Thallium-calcium-barium-copper-oxide superconductor with silver and method

Abstract: A method is disclosed for forming polycrystalline thallium system superconductors having high current carrying capacity and high magnetic field behavior. A precursor deposit is formed comprised of silver, in an amount of about 1 to 20 mole percent of metals in the deposit, and the balance oxides of calcium, barium, and copper in ratios for forming the superconductor. The precursor deposit is annealed in an oxidizing atmosphere, and in the presence of a source of thallous oxide. The source of thallous oxide is heated to a first temperature selected to incorporate thallium into the deposit in an amount of about 6 to 22 mole percent of metals in the superconductor, and the precursor deposit is heated to a second temperature to form the superconductor. Preferably, the superconductor is post-annealed in an oxidizing atmosphere at about 500° to 650°C to improve the current carrying capacity in the superconductor.

Description: This invention relates to a ceramic or oxide superconductor, and method for forming the superconductor with improved current carrying capacity. More specifically, it relates to the thallium-calcium-barium-copper oxide system of oxide superconductors, sometimes herein referred to as the thallium system superconductors. Thallium system superconductor compositions and a method of preparation were reported in "Bulk Superconductivity at 120 K in the Tl-Ca/Ba-Cu-O System," Z.Z. Sheng and A.M. Hermann, Nature Vol. 332, March 10, 1988, pp. 138-139. Briefly described, appropriate amounts of powdered Tl 2 O 3 , CaO and BaCu 3 O 4 to form a desired composition were completely mixed, ground and pressed into pellets. A tube furnace was heated to 880 to 910°C, and the pellets were placed in the furnace for three to five minutes with a flowing oxygen atmosphere. The pellets were removed from the furnace after heating, and quenched to room temperature in air, or furnace cooled to room temperature. The pellets were reaction sintered by the heat treatment, forming a superconducting oxide compound. Some quenched samples were subsequently annealed at 450°C in flowing oxygen for several hours. An interesting feature of the thallium system oxide superconductors, as well as a variety of other oxide superconductors, is that they are non-stoichiometric. In thallium system oxide superconductors, the oxygen content is adjustable. Though oxygen can vary over a small range, the variations can have a profound effect on the superconducting behavior of the compound. A convenient way of showing the composition of the oxide superconductors is by a formula showing the atomic ratio of each element, e.g., Tl 2 Ca 2 Ba 2 Cu 3 O 10+y where y is a fraction of 1. Experiments to find the useful combinations of thallium, calcium, barium, copper, and oxygen continued, and additions of lead and strontium have been tried. A number of the oxide compound compositions for the thallium system superconductors were reported in "Chemical Engineering and the Development of Hot Superconductors," R. Kumar, Chemical Engineering Progress, April 1990 pp. 17-27, and are shown by atomic ratio in Table 1 below. The critical temperature, T c , is shown for each composition. Composition of thallium system superconductors by atomic ratio Tl Pb Ca Sr Ba Cu O T c 2 - - - 2 1 6+y 80 2 - 1 - 2 2 8+y 108 2 - 2 - 2 3 10+y 125 2 - 2 - 3 4 12+y 104 1 - 1 - 2 2 7+y 80 1 - 2 - 2 3 9+y 122 1 - 2 - 3 4 11+y 120 0.5 0.5 1 2 - 2 7+y 90 0.5 0.5 2 2 - 3 9+y 122 1 - - 1 - 1 5+y 40 1 - 0.5 2 - 2 7+y 90 Since the time of the original disclosure of powder formed bulk thallium system superconducting compounds, thin films have been formed by sequential thermal evaporation, and sequential electron beam evaporation. Both the bulk sintered ceramic pellets and the thin films were treated by heating in flowing air or oxygen at temperatures of about 800° to 900°C to form the superconducting compositions. However, thallium oxide has an appreciable vapor pressure at the temperatures required to form the superconductors. As a result, thallium can be vaporized during the 800° to 900°C annealing treatments leading to loss of thallium from the superconductor. In "Superconducting Thin Films of Tl 2 Ca 2 Ba 2 Cu 3 O y and Tl 2 CaBa 2 Cu 2 O y ," D.S. Ginley et al., Physica C 156, 1988, pp. 592-598, incorporated herein by reference, it was reported that thin films of the thallium system were annealed in the presence of thallium-metal to suppress thallium loss from the superconductor. This method was found to be an unreproducible technique for forming the superconducting compositions. In another method, thin films were annealed while sandwiched between pellets of the thallium system superconductor consolidated from powders with the correct stoichiometry for the composition desired in the thin film. Thus, if a film of the superconducting composition Tl 2 Ca 2 Ba 2 Cu 3 O 10+y was being produced, the bulk ceramic pellet composition was prepared to be Tl 2 Ca 2 Ba 2 Cu 3 O 10+y . A strong correlation was found between