WO1990004856A1

WO1990004856A1 - Improvements in and relating to superconducting composite conductors

Info

Publication number: WO1990004856A1
Application number: PCT/GB1989/001240
Authority: WO
Inventors: Jan Edgar Evetts; Bartlomiej Andrzej Glowacki
Original assignee: National Research Development Corporation
Priority date: 1988-10-20
Filing date: 1989-10-19
Publication date: 1990-05-03
Also published as: GB2224276A; EP0441813A1; GB2224276B; FI911904A0; GB8923580D0; KR900702583A; GB8824630D0; CN1042023A; JPH04501188A

Abstract

A composite conductor incorporating a region of polycrystalline material capable of exhibiting superconductivity has an elongate grain structure (12) comprising grains having regions of relatively low critical current therebetween and further grains contiguous with said grains disposed to provide an alternative superconductive path to bridge said regions of relatively low critical current. Pinning centres of non-superconductive material (10) are disposed between the grains (12) to enhance the superconductivity.

Description

IMPROVEMENTS IN AND RELATING TO SUPERCONDUCTING

COMPOSITE CONDUCTORS

This invention concerns composite conductors, particularly methods for fabricating ceramic composites that exhibit superconductivity at relatively high critical temperatures and are able to carry high electric currents in high magnetic fields. Such composite conductors may be in the form of monoliths that are simply or multiply connected, or elongated in the form of wires or tapes.

An important property of a superconducting material is its ability to carry an electrical current without resistance This property may for instance be exploited in the manufacture of magnetic solenoids and machines or in monolithic material used for_. levitation or magnetic screening. If a closed superconducting circuit is formed, the resulting current (known as a supercurrent) will flow without decay. This is termed a persistent current and the circuit is said to be in persistent mode. It is found that if the electric current density in the superconducting material exceeds a particular value the current ceases to be a true supercurrent since above this current density the superconductor shows electrical resistance and ceases to be useful for applications requiring superconductivity. This limiting current density is termed the critical current density and the particular value for any material depends on the microstructure of the material, the applied magnetic field, and the temperature of the sample. A major objective in the design and optimisation of a superconducting material is to maximise the supercurrent it can carry by maximising its critical current density.

Ceramic superconductors have so far tended to possess a low critical current density particularly in high magnetic fields. This feature is a serious drawback to their widespread application. There are three major factors that can contribute to a low critical current density in these materials.

Firstly, ceramic superconductors are likely to contain narrow planar regions that are non-superconducting. In low magnetic fields these regions can still pass a small supercurrent called a Josephson supercurrent, however, this current is very strongly degraded by an applied field. Such regions are termed weak links; in polycrystalline ceramic superconductors weak links can occur at grain boundaries and at other points in the microstructure. When the material consists of an aggregate of superconducting regions separated by weak l nks it is termed a granular superconductor and the critical current it can carry is determined by the weak link network. In designing and fabricating ceramic superconductors such weak links must be eliminated or distributed spatially so as not to unduly interrupt the supercurrent flow.

A second factor that can lead to a low critical current density is the extreme critical current anisotropy often encountered in these materials. Thus in the case of the ceramic YBaCuO the critical current density in the direction of the crystallographic c-axis is much lower than that in the crystallographic a-b plane. Thus a polycrystalline material with a disordered arrangement of grains is likely to display a reduced critical current density because in some regions the current will be constrained to pass in the c-axis direction. When there is an applied magnetic field perpendicular to the direction of the current flow, the anisotropy is further complicated. If the magnetic field is parallel to the c-axis direction the critical current density decreases more rapidly with increasing applied field, than for a magnetic field parallel to the a-b plane.

A third factor which appears to determine the critical current density is the nature of the microstructure and defect structure and the way it interacts with quantised magnetic flux lines, sometimes termed flux vortices. For a material to carry a high supercurrent in a large magnetic field it is essential for the microstructure to contain features that interact strongly with the flux lines to prevent them moving. Such features are termed pinning centres, and it has long been a major objective to increase the efficiency of pinning centres in superconductors, by careful control of the microstructure. In ceramic superconductors pinning by point defects is likely to be weak because the size of the core region of the flux vortex is characteristically small. Pinning at extended planar features, and particularly at interfaces with non-superconducting regions, is likely to be stronger, because magnetic interactions will contribute to the flux vortex pinning. In the context of this specification the term grain is intended to mean a single crystal, thus polycrystalline material which is made up of a plurality of individual single crystals, can be said to be made up of grains.

