WO1989001706A1

WO1989001706A1 - Machine workable, thermally conductive, high strength, ceramic superconducting composite

Info

Publication number: WO1989001706A1
Application number: PCT/US1988/002071
Authority: WO
Inventors: James C. Garland; Joseph J. Calabrese
Original assignee: The Ohio State University
Priority date: 1987-08-14
Filing date: 1988-06-22
Publication date: 1989-02-23
Also published as: EP0376981A1; JPH03502212A; EP0376981A4

Abstract

A dispersion of ceramic superconductor particles is distributed in a matrix of a continuously connected nonferromagnetic metal. This bi-phase composite permits the material to exhibit the mechanical and thermal properties of metal and yet retains the superconducting properties of ceramic superconductors.

Description

MACHINE WORKABLE, THERMALLY CONDUCTIVE, HIGH STRENGTH, CERAMIC SUPERCONDUCTING COMPOSITE Technical Field

This invention relates generally to a bulk_, composite superconductor material which can more easily be fabricated into articles of manufacture than pure ceramic superconductor and more particularly the invention relates to fabricating metal oxide ceramic superconductors in a manner which preserves the superconductivity while providing improved thermal conductivity, higher strength, and improved machine workability, possibly including malleability and ductility in a resulting material which is considerably less brittle than pure ceramic superconductor.

Background Art

The recent discovery of superconducting metal oxide ceramics, which become superconductive at substantially higher superconductive transition temperatures than previously known materials, has created much excitement. Such ceramic superconductor materials offer the opportunity for operating superconductor components and devices at considerably less cost. In superconducting devices, the superconducting material must be formed into wires, ribbons, sheets, thin films, or other structures. With metal superconductors, as with metal conductors, the problems with fabrication are considerably less than with ceramics because metal is relatively easily shaped by deformation or machining without affecting its electrical,thermal and mechanical characteristics. Furthermore, some superconductor structures require a relatively high strength capable of withstanding substantial stress. While metals provide such high strength, superconducting ceramics do not. It is also desirable that superconductive materials exhibit a high thermal conductivity so that the heat from local heating generated by flux motion can be conducted away from the superconductor to minimize quenching. ^" Although the new ceramic superconductor materials do exhibit excellent superconductivity, unfortunately they exhibit the poor thermal and mechanical properties which are characteristic of ceramics. Superconducting ceramics have low thermal conductivity, are brittle and do not exhibit high strength. Instead, they break and chip easily and therefore cannot be conveniently machined and cannot be shaped by applying stress forces to cause plastic deformation of them into desired shapes. Thus, the ceramic superconducting materials have thermal and mechanical properties which are quite unsuitable for many superconductor applications. A variety of lamination techniques have been suggested to enable superconducting ceramics to be formed into useful shapes. For example, a cylindrical, silver tube has been filled with a superconductor ceramic powder before the powder was heat treated and then swaged or drawn into a wire. Such wire forms a fine pipe containing a core of ceramic superconductor powder. This wire can be formed into a desired shape and then heat treated to fuse the particles and make it superconductive.

Another solution is the formation of laminates or tapes. A superconductor powder, before heat treatment, is placed on a layer of metal which is then wound or otherwise formed into a laminate and then shaped into the desired structure. Following shaping, the entire structure is then heat treated to fuse the particles and initiate superconductivity. Yet another way of making superconductive wires is to pack a superconductive powder in among long microscopic chains of a suitable polymer. This mixture is then formed into the desired shape and heat treated to fuse the particles. One major problem with all these prior art solutions is that, following the sintering which fuses the ceramic particles, the structures cannot be further shaped, as by bending, drawing, or machining. Any significant change in their mechanical shape cracks and separates the fused ceramic material causing loss of continuity. There is, therefore, a need for a material which can be manufactured in a bulk form, heat treated to initiate superconductivity, and yet subsequently still be capable of being deformed into useful shapes without destroying superconductivit . There is, furthermore, a need for a superconducting material which exhibits greater strength than pure ceramic, improved thermal conductivity, and which may be machined in a manner similar to the machining of metal.

In particular, there is a need for a bulk material which retains the electrical and magnetic properties of- a superconductor which are zero electrical resistance, a high superconducting transition temperature, and good magnetic screening, and yet which has good mechanical and thermal properties, such as machinability, ductility, malleability, and high thermal conductivity.

Brief Disclosure Of Invention

The invention is essentially a solid, composite consisting of a dispersion of ceramic superconductor particles distributed in a matrix of connected, non- ferromagnetic metal which is nondestructive of superconductivity.