film quality and annealing the film in the presence of the bulk ceramic of the desired composition. Films having a thallium content slightly greater than the stoichiometric composition had improved current carrying capacity, while those that were thallium deficient had a lower current carrying capacity. High current carrying capacity in polycrystalline oxide superconductors is believed to be associated with strongly interconnected or linked grains in the polycrystalline material, and low current carrying capacity is believed to be associated with poorly connected or weak linked grains. The individual crystals in polycrystalline superconductors are sometimes herein referred to as grains. The high transition temperatures of the thallium system superconductors makes commercial use in devices such as motors, magnets, and power generation and transmission equipment operating at liquid nitrogen temperature and above possible. However, to be of practical value a ceramic superconductor must be able to carry or conduct a substantial current, at least approaching the current carrying capacity of alloy superconductors such as Nb 3 Sn. The high current carrying capacity must be evident at temperatures up to the critical temperature without quenching or loss of superconductivity as evidenced by a substantial increase in electrical resistance. In some applications the high current carrying capacity must also be evident in the presence of magnetic fields. It is known that current carrying capacity can be greatly diminished in the presence of a magnetic field, so that the superconductor decreases in current carrying capacity as magnetic field increases up to a critical magnetic field above which the superconductor is quenched. A superconductor having high magnetic field behavior is characterized by a low reduction in current carrying capacity when the superconductor is in the presence of a magnetic field. Another requirement for such commercial application of ceramic superconductors is the development of reproducible and efficient methods for the fabrication of polycrystalline forms of the thallium system. An object of this invention is a method of forming improved polycrystalline thallium system superconductors comprised of silver. Another object of this invention is a method of forming polycrystalline thallium system superconductors having strong interconnections or links between grains that provide improved current carrying capacity. Another object of this invention is a method of forming polycrystalline thallium system superconductors having improved current carrying capacity in magnetic fields. According to the invention there are provided a method of forming a polycrystalline thallium system superconductor having the process steps as in claim 1, and an oxide superconductor as in claim 14. As used herein, the term "oxidizing atmosphere" means an atmosphere comprised of oxygen and up to 80 percent of inert gas such as nitrogen or argon, however, it is preferably 100 percent oxygen. Preferably, the superconductor is post-annealed in an oxidizing atmosphere at about 500° to 650°C to improve the current carrying capacity in the superconductor. Further according to the invention there is provided an oxide superconductor according to claim 17. The following description of the invention is aided by making reference to the accompanying drawings in which:- Figure 1 is a sectional view of a side elevation of an annealing apparatus used in the method of this invention. Figure 2 is a graph showing the temperature versus time for annealing a precursor film with a thallic oxide sample by the method of this invention to form a thallium system superconductor. Fig 3 is a graph showing the current density, measured in a perpendicular magnetic field at various intensities, in a thallium system superconductor film formed by the method of this invention. Detailed Description of the Invention The polycrystalline thallium system superconductors formed by the method of this invention have improved current carrying capacity and improved high magnetic field behavior. We have found that such improved thallium system superconductors can be formed by adding silver to the precursor deposit or film, and annealing in an oxidizing atmosphere with a preselected partial pressure of thallous oxide to incorporate thallium into the film in a preselected amount at the time the superconductor is being formed. It is believed the presence of the silver in the precursor, combined with incorporation of thallium during sintering, provides a type of reaction sintering in forming the superconductor that provides strong current links between grains. The precursor deposit comprised of silver and a mixture of oxides of calcium, barium, and copper in a ratio for forming the thallium system superconductor can be formed by conventional methods known to those skilled in the art. For example, sequential electron beam evaporation can be used to deposit the metals in layers to form a film of the desired composition. The film can be oxidized either during or subsequent to the