WEAK-LINK SUPERCONDUCTOR LOOP DEVICE

Abstract: A superconductor loop device has a body formed of a substantially non-superconducting matrix which has dispersed therein regions of superconductive material such as to impart to the body the inherent properties of a weak-link superconducting loop. The body may be formed of particles of a multi-phase ceramic material wherein there is a non-superconducting and a dispersed superconducting phase. The body may be formed by compacting, compacting and sintering, or by thick film technology. Alternatively, the body may be formed of superconducting particles which are dispersed in a resin matrix.

Description: WEAK-LINK SUPERCONDUCTOR LOOP DEVICE The present invention relates to a superconductor loop device having weak-link properties and to a method of producing such loop devices. Previously proposed weak-link superconductor loop devices have been formed from a loop of superconductive material having a discrete weak-link provided therein. It has previously been proposed to provide the weak-link in the form of a point-contact, constriction or an insulator junction, and these loops have well defined supercurrent and magnetic flux characteristics. However, the point contact weak-link must be mechanically adjusted to obtain and maintain the desired characteristics of the loop, whereas the constriction and insulator junction weak-links involve relatively sophisticated fabrication techniques and are expensive to produce. A high temperature ceramic metal oxide superconductor material of the Y-Ba-Cu-0 system has been proposed in Physical Review Letters, volume 58 No. 9 pages 908 to 911. In accordance with the present invention, there is provided a superconductor loop device comprising a body formed of a matrix which is substantially non-super¬ conducting and which has dispersed therein regions of superconductive material such as to impart to the body the inherent properties of a weak-link superconducting loop. The present invention has the advantage that the material from which the loop is fabricated has inherent properties which give the loop the characteristics of a superconducting weak-link loop because the matrix is non-superconducting and so allows magnetic flux to pass through the loop without the necessity of separately providing a discrete weak-link as required in the previously proposed loops. Preferably, the loop is fabricated from ceramic material although other forms of a dispersed superconductor in a non-superconductor matrix might be used, e.g., metallic particles such as Pb, Nb or Ti in an epoxy resin matrix. It is particularly preferred that the matrix should have poor conductivity as this allows the operation of the device at radio frequencies. The conductivity range for the matrix is may be 1 milliohm.cm to 10 8 ohm.cm. References to "superconductive", "superconducting", "non-superconducting" " and "poor conductivity" relate to the properties at the intended temperature of operation of the device. The present invention provides a weak link loop which is easily reproducible. Furthermore, the use of ceramic materials allows the loop to be produced using conventional powder technology and so provides a loop which is economical to produce and which utilises materials having relatively high superconducting transition temperatures. When ceramic material is used in the present invention, it may comprise small regions of a superconducting phase dispersed within a bulk non-superconducting phase, the superconducting regions being weakly coupled. However, it is possible, and may even be preferable, for the superconducting phase to be present as the major portion of the material. Accordingly, the super¬ conducting and the substantially non-superconducting phases may be present in any mutual proportions. Preferably, the ceramic material is from the Y-Ba-Cu-O system although other ceramics or materials having a high superconducting transition temperature may be used provided that they fulfill the compositional phase requirements outlined above, eg. La or Sc based ceramic metal oxide superconductors such as La-Ba-Cu-0 system or La-Sr-Cu-0 system or other suitable ceramic materials from the rare earth-barium/strontium-Cu-0 system. A particularly preferred material comprises ( ι- X Ba x >2 Cuθ4„ ( 3 (d is undetermined) where x is 0.4 and has a superconducting phase of formula YBa2Cu3θ7_ ( 3. It was thought that such phase occupied about 1% of the material. However, it has now been ascertained that it occupies a greater proportion of the material. By varying the chemical composition, microstructure, distribution and nature of the phases of the ceramic material, it is possible to vary the weak-link characteristics of a loop formed from the material. The phase composition of the material can be varied by processing parameters such as mechanical grinding, chemical deposition, vapour transport, temperatures, atmospheres, pressures, sintering etc. The superconductive material may be present in any proportion of the body. Whilst the superconducting loop device may be formed from a multi-phase material containing a dispersed ceramic superconducting phase in a non-superconducting matrix as indicated above, the non-superconducting matrix may be a separate ceramic material or a polymer resin. It is also possible to form a cohesive body of granular multi-phase material without the use of a binder. However, the longevity of the body can be improved by use of a binder or an outer casing. Alternatively, the material can be in the form of a granular, ceramic superconductor (eg Ba2Cu3θ7«» ( -i) bound in a non-superconducting matrix, particularly a polymer resin binder (eg in an amount of about 10% by weight) . Materials of the type -described above can form loops having superconducting weak-link loop properties within the bulk of the material. Consequently, it is possible that the body can be in the form of a loop or any other non-loop configuration. The inherent property of the material to show weak link loop behaviour allows the use of the material in r.f. or d.c. SQUIDs_.. _. Where a granular multi-phase material comprising grains having both superconducting and non-superconducting phases is