PROTECTION OF SUPERCONDUCTORS FROM DEGRADATION

Abstract: A method of treating an oxide superconductor to prevent degradation thereof comprises treating the superconductor with a solution of a metal which is capable of being oxidised by the superconductor to form on those regions of the surface area of the superconductor treated with the solution an insulating, non-porous protective film of an oxide which is stable against further oxidation.

Description: PROTECTION OF SUPERCONDUCTORS FROM DEGRADATION The present Invention relates to the protection of oxide superconductors from degradation. The recently discovered oxide superconducting ceramic materials, with superconducting transition temperatures <T C ) In the range above that of liquid nitrogen (77°K), are technologically very exciting materials. However, they are not known to suffer extensive chemical degradation 1n the atmosphere, particularly 1n the presence of water vapour and carbon dioxide. In addition, they are highly oxidising and are able to oxidise metals, including gold, in bulk solution and in condensed moisture films. This reactivity will limit their eventual use and methods of eliminating the degradation must be sought prior to commercial exploitation. It 1s therefore an object of the present invention to provide a method of protecting such superconductors from degradation. According to the present Invention there 1s provided a method of treating an oxide superconductor to prevent degradation thereof the method comprising treating the superconductor with a solution of a metal which Is capable of being oxidised by the superconductor to form on those regions of the surface area of the superconductor treated with the solution an Insulating, non-porous protective film of an oxide which 1s stable against further oxidation. The Invention also provides an oxide superconductor 1n which at least a portion of the surface area of the superconductor 1s coated with an insulating, non-porous protective film of a metal oxide which is stable against further oxidation by the superconductor. The invention Is applicable particularly to the treatment of copper-containing superconducting oxides. All oxide superconductors containing copper and/or an oxidised cuprate peroxide complex will be subject to degradation as described above and will therefore be suitable for treatment. Classes of copper containing superconductors to which the invention is applicable are given in Table I, together with specific examples of each class. TABLE 1 Classes of super conductor Examples La-Ba-Cu-0 La χ 8 Sr 0 2 Cu0 _ x Y-Ba-Cu-0 Ba 2 Cu 3 0 7 . x (Pb) Bi-Sr-Ca-Cu-0 B1 2 Sr 3 _ χ Ca x Cu 2 0 8 _ x Tl-Ba-Ca-Cu-0 Tl 2 Ba 2 CaCu 3 0 10 _ χ In addition rare earth, and other, elements may dope the above. It has been found that the provision of the metal oxide film acts as an effective barrier to protect the superconductor from corrosion and environmental degradation. The oxide film 1s so effective that untreated superconductors which would normally be attacked and quickly destroyed by atmospheric moisture are (when treated In accordance with the Invention) totally stable when exposed to the atmosphere. The method of the Invention can most conveniently be effected by dipping the superconductor into the solution of the metal ions which give rise to the protective oxide film and simultaneously applying an appropriate oxidising electrochemical potential. For superconductors which have a degree of porosity, the solution will penetrate the interstices within the superconductor so that the protective metal oxide is deposited on the Interior surface area. The metal ion in the treatment solution must be one which can be oxidised on the superconductor surface to give an oxide film which 1s stable against oxidation by the highly oxidising superconductor. Suitable metal ions include Mn 2+ , Ti 3+ , Pb 2+ and Sn 2+ . In the case where the metal ion 1n an aqueous treatment solution is Mn 2+ and the superconductor contains Cu 3+ ions, the reaction process is essentially Mn 2+ + 2H 2 0 = Mn0 2 + 4H + + 2e~ In the above example, the oxidation of Mn 2+ to Mn0 2 may also be effected by an appropriate applied external potential (voltage). The preferred concentration of the metal 1on will depend on the particular metal, and the preferred ranges for Mn 2+ , T1 3+ , Pb 2+ and Sn 2 are given in Table 2 below which also Includes the preferred (addle) pH of the medium. TABLE 2 loϋ Concentration JJH Mn 2+ 0.01 to 0.5M <5 T1 3+ 10 "3 to 0.1M <2 Pb 2+ 0.01 to 0.05M <6 Sn 2+ 10" 3 to 0.01M <1 In all cases the metal 1on may be used In aqueous solution or a low water activity solvent (e.g. acetic add). Preferably the solution 1s a buffered solution, and preferably a potential of >0.8V (referred to the Standard Hydrogen Scale) 1s applied. Preferred treatment solutions are equlmolar Mn acetate and acetic acid (pH ca 4 to 5 ) , equlmolar Pb acetate and acetic add (pH ca 4 to 5), or solutions of SnCl 2 or T1C1 3 within the respective molarity ranges given above acidified by HC1 to the given pH range. Preferred electrochemical potentials are E <0.8 V S H E (most superconductors In the solutions described will take up this potential naturally). The time for which the superconductor is treated with the treatment solution will generally be in the range 1 to 10 minutes. Most preferably the treatment is effected to produce an oxide film with a thickness in the range l-10μm.

