Superconductive tube for magnetic shielding and manufacturing method therefor

Abstract: A superconductive tube for magnetic shielding has a cylindrical substrate composed of at least two members, an intermediate medium placed upon the inner and/or the outer wall surface of said substrate, and a layer containing superconductive oxide, placed upon said intermediate medium, which is integrally formed in a cylindrical shape.

Description: The present invention relates to a superconductive tube for magnetic shielding as well as to a method for manufacturing such a superconductive tube. Ferromagnetic materials such as permalloy or ferrite has been utilized to shield the space surrounded by them. In these days many devices have been introduced to shield the magnetic field by Meissner effect exerted by superconductors. For example, in Japanese Patent Laid Open JP-A-1-134998 (1989) superconductive materials was arranged to the innermost side of the space in which the magnetic field is shielded. In EP-A-0 393 932 a tube for magnetic shielding, i.e. shielding magnetic field, has been proposed by the present applicants that comprises at least two layers which include a layer of a substrate and a layer of superconductor arranged in this order from the magnetic source to be shielded. According to EP-A-0 393 932 an intermediate layer is provided between the substrate and the superconducting layer, the intermediate layer being preferably constituted by a ceramic layer and a noble metal layer, the ceramic layer being a first intermediate layer formed on the substrate and the noble metal layer being a second intermediate layer formed on the ceramic layer. However, a tube for magnetic shielding using oxide superconductor that can be used in actual practice has still remained in a developing stage at present. When superconductive materials are merely combined, magnetic field is known to leak through the joints, and high performance in shielding magnetic field cannot be achieved. This makes it necessary to form a structure of superconductive materials in an integral form. However, as a structure to be made from superconductive material becomes larger, it becomes more difficult to form a single integral structure of superconductive materials. It is not industrially desirable that the apparatus for manufacturing such structures becomes larger. On the other hand, for a large-size tube for magnetic shielding it is required to use metal or other robust material as a substrate to provide mechanical strength of the tube. When a layer of oxide superconductor is formed on a metal substrate, there remains a problem that the metal in the substrate reacts with oxide superconductor; this problem especially aggravates with compounds belonging to a Bi-Sr-Ca-Cu-O system. To solve this problem it is customary to include an intermediate medium with noble metal between the substrate and a layer of oxide superconductor to prevent the reaction. However, this solution poses another problem. It is difficult to form the intermediate medium uniformly over a large-size metal substrate, and more difficult to form a layer of oxide superconductor uniformly on the intermediate medium. As a result satisfactory superconductive properties can not be obtained. An object of the present invention is to provide a superconductive tube for magnetic shielding with good superconductive properties in shielding the magnetic field inside itself from outside or vice versa by using oxide superconductor, as well as a method of manufacturing such a superconductive tube. Another object of the present invention is to provide a large-size superconductive tube with good superconductive properties having high critical current density. According to the present invention, there is provided a superconductive tube for magnetic shielding as defined in claim 1. According to another aspect of the present invention, there is provided a method for manufacturing a superconductive tube for magnetic shielding as defined in claim 14. According to another aspect of the present invention, a method for manufacturing a superconductive tube for magnetic shielding as defined in claim 15. The invention and embodiments thereof will be further described with reference to the drawings, in which Both Fig 1 and Fig 2 show modes of joining members in the present invention. Fig.3 shows, as an example, a member with an intermediate medium comprising a glass or ceramic layer partially placed on itself. Fig. 4 shows a cylindrical shell of a noble metal that will become a layer of a noble metal in the intermediate medium in the tube. Fig. 5 shows modes of joining members to compose the tube of the present invention. Fig. 6 shows how two members and the intermediate members placed on them are joined together. The present invention is hereinafter described in detail. The present invention resides in a tube or tubes suitable for shielding the magnetic field inside itself or themselves from outside or vice versa. The tube has a cylindrical substrate, an intermediate medium carried upon the inner and/or the outer wall of the substrate, and a layer containing superconductive oxide carried upon the intermediate medium. The