WO2001056128A2 - Encapsulation de composites supraconducteurs - Google Patents

Encapsulation de composites supraconducteurs Download PDF

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Publication number
WO2001056128A2
WO2001056128A2 PCT/US2001/001762 US0101762W WO0156128A2 WO 2001056128 A2 WO2001056128 A2 WO 2001056128A2 US 0101762 W US0101762 W US 0101762W WO 0156128 A2 WO0156128 A2 WO 0156128A2
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WO
WIPO (PCT)
Prior art keywords
composite
superconducting
superconducting composite
exposing
exposed surface
Prior art date
Application number
PCT/US2001/001762
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English (en)
Other versions
WO2001056128A3 (fr
Inventor
Craig J. Christopherson
Deborah L. Ouellette
David M. Olen
Thomas De Santos
Sy-Jeng Loong
Eric R. Podtburg
David M. Buczek
Michael A. Tanner
John B. Pereira
Donald R. Parker
Peter Chryssanthacopoulous
Christine Walsh
Original Assignee
American Superconductor Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US09/488,740 external-priority patent/US6339047B1/en
Application filed by American Superconductor Corporation filed Critical American Superconductor Corporation
Priority to AU2001260972A priority Critical patent/AU2001260972A1/en
Publication of WO2001056128A2 publication Critical patent/WO2001056128A2/fr
Publication of WO2001056128A3 publication Critical patent/WO2001056128A3/fr

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Classifications

    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N60/00—Superconducting devices
    • H10N60/01—Manufacture or treatment
    • H10N60/0268—Manufacture or treatment of devices comprising copper oxide
    • H10N60/0801—Manufacture or treatment of filaments or composite wires

