Superconducting structure for magnetic shielding

Abstract: The superconducting structure for magnetic shielding according to the present invention comprises at least two layers of a superconducting layer and a substrate. This superconducting structure for magnetic shielding can be formed in a plate-like shape or a cylindrical shape. When the structure is prepared in three-layers by providing an intermediate layer between the superconducting layer and the substrate, the three-layered structure has improved superconducting properties. When a protective layer is provided on the superconducting layer to protect the layer, the resulting structure has improved thermal shock resistance. The intermediate layer preferably consists of a ceramic or a noble metal and the ceramic preferably consists of a glass.

Description: BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to a superconducting magnetic shield. More particularly, the present invention relates to a superconducting magnetic shield which can be suitably used for shielding of not only very low magnetic field such as geomagnetism or the like but also a high magnetic field of a levitated train, etc. In recent years, by utilizing a superconducting magnet made of a superconducting material having superconducting properties, there have been practically used NMR tomographs, levitated trains, etc. Further, it is being investigated to apply the high magnetic field of a superconducting magnet to development of new energy (e.g. nuclear fusion) or new energy conversion techniques (e.g. magnethydrodynamic power generation). When there is used an apparatus utilizing a superconducting magnet, such as NMR tomograph or the like, leakage of magnetic field from the apparatus occurs in some cases, giving an adverse effect to the environment. Meanwhile in measurement of very low magnetic field such as biomagnetism (α-wave), the influence of an external magnetic field such as geomagnetism or the like makes the measurement inaccurate. In order to reduce the above problems, there is desired a magnetic shielding material capable of shielding an external magnetic field. As the magnetic shielding material, there have conventionally been used soft magnetic materials having a high permeability and a low coercive force; however, these materials have too low shieldability in shielding a large magnetic force and may generate magnetism leakage in shielding low magnetic field. Therefore, in these materials, it is possible to make their volumes large to increase the shielding effect which in turn increases their weight. Hence, the object of the present invention is to solve the problems of the conventional magnetic shielding materials and provide a superconducting structure for magnetic shielding which can be suitably used for shielding of not only very low magnetic fields such as geomagnetism or the like but also high magnetic fields of levitated train, etc. EP-A-280322 describes a ceramic superconductor, useful as a wire, which comprises a laminate of layers of ceramic superconductor and stabilizing metal layers interposed between them. SUMMARY OF THE INVENTION According to the present invention, there is provided a superconducting magnetic shield, as set out in claim 1. This superconducting magnetic shield can be formed in a plate-like shape or in a cylindrical shape. In the superconducting structure of the invention, there is provided an intermediate layer between the superconducting layer and the substrate, and the resulting three-layered superconducting structure for magnetic shielding has an improved superconducting property, i.e. an improved critical current density. In the superconducting structure of the invention, there may be provided, outside the superconducting layer, a protective layer for protecting the superconducting layer, so that the resulting superconducting structure has an improved thermal shock resistance, i.e. an improved low-temperature resistance. In addition to the intermediate layer of noble metal, there may be a ceramic layer between the noble metal layer and the substrate, and this ceramic layer preferably comprises a glass, particularly a glass for porcelain enamel. In the three-layered or four-layered superconducting structure of the invention comprising a substrate, an intermediate layer and a superconducting layer or of a substrate, an intermediate layer, a superconducting layer and a protective layer, it is preferable that