In one embodiment of the invention, the volume fraction of ceramic superconductor equals or exceeds the percolation fraction for the ceramic particles, but is not greater than one minus the percolation fraction for the metal. Within that range both the metal and the ceramic particles form a continuously connected matrix, each in effect an infinite cluster. A superconductor material results which will have enhanced, metal-like thermal conductivity and strength and can be machined considerably more effectively and accurately than pure ceramic material itself and yet the material will still exhibit superconductivity through the ceramic particle matrix.

In another embodiment of the invention the volume fraction of superconductor particles is within a range so that it is less than the percolation fraction for the superconductor particles, but greater than a minimum which is needed to maintain the spacing between the ceramic particles within their tunnelling distance so that tunneling can be maintained through the metal and the interfaces between neighboring ceramic particles. Such a composite material in which the ceramic superconducting particles do not contact each other permits metal-like deformation, strength and thermal conductivity and yet the superconductivity can be maintained throughout the material by tunnelling. Brief Description Of Drawings

Fig. 1 is a graphical illustration of the properties of composites embodying the present invention over the range of relative metal and ceramic concentrations. The ranges shown are appropriate for spherical particles. Elongated powder particles would result in different ranges.

Fig. 2 is a graphical plot of resistance vs. temperature data for one embodiment of the invention.

Fig. 3 is a graphical plot like Fig. 2, but for a second embodiment of the invention.

In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

Detailed Description Small ceramic superconductor particles are prepared in accordance with the teachings of the prior art. Suitable superconductor material includes the metal oxide ceramics which have recently been developed, including all copper oxide based superconductors. For example, the superconducting ceramics contains yttrium, barium, and copper oxide such as YBa Cu O may be used. These have a layered, oxygen deficient crystal structure derived from the perovskite structure. Other such superconductors, including ones yet to be developed, can advantageously be used with the present invention if they have the mechanical and thermal deficiencies described above.

The ceramic powder or particles are micron size, for example in the range of 1 to 100 microns. Particles having a diameter of 10 microns and 50 microns have been used.

The ceramic particles are then dispersed randomly and as homogeneously as possible, in a matrix of continuously connected, nonferromagnetic metal which is nondestructive of the superconductivity of the ceramic particles. Such a dispersion may be prepared by homogeneously mixing the ceramic superconductor particles with metallic particles preferably within the same size range. The mixture is then compacted and subsequently sintered.

Selecting and controlling the relative volume fractions of the metal and ceramic particles enables the selection of the different features and characteristics which the resulting composite will exhibit.

As is known to those skilled in the art, the percolation fraction for a mixture of solid particles is the minimum volume fraction of the mixture which is necessary for a constituent to be formed into an infinite cluster which is^'a continuously connected network extending throughout the composite. For example, with substantially spherical, equal sized particles, a volume fraction of metal particles of at least approximately 16% is necessary to provide such a matrix of continuously connected metal. Similarly, the percolation fraction for the ceramic is approximately 16% so that if the volume fraction of the ceramic is at least 16%, the ceramic particles will also form such a continuously connected network extending throughout the composite.

In the present invention a continuously connected network of metal is formed extending throughout the composite. The minimum volume fraction of metal is that volume which will give such a continuously connected matrix. If the composite is formed by mixing ceramic and metal particles, the volume fraction of the metal particles must be at least equal to the percolation fraction. For a perfectly random distribution of spherical metal particles a minimum of approximately 16% volume fraction of metal is required to receive the desired metallic correctedness. The existence of this continuous metal matrix provides the bulk material with the desired metal-like thermal and mechanical properties described above.

If the metal concentration is within the range between the percolation fraction for the metal particles and one minus the percolation fraction for the ceramic particles, the ceramic superconductor phase also percolates throughout the composite to provide a continuously connected matrix or infinite cluster. This range of metal concentration is illustrated as range A in Fig. 1. Sintering of a compressed composite in range A fuses the ceramic particles into a solid superconductor network within the metal matrix. The metal matrix provides the mechanical strength and thermal conductivity which are desired, while the superconductor phase provides the superconductivity.

During machining operations, the metal matrix holds the ceramic material together and prevents the migration of cracks, thus permitting more accurate machining. The relative volume fractions or concentrations of the metal and ceramic superconductor can be adjusted within the range A to thereby adjust the mechanical properties of the composite between the more metal-like properties for higher metal concentrations and a more ceramic like properties for the lower metal concentration.

