Abstract:
A superconductor oxide composite is prepared using a press coating technique. The coated layers on various substrates exhibit good adhesion, textured microstructure, and improved J c .

Description:
The government has rights in this invention pursuant to Grant Number DE-FG02-85ER45176. 
     The United States Government retains rights to this invention based upon funding by the Department of Energy under Contract DE-FG02-85ER-45176. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to press coating and oxidation of metallic precursor alloy synthesis methods for high T c  superconducting materials. 
     Bulk, high T c  superconducting oxides suitable for power transmission applications have been synthesized with a number of processing techniques. These techniques include high temperature oxidation of metallic precursor alloys containing metallic constituents of desired superconducting oxides. Such metallic precursors can be made by any metal forming technique depending upon the required superconductor shape including melt spinning, planar flow casting, melt dipping and melt writing. Metallic precursors can contain metals stable through oxidation processing including noble metals such as silver to produce superconducting oxide/metal microcomposites with combined good mechanical and superconducting electrical properties. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, high T c  oxide superconductor composites are synthesized by pressure coating a substrate with oxidized metallic precursor alloy. 
     As an example of the invention, a Bi--Pb--Sr--Ca--Cu--O superconductor composite is synthesized by pressure coating a substrate with an oxidized Bi--Pb--Sr--Ca--Cu/noble metal precursor alloy to produce the Bi--Pb--Sr--Ca--Cu--O/noble metal superconductor composite. Other superconducting systems including Yb--Ba--Cu--O, Y--Ba--Cu--O, and Tl--Ba--Ca--Cu--O can be processed with the press coating method. 
     In some embodiments, the metallic precursor alloy can be prepared using any metals processing technique including melt spinning, melt dipping, and melt writing. The oxidized metallic precursor alloy can be pressed or rolled onto the substrate at temperatures between 25° C.-800° C. and pressures between 1 and 20 MPa. Substrate materials can be pure metals such as Ag, Au, Pt, Pd, Cu or Ni; stainless steel and nickel alloys; composites including Ag sheets on stainless steel and Ag sheets on Fe, Co, Cu, and Ni alloys, and metal/ceramic composites and ceramics. The Bi--Pb--Sr--Ca--Cu metallic precursor can contain surplus Ca or Cu. 
     In other embodiments, multiple layers whose thickness and width are controlled by adjusting processing conditions, can be applied to a substrate. Superconducting properties can be optimized by design of suitable heat treatments and multiple press/anneal sequence repetition. Selected mechanical deformation and atmospheric conditions can be combined to enhance superconductor texturing. 
     In other embodiments, the method can be used to produce high T c  superconductor coatings in a variety of geometries including large or small areas with flat, smooth surfaces and uniform thickness, long wires, and ribbons of selected thickness. Protective coatings can be pressed or rolled onto superconductor coatings using this technique. 
     Press coating can be used in combination with other superconductor preparation techniques including melt dipping and melt writing methods to further enhance superconducting and mechanical properties. Press coating can be used to join superconductors or fabricate superconductor/normal metal joints. Press coating can also be used in combination with superconducting oxides prepared by other fabrication techniques including powder processing of the individual metal oxides and pyrolysis of metal-organo precursors. 
     Compared with conventional ceramic processes or the simple metallic precursor oxidation method, the present method offers several advantages. The pressing or rolling treatment produces a dense superconductor coating. The resulting composite has good mechanical properties based on substrate strength and toughness combined with good adhesion between coating and substrate. Combined mechanical deformation and controlled atmosphere annealing produces textured microstructures, characterized by increased critical current density. This method offers considerable flexibility in fabrication of varied geometry composites including wire, narrow or wide ribbon, and small or large area coatings. Coating thickness is uniform (typically 5-200 μm), and can be accurately controlled by the pressing or rolling process. 
     The product of the oxidation and press coating method is typified by having a textured microstructure with platelike grains of superconductor phase aligned parallel to the substrate plane. The superconductor composite contains no gap between the coating and the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic illustration of the press coating process; 
     FIG. 2 is a schematic illustration of the press coating of a multi-layered superconductor composite; 
     FIG. 3 is an SEM backscattered electron (BSE) micrograph from a polished, longitudinal cross-section of multi-layered Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 21  ribbon pressed on a Ag substrate after suitable oxidation and annealing; 
     FIG. 4 is an SEM backscattered electron (BSE) micrograph from a polished longitudinal cross-section of a multi-layered Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 21  ribbon pressed on a Au substrate after suitable oxidation and annealing; 
     FIG. 5 is an SEM (BSE) cross-section micrograph for an oxidized Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 5  alloy press-coated onto a Ag substrate after suitable oxidation and annealing; 
     FIG. 6 is an SEM (BSE) cross-section micrograph for a Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 11  alloy press-coated onto a Ag substrate after suitable oxidation and annealing; 
     FIG. 7 is an SEM (BSE) micrograph for a Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 36  alloy pressed on a Ag substrate after suitable annealing; and 
     FIG. 8 is an SEM (BSE) micrograph for Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 21  alloy pressed on a Ag substrate after suitable oxidation and repeated pressing and annealing. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In a preferred embodiment, Bi--Pb--Sr--Ca--Cu--O/Ag microcomposites were fabricated on Ag, Au, Cu, Pt, Pd, Ni and silver sheet covered stainless steel substrates. 