Definitions

  • the present invention is related to methods of preparing composites for encapsulation in a sealing material.
  • High temperature superconductors have increasing utility as power conductors, fault current limiters, and other applications. Since HTS materials are generally brittle oxides, it is common to produce them in composite form, wherein superconducting filaments are supported by a noble metal matrix. The matrix is most commonly silver-based, but any metal or alloy which will withstand superconductor processing conditions and which does not act to poison the superconductor may be used.
  • noble metal matrices provide some mechanical support to superconducting filaments in a composite
  • a stronger metal may be used to mechanically reinforce a composite wire while maintaining a low cross-sectional area, or a thermally conductive material may provide improved heat transfer out of the composite.
  • the present invention provides improved methods of preparing composites for encapsulation and/or lamination.
  • a superconducting composite is to be laminated by soldering it to another material (e.g., a metal tape which can provide mechanical and/or thermal stabilization)
  • a composite is protectively encapsulated (e.g., with solder or with a polymer)
  • the encapsulating material should be free of defects that might compromise the integrity of the seal.
  • solder or another material may be used to hold the strands together, and again, a good contact and wettability between the solder and the strands should be achieved.
  • the invention comprises methods of preparing superconducting composites for lamination, encapsulation, or any other process that requires wetting of the composite by a liquid (e.g., a liquid metal).
  • the methods comprise removing a thin layer of matrix material to expose a surface having a low concentration of oxide particles.
  • the layer may be removed, for example, by chemical etching or by electrolytic cleaning. If it is removed by etching, the etchant may be, for example, nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid, ferric chloride, ammonia, a caustic solution, a mixture of these, or any other chlorinated or fluorinated solvent, and may contain other salts such as ammonium bi fluoride.
  • the thin layer may be on the order of 0.013 mm thick, or as thick as can be achieved without excessively degrading the properties of the superconductor (e.g., degrading the critical current by more than about 10%).
  • the exposed surface may have a wettability in solder of at least 250 ⁇ N/mm, 300 ⁇ N/mm, or 350 ⁇ N/mm.
  • an electrolyte such as nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid, sodium hydroxide, ferric chloride, ferrous chloride, ferric fluoride, c r ferrous fluoride may be used.
  • the removal process may leave the exposed surface at least 97% free of foreign particles, at least 99% free, at least 99.5% free, or at least 99,8% free.
  • the invention comprises methods of preparing superconducting composites for lamination, encapsulation, or any other process that requires wetting of the composite by a liquid (e.g., a liquid metal).
  • the methods comprise exposing the superconducting composite to an environment adapted to remove liquids from the exposed surface, such as a vacuum (e.g., a vacuum of about 10 millitorr) or an inert environment.
  • the environment may include an elevated temperature (e.g., a temperature between about 100°C and about 200°C).
  • the invention comprises methods of preparing superconducting composites for lamination, encapsulation, or any other process that requires wetting of the composite by a liquid (e.g., a liquid metal).
  • a liquid e.g., a liquid metal
  • the methods comprise both removing a thin layer as described above, and removing liquids by exposure to a dessicating environment as described above.
  • noble metal as it is used herein, it is meant any metal whose reaction products are thermodynamically unstable under the reaction conditions employed relative to the desired superconducting ceramic, or which does not react with the superconducting ceramic or its precursors under the conditions of manufacture of the composite.
  • a noble metal matrix may be a metal different from the metallic elements of the superconducting ceramic, such as silver, gold, or their alloys, or it may be a stoichiometric excess of one of the metallic elements of the superconducting ceramic, such as copper.
  • oxide superconductor intended for eventual use in the finished article, or any precursor thereof.
  • desired final oxide superconductor is selected for its superior electrical properties, such as high critical temperature or critical current density.
  • the desired oxide superconductor is typically a member of a superconducting oxide family which has demonstrated superior electrical properties, for example, BSCCO 2223 (including (Pb,Bi) 2 . ⁇ Sr 2 Ca 2 Cu 3 O ⁇ o+x and Pb 0 . 2 Bi 1 . 9 Sr 2 Ca 2 Cu 3 O ⁇ 0+ x) or BSCCO 2212 in the BSCCO family, or YBCO 123 in the YBCO family.
  • Precursor is meant any material that can be converted to an oxide superconductor upon application of a suitable heat treatment. Precursors may include any combination of elements, metal salts, oxides, suboxides, oxide superconductors which are intermediate to the desired oxide superconductor, or other compounds which, when reacted in the stability field of a desired oxide superconductor, produces that superconductor.
  • elements, salts, or oxides of copper, yttrium, and barium for the YBCO family of oxide superconductors; elements or oxides of copper, bismuth, strontium, and calcium, and optionally lead, for the BSCCO family of oxide superconductors; elements, salts, or oxides of copper, thallium, calcium and barium or strontium, and optionally, bismuth and lead, for the thallium (TBSCCO) family of oxide superconductors; elements, salts, or oxides of copper, mercury, calcium, barium or strontium, and optionally, bismuth and lead, for the mercury (HBSCCO) family of oxide superconductors.
  • TBSCCO thallium
  • the YBCO family of oxide superconductors is considered to include all oxide superconductors of the type comprising barium, copper, and a rare earth, such as yttrium, lanthanum, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium.
  • oxide superconductor intermediate to the desired oxide superconductor is meant any oxide superconductor which is capable of being converted to the desired oxide superconductor.
  • an intermediate may be desired in order to take advantage of desirable processing properties, for example, a micaceous structure amenable to texturing, which may not be equally possessed by the desired superconducting oxide.
  • Precursors are included in amounts sufficient to form an oxide superconductor.
  • the precursor powders may be provided in substantially stoichiometric proportion.