A material made within range A, after being sintered, can be machined but still cannot be significantly mechanically deformed as by drawing, forging, or bending because such deformation would be destructive of the fused ceramic matrix. However, if the volume fraction of ceramic superconductor particles is less than the percolation fraction but at least as great as the minimum volume fraction which is necessary to maintain the randomly distributed ceramic particles separated by a distance which does not exceed the tunnelling distance, then proximity coupling will be maintained through the metal between neighboring ceramic particles. This range is designated as range B in Fig. 1. As is known to those skilled in the art, the superconducting proximity effect causes superconductivity to be induced into a normal metal conductor in the region immediately adjacent to the superconductor because the wave function extends beyond the superconductor out into the metal. Thus, there is a distance, which is the coherence length, from each superconducting grain in the composite through which superconductivity may be induced into the metal matrix between neighboring superconducting particles. A material formulated in range B may be mechanically deformed after the ceramic has been heat treated to allow the manufacture of and bending of superconducting component parts. A composite material embodying the invention in the range B will have flow or deformation characteristics which are metal-like and yet superconductivity can be maintained principally by the proximity effect or Josephson coupling.

Embodiments of the invention may utilize any of several types of tunnelling to maintain continuous superconductivity throughout the entire composite material. As described above, superconductivity will extend into the metal by a distance on the order of the coherence length if there is a direct metal to ceramic interface with no intervening barrier. In that circumstance proximity effect or Josephson coupling will maintain the superconductivity.

If two ceramic superconductor particles are separated by a thin piece of metal, semi-conductor, or insulator, such as an insulator resulting from a fracture-or gap, then Josephson effect tunnelling will occur.

Finally, if a metal-to-ceramic superconductor interface is a semi-conductor or an insulative barrier, then quantum mechanical electron tunnelling may occur. Various types of tunnelling will occur at various sites throughout embodiments of the invention in both range A and range B.

The tunnelling distance depends upon the particular tunnelling effect being utilized to maintain superconductivity. Tunnelling distance is typically in the range from approximately 5 to 30 Angstroms to 1 micron or so.

The metals which may be used as the metal constituent in embodiments of the present invention do not include ferromagnetic metals since it is well known that ferromagnetic materials destroy superconductivity in a nearby superconductor. Other metals, e.g., silver, which exhibit the typical metal-like properties can be used. However, some metals, such as copper, share electrons with the superconductor material and this chemical reaction destroys the superconductivity. However, such reactive metals may be coated with more inert metals, such as platinum or palladium, and still be used. Particle coating would enable the use of inexpensive metals, such as copper, to minimize the cost of composites embodying the invention while requiring only a minute proportion of more expensive noble metals to form a thin coating on the outer surface of the particles, such as by chemical vapor deposition, or other coating techniques well known to those skilled in the art, which will prevent the chemical reaction and destruction of superconductivity. Bi-phase random composites embodying the invention may also be formed by mixing the ceramic particles in a liquid metal melt and then freezing the melt. In this case, the ceramic particles are heat treated to make them superconductive before being mixed with the melt. The heat treatments of the ceramic metal composite may be done in the other embodiments of the invention either before or after mixing with the metal.

The relative proportion of the two phases of the composite govern the electrical and mechanical properties of the material embodying the invention as illustrated in -Fig. 1» The metal matrix essentially governs the mechanical properties of the material reducing its brittleness, providing tensile strength, and machinability. The metal phase also provides the high thermal conductivity, which is characteristic of metals, to enable efficient cooling of superconductive material. The high thermal conductivity of the metal, as compared to the pure ceramic, serves to dissipate local heating of the superconductor resulting from flux motion, contributing to the stability of the composites and minimizing the possibility of quenching in high current density application. The high thermal conductivity of the metal matrix in the composite also allows the material to reach thermal equilibrium more rapidly than possible in a pure ceramic material. The regions of metal in the composite serve to trap lines of magnetic flux, thus stabilizing the composite in the presence of strong, magnetic fields.

A composite embodying the present invention in range B utilizes principally proximity effect or Josephson coupling to allow metal bridges to exist between the superconductor particle in which superconductivity may be induced. These metal bridges in the geometrical connected of the superconducting phase allow the simultaneous coexistence of the metal-like mechanical and thermal properties with the continuous superconductivity throughout the material. Thus, unlike single phase ceramic superconductors, the composite material is considerably more immune to microscopic flexing, mechanical defects introduced by other deformations than is single phase ceramic superconductors.

Figs. 2 and 3 illustrate composite superconductors embodying the present invention. In Fig. 2 a mixture of 18% volume fraction of palladium with YBa Cu_ O _. _sllows

that superconductivity occurs at a Tc of approximately 86 degrees K.