the material for substrate, the material for intermediate layer and the material for superconducting layer be selected so that the substrate and the intermediate layer have about the same thermal expansion coefficient as the superconducting layer. Thus, it is preferable that the thermal expansion coefficients of the substrate, the intermediate layer and the superconducting layer be kept as mentioned above. "Having about the same thermal expansion coefficient" indicates that the thermal expansion coefficients of the substrate and the intermediate layer are within about ± 6×1⊘⁻⁶ /°C relative to the thermal expansion coefficient of the superconducting layer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration showing an apparatus for measuring the magnetic shieldability of a plate-like structure. Fig. 2 is a graph showing a relationship of (a) a L/S ratio of a total length L (cm) of four sides of a magnetic shielding plate to an areas S (cm²) of one side of the plate and (b) a magnetic field ratio which is defined later. Fig. 3 is a schematic illustration showing an apparatus for measuring the magnetic shieldability of a cylindrical structure. DETAILED DESCRIPTION OF THE INVENTION The type of the superconducting layer used in the present invention is a M-Ba-Cu-O type compound [M is at least one element selected from Sc,T1, Y and lanthanide elements (La, Eu, Gd, Er, Yb, Lu, etc.] or a Bi-Sr-Ca-Cu-O type compound, in either case having a multi-layered perovskite structure. The superconducting layer comprising such a compound has a thermal expansion coefficient of about 13×1⊘⁻⁶ /°C. In this case, the Y-Ba-Cu-O type compound must have a crystal phase of YBa₂ Cu₃ O y composition and the Bi-Sr-Ca-Cu-O type compound must have a crystal phase of Bi₂Sr₂CaCu₂O x . A Bi-Sr-Ca-Cu-O type compound is particularly preferable for magnetic shielding purpose. The superconducting layer can be formed by spray coating or powder coating using a superconducting material powder or by molding a superconducting metal powder according to a doctor blade method and laminating the resulting molding (unfired) or the product obtained by firing the molding for expression of superconducting properties, onto a substrate or an intermediate layer provided on the substrate. As the superconducting material powder, there can be mentioned (1) a powder of oxide superconducting composition obtained by firing powders of metal oxides, metal nitrides, metal carbonates, metal hydroxides and metal alkoxides each of Y, Sc, La, Cu, Ba, Bi, Sr, Ca, etc., (2) a powder whose main crystal phase consists of a superconducting oxide, obtained by calcination at 8⊘⊘-95⊘°C, (3) an intermediate product powder having superconducting properties, obtained by calcination at 4⊘⊘-8⊘⊘°C and subsequent firing and (4) an oxide frit powder having superconducting properties which is obtained by melting at elevated temperatures a powder of oxide superconducting composition obtained by firing, quenching and grinding the melt, and firing the resulting powder once more. The powders (1), (2), (3) and (4) can be used alone or in combination of two or more, or in combination of (1) and (2), (1) and (3), (2) and (3), (2) and (4) or (3) and (4), or in combination of (1), (2) and (3), (1), (2) and (4) or (2), (3) and (4), or in combination of (1), (2), (3) and (4). The thickness of the superconducting layer is ⊘.1-2 mm, preferably ⊘.5 - 1 mm. When the thickness is smaller than ⊘.1 mm, the thickness is nonuniform and the supercurrent is small. When the thickness is larger than 2 mm, the adhesion of the superconductor to the substrate is poor and peeling tends to occur. The substrate is a metal, and has a given thermal expansion coefficient as mentioned above. Specifically, the metal material includes, for example, Fe, Ti, Be, Ni,stainless steel, Inconel, incoloy, Hastelloy and a steel plate with porcelain enamel. The intermediate layer provided between the substrate and the superconducting layer can be made of various noble metals (e.g. Au, Ag,Pt). The intermediate layer preferably has no reactivity with the superconducting layer. When there is used an intermediate layer having reactivity with the superconducting layer, a two-layered structure is adopted, in which said intermediate layer having reactivity with the superconducting layer is provided adjacent to the substrate and another intermediate layer having no reactivity with the superconducting layer is