used, a degree of compaction of at least 70% may be necessary to obtain the required superconductive properties which are affected by the degree of contact between the grains. The superconducting properties of the granular material are affected by the contacting surfaces of the grains of the superconducting material. Modification of the superconducting properties can be effected by modifying the contacting surfaces of the grains. The loop device may also be produced by thick film technology. Thus, in a further aspect of the present invention, there is provided a method of producing a weak-link superconducting loop device comprising the steps of depositing a suitable material onto a substrate, and processing the deposited material so as to define a loop of material having the structure and properties defined above. The weak link characteristics of a loop formed in accordance with the present invention can also be tuned by other techniques e.g. machining of the loop, placing the loop in a uniform magnetic field, passing an electrical current through the loop, providing a coil around the loop and passing a current around said coil, subjecting the loop to electrochemical processes and localised or generalised heating/cooling of the loop. Because of the weak-link characteristics of the loops according to the present invention, it is possible to incorporate such loops into devices which rely upon the detection of a variation in a magnetic field for their functioning e.g. Superconducting Quantum Interferometer Devices (SQUIDS). The superconducting properties of the loop are affected by the density and microstructure of the super¬ conducting material. This in turn can be affected by particle size, amount of binder used, degree of sintering etc. When powder technology is used to form the loop or body, density can also be affected by the pressure used to compact the material before or during sintering. Embodiments of the present invention will now be described in the following Examples. Example 1. A loop comprises a circular ring of the following dimensions: outer diameter 10mm, inner diameter 5mm, thickness 3mm. The ring is formed from ϊi.2 Ba 0.8 Cu0 4-d (d is undetermined) produced in the following manner: 5.4495g of Y2O3, 6.3512g of BaC0 3 and 3.1995g of CuO (all Analar Laboratory Reagents) are mixed together in a ball-mill-type mixer for 3 minutes. This mixture provides the correct ratios of metallic elements. The powder mixture is placed in an alumina crucible in a 950°C furnace for 45 minutes in air, the mixture being stirred after 20 minutes. The material is removed from the furnace and ground with a pestle and mortar to produce a powder having the consistency of flour. The ground material is replaced in the furnace for a further 40 minutes, stirring after 20 minutes. The material is removed from the furnace and, when cool, 1.5g of the material is compressed into the ring using a hydraulic press at approx 90001b. The ring is sintered in air at 950°C for 16 hrs in an Al crucible. Figures 1 and 2 are X_=ray diffraction spectra " of -a ' material which is similar to that outlined above. As can be seen, the X=ray analysis shows that several phases are present. Figure 2 is an enlarged view of part of the spectrum of Figure 1. In Figure 2, region A is due to the presence of the superconducting phase. A loop formed from such a material is characterised in that the magnetic flux within the loop is quantised in discrete amounts of the flux quantum, 0 Q = n /2e. _hi_ is measured using a conventional superconducting flux transformer and a conventional radio frequency SQUID (S.1E Corporation Ltd) . On increasing an external magnetic field, flux is observed to jump in and out of the ring in multiples of 0_ ι typically 50 - 100# o . This corresponds to a critical current around the loop of 1mA. These measurements can be observed at 4.2K and other superconducting behaviour is observed in similar samples at 90K. When used as a radio frequency SQUID, characteristic SQUID behaviour is observed at temperatures well above 4.2K. Example 2 A body showing weak-link body behaviour in a non-loop configuration can be formed as follows:- A 1 mm cube of a compacted multi-phase ceramic material ( γ l.2 B 0.8 CuO 4-d * n th i s Example ) is placed inside a coil of 10 turns wound over a length of 6 mm and a diameter of 3 mm. The coil is connected in parallel with a 350pF capacitor and demonstrates simple r.f. SQUID behaviour at liquid nitrogen temperatures. Such an arrangement, when attached to appropriate electronic apparatus " (eg as disclosed in Nature, 328, p.47, 1987), can be used to detect changes of ambient magnetic field of the order of InT. The performance of such a device can be optimised to such a degree that it may be used as a very sensitive magnetometer for the location of any object affecting the ambient magnetic field. Example 3 A superconducting loop can be formed by the following thick film technique:- 1. A ceramic multi-phase material of formula Yχ.2 Ba 0.8 Cu0 4-d ^ s Produced as described in Example 1 above and is powdered finely to a particle size of about 10-30 μ . This material is intimately mixed with an organic binder such as polyvinyl alcohol (PVA) . 2. The powered ceramic and binder is coated onto the surface of a 1 mm cylindrical rod of zirconia by dipping into a thinned mixture of the ceramic and binder. The thickness of the coating is selected -to give a coating of 10-100 jim after- firing. 3. The mixture is allowed to dry to allow partial evaporation of the PVA. 4 0 The coated rod is fired at 950°C for 12 hours in air or-oxygen. 5. The fired rod is cooled slowly in air or oxygen for about 12 hours. 6. The rod is then heated to 350-450°C and maintained at this temperature for 6 hours in oxygen " and is then cooled. 7. The coated cylinder is cut into discs 1 mm thick so giving loops of material having inherent weak-link superconducting properties. Example 3 is repeated using Ba2Cu3θ7_ ( 3 as a starting material. Such a material is a superconductor. However, following treatment in steps 4 to. 6 as described in Example 3, the resultant thick film contains more than one phase and provides the required weak link properties.