According to the present invention there is provided a composite conductor incorporating a region of polycrystalline material capable of exhibiting superconductivity and having an elongate grain structure comprising grains having regions of relatively low critical current therebetween and further grains

_* contiguous with said grains disposed to provide a superconducting path to bridge said regions of relatively low critical current wherein said composite conductor further incorporates elongate pinning centres of non-superconductive material, as herein defined, disposed between said grains to enhance said superconductivity. There is also provided a method of fabrication of a composite ceramic superconductor in which a material having an elongate crystalline structure is introduced so as to induce textured growth and thereby corresponding elongate grain structure in the crystallising superconducting material. The material which forms a basis for the textured growth of the superconducting material may be either non-superconducting or be a material which exhibits superconducting properties with a lower critical temeprature than that of the subsequently deposited superconductor material, and is thereby adapted to act as a pinning centre. In the context of this specification, the ter non-superconducting material is deemed to include superconducting material with a lower critical temperature, which are thereby adapted to act as pinning centres. (The flux lines are pinned at sharp differences in the material property It is therefore desirable to have a sharp interface. A normal metal or an insulator will be satisfactory as the pinning material as they are different from a superconductor. However, a superconductor will also permit strong pinning as long as its properties differ very markedly from the remainder the superconducting material.)

In this way, the grain structure and orientation of the superconducting material can be controlled so that the grains lie substantially in the direction in which the superconducting current is to flow, and the need for the superconducting current to cross grain boundaries is thereby reduced.

It is sufficient that the elongate grains of superconducting material overlap, but preferably the structure is organised so that the termination of one grain is near the central region of an adjoining grain. As a result current can transfer in a gradual manner from one elongated grain to another across a large area grain boundary aligned close to the direction of current flow. This is important since supercurrent transfer across grain boundaries is known to be a problem in these materials. The method of the invention thus ensures that the crystallographic alignment or texturing of the grain structure is such that the crystallographic direction that sustains a high current is orientated in the direction of electric current flow.

The invention is equally applicable to conductors in the form of monoliths (which may be simply or multiply connected)- or in the form of elongated wires or tapes.

The method of the invention results in a superconductor which contains a finely distributed arrangement of non-superconducting regions, which not only serve to define the crystalline orientation and morphology of the superconducting aterial during fabrication, but when the composite is superconducting, serve to pin the flux lines, thereby maximising pinning of flux vortices.

In accordance with a preferred embodiment of the invention,- the distribution of the superconducting grains is epitaxially related to the distribution and orientation of the elongate pinning centres and the crystal structure of the pinning centres is the source and stimulation of the crystal orientation of the superconducting grains during critical stages of the fabrication process.

In accordance with a preferred feature of the invention a superconducting composite includes an aggregate of particles of non-superconducting material incorporated or formed therewithin during an intermediate stage of the process of fabrication of the superconducting composite prior to heat treatment stages which determine the final grain structure of the composite.

The particles of non-superconducting material typically are filamentary i.e., are elongate in form and are like fibres or elongated flakes or plates, ribbons or tapes. The particles are at least three vortex spacings long but, preferably, they should- be much longer - up to several millimetres, or even more. (At a field of 1 tesla, the vortex spacing would be about O.lμm, which indicates a minimum length for the the particles of of the order of 1μ . ) In one specific embodiment the non-superconducting material has a precise crystal structure and each particle is preferably in the form of single crystal whose orientation is such that the surface atoms are commensurate with the atomic arrangement in the basal plane of a superconducting component which is grown thereon. The particles may be bi-crystalline or polycrystalline so long as (1) the particles spacing is small compared to the length of the individual single crystals in the bi or polycrystalline assembly (sometimes referred to as grains), and (2) the texture of the grains (sometimes referred to as crystal texture) is appropriate (i.e. the surface atoms are commensurate with the atomic arrangement of the basal plane of the superconducting component to be grown thereon.)

One example of non-superconducting material which may be used is magnesia ⁽MgO) and if this is used it should be obtained or formed as flakes having the ⁽1,0,0) plane parallel to the surface of the flake.