invention also provides a method for manufacturing such a tube. The tube of the present invention can be used to shield the magnetic field existing inside itself from external perturbation; thus inside the tube there may be only a low magnetic field with high homogeneity. Moreover, the tube of the present invention prevents the adverse effects on the magnetic field inside itself caused by perturbation of the magnetic field generated by vibration of the substrate and/or the intermediate medium. The tube of the present invention can be used to prevent a magnetic field inside itself from affecting the field outside when the source of strong magnetic field is kept inside the tube. In other words the tube can be used to shield its environment. In this specification the phrase 'a tube for shielding the magnetic field' refers to this case also. The basic structure of a superconductive tube for magnetic shielding, embodying the present invention, is composed of a cylindrical substrate, an intermediate medium placed upon the interior or exterior surface of said substrate, and a layer containing superconductive oxide placed upon the intermediate medium. The tube can have essentially any shape and size depending on its application. A cylindrical shape is preferred, though the cross section of a cylindrical shape in this specification is not limited to a circle; ellipse, truncated circle, triangle, rectangle, polygon, etc. are included. Circle is preferred as a shape of the cross section for the tube in the present invention. Likewise, the axis of the cylindrical shape according to the present invention is not restricted to be perpendicular to the top or the bottom plane of the cylindrical tube to be formed. However, the axis of the cylindrical shape is preferred to be perpendicular to the bottom plane of itself. The word 'cylindrical' is used in this broadened meaning in this specification. The tube of the present invention may have the bottom of the cylindrical tube open in some applications and may have it closed with a member or members protruding beyond the tube in other applications. As material for a substrate, any may be used that has enough mechanical strength to support itself and a plurality of layers placed on it. For example, such ceramic materials as zirconia and titania as well as noble metals, alloys and such metals as stainless steel (SUS 43⊘, SUS 31⊘, and SUS 32⊘, specified in JIS), Inconel, Incolloy, and hasteloy. There is no restriction on the thickness of a substrate. The intermediate medium plays very important roles. First it prevents oxide superconductor from reacting with a metal used as a substrate. Without the medium, when a common metal or an alloy other than noble metals and a oxide superconductor are used as materials for a substrate and an oxide superconductor layer, respectively, the metal and the oxide superconductor react significantly during firing, resulting in poor performance in superconductivity in the final product. Concerning this, the use of a noble metal as a substrate could provide a way to prevent the oxide superconductor from reacting with the substrate and solve this problem. However, this solution is not realistic due to inevitable expensive cost incurred by the use of a noble metal. Instead of a metal or an alloy, when ceramics is used as a substrate, it is not easy to obtain a substrate in a large size, though mechanical strength and the resistance against chemical reactions are satisfied. Therefore, the use of a common metal as a substrate and the presence of the intermediate medium are preferred. Therefore, for the tube according to the present invention there is provided such material as a substrate that maintains the mechanical strength up to the sintering temperature of oxide superconductor. Moreover, it is preferable that the material is easily obtained in a desired size and the shape. According to the invention there is further provided at least one layer in the intermediate medium, for instance a noble metal layer, to prevent the reaction between the substrate and a layer containing superconductive oxide. Moreover, it is also preferred that the intermediate medium has excellent adhesion onto the substrate over a wide range of temperatures from the sintering temperature of oxide superconductor to a cryogenic temperature, like that of liquid nitrogen. This function of adhesion is attained by placing another layer for adhesion in the intermediate medium between the layer to prevent the reaction and the substrate; this adhesive layer can be made out of such binding materials as glass, various kind of ceramics, metal pastes and noble metal pastes. Furthermore, it is preferred that the intermediate medium, which can attain the foregoing functions, also acts as a moderator to moderate thermal impact shocks exerted in cold/heat cycles when the tube is applied to repeated cycles between two drastically different temperatures such as cryogenic temperature and room temperature. In other words, in applications it is quite expected that the tube in the present invention undergoes drastic temperature changes in short period of time; for example, a tube in the present invention is cooled down