provided adjacent to the superconducting layer. In the present invention, it is preferable that in addition to the intermediate layer, there is a ceramic layer. The ceramic layer is formed on the substrate. The ceramic material is for example partially stabilized zirconia, stabilized zirconia, spinel, aluminia, mullite, silicon carbide and magnesia, but a crystallised glass or a glass for porcelain enemel is particularly preferable. The preferred glass constituting the ceramic layer has no particular restriction as long as it has a sufficiently large strength for adhesion to the substrate and the noble metal layer. Such a glass includes a glass for porcelain enamel, and this glass is particularly desirable for use as the ceramic layer. As the glass for porcelain enamel, there are typically mentioned, for example, a SiO₂-BaO-B₂O₃-ZnO type composition used for heat-resistant steel or heat-resistant alloy, a heat-resistant porcelain enamel consisting mainly of a SiO₂-BaO-TiO₂ type composition, a porcelain enamel for steel consisting mainly of a SiO₂-B₂O₃-Na₂O-Al₂O₃-K₂O-BaO type composition, and a glass for glass lining consisting mainly of a SiO₂-B₂O₃-Na₂O type composition. To these glass compositions for porcelain enamel may be added various elements as long as the elements gives little adverse effect to the adhesion of ceramic layer to metal as well as to the melting point of the glass compositions. The ceramic layer can be formed by various methods such as plasma spraying, gas spraying, spray coating, brush coating, dipping in slurry, sputtering and the like. Of these, spraying methods such as plasma spraying, gas spraying and the like are particularly preferable because the resulting ceramic layer has excellent adhesion to the metal substrate and can be formed as a relatively thick and stable intermediate layer. The thickness of the ceramic layer can be 1⊘ µm or more, preferably 1⊘-5⊘⊘ µm, more preferably 2⊘-2⊘⊘ µm. When the thickness is smaller than 1⊘ µm, the ceramic layer has a nonuniform thickness, making it difficult to obtain good adhesion. The noble metal layer as an intermediate layer in the present invention is formed on the ceramic layer provided on the substrate. As the noble metal constituting the noble metal layer of the present invention, there are used Ag, Au, Pd and Pt alone or in combination of two or more. As necessary, there may be used an alloy between such a noble metal and a base metal. The noble metal layer can be formed on the ceramic layer by various methods such as coating of noble metal paste, plating, press bonding of metal foil, CVD, sputtering, decomposition of noble metal compound and the like. In this case, a heat treatment may be applied as necessary in, for example, the paste-coating method. The thickness of the noble metal layer is preferably 1⊘-5⊘⊘ µm, more preferably 2⊘-2⊘⊘ µm. When the thickness is more than 5⊘⊘ µm, the effect of stabilizing the superconducting layer shows no further increase, leading to meaningless cost-up. When the thickness is smaller than 1⊘ µm, the resulting superconducting layer is nonuniform, the adhesion between the ceramic intermediate layer and the noble metal intermediate layer is low, the noble metal layer is broken, and as a result the superconducting layer reacts with the ceramic layer at the interface with the substrate, making it difficult to obtain a good superconducting layer-substrate composite. Further, when the superconducting layer-substrate composite is used by cooling at a liquid nitrogen temperature, the thickness of the noble metal layer of less than 1⊘ µm causes peeling between the noble metal layer and the ceramic layer due to their difference in thermal expansion, while the thickness of the noble metal layer of 1⊘ µm or more gives a stable composite because the stress generated by said thermal expansion difference can be absorbed by the noble metal layer. The protective layer for protecting the superconducting layer can be made of a material having an excellent thermal shock resistance (or low-temperature resistance). Therefore, the material for protective layer can be various, for example, a metal (e.g. Al), an organic material (e.g. low-temperature synthetic resin), a ceramic and a glass. The superconducting structure for magnetic shielding according to the present invention can be formed as a magnetic shielding plate or in a magnetic shielding cylinder. The magnetic shielding cylinder can be formed