  • excess or deficiency of a particular precursor is defined by comparison to the ideal cation stoichiometry of the desired oxide superconductor.
  • doping materials including but not limited to the optional materials identified above, variations in proportions and such other variations in the precursors of the desired superconducting oxides as are well known in the art, are also within the scope and spirit of the invention.
  • Figures la and lb show defects which may be observed in laminated superconducting composites
  • Figures 2a and 2b are photomicrographs of a superconducting composite before and after chemical etching treatments according to the invention
  • Figure 3 plots wetting balance of superconducting composites before and after chemical etching treatments according to the invention.
  • Figure 4 plots wetting balance of pure silver and pure copper.
  • the present invention provides improved methods of laminating a material to a superconducting composite. These methods allow solder to wet the surface of a metal matrix much more effectively, and also reduce or eliminate outgassing during soldering, thereby reducing pinholes in the solder that can allow liquid cryogen to penetrate the composite and cause ballooning.
  • Superconducting composites are often formed by first forming a precursor composite, which is then subjected to heat treatment (usually under an oxidizing atmosphere) to produce the desired superconductor composite.
  • the oxide-powder-in- tube (OPIT), metal precursor (MP), and coated conductor (CC) routes of superconductor formation are examples of such processes.
  • the precursor composite is generally converted into the final desired superconducting composite before lamination, to avoid damaging the solder layer or allowing it to poison the oxide superconductor.
  • separating agent may be, for example, a porous coating or a powder (e.g., Zr 2 O 3 , MgO, or CuO powder) which separates the adjacent metal layers to prevent sticking.
  • the separating agent is preferably a material which is relatively easy to remove from the composite surface after heat treating, small quantities of the agent often remain even after determined efforts to remove them. For example, even after cleaning with a regimen of air jets, vacuum treatments, mechanical abrasion, and ultrasonic cleaning, some particles of MgO powder have been observed to remain on the surface of the material. Partially embedded particles have also been observed after these treatments.
  • oxide and other particles may form on the surface of the composite during heat treatment.
  • a very common matrix material is silver (or silver alloys). Even high-purity silver usually contains some amount of "tramp" copper as an impurity. It has been observed that in the high temperatures and oxidating conditions used to convert precursor composites into superconducting composites, this copper may segregate to the surface to form copper oxide. In addition, silver is vulnerable to trace amounts of sulfur in the air, forming a silver sulfide skin (tarnish) on its surface.
  • any of these foreign particles or oxide/sulfide layers may be further embedded into the surface by subsequent rolling operations, rendering them even more difficult to remove without damaging the superconductor. Further, it has not previously been recognized that any need existed to remove more of the particles than can be removed by conventional methods such as water or air knives, ultrasonic cleaning, mechanical scrubbing, and the like.
  • the present invention comprehends the discovery that even a small concentration of these particles may serve as a source of "tunnel" defects in a solder layer when such a composite is laminated.
  • tunnel defects may also be associated with composite defects in which the matrix does not completely cover the superconducting filaments ("perforations"). Tunnels associated with perforations may be those most vulnerable to ballooning, since they provide a pathway for the cryogen to reach the superconducting composite at a point where the superconducting filaments are not fully protected by the metal matrix from cryogen infiltration. We have found that this type of defect can be markedly reduced by vacuum baking the composite before lamination, as described below.
  • the superconducting composite is surrounded by solder (or other encapsulating material), which in turn bonds it to stabilizing elements.
  • the stabilizing elements may be designed to provide mechanical support and/or extra thermal mass to the composite, and may comprise materials such as stainless steel, copper, nickel, or various other metals and alloys. Other configurations may be preferred in certain applications; for example, the invention may be practiced in connection with the "face-to-face" arrangement of coated conductors described in U.S. Provisional Patent Application No. 60/145,468, filed July 23, 1999, which is incorporated by reference herein.
  • FIG. la shows a "face-to-face" coated conductor configuration, comprising two coated conductors, each comprising a substrate 20, a buffer layer 22, a superconductor layer 24, and a cap layer 26. As shown, these conductors are joined by a solder layer 28 which comprises two fillets 29.
  • the fillets 29 may comprise tunnels 30, 32.
  • the tunnels 18, 30, 32 of Figures la and lb can be formed both by the presence of foreign particles on the composite surface, and by small amounts of liquid which may be trapped at perforations and/or on foreign particles as the composite enters the solder bath.
  • Figure 2a is a photomicrograph of the surface of a composite of BSCCO filaments in a silver matrix, which has been heat treated and cleaned by conventional methods. Despite cleaning procedures significantly more stringent than those normally suggested to prepare a surface for soldering, a number of foreign particles remain.
  • the large dark particles in the figure have been found to comprise primarily copper oxide, while the uneven "haze" of lighter gray material is mostly fine MgO particles. It is estimated that the total coverage of the surface by particles is on the order of 3-5%. According to the invention, a substantial portion of these particles are removed by removing a thin surface layer from the matrix.
  • the oxide particles are stripped from the surface by etching away a thin layer of the surface.
  • Figure 2b This photomicrograph shows the composite of Figure 2a after a chemical treatment according to the invention.
  • the composite was held in a 50/50 mixture of nitric acid and water containing 90 g/1 of ammonium bifluoride.
  • the etching solution was heated to 48°C and the matrix was held in the solution for a period of 5 sec, etching the silver matrix to a depth of about 0.013 mm (0.0005").
  • a comparison of Figure 2a and Figure 2b will show that a very large proportion of the foreign particles have been removed by this chemical treatment.