by liquid nitrogen to exert superconductive properties, and then the tube is left at room temperature. When the intermediate medium is composed of two layers in which glass or ceramics and a noble metal are used as materials for the adhesive layer placed on a substrate and for another layer to prevent the reaction placed on the adhesive layer, respectively, it is preferred to dispose the glass or ceramic layer partially, not entirely, over surface of a substrate, in preferable manner, for example, in stripes, numerous dots, a lattice pattern, random pattern, and any combinations of these (Fig.3). The partial coverage of the glass or ceramic layer over surface of a substrate is suitable. This local structure can moderate shocks due to thermal impact during cold/heat cycles so that the tube maintains stable performance to shield the magnetic field inside itself. The intermediate medium should be thick enough to prevent the reaction between the substrate and the layer containing superconductive oxide, and the thickness may be selected appropriately so as to minimize the overall thermal stresses affected by various physical properties such as thermal expansion coefficient, modulus of elasticity and mechanical strength, as well as the thickness of the layer containing superconductive oxide and the metal substrate. For instance, when a noble metal such as gold, or silver is used as the material for a layer to prevent the reaction, the thickness of the layer should be greater than 3⊘ µm to achieve the purpose. When a heat resistant metal such as stainless steel and inconel is used as a metal substrate, and when the intermediate medium is composed of a glass or ceramic layer on a substrate and the layer of the noble metal on the glass or ceramic layer, preferably the thickness of the layer of the noble metal is greater than 5⊘ µm in view of the thermal expansion coefficient, the modulus of elasticity and the mechanical strength of the layer containing superconductive oxide, the metal substrate, the glass or ceramic layer and the noble metal layer, since the effect as the moderating material described above can be fully attained. Furthermore, when a noble metal is used in the layer to prevent the reaction, the thickness of the layer is preferably between 5⊘ and 7⊘⊘ µm and, more preferably, between 1⊘⊘ and 6⊘⊘ µm. When the thickness of the noble metal layer is less than 5⊘ µm, the expected performance as moderating material described above can not be obtained. On the other hand, when thickness of the noble metal layer is greater than 7⊘⊘ µm, the thermal stress due to the difference of thermal expansion between oxide superconductor and the noble metal layer may become predominant rather than performing as moderating material. Moreover, the cost increases with thickness without any improvement in performance as moderating material. When the intermediate medium is composed of only one layer of glass or ceramics and another of a noble metal, the thickness of the glass or ceramic layer is preferably within a range from 5⊘ to 25⊘ µm. Preferably the thickness of the glass or ceramic layer is not beyond the above-mentioned range, since thermal stress remains in the layer generated during firing to form a layer containing superconductive oxide. Upon cooling to cryogenic temperatures such as that of liquid nitrogen such stress remained in the layer exceeds the mechanical strength of the tube, causing cracks. The oxide superconductor used in the present invention is not particularly restricted as to the material used. It can be any oxide superconductor or combination thereof such as rare earth oxide superconductors having a multi-layered perovskite structure with a M-Ba-Cu-O system in which M is one or more of rare earth elements selected from Sc, Y and lanthanides such as La, Eu, Gd, Er, Yb and Lu. Another example is oxide superconductors in a Bi series having such compositions as Bi₂ Sr₂ Ca₁ Cu₂ O x and Bi₂ Sr₂ Ca₂ Cu₃ O x . The tube according to the present invention has a structure such that a layer containing superconductive oxide is placed upon the intermediate medium that is further placed upon surface of the inner and/or the outer wall of a cylindrical substrate. The cylindrical substrate is not integrally formed initially but is made up of at least two members i.e. sub-sections joined together. A member can take any shape and size, and the invention embraces any mode of combining the members together. For instance, Figs. 1 and 2 illustrate typical examples of modes of composing a cylindrical substrate out of a plurality of members without a member closing the bottom portion. Some laminated members 8 each of which has at least one layer on it as shown in Fig. 3 are combined and joined as shown in Fig. 1. In this case the substrate is made of four identical members whose joints are parallel with the axial direction of the cylindrical substrate to be formed. Though this particular example illustrates a case in which each of the members to form a substrate has a (partial) layer of glass or ceramics provided on it before joining the members together, it should be noted that this is not an essential feature. Fig. 2 shows another mode of