Abstract:
A method of preparing a superconducting oxide by combining the metallic elements of the oxide to form an alloy, followed by oxidation of the alloy to form the oxide. Superconducting oxide-metal composites are prepared in which a noble metal phase intimately mixed with the oxide phase results in improved mechanical properties. The superconducting oxides and oxide-metal composites are provided in a variety of useful forms.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending application Ser. No. 07/547,911 filed on Jul. 3, 1990, now abandoned, which is a divisional of U.S. Ser. No. 07/061,233 filed Jun. 10, 1987; which is a continuation-in-part of U.S. Ser. No. 07/031,407, filed Mar. 27, 1987, now U.S. Pat. No. 4,826,808, issued May 2, 1989. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to superconducting materials. 
     Superconductors are materials having essentially zero resistance to the flow of electrons below a certain critical temperature, T c . It is known that certain metal oxides, e.g., La 2-x  Ba x  CuO 4-y , La 2-x  Sr x  CuO 4-y , Ba 2  YCu 3  O 9-y , etc. exhibit superconductivity. It has also been discovered by the inventors herein that a europium-barium-copper oxide compound in which the europium, barium and copper are in a 1-2-3 ratio, that is, EuBa 2  Cu 3  O x , ratio exhibits superconductivity with a transition temperature at approximately 85° K. It is desirable to provide such oxides in forms, e.g., wires or thin films, that permit practical utilization of their superconductive property. 
     SUMMARY OF THE INVENTION 
     In general, the invention features in one aspect combining the metallic elements of the desired superconducting oxide to form an alloy, and oxidizing the alloy to form the superconducting oxide. The alloy can be formed, e.g., into a wire, ribbon, sheet, rod, or ring. The solid alloys used to make these articles can be produced from the liquid state by rapid or conventional solidification processing techniques to produce, e.g., ribbons, powders, flakes, ingots, sheets, or cast shapes. Rapid solidification processing techniques include melt spinning to produce ribbons and inert gas atomization to produce powders or spray deposits. Conventional solidification processing techniques include chill casting, ingot casting, die casting, and centrifugal casting. Thermal-mechanical processing of the solid alloys can be employed to form them into final, useful shapes before oxidizing the alloys into the superconducting oxide. Thermal-mechanical processing techniques include wire drawing, extrusion, co-extrusion, hot isostatic pressing, and rolling. 
     The alloy can also be provided in the form of a relatively thick coating, e.g., on a tube, wire, rod, or shaped article such as a ring. Relatively thick coatings of the alloys can be produced by coextrusion of the alloy powder with a billet of a substrate metal or metal alloy to form wires, rods, or tubes. Alloy coatings can also be produced by plasma spraying or sputtering the alloy constituents onto a substrate metal or metal alloy that can be in many useful shapes, e.g., a tube formed into the shape of a ring. Furthermore, alloy coatings can be produced by hot dipping the substrate metal or metal alloy, e.g., in a wire form, in the liquid alloy. 
     The alloy before oxidation can also be in the form of a thin film on a metallic, insulating, or semiconductor substrate, e.g., as used in a Josephson junction device of an integrated circuit. Thin films of the alloy can be produced by chemical vapor deposition, vacuum evaporation, sputtering, molecular beam epitaxy, ion-beam mixing, and ion implantation. 
     In another aspect the invention features a superconducting oxide-metal composite in which a noble metal phase (noble in the sense that its oxide is thermodynamically unstable under the reaction conditions employed relative to the superconducting oxide that forms) is intimately mixed with a superconducting oxide phase to achieve desired mechanical properties. In preferred embodiments, the noble metal is present initially as an alloying element with the metallic elements of the oxide; the alloy is then oxidized under conditions that convert the metallic elements of the oxide to the superconducting oxide without oxidizing the noble metal. The latter precipitates as a finely divided, substantially pure metal phase (rather than as a second oxide phase) that is intimately mixed with the superconducting oxide in the final composite, the oxide phase being continuous (or nearly so) throughout the composite. The noble metal may be a metallic element different from the metallic elements of the oxide, e.g., Au, Pt, Pd, or Ag, but may also be an excess amount (stoichiometrically) of one of the metallic elements of the oxide, e.g., Cu. The superconducting oxide-metal composites exhibit improved mechanical properties (strength, ductility, etc.) because these properties are dominated by the metallic phase, rather than by the brittle oxide phase. 
     Examples of suitable oxides are described in the following references, hereby incorporated by reference and made part of this application: Chu et al. (1987) Phys. Rev. Lett. 58(4) 405-07; Cava et al. (1987) Phys. Rev. Lett. 58(4) 408-10; Wu et al. (1987) &#34;Superconductivity at 93K in a New Mixed Phase Y-Ba-Cu-O Compound System at Ambient Pressure&#34; (submitted for publication, copy attached); Hor et al. (1987) &#34;High Pressure Study of the New Y-Ba-Cu-O Superconducting Compound System&#34; (submitted for publication, copy attached); Tarascon et al. (1987) &#34;Superconductivity at 90K in a Multi-Phase Oxide of Y-Ba-Cu&#34; (submitted for publication, copy attached); Gleick, &#34;Superconductivity: A New Era of Discovery for Electricity,&#34; N.Y. Times, Mar. 10, 1987; and Tarascon et al. (1987) Science 235: 1373-76. They include, e.g., oxides of La, Ba, and Cu; La, Sr, and Cu; Lu, Ba, and Cu; Lu, Sr, and Cu; and Y, Ba, and Cu. The invention is of course applicable to any superconducting oxide. A europium-barium-copper oxide has been found to be particularly preferred, namely EuBa 2  Cu 3  O x . This invention encompasses other compounds including europium. 
     Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the invention are described by way of the following examples. 
     EXAMPLE 1 
     A superconducting oxide of La, Ba, and Cu is prepared as follows. 
     Pure La, Ba, and Cu (73.3 wt. % La, 8.1 wt. % Ba, and 18.6 wt. % Cu) are melted under vacuum in an induction melting furnace in a melt spinning apparatus. The liquid alloy is heated to approximately 800° C., and then melt spun to produce a ribbon of the alloy. 
     The alloy ribbon is oxidized by heating at a constant temperature of 450° C. in flowing pure oxygen gas until it is almost completely oxidized. The temperature is then raised to 1000° C. and held there until the sample weight is approximately constant and the oxide is homogeneous with respect to composition. The temperature is then reduced to 490° C. and maintained at this temperature for approximately 18 h. The temperature is then reduced to room temperature. 
     EXAMPLE 2 
     A superconducting oxide-metal composite, in which the oxide phase is an oxide of La, Ba, and Cu, and the metallic phase is a noble metal such as Ag, is prepared following the procedure describe in Example 1 is except that Ag metal is melted together with La, Ba, and Cu to form the alloy, the initial oxidation step is at 400° C., and the maximum oxidation temperature is less than the melting point of Ag metal (960° C.). During oxidation Ag is not oxidized, but rather precipitates out as a separate phase of substantially pure Ag. The metal phase, by being intimately mixed with the oxide phase, acts as a &#34;skeleton&#34; in the composite, resulting in improved ductility and strength. 
     EXAMPLE 3 
     A superconducting oxide-metal composite is prepared as in Example 2 except that the noble metal is excess Cu, rather than Ag metal. During oxidation, the temperature, oxygen partial pressure, and reaction time are selected to achieve the oxide stoichiometry required for superconductivity without oxidizing the excess Cu metal to Cu 2  O and/or CuO. Thus, the final composite consists of a superconducting La-Ba-Cu oxide phase and a substantially pure Cu metallic phase. 
     EXAMPLE 4 
     The alloy ribbon prepared in Example 1, 2, or 3 can be formed into a shape, e.g., a ring or coil, prior to oxidation and then oxidized to provide a superconducting oxide or oxide-metal composite in the desired shape. 
     EXAMPLE 5 
     A superconducting oxide or oxide-metal composite is prepared as in Examples 1, 2, or 3 but in the form of a wire by providing the alloy initially as a hot isostatically pressed compact of a rapidly solidified powder, ground-up rapidly solidified ribbon, or cast billet, and then forming it into a wire by wire drawing. The wire is then shaped by winding it around a metallic core and oxidized to prepare superconducting magnets. The wire can also be oxidized prior to shaping. 
     EXAMPLE 6 
     The alloy prepared in Examples 1, 2, or 3 is prepared as a rapidly solidified powder, rather than as a ribbon, and then hot isostatically pressed to form a useful shape, e.g., a ring. The shaped article is then oxidized to form a superconducting oxide or oxide-metal composite article. 
     EXAMPLE 7 
     A superconducting oxide or oxide-metal composite is prepared as in Example 1, 2, or 3 except that the alloy is prepared by melting, then atomizing the liquid alloy using inert gas atomization to form a powder. The alloy powder is coextruded with a billet of a metal or metal alloy to yield a composite wire, rod, or tube. The coextruded product consists of a metal core coated with the alloy. The tube may be coated on the inside or outside surface, or both. The coating is then oxidized to form the superconducting oxide or oxide-metal composite. In the case of the superconducting oxide-metal composites, the metal or metal alloy substrate can form a metallurgical bond with the noble metal phase of the composite, thereby promoting adhesion of the coating. 
     EXAMPLE 8 
     An alloy coating is prepared by running a metal or metal alloy wire through a bath of the molten alloy to form the coating. The hot dipping process is carried out in vacuo or under an inert atmosphere to avoid premature oxidation of the alloy. The coating is then oxidized to form the superconductive oxide or oxide-metal composite as in Examples 1, 2, or 3. 
     EXAMPLE 9 
     An alloy is prepared as in Examples 1, 2, or 3 but in the form of a coating on a substrate by spray deposition, sputtering, or plasma spraying. Oxidation produces a superconductive oxide or oxide-metal composite coating. 
     EXAMPLE 10 
     A thin film of the alloy described in Examples 1, 2, or 3 is deposited by, e.g., chemical vapor deposition, vacuum evaporation, sputtering, molecular beam epitaxy, ion beam mixing, or ion implantation, on a metallic, insulating, or semiconducting substrate. The thin film is then oxidized to form a thin film superconducting oxide or metal-oxide composite. This procedure is particularly useful in integrated circuit fabrication, and can be used to produce, e.g., Josephson junction devices. 
     Other embodiments are within the following claims.