IMPROVED SUPERCONDUCTORS

Abstract: An improved superconductor and process for manufacture of the superconductor, said superconductor comprising a precursor compound containing a metal and providing nucleation sites for Y2BaCuO5 (211) particles, said particles having a size up to 2 mu m. The superconductor has improved flux pinning, critical current density, homogeneity and an increased oxygen content.

Description: IMPROVED SUPERCONDUCTORS This invention concerns an improved superconducting material and a process for the manufacture of a thick film superconductor. It has been shown that many metals show a decrease in electrical resistance as the external temperature is reduced towards absolute zero. Below a certain critical temperature (T c ), some materials exhibit an electrical resistance which rapidly falls to zero; this phenomenon is known as "superconductivity". The cooling of superconductors is achieved practically using liquid helium (which boils at approximately 4k) and/or liquid nitrogen (which boils at 77k). The utilization of liquid nitrogen is preferred as a coolant because it is cheap, easy to handle and has a large heat flux (ie it readily transfers heat away from a surface). The superconducting state was first discovered in super-cooled merc by Onnes in 1911. Subsequent research has revealed that there are many mater systems which superconduct, and relatively recently systems based around copper oxi compounds containing such elements as bismuth, thallium, lanthanum and yttrium ha been found. These superconducting materials have been found to have so intrinsically poor physical properties including relatively low critical current densi (J c ) characteristics and poor dependence on the application of magnetic fields. It is believed that the low J c is due to problems with the intrins anisotropy of the materials, having increased superconducting properties in certa crystallographic directions; the presence of weak links at grain boundaries; a localised inhomogeneities and low density of sintered bulk superconductors. The have been attempts made to increase J c by using processing techniques such melt-textured growth (MTG), quench and melt growth and melt-powder-melt-grow (MPMG). The aim of these techniques is to increase J c by crystallographic texturin of the superconducting matrix and to introduce controlled size flux pinning centres int the matrix. The inherent disadvantage with these techniques is that the melt-quenchin process is difficult to control and is technically impractical for the preparation of larg bulk materials and other material formats. The possible practical uses of superconductors are varied and includ medical technology, microwave devices, transportation, magnetic energy storage power generation and transmission, fault current limiters and electronics applications such as contaα materials and printed circuit boards. Their use would lead to significan energy savings and reductions in equipment size due to increased efficiency. Superconductors may be utilized in many formats including bulk material, wires and tapes, thin films, thick films and single crystals. In an yttrium-based superconducting system (YBCO) it is known that when a small amount of platinum is added to the YBa 2 Cu 3 0 7 . 0 (123) superconductor before thermal processing, the inherent Y 2 BaCuO s (21 1) precipitates, which are present in the system, are highly refined and uniformly distributed throughout the matrix. The refinement of the 211 phase leads to an increase in the J c values obtainable. It has also been found that a fine dispersion of 211 particles suppresses the formation of cracks, increases the oxygen diffusion rate and may act as and/or introduce flux pinning centres which are beneficial for increasing J c in the material. The mechanism by which the refinement of the 211 phase occurs is not fully understood, however, it is believed that platinum (present as the complex compound Ba 4 CU |+ -.Pt 2 _ x 0 9 _ z ) acts as a site for the heterogeneous nucleation of the 211 phase during the melt texturing of the material. There are two other phases commonly present in the YBCO namely BaCu0 2 and CuO which when present as majority phases are detrimental to the properties of the superconductor. A paper by Yoshida M et al (Physica C 185-189 (1991) 2409-2410) reports the effects of the addition of platinum group metals (PGMs) to YBCO (123). Platinum and rhodium were found to increase J c but the other metals had negligible, if any, effect. The PGMs were added to YBCO (123) in amounts up to 1.0wt% to bulk material and subjected to a partial melt and slow cooling process. The 211 particles which were present were observed to have a highly refined morphology. It was suggested that the Ba 4 CuPt,O 9 (0412) compound may act as a suitable site in the matrix for the nucleation of 211 particles because it exhibits close lattice matching with the 211 phase. A recently published paper by Jong-Hyun Park et al (Journal of Materials Science: Materials in Electronics 4 (1993) 77-82) reports work to establish whether platinum alone provides the nucleation sites or whether it is platinum present as the compound Ba 4 CuPt 2 θ 9 . The latter was proposed to act as a nucleation site for 211 particles. The process used 1.0wt% Ba 4 CuPt 2 θ 9 added to bulk specimens of YBCO (123) in a partial melting process. Published European patent application EP 0493 007 Al (International Superconductivity Technology Center) discloses a superconducting material comprising a rare earth oxide superconducting material, eg YBa 2 Cu 3 O y , and which includes