combining members into a substrate; the joints of the four members are perpendicular to the axial direction of the cylindrical substrate to be formed. Fig. 5 (a)-(i) further illustrate modes of combining members into a substrate. In Fig. 5 (a), (b), (c) and (h) the joints between the members extend parallel to the axial direction of the cylindrical substrate to be formed from them, while in Fig. 5 (d) and (e) they are perpendicular to the axial direction of the cylindrical substrate to be formed from them. Fig. 5 (g), (h), and (i) illustrate a general structure that contains one opening at the bottom end plane of the tube and another with a smaller diameter than the first at the top of the structure. This smaller opening could be used to introduce wiring for, for example, a sensor. In Fig. 5 (g) and (i) the joints between the members are perpendicular to the axial direction of the cylindrical substrate to be formed while in Fig. (h) they are parallel to the axial direction. One possible shape of a member integrally contains both a component portion that becomes part of the cylindrical side of the substrate and a portion which becomes part of the top of the cylindrical substrate. As an alternative possibility a member may provide only a part of the cylinder or only the top portion of the cylindrical substrate to be formed; in other words, the cylinder itself and the top of the cylindrical substrate are made independently and then joined. Fig. 5 (a) and (h) show the former case. In contrast Fig. 5 (b), (c), and (h) show the latter case. In Fig. 5 (b) the top portion of the cylindrical substrate to be formed is made up from more than one member i.e. subsection while in Fig. 5 (c) it is provided by a single integral member. In Fig. 5 (f) the joints in the cylindrical side and the top portion of the cylindrical substrate to be formed are perpendicular and parallel to the axial direction of the cylindrical substrate to be formed, respectively. As shown above, various modes of combining members into a cylindrical substrate are available, and a preferable mode can be chosen depending on its application. The factors to consider for the choice of mode include the purpose of the application, actual conditions of the application, the intermediate medium, oxide superconductor in a layer, methods of binding members, methods of placing the intermediate medium upon the substrate, etc. There are numerous variations in shapes and sizes for members to form a substrate and in surface treatments applied before joining them. For instance, Fig. 6 shows typical modes of joining members. In Fig. 6(a) - (c) a member 1 includes flanges, and the flanged portions 4 of two members are joined by nuts 5 and a bolt 6. With a flange as part of a member, it is preferred to cut the corner into curvature between the flat plane of the member and the flanged portion. After joining the Ag layer later, the curved portions around the joint thus formed help to reduce the protrusion of the Ag layer along the joint. In Fig. 6 (d) and (e) members do not contain a flange. The joint 7 between two members is abutted against each other and joined by welding or other appropriate methods when the members are made of metal or by glass when the members are made of ceramics. In Fig. 6 (e) the joint 7 between two adjacent members needs to be welded along with that between the two corresponding adjacent layers of a noble metal 3 on the two members 1. In such a case the melting point of the noble metal may differ greatly from that of the metal used as the substrate, and the one with the lower melting point may partially melt upon welding of the other. When this incidence happens, it is not necessary to weld the entire cross section of the joint of the metal with the higher melting point; build-up welding or other preferable methods may suffice as long as the practical mechanical strength is satisfied. In the present invention a layer in the intermediate medium may be depositted on members before they are joined to form a cylindrical substrate. When the height of the cylindrical substrate, being parallel to the direction of the principal axis, is relatively short such as those less than 5⊘⊘ mm, the members may be joined into a cylindrical substrate and then shaped integrally. Usually, it is preferred to form the medium on the members before joining them. The intermediate medium can be formed by any of known methods, which can be selected properly depending on the intermediate medium. The glass layer or the ceramic layer can be formed, usually, by a coating method such as a spray coating using slurry composed of the ingredients for a layer in the intermediate medium or appending thin films formed in accordance with the shape for each of the members. A layer of noble metal made of, for example, silver and gold is usually formed by connecting foils upon the glass layer on a member or a cylindrical substrate Alternatively a shell of noble metal in a cylindrical shape may be independently formed by connecting foils. One method of manufacturing a tube with the intermediate medium composed of two layers is shown below. A layer made out of glass is formed on members before joining