A method of preparing a superconducting oxide and superconducting oxide metal composites

Abstract: A superconducting oxide is prepared 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: 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 been discovered 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 , exhibits super-conductivity 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. In the past such superconducting oxides were prepared by mixing the individual metal oxide particles to a powdered mixture, forming said mixture into a desired shape, especially into a wire or a ribbon, and sintering and annealing said shaped body in a controlled atmosphere. An essential disadvantage of these superconductive oxide products resides, however, in the mechanical properties thereof, especially in their extremely high brittleness and hardness which considerably impede processing and forming and, thus, are an obstacle to a universal application. In the sintering process and especially in the subsequent cooling microcracks come into existence which, during application, hinder the current passage in a similar way as in the case of inhomogeneous areas in the sintered body, and which lead, due to the increased electrical resistance during operation, to a local heating beyond the critical temperature T. In a portion of non-prepublished EP-A-0 283 024 based on JP Patent Application 67 647/87 there is disclosed a method for manufacturing a superconductor containing a ceramic superconductive oxide. In this method the metal elements of the superconductive oxides will be melted to an alloy and this alloy will be formed to a fine wire of about 100 µm diameter or to a thin ribbon of about 100 µm thickness by passing through a fine hole or nozzle. For yielding an amorphous structure the wire or ribbon will be injected into a liquid cooling medium. Then, the rapidly solidified amorphous wire or ribbon is oxidized in a furnace for a time period of 2 hours at a high temperature of more than 1000 °C in a controlled oxigen atmosphere in order to obtain a superconductive conductor containing a ceramics superconductive material. Further, in the non-prepublished EP-A-0 282 839 there is disclosed the preparation of a film of superconductive oxides by vapour deposition processes in which multiple metal sources are used. The pure metal vapours of each of these sources travel to a heated substrate which is exposed to an oxygen ambient for forming a metal oxide film of 1000 Å to 1 µm thickness on the substrate. Afterwards this film will be annealed at a high temperature of about 900 °C in an oxygen atmosphere for obtaining superconductive properties. Further in a portion of the non-prepublished EP-A-0 282 286 based on JP-A-56 856/87 there is disclosed a method for producing a compound superconductive silver wire containing a predetermined amount of superconductive oxides in the silver matrix. In said method a melted silver alloy containing 70 to 90 wt.% silver and the elements that form a superconductive oxide will be formed to a wire or ribbon by preferably swaging, wire drawing or rolling. The prepared silver alloy wire is then heated for a predetermined time period at a temperature between 500 and 900 °C in an oxidizing atmosphere to form perovskite-type superconducting oxides in the silver matrix. According to a first aspect of the invention a method for preparing superconductive oxides is defined in claim 1. According to a second aspect of the invention a method for preparing superconductive oxide-metal composites is defined in claim 2. 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) "Superconductivity at 93K in a New Mixed Phase Y-Ba-Cu-O Compound System at Ambient Pressure" (submitted for publication, copy attached); Hor et al. (1987) "High Pressure Study of the New Y-Ba-Cu-O Superconducting Compound System" (submitted for publication, copy attached); Tarascon et al. (1987) "Superconductivity at 90K in a Multi-Phase Oxide of Y-Ba-Cu" (submitted for publication, copy attached); Gleick, "Superconductivity: A New Era of Discovery for Electricity," N.Y. Times, Mar. 10, 1987; 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 present invention encompasses other compounds including europium. The invention is hereinafter more particularly described by way of example only with reference to 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 18h. 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 described in Example 1 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 "skeleton" 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 vapour 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. European Patent Application 0 283 024 in the name of Sumitomo Electric Industries Limited has been drawn to our attention. This Sumitomo application designates DE, FR and GB, and was published on 21 September 1988, which is after the filing date of this current application, and claims priority from Japanese Patent Applications JP 67647/87 filed on 20 March 1987 and JP 81817/87 filed on 2 April 1987. JP 67647/87 which was filed before the priority date of this present application describes a method of manufacturing a superconductive conductor containing a ceramics superconductive material generally expressed by a formula AaBbCc, where A represents at least one element selected from groups IIa and IIIa of the periodic table, B represents at least one element selected from groups Ib, IIb and IIIb of the periodic table, and C represents at least one element selected from oxygen, carbon, nitrogen, fluorine and sulphur said method comprising: a step of melting said AaBb and injecting the same into a cooling medium through a hole to cool and solidify the same; and a step of heating the same in an atmosphere containing said C. Also disclosed is heating of La 1.66 Sr 0.34 Cu in a crucible to prepare a tape-like body of 30µm thickness and 60mm width by the so-called roll quench method. This substance is then heated in a furnace of 1000°C under oxygen partial pressure of 200 Torr for two hours, to obtain a superconductive conductor.