Claims: CLAIMS : 1. A superconductor loop device comprising a body formed of a matrix which is substantially non-superconducting and which has dispersed therein regions of superconductive material such as to impart to the body the inherent properties of a weak-link superconducting loop. 2. A device as claimed in claim 1, wherein the superconductive material is a ceramic superconductor. 3. A device as claimed in claim 1 or 2, wherein the substantially non-superconducting matrix is a ceramic material. 4. A device as claimed in claim 1, formed of a multi-phase material comprising a non-super¬ conducting phase providing said matrix and a dispersed superconducting phase providing said regions of superconductive material. 5. A device as claimed in claim 4, " wherein the multi-phase material is a ceramic material. 6. A.device as claimed in any preceding claim, wherein the superconductive material is a superconducting ceramic material selected from the rare earth-barium/strontium-Cu-O system. 7. A device as claimed in claim 1 or 2, wherein the substantially non-superconducting matrix is provided by a resin. 8. A device as claimed in any one of claims 1 to 6, wherein the body is a sintered body. 9. A device as claimed in any one of claims 1 to 6, wherein the body is unsintered and formed of compacted particles. 10. A device as claimed in any one of claims 1 to 6, wherein the body is formed by a thick film technique on a substrate. 11. A device as claimed in any preceding claim, wherein the matrix has poor conductivity. 12. A method of producing a superconductor loop device as claimed in claim 1, comprising the step of forming a body from particles of a material which has a non-super¬ conducting phase and a dispersed super- conducting phase. 13c A method as claimed in claim 12, wherein the body is formed by compacting. 14. A method as claimed in claim 12, wherein the body is formed by compacting and sintering. 15. A method as claimed in claim 12, wherein the body is formed by thick film technology on a substrate. 16. A method as claimed in claim 15, wherein the body is formed by steps which include mixing said particles with a binder, coating the mixture onto the substrate, and firing the coated substrate. 17. A superconductor loop device comprising a body formed of regions of superconducting material and regions of substantially non-superconducting material which are mutually arranged so as to impart to the body the inherent properties of a week-link superconducting loop. 18. A method of producing a superconductor loop device as claimed in claim 17, comprising the steps of mixing particles which consist of or contain a superconductor with a binder, and shaping the resultant mixture to form a body. REQUESTFORRECTIFICATIONUNDERRULE91.1(f) It has cose to my attention that, because ©f a clerical error in a letter of instruction to me, Italy was inadvertently omitted free the list of countries in which protection waβ to be sought. I enclose a copy of such letter of instruction (dated 29th March 1988) to ne together with a copy of a draft letter which was not sent to ae but which clearly shows the intention of the Applicants to include Italy * If necessary, a signed statement to this effect can be obtained froa the signatory to the letter of 29th March 1988. However, it is believed that this is not necessary in the circumstances, at least at this stage.

Inventors: ABELL JOHN STUART, GOUGH COLIN EDWARD, MUIRHEAD CHRISTOPHER MICHAEL, ABELL, JOHN, STUART, GOUGH, COLIN, EDWARD, MUIRHEAD, CHRISTOPHER, MICHAEL
Applicants: UNIV BIRMINGHAM, THE UNIVERSITY OF BIRMINGHAM
Application year: 1988
Main classification: H01L39
IPC codes: G01R33, H01L39