them. Then a cylindrical substrate with the glass layer is formed by joining the members. Independently a shell of a noble metal is prepared such that this shell matches with the surface of the glass layer in the cylindrical substrate. Finally the shell of the noble metal is placed upon the cylindrical substrate. It is especially preferable to place a cylindrical shell of a noble metal independently prepared upon a cylindrical substrate having partial coverage of the glass or ceramic layer on its outer and/or inner surface. In this green tube a layer of a noble metal is attached to a substrate only through the partial glass or ceramic layer, and there leaves space between the the layer of a noble metal and the substrate in the area that the glass or ceramic layer is absent. Upon the completion of the tube in this invention this space is considered to play a very important role in absorbing different thermal expansions between the substrate and a layer containing oxide superconductor. Various kinds of structures that also relate to manufacturing methods can be adopted for the intermediate medium placed upon members. For instance, in Fig. 6, the intermediate medium is composed of the glass or ceramic layer 2 placed on the member 1 and the Ag layer 3 placed on the glass or ceramic layer. In Fig. 6 (d) and (e) members with the intermediate medium 8 that are joined do not contain a flange, and the glass or ceramic layer 2 is formed on the member 1 and the silver layer 3 is on the glass or ceramic layer. In contrast, the other three figures in Fig. 6 contains flanges. In Fig. 6 (a) and (b) no layer is formed on the portion above the flange 4. In detail, the glass layer 2 of about 1⊘⊘ to 2⊘⊘ µm thickness is formed on the member 1 except for the flanged portion 4. Another silver layer is placed upon this glass layer; as a result this silver layer does not form on the portion above the flange 4, either. In Fig. 6 (c), however, the silver layer is formed directly upon surface of the portion above the flange 4. Like Fig. 6 (a) and (b), the glass layer is formed on the member 1 except for the flanged portion. The Ag layer 3 is formed on the the glass layer and also on the portion in members of the flange 4. The intermediate medium formed on each of the members as described above is also joined together along with the members. Therefore, both the intermediate medium and the substrate constitute a tube with cylindrical shape as a whole. In this case, the intermediate medium formed on the members may be joined in accordance with the joining mode of the substrate. For instance, in the mode shown in Fig. 6 (a) and (d), in which the Ag layer 3 is placed on the glass or ceramic layer 2 placed on the member 1, the edge of the Ag layer on one member extends above the edge of the Ag layer on another member to result in stacking two Ag layers along the joint. Then the contact 9 between the top end of the upper Ag layer and the lower Ag layer are joined by welding or using an Ag paste. In another mode shown in Fig.6 (b), (c) and (e), they are joined at the contact 9 in the Ag layer formed on the substrate by welding or using an Ag paste. After the Ag paste is coated to join two adjacent Ag layers, firing was applied between about 8⊘⊘ and 9⊘⊘°C to complete to join them. Then it is preferred to polish and smooth the portion welded or coated by the Ag paste by grinder or other appropriate known methods because the smooth surface on the intermediate medium help a layer containing superconductive oxide placed upon it become smoother. Fig. 4 shows a mode of manufacturing the tube in the present invention. In this mode the intermediate medium is composed of two layers, and one of them is independently formed in advance into a cylindrical shell out of a noble metal. In Fig. 4(a), for example, silver foils of thickness ranging from 3⊘⊘ to 5⊘⊘ µm are overlapped to some extent or abutted against each other and then joined to form a silver shell 3 in cylindrical shape. The method in which the silver foils are joined is basically the same as that used in a silver layer formed on members. In Fig. 4(b), silver foils are joined at the contact 9 in the foils by either welding or Ag pastes. In Fig. 4(c) a small portion of one end of a silver foil overlaps that of another and then the area of the contact 9 is joined by either welding or Ag pastes. Then the silver cylindrical shell 3 thus formed is supported by core material made of any kind of appropriate materials till it is placed upon a cylindrical substrate that with at least one layer in the intermediate medium. Fig. 1 shows an example of placing the silver foil 3 upon a cylindrical shell with members 8 that have a sole layer of glass 2 as part of the eventual intermediate medium. First the four members with a glass layer are placed along the periphery of the silver cylindrical shell 3 in such a way that the four members are arranged to become a cylindrical substrate upon firing. Then this whole structure is fired at firing temperature to place the silver cylindrical shell 3 upon the glass layer 2 on the members 8. Finally the flanges 4 on the members 8 are bound by bolts and nuts to form a tube 1⊘ in a cylindrical