Whilst the crystal structure and orientation of the non-superconducting single crystal are preferably such as to induce the desired m crostructure in the composite after appropriate heat treatment, the single crystals are preferably distributed in an aligned manner, preferably such that neighbouring particles lie parallel to one other and are arranged so that the end of each particle lies close to the central portion of each neighbouring filament. Preferably the direction of the longer axis of each non-superconducting particle lies in the direction in which a supercurrent is to flow within the final composite.

In certain monolithic composite conductor designs the mean non-superconductor particle direction has to vary from point to point within the conductor.

By choosing the minimum particle dimension (i.e. thickness), so the flux pinning at the chosen magnetic field for operation of the superconductor can be optimised.

The particle spacing of the non-superconducting material should be sufficiently close to ensure epitaxial growth of the superconductor throughout the space between particles and in a preferred microstructure in accordance with the invention each particle is surrounded by superconductor with an orientation determined by the atomic structure of the particle and there is either a continuity of epitaxy to the neighbouring particles of non-superconductor or a maximum of one grain boundary separating superconductor adjacent to neighbouring particles of non-superconductor.

The particle of non-superconductor must not degrade the superconductor by interdiffusion or reaction and the material selected should preferably have elastic properties and a thermal expansion coefficient which are compatible with the superconductor composite geometry.

A preferred material for the non-superconducting material is magnesia (MgO). Magnesia is suitably inert with respect to most ceramic superconductors and in addition it has a suitable crystal structure for the epitaxial growth of appropriately oriented superconducting grains.

Alternatively the non-superconducting material may be monocrystall ine material such as sapphire in the form of fibres coated with an epitaxial buffer layer of magnesia, for example by the method described in BPA8812038.1.

In an alternative scheme magnesium metal precursor to the magnesia, may be coated with silver prior to fabrication, and filaments may be formed (prior to assembling the composite) by swaging and drawing the silver clad magnesium into the form of a wire. The magnesia may then be synthesised by diffusing oxygen through the silver coating during a low temperature anneal and the filaments thus prepared may be incorporated into a composite either with the silver coating or after the coating has been removed.

It is also possible to form the filaments described, in situ, during deformation of the powder composite, in which event the magnesia is formed in the same way during a low temperature pre-anneal in oxygen.

It is to be understood that the invention is not restricted to the use of magnesia or magnesia coated materials. Suitable alternative materials would be strontium titanate, yttria, yttrium barium copper oxide (Y₂BaCu0₅ - the so-called green phase), yttria-stabi 1 ised zirconia, lanthanum gallate, silver or magnesium fluoride.

Fabrication may involve combining the desired particles of non-superconducting material and one or more other materials which are to form the superconducting matrix, to form the composite, at the outset, and then reacting the composite to obtain the superconducting phase.

Where the particles are synthesised prior to the formation of the composite, a number of different methods may be used to incorporate them into the composite in a suitably al gned state. In one method, the particles are mixed with powdered material which is to form the superconducting matrix and placed in a suitable ductile container, for example silver-palladium alloy, which may itself be clad with stainless steel. This preform may be reduced by swaging, drawing, extrusion, forging or rolling to the required dimensions.

In a second method the powdered superconductor may be in the form of mixed precursor powders which may be oxides, nitrates, carbides, or metallic powders.

In a third method the matrix powders may be mixed with a viscous binder material and processed by viscous processing techniques to produce the necessary aligned microstructure.

In a fourth method the particles may be incorporated in the liquid during sol-gel processing and aligned by a flow-coating or spin-coating process on a suitable substrate and this process may be repeated as often as necessary to build up the required thickness of material.

In a fifth method the particles may be sprayed onto a substrate at the same time as the liquid precursor either separately or through a common nozzle. The spraying process may be thermal spraying or a plasma spray process.

In a sixth method the composite may be formed by a liquid infiltration method whereby the liquid superconductor or precursor is infiltrated into a fibrous mat in the form of a wire, tape or sheet. Alternatively fabrication may involve the use of precursor materials which after processing will form the desired particles. Examples of such precursor materials are magnesium metal or magnesium coated sapphire fibres. Where precursor material (s) are employed for forming the particles, it is necessary to react the precursor material(s) and form the particles before the reaction which is to form the superconducting matrix.