in two types, i.e. a type wherein the superconducting layer is positioned at the outside of the cylinder so as to shield a magnetic field from an external magnetic source and a type wherein the superconducting layer is provided at the inside of the cylinder so as to shield a magnetic field from an internal magnetic source. In the magnetic shielding plate, the plate desirably has a L/S ratio of ⊘.4 cm⁻¹ or less when L (cm) is defined as the total length of sides of the plate and S (cm) is defined as the area of one surface of the plate, in order to minimize the magnetic field leakage from the joints of a large area magnetic shielding plate obtained by jointing. The shape of the plate is preferably a regular hexagon or a regular square because such a shape can give a large area jointed plate for magnetic shielding without creating gaps at the joints. The regular hexagon or regular square, in particular, has a smaller L/S value than other shapes when compared on the same area basis, and can minimize magnetic field leakage. In the magnetic shielding plate, it is desirable that the substrate be not covered by the superconducting layer at the four ends at a width of about 1 mm or more because such a structure (the area of the substrate is larger than that of the superconducting layer) can suppress the peeling of the superconducting layer and the formation of cracks. Meanwhile in the magnetic shielding cylinder, it is desirable that the ratio of the length to the inside diameter be 1.5 or more because such a ratio can increase magnetic shielding at the cylinder center. Further in the magnetic shielding cylinder, it is desirable that the superconducting layer is coated at the inside or outside of the substrate with the both ends of the substrate uncoated at a width of about 5 mm or more, preferably 1⊘-5⊘ mm because such coating can suppress the peeling of the superconducting layer and the formation of cracks. Next, the process for producing the superconducting magnetic shielding structure of the present invention is explained on an example of a magnetic shielding plate comprising a substrate, a ceramic layer provided thereon, a noble metal layer provided on the ceramic layer and a superconducting layer provided on the noble metal layer. A superconducting layer is formed on the ceramic layer and the noble metal layer, provided on a substrate; then, drying and firing are effected; thereby, a magnetic shielding plate can be obtained which is a composite of the substrate, the ceramic layer, the noble metal layer) and the superconducting layer. In this case, when the formation of the superconducting layer is made by slurry coating or the like using an organic binder or an organic solvent, it is preferable that a heat treatment be conducted as a pretreatment for firing, that is, the coated superconducting layer be heat-treated at 5⊘⊘-93⊘°C in an oxygen-containing atmosphere for a given length of time to reduce the carbon residue to ⊘.5% by weight or less. In the present invention, the firing is effected in oxygen or air. The firing temperature and time can be determined appropriately depending upon the type of superconducting material used and the type of desired superconducting structure, but can be generally 85⊘-95⊘°C and about ⊘.5-2⊘ hours. This oxide superconducting laminate of the present invention has an integral structure obtained by forming the ceramic layer and the noble metal layer on a metal substrate and forming an oxide superconducting layer. This two-layered structure consisting of a ceramic layer and a noble metal layer has a synergistic effect on both of the metal substrate and the oxide superconducting layer. Therefore, in the oxide superconducting laminate of the present invention, each layer on the metal substrate is stabilized, and causes no peeling or cracks when subjected to repeated use in liquid nitrogen for expression of superconducting properties. Further in the laminate, there occurs no reaction between the metal substrate and the superconducting material during the firing, irrespective of the type of the superconducting material. Furthermore, the laminate has higher superconducting properties than conventional laminate comprising a single intermediate layer. The reasons for these advantages are not clear but are presumed to be that the noble metal layer diffuses into both of the ceramic layer and the superconducting layer and thereby contributes to stabilization and