  • etching be to a depth of at least half of the typical particle diameter of the surface particles, and etching may be as deep as desired without damaging a large fraction of the superconducting filaments. Some damage to surface filaments is acceptable, as long as it does not cause an unacceptable degradation in the desired properties of the superconducting composites (e.g., a degradation of more than 10% in the total critical current).
  • the inventors have discovered the surprising result that the wettability of the matrix (e.g., by solder) is remarkably enhanced.
  • the matrix is generally thinner and more uniform after etching.
  • the enhanced wettability of the etched matrix is due to removal of the foreign particles.
  • the concentration of particles on the surface before treatment is relatively small (well within normal guidelines for a surface "clean" enough to solder), they appear to have a significant effect upon wettability of the matrix. This conclusion is supported by the determination that pure silver has a wettability similar to that of the treated samples (see Figures 3 and 4).
  • Figure 3 plots measurements of the wetting of matrix material which has been treated according to the invention. These plots were generated by measuring the force on a strip of composite material as it was dipped into a pot of molten solder, according to MIL-STD-883/TM2022 (incorporated herein by reference). Lines 110 show force (in ⁇ N/mm) vs. time (in seconds) for untreated specimens, while lines 112 show similar measurements for specimens treated according to the invention.
  • the untreated specimens have been found to exhibit small solder tunnels and consequent ballooning, despite the fact that they pass many conventional solderability tests, including the MIL-STD-883/TM2022 and DeVore's Solderability Index (both of which are discussed in Russell, et al, "Component Solderability Guide,” Electronic Industries Association, incorporated herein by reference).
  • the specimens treated according to the invention exhibited significantly different behavior. There was essentially no initial resistance to insertion of the specimens into the solder at the start of the test, and the capillary force built up to a much higher level, indicating that the surface of the specimens was much more readily wetted by the solder. In addition, the instability was much reduced for two of the three treated specimens.
  • the wettability data for the treated and untreated composites were compared with wettability data for pure silver 118 and for pure copper 120, as shown in Figure 4. While the copper and silver specimens required a greater force to break the initial surface tension of the liquid solder, because of their larger cross-sectional area, the wetting curve for pure silver is very similar to that of the treated specimens, lending support to the idea that the treatment works by exposing an uncontaminated silver (or alloy) surface.
  • about 10-13 weight % Ga (with respect to Ag content) was deposited on the surface of a superconducting tape comprising BSCCO filaments in a silver sheath, using electroplating techniques and a reel to reel electroplating line. After plating, the Ga was diffused into the sheath, making a high resistance Ag-Ga solid solution alloy. After the diffusion process, a gallium oxide skin was present on the surface of the tape. This oxide layer was chemically removed prior to subsequent lamination processing, using a 50% nitric acid / 50% deionized water solution containing 90 g/1 of ammonium bifluoride.
  • the etching solution was heated to 48°C and the diffused tape was run through the solution (maintaining a 5 sec residence time) using a reel to reel etching line. Once the tape was chemically stripped of its gallium oxide layer, it was readily soldered during the lamination process.
  • the lamination process was used to provide both mechanical and thermal stabilization for the strand.
  • the gallium post processed wire was bonded to a stainless steel stabilizer using a 50/50 lead-indium solder.
  • the chemical etching treatments described above provided substantial improvements to the wettability of the superconducting composite during subsequent lamination steps.
  • the formation of tunnel defects can also be reduced by vacuum baking the composite prior to coating it with solder.
  • vacuum baking For example, exposure of a composite comprising BSCCO fibers embedded in a silver matrix to a temperature of about 100-200°C and a pressure of about 10 millitorr for about three to twelve hours has been found to significantly reduce the frequency of tunnels through solder fillets associated with perforations.
  • the pressure chamber may be backfilled with an inert gas such as argon or nitrogen while the composite cools.
  • the exposed ceramic material at perforations, and/or any remaining foreign particles, "getter” or absorb liquid either during solution treatment steps (e.g., ultrasonic cleaning to remove surface particles, or the chemical etching steps described above), from exposure to a humid environment, or from a combination of these factors. If this liquid is not removed, it vaporizes or outgasses when the superconducting composite is immersed in liquid solder, leaving a tunnel defect in the fillet. Vacuum baking prior to lamination evaporates the liquid so that fillets form without tunnels.
  • vacuum baking is not the only treatment step appropriate for removing residual liquid; for example, a bake in an inert atmosphere such as nitrogen or argon gas could also be used, as could a room-temperature dessicating environment.
  • This vacuum bake step is preferably the last step before soldering, or at least is subsequent to all solution processing steps, so that all water or other liquid is removed before soldering. It is preferred that the vacuum bake conditions not be conducive to the formation of foreign particles on the surface that might interfere with wettability of the composite with the solder.
  • oxygen activity should be kept low, especially if metallic copper, gallium, or other oxidizable metals are present at the surface of the composite, to avoid forming new oxide particles after the chemical cleaning step of the invention has been completed.
  • care should be taken that silver sulfide does not form on the surface of the superconducting composite, for example by bagging the wires in an inert atmosphere.
  • the vacuum baking described herein may also be beneficially used with other techniques that may leave traces of water or other liquids on the surface of a superconducting composite, such as some of the reducing techniques described in commonly assigned “Separating layer for Heat Treating Superconducting Wire,” (attorney docket 0019696-0155), filed on even day herewith and incorporated herein by reference.

