GB2204033A - Method of forming superconductive ceramic material - Google Patents

Abstract

Certain ceramic materials produced by specified heat treatment have been found to exhibit superconductivity at temperatures considerably higher than the liquid helium temperatures previously necessary for superconducting alloys. The forming of such superconducting ceramics into useful shapes and components presents difficulties owing to the inherent inflexible characteristics of ceramics. The present invention overcomes this problem by forming the superconductive ceramic from organic solutions of organic compounds of suitable cations. Such solutions may yield a superconducting ceramic powder directly by pyrolysis or more usefully the solutions may be evaporated to produce a gel which may be applied as a coating on a substrate, reduced to a powder by rapid heat treatment or evaporated slowly to produce a monolithic ceramic. The powder may be combined with an organic binder, and flowed into a sheet which is dried and may be cut into tape. The tape may then be wound to the required shape and fired.

Description

A Method of forming Superconductive Ceramic Material This invention relates to a method of forming superconductive ceramic material and to electrical components and equipment incorporating such material formed by this method.

Until comparatively recently superconducting materials have consisted of metal alloys, all of which had the considerable disadvantage of requiring liquid helium for their operation, since the transition to the superconducting state is below the boiling point of other coolants. Liquid helium is expensive and requires elaborate and costly cryostatic equipment for its utilisation. Recently, superconductivity has been reported in ceramic materials at or about liquid nitrogen temperatures (77K). This offers a significant increase in scope for superconducting devices. At the same time, however, the development of such refractory materials places restrictions on the use of conventional fabrication methods since ceramics are non-ductile materials, difficult to shape into the thin sheet and wire forms required for practical devices. (See Bednorz J.G., Muller K.A.Z Phys. B64, 189, 1986).

Ceramic superconductors are found in, for example, oxygen deficient perovskite structures, in systems such as Bax Lay Cuz O6 or Bax Yy Cuz O & ; the superconducting transition temperature Tc being dependent largely on 6 , and x, y, z being related. These ceramics have been prepared by conventional powder routes employing, for instance, the carbonates of alkaline earth metals (Ba, Sr, Ca) and oxides of other constitutents, (La, Cu, Y etc).

Some disadvantages of these known methods are: (i) Extensive milling and attrition is required for mixing and particle size reduction, resulting in impurity pick-up.

(ii) Particle size dependence.

(iii) There is a practical lower limit to particle size resulting from a non-linear increase in cost with decreasing size.

This lower limit influences devices that can be produced.

(iv) Difficulty of producing uniform thin films, and coatings on substrates.

An object of the present invention is therefore to provide a method of manufacturing, and in particular, forming, superconductive ceramic materials which does not suffer from the above disadvantages of the conventional method.

According to one aspect of the present invention, a method of forming superconductive ceramic materials comprises the steps of providing organic compounds of constituent metals of superconductive ceramics, dissolving these organic compounds in a volatile organic solvent, heating the solution to remove extraneous constituents and shaping and firing the resultant material to form a superconductive ceramic.

The heating of the solution may include a gelling stage in which the solution is converted to a gel. The shaping step may comprise forming the gel into a mass of predetermined shape, the resulting gel mass being then evaporated with a slow increase of temperature to produce a monolithic ceramic body. The slow increase of temperature may extend over a number of days up to a maximum temperature of 100000 in an atmosphere of excess oxygen.

Alternatively, the gel may be rapidly heated to a maximum temperature of 100000 in an atmosphere of excess oxygen to produce a finely divided powder.

The gel may be applied as a coating to a metal or insulator component. In this case, the coating may be locally fired e.g. by a laser or electron beam heat treatment, to produce superconductive tracks.

In another alternative the solution may be pyrolysed in alcohol by ignition at a temperature in excess of 70000 to produce a finely divided powder. In this case the pyrolysed products are preferably continuously agitated in excess air to remove carbonaceous material.

A fluid organic binder may be added to the ceramic powder, the powder/binder mix may be spread onto a film, and the resulting sheet dried and cut into sheet or strip form. The film may alternatively be arranged to move as the mix is applied to it, the thickness of the resulting sheet being controlled by a doctor blade.

Flexible tape may thus be produced in an un-fired state, the tape then being wound and fired to form a superconducting electrical component.