shape composed of a substrate with the intermediate medium on it. Without flanges, by welding members can be joined together to become an integral tube in a cylindrical shape. In the tube according to the present invention, the layer containing superconductive oxide described above is integrally formed on the intermediate medium so that the resultant overall structure comprises a substrate, the intermediate medium, and a layer containing superconductive oxide. The layer containing superconductive oxide binds onto the intermediate medium by any of known methods such as coating, spraying, and doctor blading. Usually spray coating is applied to the surface of the intermediate medium, followed by sintering at about 8⊘⊘ to 115⊘°C to form a layer containing superconductive oxide. No particular restriction applies to the thickness of a layer containing superconductive oxide. Depending on its application, the thickness of the layer can be properly chosen to satisfy required performance to shield the magnetic field inside the tube by considering superconductive properties, materials used as superconductive oxide, etc. When the tube in the present invention contains bottom members, it is preferable that the bottom members are connected to rim of the tube with smooth curvature or blunt angles. Here a blunt angle and an acute angle are meant to any angle ranging from 9⊘ to 18⊘ and ⊘ to 9⊘ degrees, respectively, and this angle shall be measured from the inside the tube. When a bottom member is connected to rim of the tube with the right angle or an acute angle, or when the radius of the curvature is less than 5 mm at the connection part of rim, the properties of the tube to shield the magnetic field inside itself significantly deteriorate due to cracks around the connected portion caused by mechanical stress generated by the thermal shock by the cold/heat thermal cycle mentioned before. Any member that composes the bottom plane of the tube is preferably joined in a blunt angle due to the same reason. Preferably the tube in the present invention contains two openings with different inner diameters as shown in Fig.5(g)-(h), because the opening with the smaller diameter is quite convenient in introducing lines of sensors etc. Examples The present invention will now be described more in details but it shall not be limited to the following examples. (Example 1) Four members, i.e. sub-sections, each having flanges at two edge positions were manufactured out of inconel such that a tube of 500 mm in diameter and 1⊘⊘⊘ mm in height in the direction parallel to the main axis was formed by combining them along joints extending in parallel with the axial direction of the tube as shown in Fig. 1. Each member was initially given a surface treatment by sand blasting. Then, except for the flange portion glass slurry was spray coated onto each of the members, using masking with paper tape to form a lattice-like pattern each of 3⊘ mm interval, followed by firing between about 8⊘⊘ and 9⊘⊘°C for one hour to form a glass layer of a lattice-like pattern with thickness of 1⊘⊘ µm. Subsequently, Ag foils with thickness of 3⊘⊘ µm were placed over the glass layer on the inconel substrate such that the Ag foil extended slightly beyond the side of the flange. Then the glass layer and the Ag foil were bound by heating at 85⊘ to 9⊘⊘°C for one hour. Subsequently, respective flanges of the four members laminated with the intermediate medium abutted against each other and fixed by means of bolts and nuts in the mode shown in Fig. 6 (a), to form a cylindrical tube. The contact on the overlapped portion of the Ag foils was connected by welding, and then the connected portion was smoothed out by a hand grinder. On the Ag layer on the cylindrical green tube thus obtained, slurry containing oxides that give Bi₂Sr₂Ca₁Cu₂O x upon firing was spray coated, and the resultant tube was heated under oxygen atmosphere to 885°C for 3⊘ min to form and partially melted the superconductive oxide. Subsequently the tube was gradually cooled down, or annealed, to 85⊘°C in the rate of 1°C/min, and was stood at 85⊘°C for 15 hr to crystallize the superconductive oxide. Then after changing the oxygen atmosphere into that of nitrogen the tube was treated at about 7⊘⊘°C for 1⊘ hr to result in the tube coated with Bi(bismuth)-system oxide superconductor with thickness between about 25⊘ to about 35⊘ µm. The tube obtained has satisfactory appearance by visual observation, and was evaluated for the cold/heat cycle. Even after the cold/heat cycle both the appearance and the evaluation for the performance are satisfactory. The results are shown in Table 1. As evaluation for the cold/heat cycle, the tube at room temperature was first immersed in liquid nitrogen. Then after the temperature of the tube itself had fallen to the liquid nitrogen temperature it was maintained at this temperature for 30 min and the performance to shield magnetic field inside itself was measured in this liquid. Subsequently the tube was taken out of the liquid and left at room temperature. Finally there was a period of 30 min after the temperature of the entire tube reached room temperature. This whole process completes