     Precursor alloy ribbons were produced by vacuum melting and melt spinning rapid solidification of alloys with nominal compositions (atomic proportions) 1.4 Bi--0.6 Pb--2 Sr--3 Ca--4 Cu with 5, 11, 21 and 36 Ag, corresponding to 35-80 wt. % Ag. Surplus Ca and Cu in the alloys is necessary for obtaining a well developed &#34;2223&#34; superconducting phase. Ribbon thickness and width ranged typically from 50-70 μm and 2 to 3 mm respectively. Ribbons were fully oxidized at 500°-600° C. in 100% O 2  for 40 h and annealed at 820° C. in 5% O 2  +95% Ar for 8 h prior to press coating. Pure Ag sheets (≧99.9% Ag) 0.025-0.25 mm thick, Au, Pt, Pd, Ni, Cu sheets 0.051 mm thick, and stainless steel plates (≃0.3 mm) covered by 0.025 mm thick Ag sheet were used as substrates. Metallic substrates were used as received without further polishing. Substrates were cleaned with dry methanol and distilled water. 
     Press coating was conducted in a PR-22 Pneumatic Mounting Press under 2-10 MPa pressure at 160° C. Substrates of approximately 3×10 mm dimensions and ribbon were positioned between two steel dies. FIG. 1 shows one piece of ribbon 30 being pressed as indicated by arrow 31 on substrate 32 by dies 34. Alternately, substrates 10 and five layers of ribbon 12, 14, 15, 16, and 18 were positioned between dies 20 with or without lubricating teflon films 22 as shown in FIG. 2. Pressure was applied slowly in the direction given by arrow 24, held for at least 4 minutes, and released. 
     Press-coated layers were flat, with smooth, shiny surfaces and uniform thicknesses. The degree of adhesion between coating and substrates depends upon substrate properties, applied pressure and processing temperature. Substrate/coating adhesion has been qualitatively classified by visual inspection after cooling, heat treatment and slow bending to approximately 20 degree angles. 
     Coatings on Ag substrates exhibited the best adhesion, followed by those on Au, Cu and Pd substrates. Coating adhesion on Ag and Au was satisfactory, without spalling or delamination even after subsequent annealing, cooling and bending to 20 degrees. Coatings on Pt, Ni and 304 stainless steel were not adherent. Coatings on Pt, Ni or stainless steels were made adherent by pressing two substrate sheets with teflon films on both sides of the coatings, as shown in FIG. 2. 
     Pressing caused coating and substrate deformation. The extent of deformation depended on applied pressure, coating and substrate materials, processing temperature and lubrication. A thickness reduction ratio R was used to describe the deformation, as 
     
         R=t.sub.2 /t.sub.1 
    
     where t 1  and t 2  are the coating thickness before and after pressing, respectively. Table I shows thickness reduction ratio R, applied pressure, coating and substrate materials, and adhesion behavior of coated layers. 
     As shown in Table I, ratios R vary from 0.30 to 0.45 for Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 21  coatings on different substrates subject to identical pressing conditions of 8 MPa without lubrication. Deformation decreased in order for Au, Ag, Cu, Pd, Pt and Ni substrates consistent with the different hardness, ductility and contact friction characteristic of these metals. Lubricant teflon films reduced contact friction and increased deformation by 30-40%, and made behavior of different substrates more uniform. 
     Adhesion and deformation of coatings also depended on applied pressure. A ≧4 MPa pressure was needed for good coating adhesion on Ag, while pressure ≧10 MPa caused large deformation and sample edge cracking. Coating material composition did not affect the deformation ratio significantly. Coatings with different Ag concentrations exhibited similar thickness after pressing, as shown in Table I. 
     Press-coated specimens required a second anneal to ensure good superconducting properties. Such anneals removed microcracks created by press/deformation processes which interrupt superconducting grain continuity and adversely affect superconducting properties. Anneals were conducted in 5% O 2  +95% Ar atmospheres. Deformation processing and annealing were combined to optimize superconducting properties. Resulting superconducting onset temperature, T on , zero resistance temperature, T R ═O, and critical current density, J c , at 77K in zero field are listed in Table II, together with sample pressing and annealing conditions. 