improved superconducting properties. The present invention is described in more detail below by way of the Examples. However, the present invention is in no way restricted to these Examples. Comparative Examples 1-4 On the surface of a metal substrate having a thickness of 1 mm and a regular square shape of 12⊘ mm in side length was spray-coated a powder whose crystal phase was composed mainly of Bi₂Sr₂CaCu₂ O x so that the resulting film has a thickness of about ⊘.1-4 mm after drying and the metal substrate had an uncovered portion of ⊘-1⊘ mm in width at the four sides (i.e. the length of one side of the film was 12⊘-1⊘⊘ mm). Then the film was dried, followed by firing at 9⊘⊘°C for ⊘.5 hour, to obtain various metal plates each comprising a metal substrate and a superconducing ceramic layer formed thereon whose crystal phase was composed mainly of Bi₂Sr₂CaCu₂O x and which had a thermal expansion coefficient of about 13 × 1⊘⁻⁶/°C. As the metal substrate, there were used various materials from Kovar having a thermal expansion coefficient of 4.7 × 1⊘⁻⁶/°C to copper having a thermal expansion coefficient of 19.7 × 1⊘⁻⁶/°C. As shown in Table 1, peeling occurred in the case of Kovar having a thermal expansion coefficient of 4.7 × 1⊘⁻⁶/°C; good adhesion was obtained in the cases of nickel of 13.3 × 1⊘⁻⁶/°C; peeling occurred in the cases of copper of 19.7 × 1⊘⁻⁶/°C. When the distance from each end of substrate to corresponding end of superconducting layer, i.e. the width of the uncovered substrate portion was less than 1 mm, there occurred the peeling of superconducting ceramic layer from substrate. Comparative Examples 5-6 On the surface of a ceramic substrate having a thickness of 5 mm and a regular square shape of 12⊘ mm in side length was spray-coated a powder whose crystal phase was composed mainly of YBa₂Cu₃O y so that the resulting film had a thickness of about 1 mm after drying and the ceramic substrate had an uncovered portion of 5 mm in width at the four sides (i.e. the length of one side of the film was 11⊘ mm). Then the film was dried, followed by firing at 95⊘°C for 1⊘ hour, to obtain various ceramic plates each comprising a ceramic substrate and a superconducting ceramic layer formed thereon whose crystal phase was composed mainly of YBa₂CU₃O y and which had a thermal expansion coefficient of about 13 × 1⊘⁻⁶ /°C. As the ceramic substrate, there were used zircon having a thermal expansion coefficient of 4.2 × 1⊘⁻⁶/°C and mullite having a thermal expansion coefficient of 5.3 × 1⊘⁻⁶/°C. As shown in Table 2, peeling occured in these comparative examples. Example 1 On the surface of a metal substrate having a thickness of 1 mm and a regular square shape of 12⊘ mm in side length was formed, as an intermediate layer, platinum, thickness of about 2⊘⊘ µm. On this intermediate layer was spray-coated, as a superconducting ceramic layer, a powder whose crystal phase was composed mainly of Bi₂Sr₂CaCu₂O x so that the resulting layer had a thickness of about 1 mm after drying and the four-side portion of the substrate was not coated at a width of 5 mm (the length of one side of the ceramic layer was 11⊘ mm). Then the ceramic layer was dried, followed by firing at 9⊘⊘°C for ⊘.5 hours, to obtain various metal plates each comprising a metal substrate, an intermediate layer formed thereon and a superconducting ceramic layer formed on the intermediate layer whose crystal phase was composed mainly of Bi₂Sr₂CaCu₂Ox. As the substrate material, there was used titanium or nickel. As shown in Table 3, the intermediate layer showed good adhesion, and the critical current density characterizing superconductivity in liquid nitrogen was high. The magnetic shielding plates prepared in Example 1 and Comparative Examples were measured for magnetic shieldability, using an apparatus for measurement of magnetic shieldability, shown in Fig. 1. That is, a magnetic shielding plate 2 was placed in a container 1 for liquid nitrogen; the container 1 was filled with liquid nitrogen; an external magnetic field was applied from an electromagnet 3; and using a Gauss meter 4 provided so as to face the electromagnet 3 via the container 1, there was measured a maximum external magnetic field (magnetic shieldability) at which the magnetic field began to exceed the background. Example 2 In the metal plate comprising a metal substrate, an intermediate layer formed thereon and a superconducting ceramic layer formed on the