platinum or rhodium in the range 0.01-5.0wt%. The raw materials Y, Ba and Cu are combined, together with platinum or rhodium, and subjected to a process of melting, slow cooling and then a further heat treatment at 500°C for 24 hours. J c values were calculated to reach 1.6xl0 4 Acm ~2 . EP 0537363 Al (International Superconductivity Technology Center) describes a bulk superconductor produced using a MPMG method in which platinum is added to the raw materials to form Ba 4 Cu 2 PtO y . The platinum powder has an average particle diameter in the range O.l-lOμm and is added in an amount of 0.2- 2.0wt%. J, values of 1.5xl0 -3.5x l0 4 Acm "2 were achieved. Another method of producing a bulk superconductive material is disclosed in EP 0 587 326 Al. In this method platinum, either as a metal powder or an oxide, was added to powdered YBa 2 Cu 3 0 7 . x and Y 2 BaCu0 5 . A small amount of silver may also be included which provides resilience to crack formation. Ba 2 CuPt- ; 0 9 was added to YBCO in the range 2-6wt%; alternatively rhodium could replace platinum. J c was determined to be 1.7xl0 4 and 1.8xl0 Acm "2 for platinum and rhodium respectively. The method includes a partial melt step. Although, as shown above, it has been possible to increase the J c value for bulk superconductors to a certain extent, the same is not true for thick film superconductors. Many previous studies into the superconducting properties of YBCO thick films deposited onto a variety of substrates have shown that the characteristic properties are highly dependent upon the substrate material and the processing conditions of the film. YBCO thick films deposited and processed on alumina substrates have been observed to have poor properties with respect to bulk melt processed material, possessing J c values in the region of lOOAcm '2 and transition temperatures of around 90K (T C Shields et al, Supercond Sci Technol 5 (1992) 627- 633). The superconducting properties of YBCO thick films are significantly enhanced by processing on yttria-stabilised zirconia (YSZ) substrates, with J c 's of l.S lG^Acm 2 and T c 's of 92K being readily achievable (Y J Bi et al Mat Sci Eng B21 (1993) 19-25). Certain additions made to thick films of YBCO have been shown to increase the superconducting characteristics of the material. For example BaSnO 3 additions have been shown to increase transport J c 's to 2x10 Acm " (T C Shields et al, Physica C 249 (1995) 387-395), and additions of controlled concentrations of silver have increased J c 's up to 3xl0 3 Acm "2 (M J Day et al, Physica C 185-189 (1992) 2395- 2396). These additions are observed to modify the microstructure of the films, by different means to PGM additions, as to increase the flux pinning capabilities of the material. As yet these have been the largest transport J c values reported in thick films of YBa 2 Cu 3 0 7 . Λ . It is the aim of the present invention to provide an improved superconducting material with an increased J c value. The present invention provides a thick film superconductor comprising a superconductor matrix, nucleation sites provided by a precursor phase and particles of Y 2 BaCuO s (211) nucleated by said precursor phase, wherein said particles have size up to 2μm along one axis. The advantages of the presence of 211 particles of this size is that they will precipitate more homogeneously within the matrix. In addition these smaller precipitates have an increased surface curvature and therefore a greater number of defects associated with the 211/123 interface, with the result that the precipitates enhance flux pinning and increase J c . Preferably the superconducting matrix is YBa : Cu 3 O 7 . x (123). The precursor phase may be Ba 4 CuM 0 where M is one or more PGM's. The precursor material acts as a nucleation site for the deposition of the 211 phase due to the close lattice matching of the two phases. Preferably M is selected from platinum, rhodium or palladium or a combination thereof. Preferably the Y 2 BaCu0 5 (211) particles have a high surface curvature. The particles may be needle shaped due to preferential growth in certain crystallographic axes from the nucleation site; alternatively smaller particles are approximately spherical in nature. There is also provided a process for the manufacture of a superconductor comprising the steps: i) combining BaC0 3 , CuO and Y 2 0 3 and drying; ii) combining BaO, CuO and the PGM in a cationic ratio of 4: 1 :2, calcining at 800°C for 36 hours and optionally grinding at intervals; iii) combining the products of i) and ii); iv) depositing the superconductor on a substrate, and v) melt texturing the superconductor. Preferably, the process comprises the following steps: i. intimately combining and reacting BaC0 3 , CuO and Y 2 0 3 by (a) ball milling in sealed polyethylene jars with ethanol, utilising hig purity yttria-stabilised zirconia (YSZ) milling media, drying at 100° for 24 hours and subsequendy reacting by a solid state calcination technique at 900°C for 24 hours with intermediate grinding steps; (b) spray drying from a nitrate solution and reacting; ii. combining BaO, CuO and PGM in a cationic ratio of 4:1:2 by ball milling in polyethylene jars and milling with YSZ milling media in ethanol, drying at 100°C for 24 hours and calcining at 800°C for 36 hours with intermediate grinding intervals to ensure complete reaction; iii. combining the products of 1 and 2; iv. screen printing the superconductor onto a suitable substrate; and v. melt-texturing the superconductor in a flowing oxygen atmosphere. Suitably the superconductor is combined with an