     T c  and J c  were measured using a standard four-point probe technique. In J c  (77) measurements, a criterion of 1 μv/cm was used to define the critical current I c  and the entire coating cross section was used to calculate the critical current density J c  (77) Special attention was given to current passing through the Ag substrate since Ag is an excellent electrical conductor. At 77K, a pure Ag substrate showed a linear plot of voltage versus current at a scale of 1 mA, with a slope of order 0.1-1 μV/mA, while a superconducting coating on a Ag substrate did not show any voltage until the current reached I c  (approximately 1A), when a sharp transition occurred. Hence, it follows that I c  measurements were not significantly affected by the Ag substrates. 
     
                                           TABLE I__________________________________________________________________________Deformation Ratio and Pressed Coating Quality             Applied                  DeformationCoating     Substrate &amp;             Pressure                  Ratio  Surface QualityMaterial    Lubricant             (MPa)                  R(= t.sub.2 /t.sub.1).sup.*1                         &amp; Adhesion.sup.*2__________________________________________________________________________Bi.sub.1.4 Pb.sub.0.6 Sr.sub.2 Ca.sub.3 Cu.sub.4 Ag.sub.21       Ag/NL.sup.*3             8    0.35   adhesion good600C/48h + 820/8h             smooth surface       Au/NL 8    0.30   adhesion good                         smooth surface       Cu/NL 8    0.36   adhesion fair                         smooth surface       Pd/NL 8    0.38   adhesion fair                         smooth surface       Pt/NL 8    0.42   no adhesion       Ni/NL 8    0.45   no adhesionAs above    Ag/TF.sup.*4             8    0.24   adhesion good                         smooth surface       Cu/TF 8    0.25   edge cracking       Ni/TF 8    0.27   smooth surfaceAs above    Ag/NL 2    0.80   barely adhered       Ag/NL 4    0.70   adhesion fair                         smooth surface       Ag/NL 6    0.40   adhesion good                         smooth surface       Ag/NL 10   0.32   adhesion good                         smooth surface                         edge crackingBi.sub. 1.4 Pt.sub.0.6 Sr.sub.2 Ca.sub.3 Cu.sub.4 Ag.sub.3       Ag/NL 8    0.33   adhesion fair400C/48h + 820/8h             surface roughnessBi.sub.1.4 Pb.sub.0.6 Sr.sub.2 Ca.sub.3 Cu.sub.4 Ag.sub.11       Ag/NL 8    0.35   adhesion fair600C/48h + 820/8h             smooth surfaceBi.sub.1.4 Pb.sub.0.6 Sr.sub.2 Ca.sub.3 Cu.sub.4 Ag.sub.36       Ag/NL 8    0.35   adhesion good600C/48h + 820/8h             smooth surface__________________________________________________________________________ .sup.*1 Obtained by pressing the coating with two substrates on both sides; t.sub.1 and t.sub.2 are the coating thickness before and after pressing. .sup.*2 Obtained by pressing the coating to a metal substrate. .sup.*3 NL indicates pressing without lubrication. .sup.*4 TF indicates pressing with teflon sheets as lubricant. 
    
     
                                           TABLE II__________________________________________________________________________Pressing, Oxidation and Annealing Conditions With ResultantSuperconducting Properties                               T.sub.on /T.sub.R=0                                    J.sub.c (77)Alloy   Substrate         Treatment             K    A/cm.sup.2__________________________________________________________________________Bi.sub.1.4 Pb.sub.0.6 Sr.sub.2 --   Ag    melt writing, without pressing,                               116/106                                    500Ca.sub.3 Cu.sub.4 Ag.sub.21         600/40h + 820/72h   Ag    600/40h + 820/80h + P.sub.1 .sup.*1 + 820/72h                               116/105                                    800   Au    same as above         114/102                                    400   Pt    same as above         70/--                                    --   Pd    same as above         110/75                                    --   Cu    same as above         80/72                                    --   Ni    same as above         76/71                                    --   Ag/S.S..sup.*2         same as above         114/102                                    450   Ag    600/40h + 820/80h + P.sub.2 .sup.*3 + 820/72h                               116/107                                    1200   Ag    600/40h + 820/80h + P.sub.2 + 820/72h + P.sub.2                               116/107                                    2000         800/48hBi.sub.1.4 Pb.sub.0.6 Sr.sub.2 --   Ag    500/40h + 820/80h + P.sub.1 + 830/72h                               116/100                                    500Ca.sub.3 Cu.sub.4 Ag.sub.5Bi.sub.1.4 Pb.sub.0.6 Sr.sub.2 --   Ag    500/40h + 820/80h + P.sub.1 + 820/72h                               114/104                                    700Ca.sub.3 Cu.sub.4 Ag.sub.11Bi.sub.1.4 Pb.sub.0.6 Sr.sub.2 --   Ag    600/40h + 820/80h + P.sub.1 + 815/72h                               116/104                                    750Ca.sub.3 Cu.sub.4 Ag.sub.36__________________________________________________________________________ .sup.*1 P.sub.1 = Pressing with two substrates on both sides at 150-200C, pressure = 8MPa. .sup.*2 Ag/S.S. = Ag sheets covered stainless steel (304). .sup.*3 P.sub.2 = Pressing with two substrates on both sides at 150-200C, with teflon films between dies and substrates, pressure = 8MPa. 