one cycle, and five cycles were carried out. The performance to shield the magnetic field inside itself from outside after the fifth cycle is compared to that after the first cycle by the following equation and evaluated as "○" for more than 8⊘%, "Δ" for more than 5⊘% and "x" for less than 5⊘%. Evaluation(%) = Performance to shield magnetic field after the fifth cold/heat cycle Performance to shield magnetic field after the first cold/heat cycle x 1⊘⊘ (Examples 2 to 25) The tubes in the present invention were obtained in the same procedures as those in Example 1, except for the conditions shown in Table 1, i.e., the presence or the absence of a bottom member in a tube, shape of members, mode of joining members, thicknesses of the glass layer and of the Ag layer, and a mode of joining the glass layers and/or the Ag layers, the sintering temperature under oxygen atmosphere, and heat treatment temperature under nitrogen atmosphere. Each of the tubes obtained was evaluated for the visual appearance and the performance after cold/heat cycles. The results are shown in Table 1. (Comparative Examples 1 to 8) Each of the tubes was obtained in the same procedures as those in Example 1, except for forming cylindrical substrates integrally, changing the thickness of the glass layer and the Ag layer in the intermediate medium, the sintering temperature under oxygen atmosphere, and the heat treatment temperature under nitrogen atmosphere, as shown in Table 2. Each of these tubes was evaluated for the visual appearance and the performance after the cold/heat cycles. The results are shown in Table 2. In the evaluation for the visual appearance in the table, X represents local unevenness in the binding portion between the glass and the Ag layer. As apparent from the examples and the comparative examples described above, when a tube is relatively small with the height of up to 5⊘⊘ mm, both of a tube with a cylindrical substrate formed integrally by the conventional method and of a tube with a cylindrical substrate made out of members in present invention were evaluated as satisfactory for the visual appearance and the performance after the cold/heat cycles. On the other hand, when a tube has the height greater than 1⊘⊘⊘ mm, it was difficult to uniformly form the glass layer being satisfactorily adhesive to the integral cylindrical substrate. Local unevenness was observed in the bound portion between the lattice-like glass layer and the Ag layer, reducing superconductive properties and greatly affecting the performance adversely to shield magnetic field after the cold/heat cycles. In contrast, when a tube with the height greater than 1⊘⊘⊘ mm was manufactured with members according to the present invention, either the intermediate medium or the layer containing superconductive oxide became highly homogeneous, and satisfactory results were obtained both of the visual appearance and of the performance to shield magnetic field after the cold/heat cycles. (Example 26) A lattice-like glass layer was formed on each of the members, except for a portion of the flange to obtain the laminated members 8 with the glass layer in the same procedures as those in Example 1; the only exception in this procedure is the size of a tube to be formed:it had the diameter of 1⊘⊘ mm and the height of 45⊘ mm. A silver cylindrical shell 3 with the diameter of 1⊘⊘ mm was independently formed by abutting and welding Ag foils with the thickness of 3⊘⊘ µm, and then it was reinforced and supported by a core material made of metal. Then after combining and arranging each of the laminated and joined members 8 so as to wind the periphery of the thus obtained Ag foil cylindrical shell 3, they were heated between about 85⊘ and about 9⊘⊘°C for one hour to bind the glass layer 2 and the Ag foil cylindrical shell 3. Subsequently, in the same manner as in Example 1, each of the members 1 was fixed by flanges in the mode shown in Fig. 6 (a) to fabricate a cylindrical laminate 1⊘. A layer containing superconductive oxide with thickness of 25⊘ µm was formed integrally on the Ag layer of the resultant cylindrical laminate 1⊘ in the same manner as in Example 1, to obtain a tube. The appearance of the tube thus formed was visually observed and the cold/heat cycle was evaluated in the same manner as in Example 1. The results are shown in Table 3. (Examples 27 to 4⊘) The tubes in the present invention were obtained in the same procedures as those in Example 26, except for the conditions shown in Table 3, i.e., the presence or the absence of a bottom member in a tube, shape of members, mode of joining members, thickness of the Ag layer, and a mode of joining the Ag layers. Each of the tubes obtained was evaluated for the visual appearance and the performance to shield magnetic field after the cold/heat cycles. The results are shown in Table 3. (Comparative Examples 9 to 18) The tubes in the present invention were obtained in the same procedures as those in Example 26, except for integrally forming a cylindrical substrate and changing the thickness of the Ag layer as shown in Table 4. Each of the tubes obtained was evaluated for the visual appearance and the performance to shield magnetic