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  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Superconductor Devices And Manufacturing Methods Thereof (AREA)
  • Inorganic Compounds Of Heavy Metals (AREA)

Abstract

L'invention concerne des procédés de préparation de composites supraconducteurs destinés à la réalisation d'une fermeture hermétique. On a découvert que ces composites supraconducteurs peuvent ne pas être entièrement mouillés par la brasure en cours de fabrication, et que leur mouillabilité peut être augmentée par gravure chimique de surface. Par ailleurs, on a découvert que l'étuvage sous vide de ces composites avant le brasage peut améliorer les propriétés de revêtement.
PCT/US2001/001762 2000-01-20 2001-01-18 Encapsulation de composites supraconducteurs WO2001056128A2 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU2001260972A AU2001260972A1 (en) 2000-01-20 2001-01-18 Pre-treatments for the encapsulation of superconducting composites

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US09/488,740 US6339047B1 (en) 2000-01-20 2000-01-20 Composites having high wettability
US09/488,740 2000-01-20
US54821500A 2000-04-12 2000-04-12
US09/548,215 2000-04-12

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WO2001056128A3 WO2001056128A3 (fr) 2002-10-03

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Cited By (2)

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Publication number Priority date Publication date Assignee Title
WO2005062029A2 (fr) * 2003-12-09 2005-07-07 Superpower, Inc. Systeme de fabrication de bande
WO2013148811A3 (fr) * 2012-03-30 2014-10-16 American Superconductor Corporation Conducteur électrique large à rigidité élevée dans l'axe c

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US5507924A (en) * 1993-02-01 1996-04-16 Sumitomo Electric Industries, Ltd. Method and apparatus for adjusting sectional area ratio of metal-covered electric wire
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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2005062029A2 (fr) * 2003-12-09 2005-07-07 Superpower, Inc. Systeme de fabrication de bande
WO2005062029A3 (fr) * 2003-12-09 2005-09-09 Superpower Inc Systeme de fabrication de bande
US7146034B2 (en) 2003-12-09 2006-12-05 Superpower, Inc. Tape manufacturing system
US7805173B2 (en) 2003-12-09 2010-09-28 Superpower, Inc. Tape manufacturing system
WO2013148811A3 (fr) * 2012-03-30 2014-10-16 American Superconductor Corporation Conducteur électrique large à rigidité élevée dans l'axe c
KR20150003227A (ko) * 2012-03-30 2015-01-08 아메리칸 수퍼컨덕터 코포레이션 C-축 강도가 높은 광폭 전기 전도체
KR101641013B1 (ko) 2012-03-30 2016-07-19 아메리칸 수퍼컨덕터 코포레이션 C-축 강도가 높은 광폭 전기 전도체

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AU2001260972A1 (en) 2001-08-07

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