Similarly, Fig. 3 illustrates a composite embodying the present invention having a 19% volume fraction of platinum and 81% ceramic superconductor. It demonstrates that superconductivity at a Tc of approximately 63 degrees K. Other materials may be added to the composite to obtain other characteristics and features.

While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications may be adopted without departing from the spirit of the invention or scope of the following claims.

Claims

1. A superconductor comprising: a dispersion of ceramic superconductor particles distributed in a matrix of continuously connected nonf erromagnetic metal which is nondestructive of superconductivity .

2. A superconductor in accordance with claim 1 wherein the volume fraction of ceramic superconductor particles is less than the percolation fraction for the ceramic particles and at least as great as the minimum fraction which is necessary to maintain the ceramic particles separated by a distance not exceeding the tunnelling distance in order to maintain tunnelling through the metal between neighboring ceramic particles.

3. A superconductor in accordance with claim 1 wherein the proportion of ceramic superconducting particles is substantially in the range of 14% to 16%.

4. A superconductor in accordance with claim 1 wherein the volume fraction of ceramic particles is at least as great as the percolation fraction for the ceramic particles and not greater than one minus the percolation fraction for the metal.

5. A superconductor in accordance with claim 1 or 2 or 3 or 4 wherein the metal phase comprises a plurality of metal particles in a volume fraction at least equal to the percolation fraction of the metal particles.

6. A superconductor in accordance with claim 5 wherein the metal particles have an exterior coating of a nonf erromagnetic metal which is non-destructive of superconductivity.

7. A superconductor in accordance with claim 5 wherein the ceramic particles have a width substantially in the range of 1 micron to 100 microns.

8. A superconductor in accordance with claim 7 wherein the metal particles have a width substantially in the range of 1 micron to 100 microns.

5 9. A superconductor in accordance with claims 1 or 2 or 3 or 4 wherein the ceramic particles have a width substantially in the range of 1 micron to 100 microns.

10. A superconductor in accordance with claim 1 or 2 or 3 or 1.0. 4 wherein the superconductor particles comprise metal oxide ceramic particles.

11. A superconductor in accordance with claim 10 wherein the ceramic particles comprise M Ba, CU_ 0 where x = 7-<5, and

15 where M = Y, or metals from the rare earth series.

12. A superconductor in accordance with claim 10 wherein the metal in contact with the ceramic superconductor particles is a noble metal selected from the group consisting of platinum,

20 palladium, silver, and gold.

13. A superconductor in accordance with claim 1 or 2 or 3 or 4 wherein the metal in contact with the ceramic superconductor particles is a noble metal selected from the group consisting of platinum, palladium, silver, and gold.

14. A method for forming a bulk superconductor material, the method comprising:

(a) preparing ceramic superconductor particles; and

(b) dispersing the ceramic particles in a matrix of continuously connected non- ferromagnetic metal which is non-destructive of superconductivity to fabricate a metal/ ceramic composite.

15. A method in accordance with claim 14 wherein the volume fraction of ceramic superconductor particles is less than the percolation fraction for the ceramic particles and at least as great as the minimum fraction which is necessary to maintain the ceramic particles at a distance not exceeding the coherence length in order to maintain proximity effect tunnelling through the metal between neighboring ceramic particles.

16. A method in accordance with claim 14 wherein the proportion of ceramic superconducting particles is substantially in the range of 14% to 16% .

17. A method in accordance with claim 14 wherein the volume fraction of ceramic particles is at least as great as the percolation fraction for the ceramic particles and not greater than one minus the percolation fraction for the metal.

18. A method in accordance with claim 14 or 15 or 16 or 17 wherein the composite is formed by mixing particles of said metal with said ceramic superconductor particles to form a random dispersion and sintering the particles.

19. A method in accordance with claim 18 wherein the metal particles comprise nonferromagnetic metal particles coated with a layer of a metal which does not destroy the superconductivity.

20. A method in accordance with claim 15 wherein subsequent to heat treatment of the ceramic superconductor and after fabricating the metal/ceramic composite, a stress is applied to the composite to deform it into a useful object.

21. A method in accordance with claim 20 wherein the composite is extruded to deform it into a wire, rod, or tube.

22. A method in accordance with claim 17 wherein subsequent to heat treatment of the ceramic superconductor and after • fabricating the metal/ceramic composite, the composite is machined into a useful object.

23. A method in accordance with claim 14 or 15 or 16 or 17 wherein the composite is fabricated by mixing the ceramic particles in a liquid phase metal and then solidifying the metal.

24. A method in accordance with claim 23 wherein the ceramic particles are amalgamated.