Claims: CLAIMS 1. A method of treating an oxide superconductor to prevent degradation thereof characterised in that 1t comprises treating the superconductor with a solution of a metal which is capable 5 of being oxidised by the superconductor to form on those regions of the surface area of the superconductor treated with the solution an insulating, non-porous protective film of an oxide which is stable against further oxidation. 2. A method of treating an oxide superconductor to prevent 10 degradation thereof as claimed in claim 1 characterised 1n that 1t comprises dipping the superconductor Into a solution of metal Ions which give rise to a protective oxide film and simultaneously applying an appropriate oxidising electrochemical potential. 153. A method of treating an oxide superconductor to prevent degradation thereof as claimed 1n claim 2 characterised in that the metal ions are selected from the class including Mn 2+ , T1 3+ , Pb 2+ and Sn 2+ . 4. A method of treating an oxide superconductor to prevent 20 degradation thereof as claimed In claim 3 characterised in that the metal ions are Mn 2+ In a solution of concentration 0.01 to 0.5M. 5. A method of treating an oxide superconductor to prevent degradation thereof as claimed in claim 4 characterised 1n that 25 the pH of the solution Is <5. 6. A method of treating an oxide superconductor to prevent degradation thereof as claimed 1n claim 4 characterised 1n that the treatment solutions are equimolar Mn acetate and acetic add. 7. A method of treating an oxide superconductor to prevent 30 degradation thereof as claimed 1n claim 3 characterised in that the metal ions are Pb 2+ in a solution of concentration 0.01 to 0.05M. 8. A method of treating an oxide superconductor to prevent degradation thereof as claimed 1n claim 7 characterised 1n that 35 the pH of the solution is <6. 9. A method of treating an oxide superconductor to prevent degradation thereof as claimed in claim 4 characterised in that the treatment solutions are equlmolar Pb acetate and acetic add. 10. A method of treating an oxide superconductor to prevent degradation thereof as claimed in claim 3 characterised in that the metal ions are Sn 2+ in a solution of concentration 10~ 3 to 0.01M. 11. A method of treating an oxide superconductor to prevent degradation thereof as claimed 1n claim 10 characterised In that the pH of the solution 1s <1. 12. A method of treating an oxide superconductor to prevent degradation thereof as claimed 1n claim 11 characterised in that the treatment solutions are SnCl 2 and hydrochloric add. 13. A method of treating an oxide superconductor to prevent degradation thereof as claimed 1n claim 3 characterised In that the metal Ions are T1 3+ 1n a solution of concentration 10" "3 to 0.1M. 14. A method of treating an oxide superconductor to prevent degradation thereof as claimed 1n claim 13 and the pH of the solution 1s <2. 15. A method of treating an oxide superconductor to prevent degradation thereof as claimed 1n claim 11 characterised In that the treatment solutions are T1C1, and hydrochloric add. 16. A method of treating an oxide superconductor to prevent degradation thereof characterised in that 1t comprises treating the superconductor with a solution of a metal which is capable of being oxidised by the superconductor and applying an external voltage to oxidise said metal. 17. A method of treating an oxide superconductor to prevent degradation thereof as claimed in claim 16 characterised in that a potential of >0.8V (referred to the Standard Hydrogen Scale) 1s applied. 18. A method of treating an oxide superconductor to prevent degradation thereof as claimed in any one of the preceding claims characterised in that the time for which the superconductor 1s treated with the treatment solution is in the range 1 to 10 minutes. 19. An oxide superconductor in which at least a portion rf the surface area of the superconductor is coated with an insulating, non-porous protective film of a metal oxide which 1s stable against further oxidation by the superconductor. 20. An oxide superconductor as claimed in claim 19 coated with an oxide film with a thickness in the range l-10μm.

Inventors: HEPBURN BEVERLEY JUDITH, LYON STUART BREINGAN, HEPBURN, BEVERLEY, JUDITH, LYON, STUART, BREINGAN
Applicants: UNIV MANCHESTER, THE UNIVERSITY OF MANCHESTER INSTITUTE OF SCIENCE
Application year: 1990
Main classification: H01L39
IPC codes: C01B13, C01G1, C04B41, C25D9, H01L39