    
     As shown in Table II, the pressed coatings on all the substrates used in the present work exhibited superconductivity after annealing. Those on Ag and Au substrates showed T R ═O ≧100K, indicating that a well developed &#34;2223&#34; superconductor phase was formed. The coating on Pd showed T on  ═˜110K, but T R ═O ═70-80K, reflecting co-existence of &#34;2223&#34; and &#34;2212&#34; superconducting phases. Coatings on Cu and Ni showed superconducting transition temperatures of 70-80K, indicating that the superconducting phase was mainly the &#34;2212&#34; phase. Coatings on Pt exhibited superconducting onset temperatures around 70K, with no zero resistance temperatures. Reactions between coatings and certain of these substrates during annealing affected the formation of the superconducting phases. The &#34;2223&#34; phase did not form after coating reaction with Pt, Cu and Ni substrates. The &#34;2212&#34; superconducting phase, however, survived the reactions, probably because of its greater stability. 
     Substrate/coating reactions can be avoided by covering substrates with Ag sheets. Coatings pressed on Ag covered stainless steel(304) substrates exhibited T R ═O ≧100K and J c  (77)=450 A/cm 2 , comparable to results with Ag and Au substrates. 
     Microstructural observation and microanalysis were performed with a JEOL Superprobe 733 Microanalyzer equipped with Tracor Northern 5500-5600 WDS and EDS systems. Backscattered electron images (BSE) show contrast between phases of differing chemical composition. FIGS. 3 and 4 show two BSE micrographs of longitudinal cross sectional microstructure of multilayer ribbons of Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 21  pressed on Ag and Au substrates. The ribbons were oxidized at 600° C. in 100% O 2  for 40 h, then annealed at 820° C. in 5% O 2  +95% Ar for 8 h before pressing. Pressing was conducted at 8 MPa and both sides of the coating were covered by substrates. Specimens were reannealed at 820° C. in 5% O 2  +95% Ar for 72 h. Substrates are marked to indicate Ag or Au, bright areas are Ag, plate-like, light gray grains are &#34;2223&#34; superconducting phase, and the dark grains are non-superconducting oxides. 
     As shown in FIG. 3, no gap is visible between the coating and Ag substrate, indicating that there is no coating/substrate reaction. Coating adhesion is very good. In FIG. 4, a dark area close to the Au substrate indicates that some reaction took place at that interface. The affected area was thin (5-10 μm wide), and did not compromise superconducting properties significantly. 
     FIGS. 5, 6 and 7 are BSE micrographs of Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 4 , Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 11 , and Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 36  coatings on Ag substrates oxidized sequentially at 500° C., 550° C. and 600° C. in 100% O 2  for 40 h, annealed at 820° C. in 5% O 2  +95% Ar for 8 h, pressed with 8 MPa and both sides covered with Ag, and reannealed at 820° C. in 5% O 2  +95% Ar for 72 h. The Ag, &#34;2223&#34; superconducting phase, and non-superconducting phases have contrasts similar to those in FIGS. 3 and 4. The &#34;2223&#34; superconducting phase (platelike, gray grains) was well developed in all three specimens consistent with the T c  and J c  listed in Table II. The microstructure shows apparent texture with Ag and plate-like &#34;2223&#34; grains aligned parallel to the substrate plane, especially for the higher Ag content specimens shown in FIGS. 6 and 7. Texturing produced by pressing deformation enhanced J c . 
     FIG. 8 is a BSE micrograph for a Bi 1 .4 Pb 0 .6 Sr 2  Ca 3  Cu 4  Ag 21  coating pressed on a Ag substrate by repeated pressing and annealing. Processing included oxidation at 600° C. for 40 h, annealing at 820° C. for 8 h, pressing at 8 MPa with Ag sheets and teflon films, annealing at 820° C. for 72 h, pressing under the same conditions, and reannealing at 800° C. for 48 h. The &#34;2223&#34; phase exhibited better developed texture than is visible in FIGS. 3, 4, 5, 6 or 7. The coating was thin (approximately 20 μm), and J c  (approximately 2000 A/cm 2 ) was further improved.