film formation. The electron beam evaporation technique is discussed in "Superconducting Thin Films of Tl 2 Ca 2 Ba 2 Cu 3 O y and Tl 2 CaBa 2 Cu 2 O y ," D.S. Ginley, et al. referenced above. A spin-coating technique for forming the precursor film is described in, "Preparation of Tl 2 Ba 2 Ca 2 Cu 3 O y thick films from Ba-Ca-Cu-O films," R. Sugise, et al., Japanese Journal of Applied Physics, Vol. 27, No. 12, December, 1988, pp. L2314-L2316, incorporated herein by reference. Films of oxide superconductors have also been prepared using a spray pyrolysis technique based on the pyrolysis of an ultrasonic-generated aerosol containing the metals in the precursor. A spray pyrolysis method of forming precursor films is disclosed in "High T c Superconducting Films Prepared by Pyrolysis of an Ultrasonic-Generated Aerosol," M. Langlet et al., Journal of the Less-Common Metals, Vol. 151 (1989) pp. 399-406, incorporated herein by reference. Additional information about thallium system superconductor compositions and methods of preparation can be found in U.S. Patents 5,073,536, and 4,870,052, incorporated herein by reference. Precursor films can be deposited on substrates resistant to chemical reaction with the precursor film such as; the metals silver, gold, platinum, palladium, or alloys of the metals; and ceramics lanthanum aluminate, strontium titanate, magnesium oxide, and yttria stabilized zirconia. Bulk forms of the superconductor, such as pellets or rods, can be prepared by solid state reaction of powders known in the art. For example, the article by Sheng et al., Applied Physics Letters , May 16, 1988 describes powder preparation of compositions of the thallium system superconductors. The method of this invention provides a select partial pressure of thallous oxide over the precursor deposit, while at the same time the precursor is heated to a preselected temperature where the thallium system superconductor forms. Silver in the deposit improves the reaction sintering formation of the oxide superconductor, and the incorporation of thallium into the precursor during the formation of the superconductor is controlled to provide a polycrystalline structure having a high current carrying capacity. Control of volatile and toxic thallium oxide in the thallium system superconductor compositions has been a significant problem in known annealing methods, so that resort has been made to elaborate processes of annealing precursor films sandwiched between pellets comprised of the superconducting composition desired in the film. In the method of this invention, improved polycrystalline thallium system superconductors having strong current links between grains are formed without use of such pellets. The method of this invention can be further shown by making reference to the apparatus shown in Figure 1. Apparatus 2 is comprised of an annular multizone furnace 4 having a first heating zone 6 and a separate second heating zone 8. A suitable multizone furnace would be the series 31102 furnace from Applied Test Systems, Inc., about 41 cm in length and 30 cm in diameter. Resistance heating coils 7 and 9 surround heating zone 6 and 8 respectively, and provide the heat source for each zone. Resistance heating coils 7 and 9 are made, for example, from NICHROME resistance wire. Annular shields 10 approximately 10 cm in length and 3 cm in diameter are located within heating zones 6 and 8, and axially to furnace 4. The shields serve as heat pipes that evenly distribute the heat from resistance heating coils 7 and 9 within each heating zone. Heat shields 10 can be made from a heat resistant alloy, for example high nickel and chromium stainless steels having a minimum chromium plus nickel content of 20 weight percent, or INCONEL 600 nickel based superalloy. At each end of furnace 4 are auxiliary resistance coil heaters 12. The auxiliary heaters reduce the temperature gradient between the heating zones and the atmosphere outside furnace 4. In this way, the temperature within at least a central portion of about 4 cm of each heating zone can be controlled to within about plus or minus 2°C. Axially located within annular multizoned furnace 4 is a reactor assembly 20. Reactor assembly 20 is comprised of a combustion chamber 22 having atmosphere inlet 24 and atmosphere outlet 26. Combustion chamber 22 is made from a material suitable for containing an oxidizing atmosphere at elevated temperatures up to about 950°C, for example, a quartz tube of about 100 cm in length by 2.5 cm in diameter. The combustion chamber is slidably located in furnace 4, extending through heat shields 10. Inlet 24 is connected to an oxidizing gas supply 28 that is controlled by valve 30. Outlet 26 is connected to a back diffusion gas trap 32. A suitable trap 32 is a silicon oil gas bubbler comprised of a flexible tube connected to outlet 26 that extends into an open container partially filled with silicon oil. Gas passing through the tube is bubbled through the oil before passing into the atmosphere so that the ambient atmosphere cannot diffuse backwards through the tube