intermediate layer, obtained in Example 1, there was formed a protective layer on the superconducting ceramic layer. The resulting plate was placed in liquid nitrogen to evaluate its thermal shock resistance. As the protective layer, there was used a low-temperature resistant synthetic resin. As shown in Table 4, the protective layer showed good thermal shock resistance in the immersion-in-liquid-nitrogen test. Example 3 and Comparative Example 7 On various substrates having a thickness of 1-5 mm and a regular square or regular hexagon shape of different size was formed a superconducting ceramic layer to obtain various unit plates for magnetic shielding. A plurality of the same unit plates were combined to prepare various square panels for magnetic shielding each having a side length of 1 m. The panels were measured for magnetic shieldability. As shown in Table 5, in the magnetic shielding panels whose L/S ratios [L (cm) was the total length of sides (i.e. peripheral length) of each unit plate constituting a magnetic shielding panel, and S (cm²) was the surface area of one side of the unit plate] were ⊘.4 cm⁻¹ or less, the applied magnetic field was reduced to 1/1⊘ or less and a sufficient magnetic shieldability was obtained. Fig. 2 is a graph showing the relationship of L/S and magnetic field ratio. As is clear from Fig. 2, when the L/S was less than ⊘.4 cm⁻¹, the applied magnetic field was reduced to 1/1⊘ or less and there was obtained a sufficient magnetic shieldability. Example 4 and Comparative Examples 8-9 On the outer surfaces of various cylindrical substrates made of various metals and having a thickness of 1-5 mm and various sizes was formed a superconducting ceramic layer to obtain cylindrical structures for magnetic shielding. The cylindrical structures were measured for magnetic shieldability. As shown in Table 6, when the ratio of cylinder length to cylinder inner diameter was 1.5, the magnetic field at the cylinder center was 1/1⊘⊘ or less of the applied magnetic field and there was obtained a sufficient magnetic shieldability. Also, when the distance from each end of the cylindrical substrate to each corresponding end of the superconducting ceramic layer was 1⊘ mm or more, there was no peeling or crack at the ends of the superlconducting ceramic layer and thereby magnetic leakage could be prevented. Example 5 The surface of a plate-like SUS 43⊘ stainless steel substrate of 1⊘⊘ mm × 1⊘⊘ mm × 1.⊘ mm was sand blasted with alumina grinding particles. On the resulting roughened surface was plasma-sprayed a PSZ (partially stabilized zirconia) powder to form a PSZ intermediate layer of 2⊘⊘ µm. On this layer was coated a Ag paste, followed by drying (1 hour at 8⊘°C) and subsequent heat treatment (1⊘ minutes at 94⊘°C) to form a Ag intermediate layer of about 3⊘ µm to obtain a metal substrate having thereon two intermediate layers consisting of a PSZ layer and a Ag layer. Then, powders of Bi₂O₃, SrCO₃, CaCO₃ and CuO each having an average particle diameter of 3 µm were mixed at a molar ratio of 1 : 2 : 1 : 2. Water was added to the mixture, and they were calcinated at 8⊘⊘°C for 1⊘ hours in air. The calcinated product was ground with ZrO₂ boulders in ethanol for 15 hours to obtain a superconducting sintered powder whose main crystal phase had a composition of Bi₂Sr₂CaCu₂O x . To this poweder were added 15 g of toluene and ⊘.2 g of polyvinyl butyral to prepare a slurry. This slurry was spray-coated on the above second intermediate layer (Ag layer). Drying was effected at 8⊘°C for 1 hour. Then, the spray coating and drying was repeated four times to form a film of about 3⊘⊘ µm on the Ag layer. The thus obtained laminate comprising a metal substrate, two intermediate layers formed thereon and a superconducting calcinated powder layer formed on the second intermediate layer was dried at 1⊘⊘°C for 1 hour and then fired at 91⊘°C for 1⊘ minutes in an electric furnace to obtain a Bi-based oxide superconducting laminate. A test piece was cut out from the laminate and measured for critical current density (Jc) in liquid nitrogen according to a DC four-probe method. The Jc was 865 A/cm². Another test piece was subjected to a repeated procedure of immersion (cooling) in liquid nitrogen and returning to room temperature. Both the oxide superconducting layer and the intermediate layer of the laminate caused no crack or peeling, and the laminate had good adhesion. Example 6 A PSZ powder was plasma-sprayed on a plate-like SUS 43⊘ stainless steel substrate of 1⊘⊘ mm × 1⊘⊘ mm × 1.