organic binder to produce an ink and deposited onto an yttria-stabilised zirconia substrate by screen printing. Other deposition methods well known in the art can be used. The invention will be described by way of example which is not intended to be limiting of the invention. EXAMPLE 1 Platinum and/or Rhodium Powder Addition to YBCO powder A superconducting ceramic YBCO powder was synthesised utilizing a spray drying technique. Platinum and/or rhodium powder additions (0.8-2.5μm) were made to the YBCO (123) powder in the proportions 0.05, 0.1, 0.15, 0.2, 0.3, 0.5, 1.0 and 5.0 wt% and each sample intimately mixed. Differential thermal analysis (DTA) was performed on the resultant powders under flowing oxygen. The results showed that as platinum and/or rhodium is added to the YBCO powder a reaction is observed to occur with the YBCO at about 670°C. This reaction is dependent upon the concentration and particle size of the platinum and/or rhodium added. This reaction is thought to be the one responsible for the formation of the 0412 compound and consequently leads to the generation of other reaction products rich in copper (CuO, BaCu0 2 ) within the matrix. The doped powders were used to produce thick films and processed in a tube furnace in an atmosphere of flowing oxygen to a temperature of 1050°C. Heating rates of 5°C min "1 were used and cooling was achieved using 4°C min "1 to 900 °C and 2°C min '1 to room temperature. An increased amount of second phases were produced as increasing amounts of platinum and/or rhodium were added to the system. Maximum increases in J c were obtained using 0.15wt% platinum addition, with transport J c 's in excess of 4x10 Acm " at 77k. Doping above this level was observed to give poor superconducting properties due to the production of high volumes of second phases. An increase in J c value to 2.8-3 l0 3 Acm "2 was obtained by rhodium addition. EXAMPLE 2 Synthesis and Addition Ba,CuM : O ? Where M is a PCM Powders of BaO, CuO and PGM powder (one or more of platinum, rhodium and palladium) with a cationic ratio of 4: 1 :2 were mixed in polyethylene containers together with ethanol and YSZ milling balls for 1 hour, dried at 100°C for 12 hours, and calcined together at 800 °C for 36 hours with intermediate grinding stages every 12 hours to ensure complete reaction. Two batches of material were produced, one processing in air and one in oxygen. The material calcined in oxygen was more fully reacted than in air and the PGM-containing phase that was produced was of the composition Ba 4 Cu, +) .M 2.x 0 9 . z (0412). The reaction is thought to be diffusion controlled as there is no liquid phase present at this temperature and the reaction is observed to increase with reduced particle size (ie increased surface area) of PGM added. Furthermore characterisation of the processed powders using electron microphobe analysis (EPMA) showed that 0412 synthesised with rhodium, and increasingly so with palladium, were less homogeneous in nature after calcination. A first portion of this phase was added to 123 YBCO powder and DSC/DTA analysis was performed. No 670°C reaction was observed in these doped materials. Thick films were then produced and characterised. An optimsed transport critical current density in excess of 6xl0 3 Acm " " at 77K and zero applied field with an approximate addition of 0.4wt% synthesised with platinum was obtained. A second portion of the as-synthesised 0412 phase powder (d 50 =3.7μm) was subjected to one hour of rigorous grinding in a high speed zirconia ball mill. The powder was re-sized with diameters being considerably smaller (d 50 =0.4μm). Optimised J c 's in excess of 7xl0 3 Acm "2 at 77 and zero applied field with an approximate addition of 0.3wt% 0412 synthesised with platinum were obtained.

Claims: CLAIMS 1. A thick film superconductor comprising a superconductor matrix, nucleation sites provided by a precursor phase and particles of Y 2 BaCu0 5 (211) nucleated by said precursor phase, wherein said particles have a size up to 2μm along one axis. 2. A superconductor according to claim 1 wherein the matrix is YBa,Cu 3 0 7 . x (123). A superconductor according to claim 2 wherein the precursor phase is Ba 4 CuM 2 0 9 and M is one or more platinum group metal (PGM). 4. A superconductor according to claim 3 wherein M is selected from platinum, rhodium or palladium or a combination thereof. 5. A superconductor according to any preceding claim wherein the Y 2 BaCuO s (211) particles have a high surface curvature. 6. A superconductor according to claim 5 wherein the particles are substantially needle shaped or approximately spherical. 7. A process for the manufacture of a superconductor according to any of claims 1-8 comprising the steps: i) combining BaC0 3 , CuO and Y 2 0 3 and spray drying; ii) combining BaO, CuO and platinum in a cationic ratio of 4: 1:2, calcining at 800°C for 36 hours and optionally grinding at intervals; iii) combining the products of i) and ii); iv) depositing the superconductor on a substrate, and v) melt-texturing the superconductor. 8. A process according to claim 9 wherein the superconductor is combined with an ink and is deposited onto an yttria-stabilised zirconia substrate by screen printing.

Inventors: ABELL JOHN STUART, LANGHORN JASON BARNABAS, ABELL, JOHN, STUART, LANGHORN, JASON, BARNABAS
Applicants: ABELL JOHN STUART, JOHNSON MATTHEY PLC, LANGHORN JASON BARNABAS, ABELL, JOHN, STUART, JOHNSON MATTHEY PUBLIC LIMITED COMPANY, LANGHORN, JASON, BARNABAS
Application year: 1995
Main classification: H01L39
IPC codes: C01G1, C01G3, H01B12, H01B13, H01L39