into combustion chamber 22. A reaction vessel 16 is located within combustion chamber 22, and is made from a material resistant to chemical attack from thallous oxide such as fused alumina, gold, or zirconia, and is suitable for containing an oxidizing atmosphere. Reaction vessel 16 is located within combustion chamber 22 so that a first zone 6' within reaction vessel 16 is approximately centrally located within heating zone 6 of tubular furnace 4, and a separate second zone 8' within reaction vessel 16 is approximately centrally located within heating zone 8 of tubular furnace 4. Reaction vessel 22 can be formed, for example, from a fused alumina tube about 25 cm long by 1.25 centimeters in diameter, having one open end 18. A holder 14 is made from a material chemically inert to attack from thallous oxide, such as gold, and is used to hold a source of thallous oxide. A suitable source of thallous oxide is, for example, thallic oxide, thallous oxide, or oxide compounds of thallium. The source of thallous oxide is placed within holder 14, for example a deposit of 1 gram of thallic oxide is suitable for several annealing procedures for small samples of precursor films. A tray 21 is made from one of the materials chemically resistant to attack from thallous oxide. Substrate 23, having the precursor film deposited thereon, is placed on tray 21. Reaction vessel 16 is prepared for use in the method of this invention, by first placing the thallous oxide in holder 14 within first zone 6' and the precursor film on substrate 23 on tray 21 within second zone 8' of reaction vessel 16, and sealing an oxidizing atmosphere within reaction vessel 16. A suitable procedure for sealing an oxidizing atmosphere within reaction vessel 16 comprises placing the reaction vessel within a gas impermeable container having one open end, such as a plastic bag, and loosely placing a cap 40 over the open end 18 of reaction vessel 16. Cap 40 is made from material resistant to chemical attack from thallous oxide such as silver foil. A tubular cylinder, having a larger diameter than reaction vessel 16, is placed in the bag and reaction vessel 16 is positioned within the cylinder so that the cap is inside the cylinder. The open end of the bag is then sealed around a tube that is alternately connected to a vacuum and then a source of oxidizing gas. The cylinder prevents the bag from collapsing over the open end of the reaction vessel during evacuation. The container is evacuated, and backfilled with an oxidizing atmosphere several times to establish the desired oxidizing atmosphere within reaction vessel 16. A flow of the oxidizing gas is maintained in the container while a slit is made in the container near the cap. One or two wire loops are passed through the slit and placed around the portion of cap 40 that extends over reaction vessel 16, and the ends of the wire loops are twisted together so that the wire loops 62 seal the cap over the open end 18 of reaction vessel 16. Combustion chamber 22 is opened and closed at ground glass joint 60. Thermocouples 42 and 44 are positioned within combustion chamber 22 to measure the temperature in first vessel zone 6' and second vessel zone 8' respectively. A conventional heater control means 50 controls the temperature of heating coils 7 and 9. Thermocouples 42 and 44 enter combustion chamber 22 through seal 36, and are positioned so that the temperature sensing ends of the thermocouples are centrally located in first zone 6 and second zone 8 respectively. Combustion chamber 22 is opened at ground glass joint 60, and reaction vessel 16 is placed within combustion chamber 22 so that first vessel zone 6' and second vessel zone 8' are centrally located within heating zones 6 and 8 respectively. Combustion chamber 22 is closed and sealed at ground glass joint 60. Valve 30 is opened and chamber 22 is flushed with the oxidizing atmosphere within reaction vessel 16 so that the oxidizing atmosphere previously sealed within vessel 16 is maintained. Heater control 50 is activated to heat zones 6 and 8. First vessel zone 6' is heated to a first temperature, for example to a temperature of about 730°-800°C, selected to incorporate thallium into the precursor film in an amount of about 6 to 22 mole percent of the metals in the superconducting film. Second vessel zone 8', and the precursor film, are heated to a second temperature that forms the thallium system superconductor, for example about 855°-890°C, preferably about 855°-860°C. For example, the TlCa 2 Ba 2 Cu 3 O 8.5+y thallium system superconductor comprised of about 5 mole percent silver is formed by incorporating about 6 to 12 mole percent, preferably, about 9 mole percent thallium based upon the total moles of metals in the superconducting film. In another example, the Tl 2 Ca 2 Ba 2 Cu 3 O 10+y thallium system superconductor comprised of about 5 mole percent silver is formed by incorporating about 12 to 22 mole percent, preferably, about 16 mole percent thallium based upon the total moles of metals in the superconducting film. The heating rate of first vessel