⊘ mm in the same manner as in Example 5 to form a PSZ intermediate layer on the substrate. On this intermediate layer was sprinkled a silver carbonate powder, and a heat treatment of 85⊘°C for 3⊘ minutes was applied to form, as a second intermediate layer, a Ag layer of 25 µm in thickness on the PSZ intermediate layer (first intermediate layer). Then, powders of Bi₂O₃, SrCO₃, CaCO₃ and CuO each having an average particle diameter of 3 µm were mixed at a molar ratio of 1 : 2 : 1 : 2. To 1⊘⊘ g of the mixed powder were added 2⊘ g of distrilled water and ⊘.2 g of polyvinyl alcohol to prepare a slurry. This slurry was spray-coated on the above second intermediate layer (Ag layer), and drying was effected at 1⊘⊘°C for 3⊘ minutes. Then, the spray coating and drying was repeated 7 times to form a film of about 6⊘⊘ µm on the second Ag intermediate layer. The resulting article comprising a substrate, two intermediate layers formed thereon and a superconducting calcinated powder layer formed on the second intermediate layer was dried at 1⊘⊘°C for 1 hour and then fired at a maximum temperature of 9⊘⊘°C for 1⊘ minutes in an electric furnace to obtain a Bi-based oxide superconducting laminate. The laminate was measured for Jc in the same manner as in Example 5. The Jc was 72⊘ A/cm². Example 7 A spinel powder was plasma-sprayed on a plate-like SUS 43⊘ stainless steel substrate of 1⊘⊘ mm × 1⊘⊘ mm × 1.⊘ mm in the same manner as in Example 5 to form a spinel layer of 3⊘⊘ µm in thickness. Onto this spinel layer was press-bonded a Au foil of 3⊘ µm in thickness with heating to form a Au layer. A Bi-based superconducting powder of Bi : Sr : Ca : Cu = 2 : 2 : 1 : 2 was molded into a tape shape by a doctor blade method. This green tape was placed on the above Au layer, and firing was effecte at 92⊘°C for 1⊘ minutes to obtain a Bi-based oxide superconducting laminate. The laminate was measured for Jc in the same manner as in Example 5. The Jc was 82⊘ A/cm². Examples 8-13 A PSZ powder was plasma-sprayed on a plate-like SUS 43⊘ stainless steel substrate of 1⊘⊘ mm × 1⊘⊘ mm × 1.⊘ mm in the same manner as in Example 39 to form a PSZ layer of 2⊘⊘ µm as a first intermediate layer. Then, on the PSZ layer was formed a Ag layer as a second intermediate layer according to a plating method or a thermal press-bonding method. On the second intermediate layer was formed a Bi-based superconducting layer in the same manner as in Example 5, whereby six oxide superconducting laminates were obtained. The constitution and critical current density at 77 K of each oxide superconducting laminate are shown in Table 9. As is clear from Table 7, each laminate showed good superconducting properties. Comparative Examples 1⊘-13 A PSZ powder was plasma-sprayed on a plate-like SUS 43⊘ stainless steel substrate of 1⊘⊘ mm × 1⊘⊘ mm × 1.⊘ mm in the same manner as in Example 5 to form a PSZ first intermediate layer of 2⊘⊘ µm. On this PSZ layer was formed a Ag second intermediate layer by plating in the same manner as in Examples 8-13 The thickness of the Ag layer was ⊘, 1, 5 or 8 µm. On the Ag layer was formed a Bi-based superconducting layer in the same manner as in Example 5, whereby four oxide superconducting laminates were obtained. The constitution and critical current density at 77 K of each laminate are shown in Table 7. As is clear from Table 7, none of the laminates showed good superconducting properties. The laminates obtained in Examples 8-13 and Comparative Examples 1⊘-13 were subjected to a thermal cycle test. This test was conducted by immersing a laminate of room temperature (2⊘°C) in liquid nitrogen for 1⊘ minutes, taking it out and leaving at room temperature, repeating this cycle 5 times and examining the occurrence of peeling. The results are shown in Table 7. As is clear from Table 7, peeling occurred in Comparative Examples 1⊘-13 wherein the Ag intermediate layer had a thickness of 8 µm or less, while no peeling occurred in all of Examples 8-13. Example 14 On the outer surface of a cylindrical substrate made of incoloy 825 and having a diameter of 5⊘ mm, a height of 15⊘ mm and a thickness of 1 mm was spray-coated, at a thickness of 1⊘⊘ µm, a slurry obtained by dissolving a glass powder for porcelain enamel (45 wt.% of SiO₂, 2⊘ wt.% of TiO₂, 15 wt.% of B₂O₃, 1⊘ wdt.% of Na₂O, 5 wt.% of NiO, 2.5 wt.% of K₂O, 2.5 wt.