Claims: A method of preparing a superconducting oxide, characterized by combining the metallic elements of said oxide to form an alloy, forming said alloy into a desired shape, and oxidizing the formed alloy in an oxidizing step at a first temperature in oxygen or in an oxygen containing atmosphere, raising the temperature to a second temperature higher than the first temperature to form the superconducting oxide phase in a second heating step at that second temperature until the weight of the sample is constant. A method of preparing a superconducting oxide metal composite, characterized by combining the metallic elements of said oxide and a noble metal to form an alloy; forming said alloy into a desired shape, and oxidizing the formed alloy in an oxidizing step at a first temperature in oxygen or in an oxygen containing atmosphere, raising the temperature to a second temperature higher than the first temperature and lower than the melting temperature of the noble metal to form the superconducting oxide phase intimately mixed with the noble metal phase in a second heating step at that second temperature until the weight of the sample is constant. Method according to claim 2, characterized in that said noble metal comprises a stoichiometric excess of one of said metallic elements of said oxide, preferably Cu. Method according to any of the claims 1 to 3, characterized in that said alloy is formed by melting said metallic elements together, followed by a rapid solidification processing. Method according to claim 4, characterized in that said rapid solidification processing comprises melt spinning or inert gas atomisation. Method according to claim 5, characterized in that the powder prepared by said inert gas atomisation is coextruded with a metal billet to yield a composite wire, rod or tube. Method according to any of the claims 1 to 3, characterized in that said alloy is formed by melting said metallic elements together, followed by a casting process. Method according to any of the claims 1 to 7, characterized in that said alloy is formed into said shape of preferably a wire, ribbon, sheet, rod or ring, by wire drawing, extrusion, coextrusion, hot isostatic pressing or rolling. Method according to claim 8, characterized in that the wire is wrapped around a metallic core in the preparation of a superconducting magnet. Method according to any of the claims 1 to 3, characterized in that said alloy is formed into a thin film by chemical vapour deposition, vacuum evaporation, spattering, molecular beam epitaxy, ion-beam mixing or ion implantation. Method according to any of the claims 1 to 10, characterized in that said metallic elements are chosen from the group consisting of Lu, La, Ba, Cu, Y and Sr. Method according to claim 11, characterized in that said metallic elements comprise La, Ba and Cu; or La, Sr and Cu; or Y, Ba and Cu. Method according to any of claims 1 to 12, characterized in that said metallic elements comprise Lu, Ba and Cu; or Lu, Sr and Cu; or Eu, Ba and Cu, preferably in a 1-2-3-ratio. Method according to any of claims 1 to 13, characterized in that the metallic elements are melted under vacuum to an alloy, the liquid alloy is heated to approximately 800 °C and then formed, the formed alloy is oxidized by heating at a constant temperature of 400 to 450 °C in flowing oxygen gas until it is almost completely oxidized and the temperature is then raised to no greater than 1000 °C or in the case of the presence of noble metal to a temperature less than the melting point of the noble metal, said raised temperature is held until the sample weight is approximately constant and the superconducting oxide is homogeneous with respect to the composition, and the temperature is then reduced to an intermediate value for a predetermined time and then reduced to room temperature.

Inventors: VANDERSANDE JOHN B, YUREK GREGORY J, VANDERSANDE, JOHN B., YUREK, GREGORY J.
Applicants: MASSACHUSETTS INST TECHNOLOGY, MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Application year: 1988
Main classification: H01L39
IPC codes: H01L39, C01G1, H01B12, C01G3, C01B13, C04B35, H01B13, C23C16