zone 6' is controlled so that the first temperature is reached after the precursor film has been heated in second vessel zone 8' to the second temperature where the polycrystalline thallium system superconductor forms. For example, the first vessel zone 6' is heated to about 40°C below the first temperature, and when second vessel zone 8' has been heated to the second temperature, first vessel zone 6' is heated to the first temperature. If first zone 6' is heated to the first temperature before the second zone 8' is heated to the second temperature, an excessive amount of thallium can be incorporated in the film. In other words, the heating rates in zones 6' and 8' are selected so that the amount of thallium incorporated into the film, at any time during the annealing process, does not exceed the desired final amount. The resulting superconducting film has a high current carrying capacity and high magnetic field behavior. The precursor film is heated for a period of time in which the superconducting phase forms but before there is significant reaction with the substrate. Preferably, the precursor film is heated for about 10 to 60 minutes. Barrier layers, for example, of barium aluminate or barium zirconate deposited on the substrates prior to depositing the precursor film help minimize interaction between the film and substrate during heating. The use of such barrier layers would allow increased time periods for annealing in which reaction between the precursor film and substrate is minimized. After annealing is complete, the combustion chamber is allowed to cool in the furnace with the the oxidizing atmosphere flowing through combustion chamber 22. Preferably, the superconducting film is post-annealed in an oxidizing atmosphere at about 500°-650°C, preferably about 500°-600°C to provide further improvement in the current carrying capacity of the superconductor. The oxide superconductor formed is comprised of about 1 to 20 mole percent silver, based upon metals in the superconductor, in an oxide compound comprising about 72 to 75 mole percent metals and the balance oxygen, the metals comprising, in mole percent of metals in the compound, about 4 to 12 percent thallium, about 11 to 13 percent barium, about 11 to 13 percent calcium, and about 16 to 19 percent copper. The preferred oxide superconductor is comprised of about 1 to 20 mole percent silver, based upon metals in the superconductor, in an oxide compound having the formula Tl x Ca 2 Ba 2 Cu 3 O 8.5+y where x varies from about 0.7 to 1, and y is a fraction of 1. Examples Precursor film samples were formed on single crystal and polycrystalline yttria stabilized zirconia substrate chips about 8 x 12.5 x 0.5 mm by the spray pyrolysis technique. Briefly described, a metal nitrate solution was formed by dissolving about 5 mole percent silver, based upon the total metals in the precursor film, and CuO, CaCO 3 , and BaCO 3 in the ratios for forming thallium system superconductors having the composition of about Tl x Ca 2 Ba 2 Cu 3 O 10+y in a nitric acid solution. The substrate chips were cleaned by ultrasonic agitation in trichlorethylene, followed by ultrasonic agitation in acetone, followed by rinsing in methanol and blowing dry. The cleaned substrate chips were mounted on a heating block and heated to about 275°C. The nitrate solution was ultrasonically nebulized and sprayed onto a substrate chip to form a 3 to 5 micron coating of the metal nitrates. The coated substrate was heated to about 650°C to pyrolize the precursor film, decomposing the nitrates and oxidizing the film. The substrate chips were cooled, removed from the heater block, transferred to a combustion tube furnace at 500°C, and heated to about 845°C in oxygen in about 30 minutes. The films were cooled in the oxygen atmosphere and stored in a desiccator. The final precursor film samples were about 2 to 4 microns thick. The precursor film samples were annealed by the method of this invention to form polycrystalline Tl x Ca 2 Ba 2 Cu 3 O 10+y superconductive films where x varies from about 1.3 to 2, or polycrystalline Tl x Ca 2 Ba 2 Cu 3 O 8.5+y superconductive films where x varies from about 0.7 to 1. Precursor film samples were heated in the reaction vessel in an oxygen atmosphere to temperatures of about 855-860°C while at the same time heating a sample of thallic oxide to various temperatures between 735°C to 800°C, and holding at the annealing temperature for about thirty minutes. An example of the heating schedule for the thallic oxide and precursor film is shown in Fig. 2. Fig. 2 is a graph of the temperature versus time recorded during annealing of a precursor film in zone two with a piece of thallic oxide in zone 1 of the apparatus in Fig. 1. The films were furnace cooled to 500°C, cooled at about 20°C per minute to 100°C, and removed from the combustion chamber. The superconducting films were given a post-anneal in an oxygen atmosphere at about 600°C for 8 hours. The thallium content in the annealed films was determined from the weight gain in the samples after the anneal. A technique well