In one such fabrication route the particle precursors are formed by incorporating a ductile metallic powder such as magnesium into the composite preform prior to deformation. Highly textured metallic filaments may then be formed by swaging, drawing, extrusion or rolling, and the filaments may then be oxidised to form the particles in a heat treatment stage prior to the final reaction stage that gives the superconductor microstructure with the desired morphology and crystalline texture.

Whatever the route employed to achieve the desired alignment before final reaction, the final heat treatment stages are designed to react and modify or control the microstructure of the material which is to constitute the superconductor so as to produce the desired microstructure morphology and crystalline texture. As previously mentioned, this depends on solid state epitaxy, tacto-epitaxy or melting or partial melting and recrystallisation and grain growth that is nucleated and controlled by the particle crystal structure and distribution. The precise combination of heat treatment times and temperatures has been found to vary from one ceramic superconductor to another and also depends on the particle type and dimensions of the composite. For example, in some instances it is advantageous to carry out the reaction in a temperature gradient or magnetic field or under a pre-determined loading condition. In the final stages of this reaction process it is likely that low temperature anneals will be required in oxygen, nitrous oxide or other gases. The methods described have been applied to ceramic superconductors based on rare earth metals (e.g. YBaCuO) , bismuth metal (e.g. BiPbSrCaCuO) and thallium metal (e.g. TIBaCaCuO).

The invention may also include the addition of selected additives in particular silver, gold or hafnium to control the grain structure and distribution of impurities within the composite.

It is also envisaged that a composite constructed in accordance with the invention incorporate electrodes and solid state electrolytes to enable titration of chemical species as described in previous Patent Applications Nos. 8710113, 8714993 and 8717506, to be accomplished.

The invention will now be described by way of examples with reference to the accompanying drawings in which are shown cross-sections of superconducting composite wire and details of the microstructure or grain structure, and in which:-

Figure 1 is a cross-section of a superconducting composite wire precursor;

Figure 2 is a cross-section showing the microstructure of the composite after processing;

Figure 3 is a cross-section of the composite after subsequent coatings have been applied; and Figure 4 shows the microstructure of the composite wire. In a method in accordance with a specific embodiment of the invention powdered YBa₂Cu₃0₇ 11 with mean particle size 0.5μm is mixed with filamentary particles 10 in the form of elongated flakes of -either SrTi0₃ or MgO with average dimension 4x2x500μm. The volume fraction of filament is 20%. The mixture is well dispersed in a viscous mineral oil to form a colloidal suspension and then viscous processed to produce suitable alignment of the filaments. Finally it is formed to produce a tape with cross-section 5x0.5mm. The mean spacing between the filaments is 2μm (see Figure 1).

The sample is then heat treated in flowing oxygen at 500°C for 12 hours to pyrolyse the binder material. Then at 850°C for 1 hour to drive off residual carbonaceous traces. Finally it is sintered for up to 16 hours at 980°C in air. During this stage nucleation of the preferred microstructure occurs. The sample is then cooled in oxygen at .25°C/min to 880°C and then at l°C/min to 500°C where it is held for an isothermal oxygen anneal for 200 hours before finally cooling at 1°C per minute to ambient temperatures. The final microstructure comprising single-crystal grains 12 with boundaries 13 is depicted in Figure 2. In an alternative fabrication route embodying the invention the same filaments are suspended in a liquid solution 15 of the superconductor in the form of mixed nitrates. A 0.5ml measure of the solution is spun coated on to an MgO wafer substrate 14 to produce a coating 5-10μm thick. After drying at 75°C for 10 minutes subsequent coatings are spin coated on to the same sample until the composite thickness is 0.1mm (see Figure 3).

The heat treatment is as follows. The sample is heated slowly to 870°C and held for 30 minutes in oxygen. Then the sample is sintered for 2-5 minutes at 980°C in air and then cooled at 25°C/min to 880°C and then at l°C/min to 500°C where it is held for an isothermal oxygen anneal for 100 hours before finally cooling at 1°C per minute to ambient temperatures. The final microstructure is again as depicted in Figure 2.