% of CuO) in isopropyl alcohol. Thereon was press-bonded a Ag foil of 1⊘⊘ µm in thickness, and the resulting article was fired at 9⊘⊘°C for 1 hour in the atmosphere to joint the substrate, the glass powder and the Ag foil. On the Ag foil was spray-coated, at a thickness of 5⊘⊘ µm, a slurry obtained by dissolving a Bi₂Sr₂CaCu₂O x powder in isopropyl alcohol. The coated slurry was partially melted at 89⊘°C for 3⊘ minutes in an oxygen atmosphere, then annealed to 85⊘°C at a rate of ⊘.5°C/min, and allowed to stand at 85⊘°C for 15 hours to give rise to crystallization. Thereafter, the resulting article was heat-treated at 4⊘⊘°C for 2⊘ hours in a nitrogen atmosphere. The thus obtained cylinder was measured for magnetic shieldability using an apparatus for measurement of magnetic shieldability, shown in Fig. 3. In Fig. 3, the above cylinder 11 was placed in a container 1⊘ for liquid nitrogen; the container 1⊘ was filled with liquid nitrogen; an external magnetic field was applied from an electromagnet 12; using a Gauss meter 13 placed in the cylinder 11, there was measured a maximum external magnetic field (magnetic shieldability) at which the magnetic field in the cylinder began to exceed the background. The magnetic shieldability was 1⊘ G. The cylinder was observed after the measurement of magnetic shieldability, but there was no defects such as crack, peeling and the like.

Claims: A superconducting magnetic shield comprising a superconducting layer, a metal substrate and a discrete intermediate layer between the superconducting layer and the metal substrate, wherein the superconducting layer is composed of at least one metal oxide having a multi-layered perovskite structure selected from M-Ba-Cu-O type compounds, where M is at least one of Sc, Tl, Y, La, Eu, Gd, Er, Yb and Lu, and Bi-Sr-Ca-Cu-O type compounds and has a thickness in the range 0.1 to 2 mm, the intermediate layer is composed of noble metal, has a thickness in the range of not less than 10 µm and is in contact with the superconducting layer, the metal substrate providing support for the superconducting layer and intermediate layer formed on it and being thicker than the intermediate layer. A shield according to claim 1 further having a layer of ceramic material between the intermediate layer and the substrate. A shield according to claim 2 wherein the ceramic material comprises glass. A shield according to any one of claims 1 to 3, further comprising a protective layer on the side of the superconducting layer opposite to its side towards the substrate. A shield according to claim 4, wherein the protective layer comprises aluminum or a low-temperature resistant synthetic resin. A shield according to any one of claims 1 to 5, which has a plate-like shape. A shield according to claim 6, which has a L/S ratio of 0.4 cm⁻¹ or less where L (cm) is the total length of the sides and S (cm²) is the area of one surface. A superconducting shielding magnetic plate, comprising at least two superconducting shields according to claim 6 or claim 7. A shield according to claim 6 or claim 7, wherein the superconducting layer is arranged on the substrate, which is rectangular, so that the portions of the substrate adjacent to its four sides having a width of 1 mm or more are not covered by the superconducting layer. A shield according to any one of claims 1 to 5, which has a cylindrical shape with the superconducting layer positioned at the outer side so as to shield from a magnetic field emitted from an external magnetic source. A shield according to any one of claims 1 to 5, which has a cylindrical shape with the superconducting layer positioned at the inner side so as to shield the interior from a magnetic field emitted from the substrate and the intermediate layer, as well as an external magnetic field. A shield according to claim 10 or claim 11 which has a length-to-inside diameter ratio of 1.5 or more. A shield according to any one of claims 10 to 12, wherein the superconducting layer is arranged on the substrate so that the portion of the substrate having a width of 5 mm or more adjacent to each end, is not covered by the superconducting layer.

Inventors: OKI SHUICHIRO, SAKAI HITOSHI, WATANABE KEIICHIRO E COOP NAKA, YOSHIDA HITOSHI, YOSHIDA MANABU, OKI, SHUICHIRO, SAKAI, HITOSHI, WATANABE, KEIICHIRO, 4E COOP. NAKANOMACHI HOUSE, YOSHIDA, HITOSHI, YOSHIDA, MANABU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1990
Main classification: H01L39
IPC codes: H01L39