A fluid organic binder may be added to the ceramic powder and the powder/binder mix extruded to produce a wire-like material for winding and firing to form a superconductive electrical component.

The un-fired ceramic material is preferably fired at a temperature between 90000 and 103000 to induce a superconductive state.

Several methods of forming superconductive ceramic material in accordance with the invention will now be described, by way of example.

The essence of the method is the use of a solution of organic compounds of the constitutents of a superconductive ceramic.

While finely divided superconductive ceramic powders can be produced directly from the solution, as will be explained, the development of a gel from the solution can provide a very versatile material applicable to a wide range of applications. Examples of this 'sol-gel' process will now be given.

As explained above, oxygen deficient perovskite structures have been found to provide ceramic superconductors. Examples of suitable cations for oxygen deficient perovskite structures are La, Ba, Ca, Si, Nd, U, Ce, Y, and Sc. Copper oxide or another metal oxide which offers several simultaneous coordination states and allows for intercalation of oxygen is a necessary ingredient. Nb203 may be present to assist in grain size control at the later firing steps.

Suitable organic precursors to enable stable 'sols' to be formed are, e.g., Yttrium acetate Y(OOCCH3)3 4H20 Yttrium oxalate Y(C204)3 4H20 Barium acetate Ba(0OCCH3)2 Barium ethoxide Ba(0C2H5)2 Copper ethoxide Cu(0C2H5)2 these compounds being selected for solubility in a volatile organic solvent, e.g. isopropanol. Similar organic compounds for the other cations mentioned above will also be suitable.

A typical formulation for a sol would be: Yttrium acetate 35g Barium ethoxide 309 Copper ethoxide 259 Isopropanol 1000ml, reacted at c.3O0C.

This sol is subjected to acidification by 0.5 ml of 0.1M HC1 to adjust the pH value and hence the rate of gel formation. With the further addition of 100ml H20, the sol is allowed to evaporate to the onset of gelation, as shown by increasing viscosity. The rate of gelation may be controlled by monitoring viscosity changes with time and altering evaporation rates accordingly through temperature and atmosphere control. The gelled material is then treated in any or all of the following manners: (1) To produce a monolithic ceramic The gel is evaporated over a period of 10 to 30 days with slow increase of temperature.Above 500 C decomposition and loss of carbon occurs; from 500 C up to 10000C maximum the 0 content of the atmosphere is maintained at between 20X and 100%, to ensure uptake into the structure. Slow solvent removal is essential to maintain a coherent gel, and thus to produce a monolithic ceramic.

(2) To produce a powder The gel is subjected to rapid drying and heating (atmosphere as in 1) to produce a finely divided powder.

(3) To produce a coating or film The gel is applied as a low viscosity coating to suitable substrates, for example copper bars, wires, or insulating materials by methods such as spraying, dip coating and, for flat substrates, screen printing and spin coating. Deposited films can be conventionally heat-treated by furnacing and/or controlled drying as in (1).

The coatings may, at certain stages of their production, be sufficiently flexible to permit winding of coils after coating application. Alternatively the coatings may be applied after coil winding operations are complete.

In an alternative to the complete sol-gel process the sol is formed as above but, without producing a gel, is subjected to pyrolysis by igniting the sol at greater than 7000C in propanol.

Continuous agitation of the products ensures removal of carbonaceous material by exposure to excess air. A suitable inert agitator, e.g. a recrystallised alumina rod, has been found suitable. A finely divided powder again results.

Powders of the Cu-perovskite structures prepared as above, i.e., b3. a sol-gel route or by pyrolysis, are further processed by one of the following methods: (a) Tape Casting.

A fluid organic binder is added to the powder, and the mix flowed uniformly on a suitable moving carrier film under a doctor blade to control thickness of the sheet so formed. Drying of the sheet results in a flexible "tape" in an unfired state. This may be cut and/or wound to the required configuration of the electrical device.

(b) Extrusion Extrusion of the powder suspended in a binder through a nozzle produces flexible, wire-like material which may be wound or shaped in the unfired state to the appropriate configuration.

(c) Screen Printing A viscous suspension of the powder with, e.g., polyvinyl alcohol is made and applied to a substrate by a screen printing process using a screen of 120 - 240 apertures/inch. The pattern so printed is dried and fired in a manner as described above. One application of such a pattern would be electronic switching devices.