In a further alternative fabrication route embodying the invention powdered YBa₂Cu₃0₇ with mean particle size l-2μm is. mixed with 20% volume fraction of magnesium powder with mean particle size 20μm. The mixture is inserted in a cylindrical silver preform 16 of outer diameter 15mm and inner diameter 12mm and 20cm long. The composite is then hydrostatical ly extruded to 1mm outer diameter. The heat treatment is for 200 hours at 300°C in oxygen followed by 2-5 minutes at 980°C and cooling at 25°C/minute to 500°C where it is held for an isothermal oxygen anneal for 100 hours before finally cooling at 1°C per minute to ambient temperatures. The microstructure comprising filaments 17 in a pre-cursor matrix 18 after extrusion but before heat treatment is depicted in Figure 4.

Claims

1. A composite conductor incorporating a region of polycrystall ne material capable of exhibiting superconductivity and having .an elongate grain structure (12) comprising grains having regions of relatively low critical current therebetween and further grains contiguous with said grains disposed to provide a superconducting path to bridge said regions of relatively low critical current characterised in that said composite conductor further incorporates elongate pinning centres (10) of non-superconductive material, as herein defined, disposed between said grains ⁽12⁾ to enhance said superconductivity.

2. A composite conductor as claimed in claim 1 characterised in that the elongate pinning centres (10) are distributed in an aligned manner with the longer axis of neighbouring elongate pinning centres substantially parallel to one other and the end of each elongate pinning centre lies adjacent to the central portion of each neighbouring filament.

3. A composite conductor as claimed in claim 2 characterised in that the direction of the longer axis of each elongate pinning centre (10) lies in the direction in which a supercurrent is to flow within the final composite.

4. A composite conductor as claimed in claim 1 characterised in that the distribution of the grains (12) is epitaxially related to the distribution and orientation of the elongate pinning centres.

5. A composite conductor as claimed in any one of the preceding claims characterised in that the elongate pinning centres (10) are at least three vortex spacings long.

6. A composite conductor as claimed in claim 5 characterised in that the length of the elongate pinning centres is at least lμm.

7. A composite conductor as claimed in any one of the preceding claims characterised in that the non-superconductive material includes at least one of the class including magnesia, strontium titanate, yttria, yttrium barium copper oxide, yttria-stabilised zirconia, lanthanum gallate, silver and magnesium fluoride.

8. A composite conductor as claimed in claim 7 characterised in that the non-superconducting material is magnesia (MgO).

9. A composite conductor as claimed in claim 8 characterised in that the magnesia is deposited on a substrate of monocrystal 1 ine materi l .

10. A composite conductor as claimed in claim 8 characterised in that the monocrystal line material is sapphire.

11. A method of fabrication of a composite conductor incorporating grains of superconductive material characterised in that particles of a material having an elongate structure is introduced so as to provide elongate pinning centres between said grains of superconductive material.

12. A method of fabrication of a composite conductor as claimed in claim 11 characterised in that the particles are mixed with powdered material which is to form the grains of superconductive material and placed in a suitable ductile container to create a preform which is then reduced to a required dimension.

13. A method of fabrication of a composite conductor as claimed in claim 12 characterised in that the powdered material is in the form of mixed precursor powders.

14. A method of fabrication of a composite conductor as claimed in claim 13 characterised in that the powdered material includes at least one from the class which includes oxides, nitrates, carbides, or metallic powders.

15. A method of fabrication of a composite conductor as claimed in claim 11 characterised in that the powdered material is mixed with a Viscous binder material and processed by viscous processing techniques to produce an aligned microstructure.

16. A method of fabrication of a composite conductor as claimed in claim 11 characterised in that particles of the powdered material are incorporated in a liquid during sol-gel processing and aligned by a coating process on a substrate.

17. A method of fabrication of a composite conductor as claimed in claim 11 characterised in that particles are sprayed on to a substrate at the same time as a liquid precursor.

18. A method of fabrication of a composite conductor as claimed in claim 11 characterised in that a composite is formed by a liquid infiltration method in which a liquid superconductor or precursor is infiltrated into a fibrous mat.

19. A method of fabrication of a composite conductor as claimed in claim 11 characterised in that an additional material is added to the composite conductor material to control the grain structure and distribution of impurities therein.

20. A method of fabrication of a composite conductor as claimed in claim 19 characterise in that said additional material is chosen from the class including gold, silver and hafnium.