known in the art as four point probe DC measurement was performed at various temperatures to measure the critical temperature, T c , in the films. Film samples were patterned with a bridge structure about 0.4 millimeter long and 100 microns wide by a standard mask and chemical etch process for measuring the critical current, J c , in the film. The processing temperatures, thallium content, critical current, and critical temperature of some of the superconducting film samples are shown below in Table II. Sample No. Tl Temp. (°C) Film Temp. (°C) Tl x T c (K) J c (A/cm 2 ) 1 735-750 855-860 0.70 107 62,000 2 735-750 855-860 1.00 107 13,300 3 735-750 855-860 0.70 106 23,900 4 735-750 855-860 0.80 105 21,000 5 735-750 855-860 0.75 104 105,000 6 735-750 855-860 0.70 105 12,600 7 735-750 855-860 0.75 105 18,800 8 735-750 855-860 0.75 105 8,800 9 735-750 855-860 0.80 104 22,000 10 735-750 855-860 0.75 105 42,000 11 750-800 855-860 1.35 115 12,700 12 750-800 855-860 1.33 112 10,100 The critical current of a number of the superconducting film samples was measured before and after the post-annealing treatment, and are shown below in Table III. Sample No. J c (A/cm 2 ) After Post-Annealing J c (A/cm 2 ) 13 14,000 30,500 14 350 1,500 15 2,300 7,000 16 3,800 10,300 17 3,400 7,500 18 19,000 62,000 19 3,700 6,700 20 6,900 12,700 21 1,250 1,800 22 8,100 14,200 23 5,900 10,200 24 1,100 3,300 25 350 1,055 X-ray diffraction patterns of the samples were obtained on a Siemens D500 diffractometer using copper K-alpha radiation. An incident beam collimator was employed to restrict the X-ray beam to the surface of the sample. The X-ray diffraction patterns showed the presence of a well crystallized Tl x Ca 2 Ba 2 Cu 3 O 8.5+y phase where x is between about 0.7 to 1, or Tl x Ca 2 Ba 2 Cu 3 O 10+y phase where x is between about 1.3 to 1.6, with only a trace of the Tl 2 CaBa 2 Cu 2 O 8+x phase. The relative peak intensities of the patterns showed that the C-axis of the unit cell of the primary phase was oriented perpendicular to the substrate surface, with a high degree of alignment of the Tl x Ca 2 Ba 2 Cu 3 O 8.5+y or Tl x Ca 2 Ba 2 Cu 3 O 10+y phase. Scanning electron microscope micrographs were obtained using a Hitachi S-800 field emission scanning electron microscope. The sample microstructures consist of plate-like grains elongated in the direction parallel to the substrate. The thallium content of samples 1 and 3-10 was determined to be significantly less than that expected for the stoichiometric composition of the Tl 1 Ca 2 Ba 2 Cu 3 O 8.5+y film. The thallium content of sample films 11, and 12 was found to be significantly less than that expected for the formation of stoichiometric Tl 2 Ca 2 Ba 2 Cu 3 O 10+y . The magnetic field dependence of the current carrying capacity of sample 5 was measured between about 4.2 to 90 Kelvin. The superconducting film of sample 5 was placed in a magnetic field that was perpendicular to the direction of current flow in the film, and was varied from 0 up to about 10 tesla while the current density in the film was measured. The current density versus applied magnetic field is shown in Fig. 3. Even under this most severe condition of magnetic field, the film exceeded 100,000 A/cm 2 at 20 K out to fields of 10 tesla. It is believed this is the highest magnetic field behavior ever measured in a polycrystalline thick film Tl 1 Ca 2 Ba 2 Cu 3 O 8.5+y superconductor. The Examples above were made with a precursor film containing silver, and formed by spray pyrolysis. Depending upon the initial form of the precursor film or deposit the processing conditions for forming the thallium system superconductor having strong current links between grains can change. For example, the densely formed highly reactive films formed by sequential electron beam evaporation are expected to form the superconductor at temperatures similar to the annealing temperatures in the Examples. However, silver containing precursor oxide films prepared by any of the deposition techniques can be converted to thallium system superconducting films with properties comparable to those of the films prepared by the annealing processes of this invention. The processing conditions required will depend upon the composition of the precursor oxide film, and the film deposition process. By performing a few simple experiments as in the Examples above, using the method of this invention, the annealing temperature ranges in the zones for the source of thallous oxide and the superconducting precursor can be determined for forming the precursor films or deposits into the thallium system superconductor having strong current links between the grains. The use of silver additions in the synthesis of bulk forms of the thallium system superconductors from precursor oxide powder mixtures will also provide improvements in the current links between grains. However, because of the relatively slow rate of diffusion of thallium it is unlikely that very thick samples could be prepared without the deliberate addition of thallium oxide powder to the precursor mixture.