Linear tracks/components of dimensions 10 - 40 microns are quite practicable. ~ The materials and devices formed by the above methods are ceramics of defined shape or form but have not been fired and are not therefore superconductive. Firing of the shaped materals at temperatures between 9000C and 10300C produces ceramics with the necessary superconductivity.

In the process described above in which a gel is applied as a continuous coating to a substrate a specific advantage is that the coating can be locally heat processed by, for example, laser or electron beam treatment so as to write superconductive tracks into the deposited material. Unwanted material may be removed by a suitable treatment, e.g., solvent washing. An optional secondary treatment of the above treated films can also be employed in order to develop a desired property.

Where coatings of these materials are in danger of 'quenching', i.e., reverting to the normal conducting state, a metallic or otherwise normally electrical conductive substrate could be used as a route for the normal (i.e. non superconductive) electrical current.

The materials described above have numerous applications: Electrical Engineering - transformers ) both utilising tape cast or - generators ) extruded superconducting ceramics - DC magnets ) for accelerators) tape cast, extruded coatings - tomographs ) and monoliths levitation ) energy storage ) Electronics - computers ) coatings - quantum devices ) cast sheets (SQUIDS) - switching devices )

Claims (20)

1. CLAIMS 1. A method of forming superconductive ceramic materials comprising the steps of providing organic compounds of constituent metals of superconductive ceramics, dissolving these organic compounds in a volatile organic solvent, heating the solution to remove extraneous constituents and shaping and firing the resultant material to form a superconductive ceramic.
2. 2. A method according to Claim 1, wherein said step of heating the solution includes a gelling stage in which the solution is evaporated to provide a gel.
3. 3. A method according to Claim 2, wherein the shaping step comprises forming said gel into a mass of predetermined shape, the resulting gel mass being further evaporated with a slow increase of temperature to produce a monolithic ceramic body.
4. 4. A method according to Claim 2 or Claim 3, wherein said slow increase of temperature extends over a number of days up to a maximum temperature of 10000C in an atmosphere of excess oxygen.
5. 5. A method according to Claim 2, wherein the gel is rapidly heated to a maximum temperature of 10000C in an atmosphere of excess oxygen to produce a finely divided powder.
6. 6. A method according to Claim 2, wherein said gel is applied as a coating to a metal or insulation component.
7. 7. A method according to Claim 6, wherein said coating is locally fired to produce superconductive tracks.
8. 8. A method according to Claim 7, wherein the local firing is performed by a laser or by electron beam heat treatment.
9. 9. A method according to Claim 1, wherein the solution is pyrolysed in alcohol by ignition at a temperature in excess of 7000C to produce a finely divided powder.
10. 10. A method according to Claim 9 in which the pyrolysed products are continuously agitated in excess air to remove carbonaceous material.
11. 11. A method according to any of Claims 5, 9 & 10, wherein a fluid organic binder is added to the ceramic powder, the powder/binder mix is spread onto a film, and the resulting sheet is dried and cut into sheet or strip form.
12. 12. A method according to Claim 11, wherein said film is arranged to move as said mix is applied to it, the thickness of the resulting sheet being controlled by a doctor blade.
13. 13. A method according to Claim 11 or Claim 12, for producing flexible tape in an un-fired state, winding the tape, and firing the wound tape to form a superconducting electrical component.
14. 14. A method according to any of Claims 5, 9 & 10, wherein a fluid organic binder is added to the ceramic powder and the powder/binder mix is extruded to produce a wire-like material for winding and firing to form a superconductive electrical component.
15. 15. A method according to any of Claims 5, 9 & 10, wherein the ceramic powder is added to an organic liquid to produce a viscous suspension, the suspension is applied to a sheet or substrate by a screen printing process,and the pattern so printed is dried and fired.
16. 16. A method according to any preceding claim wherein the un-fired ceramic material is fired at a temperature between 9000C and 10300C.
17. 17. A superconductive ceramic component made by a method according to any preceding claim.
18. 18. Electrical equipment including a superconductive ceramic component made by a method according to any of Claims 1 to 15.
19. 19. A superconductive ceramic component made by a method as hereinbefore described.
20. 20. A method of forming a superconductive ceramic component as hereinbefore described.