sintering E = 2 1 " * sintering ♦ = 3 Id sintering In figs. 4 and 5, symbols marked with an ', e.g. ♦ ' are for tapes containing 1.0 o alkali-halide, symbols without are for tapes containing 0.5 % alkali-halide. Maximum Ic value for the reference tape after the 3 l i sintering was about 17.5 A, whereas Ic values of 30 A and 25.5 A were obtained for the tapes containing KCl and CaF . , respectively. It is therefore evident, that the tapes containing alkali-halides represent a substantial improvement m comparison with the reference tape.

Claims: CLAIMS : 1. A method of producing a superconducting article based on a bismuth (lead) -strontium-calcium-copper-oxide powder composite material (BSCCO material) comprising the steps of: a) providing a BSCCO precursor, b) mixing the BSCCO precursor with from 0.1-5 % (w/w) of one or more alkali-halides or alkaline earth-halides or a mixture of them to obtain a modified BSCCO precursor, c) preparing at least one precursor filament by surrounding the modified BSCCO precursor with at least one constraining member to obtain at least one green singlefllamentary article, d) deforming and texturing at least one green smglefllamentary article, preferably by rolling and/or drawing and/or pressing, m one or more steps to obtain at least one deformed smglefllamentary article; e) if there is more than one singlefllamentary article, bundling the singlefllamentary articles by surrounding the BSCCO precursor with at least one constraining member to obtain a multifllamentary article, and optionally deforming and texturing the multifllamentary article, f) sintering the deformed ulti- or s glefllamentary article m one or more steps by heat treatment, g) optionally subjecting the article to further steps of deforming and/or sintering. 2. A method of producing a superconducting article according to claim 1, where the alkali-halide is selected from a group consisting of LiF, LiCl, NaF, NaCl, KF, KCl, RbF, RbCl, CsF, CsCl, gF_, MgCl_, CaF_, CaCl_, SrF_, SrCl_, BaF_, and BaCl_. 3. A method of producing a superconducting article according to claims 1 or 2, where the amount of added alkali-halide is m the range of 0.1-4.0 " (w/w), preferably 0.2-3.0 % (w/w) and more preferably 0.4-2.0 i (w/w) . 4. A method of producing a superconducting article according to claims 1-3, where said BSCCO precursor comprises a major 2212 phase, preferably at least 60 % (w/w) . 5. A method of producing a superconducting article according to claim 4, wherein said BSCCO precursor has the nominal composition of 1.8±0.5 : 0.3+0.2 : 2±0.4 : 2±0.4 : 3±0.4 (Bi : Pb : Sr : Ca : Cu) . 6. A method of producing a superconducting article according to any of the previous claims, wherein the superconducting article is a superconducting wire or a superconducting tape. 7. A method of producing a superconducting article according to any of the previous claims, wherein said superconducting article is a smglef lamentary article. 8. A method of producing a superconducting article according to claim 7, wherein said singlefilamentary article nas an average transverse thickness less than 500 μm, and an average variation thickness along its length of less than about 20%. 9. A method of producing a superconducting article according to claims 1-6, wherein said superconducting article is a multifllamentary article comprising from 2- 1000 filaments, preferably from 7-200. 10. A method of producing a superconducting article according to claim 9, wherein each of said filaments has an average transverse thickness less than 500 μm, and an average variation m thickness along its length of less than about 20%. 11. A method of producing a superconducting article according to any of the previous claims, wherein the constraining member or constraining members are of a noble metal selected among Ag, Au, Pd, and Pt, or alloys based on said elements. 12. A method of producing a superconducting article according to claim 11, wherein the constraining member or constraining members are of Ag or of an Ag-alloy. 13. A method of producing a superconducting article according to any of the previous claims, wherein the deforming and texturing of the green singlefllamentary is performed by a rolling process. 14. A method of producing a superconducting article according to any of the previous claims, wherein sintering of the deformed single- or multifllamentary article is performed by one or more heat treatments at a temperature the range of 400-950 °C, preferably at a temperature m the range of 600-875 °C and more preferably at a temperature m the range of 750-840 °C . 15. A method of producing a superconducting article according to any of the previous claims, wherein the article is deformed, e.g. by rolling. 16. A method of producing a superconducting article according to any of the previous claims, wherein the sintering heat treatment (s) are performed at an oxygen partial pressure of from 7-21 % . 17. A method of producing a superconducting article according to claims 1 - 16, where one at least of the coumpounds : BiF3, CuF2 or PbF2 is mixed with the precursoe powders . 18 A method of producing a superconducting article according to claim 17, where the amount of added BiF3, CuF2 or PbF2 or mixture thereof is in the range of 0.1- 4.0 (w/w), preferably 0.2-3.0% (w/w) and more preferably 0.4-2.0% (w/w) . 19. A method of producing a superconducting article according to claim 1, where a fluorine containing compound or several of them is a constituent of the provided BSCCO precursor. 20 A method of producing a superconducting article according to claimm 19, where the amount fluorine containing compound (s) in the provided BSCCO precursor is in the range of 0.1-550.0% (w/w) , preferably 0.2- 25.0&(w/w) and more preferably 0.4 - 10.0% (w/w). 21. A superconducting article produced according to the method of each of the preceding claims 1-20. 22. A modified BSCCO precursor for producing a superconducting article characterised in that it comprises from 0.1-5 % (w/w) of an alkali-halide. 23. A superconducting article produced from a modified BSCCO precursor according to claim 22.

Inventors: GRIVEL JEAN-CLAUDE, GRIVEL, JEAN-CLAUDE
Applicants: GRIVEL JEAN CLAUDE, NORDIC SUPERCONDUCTOR TECH AS, GRIVEL, JEAN-CLAUDE, NORDIC SUPERCONDUCTOR TECHNOLOGIES A/S
Application year: 2000
Main classification: H01L39
IPC codes: C01G1, C01G29, C04B35, H01B13, H01L39