Claims: A method of forming a polycrystalline thallium system superconductor comprising the steps of: a) forming a precursor deposit including oxides of calcium, barium, and copper in ratios for forming the superconductor; and b) annealing the precursor deposit in an oxidizing atmosphere while in the presence of a source of thallous oxide, the source of thallous oxide being heated to a first temperature selected to incorporate thallium into the deposit in an amount of about 6 to 22 mole percent of metals in the deposit, and the deposit being heated to a second temperature, greater than the first temperature, to form the superconductor; characterised in that said precursor deposit further includes silver, in an amount of about 1 to 20 mole percent based upon the total moles of metals in the deposit. The method of claim 1, wherein the thallium is incorporated into the deposit in the amount of about 6 to 12 mole percent, preferably about 9 percent. The method of claim 1, wherein the thallium is incorporated into the deposit in the amount of about 12 to 22 mole percent, preferably about 16 percent. The method of any one of claims 1 to 3, wherein the amount of silver is about 5 mole percent. The method of any one of the preceding claims, wherein the deposit is heated to the second temperature before the source of thallous oxide is heated to the first temperature. The method of any one of the preceding claims comprising disposing the precursor and the source of thallous oxide in a vessel that is resistant to chemical reaction with thallous oxide, the vessel having separate heating zones with communication of atmosphere between zones, the source of thallous oxide being disposed in a first heating zone within the vessel, and the precursor being disposed in a second heating zone; and sealing the vessel so that it contains the oxidizing atmosphere. The method of any one of the preceding claims comprising post-annealing the superconductor in an oxidizing atmosphere at a temperature from 500° C to 650° C. The method according to any one of the preceding claims wherein the first temperature is from 730° C to 800° C or from 735° C to 750° C. The method according to any one of the preceding claims wherein the second temperature is from 855° C to 890° C. The method according to any one of the preceding claims wherein the oxidizing atmosphere is oxygen. The method according to claim 10 wherein crystallites of the polycrystalline deposit are aligned with a c-axis perpendicular to the surface of substrate, on which the deposit is formed. The method according to any one of the preceding claims wherein the precursor is formed by nitrates of calcium, barium and copper and an effective amount of silver nitrate; and heat treating the precursor in an oxidizing atmosphere to convert the nitrates to oxides of calcium, barium, copper and silver. The method according to any one of claims 1 to 11 wherein the precursor is formed by applying to a surface of the substrate a liquid coating of an aqueous solution containing dissolved oxides of calcium, barium, copper and silver in stoichiometric ratio to form the superconductor; heating the liquid coated substrate to dry the coating; and then heat treating the coated substrate in an oxidizing atmosphere to form the precursor comprising the oxides of calcium, barium, copper and silver. An oxide superconductor including a compound of the formula Tl x Ca 2 Ba 2 Cu 3 O 8.5+y where x varies from 0.7 to 1 and y is a fraction of 1, the compound comprising from 72 to 75 mole percent metals and the balance oxygen, the metals comprising, in mole percent of metals in the compound, from 4 to 12 percent thallium, from 11 to 13 percent barium, from 11 to 13 percent calcium. and from 16 to 19 percent copper. characterised by the inclusion of about 1 to 20 mole percent silver, based upon metals in the superconductor.

Inventors: DELUCA JOHN ANTHONY, KARAS PAMELA LYNN, DELUCA, JOHN ANTHONY, KARAS, PAMELA LYNN
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1993
Main classification: H01L39
IPC codes: C01G15, H01B13, H01L39, H01B12, C01G1