field after the cold/heat cycles. The results are shown in Table 4.In the evaluation for the visual appearance in the table, X represents local unevenness in the binding portion between the glass and the Ag layer and Y represents insufficient welding. These examples and comparative examples show that the tubes of the present invention are superior to those made in prior art. A method of manufacturing the tube of the present invention in which a cylindrical shell of silver independently made was place upon a glass layer on the cylindrical substrate, led to a tube with satisfactory appearance and performance to shield magnetic field. Another method of manufacturing the tube of the present invention in which a cylindrical shell of silver independently made was placed upon laminated members with glass layers arranged in the periphery of the shell of silver, and then in which the members with glass layers thus placed against the shell were joined among themselves, led to a tube with satisfactory appearance and performance to shield magnetic field. In the comparative examples in which a cylindrical shell of silver is placed upon a cylindrical substrate with a glass layer, the heights of the cylindrical structures were found to be irrelavant to obtain a highly homogeneous layer containing superconductive oxide. Joint portions between silver layers and those between the silver layer and the glass layer did not achieve satisfactory homogeneity, which adversely affect the homogeneity in the layer containing superconductive oxide placed upon them. As a result the performance to shield magnetic field significantly deteriorated after the cold/heat cycles.

Claims: A superconductive tube for magnetic shielding, comprising a cylindrical substrate (1) composed of at least two members (8); an intermediate medium (2,3) carried upon the inner surface and/or the outer surface of said substrate (1); and a layer containing superconductive oxide carried upon said intermediate medium (2,3); wherein the intermediate medium comprises a first layer (3) for inhibiting reaction between the substrate (1) and the superconductive oxide and a second layer (2) between at least a part or parts of the first layer and the substrate (1); at least said layer containing said superconductive oxide being a unitary layer in integral form extending over said at least two members (8) of said substrate (1). A superconductive tube for magnetic shielding as recited in claim 1, wherein joints between at least some of said members (8) are parallel to the axial direction of said substrate (1). A superconductive tube for magnetic shielding as recited in claim 1, wherein joints between at least some of said members (8) are perpendicular to the axial direction of said substrate (1). A superconductive tube for magnetic shielding as recited in claim 1, wherein joints between at least some of said members (8) extend in a direction slanted relative to the axial direction of said substrate (1). A superconductive tube for magnetic shielding as recited in claim 1, further comprising one or more bottom sections connected to a bottom rim of a cylindrical side portion of said substrate. A superconductive tube for magnetic shielding as recited in claim 5, wherein the whole bottom rim is connected to one or more said bottom sections. A superconductive tube for magnetic shielding as recited in claim 6, wherein each said bottom section is connected to the cylindrical side portion of the substrate with an obtuse angle and/or a radius of curvature greater than 5 mm seen from inside of said substrate, at the connection to the bottom rim. A superconductive tube for magnetic shielding as defined in any one of the preceding claims wherein said first layer (3) of said intermediate medium comprises a layer of a noble metal that contacts said layer containing superconductive oxide. A superconductive tube for magnetic shielding as defined in claim 8 wherein said layer (3) of a noble metal extends over each joint of said at least two members (8) of said substrate (1). A superconductive tube for magnetic shielding according to any preceding claim, wherein said substrate (1) is metal and the second layer (2) essentially consists of ceramics. A superconductive tube for magnetic shielding according to any preceding claim, wherein said second layer (2) contains glass. A superconductive tube for magnetic shielding according to any one of claims 1 to 10 wherein said second layer (2) essentially consists of glass. A superconductive tube for magnetic shielding as defined in claim 9, claim 10 or claim 11 wherein said second layer (2) only partially covers the inner and/or outer surface of said substrate (1). A method for manufacturing a superconductive tube for magnetic shielding which has a substrate (1) and a layer containing superconductive oxide thereon, which method comprises: forming an intermediate medium (2,3) upon the inner and/or the outer surface of a plurality of members (8), wherein the intermediate medium comprises a first layer (3) for inhibiting reaction between the substrate (1) and the superconductive oxide and a second layer (2) between at least a part or parts of the first layer and the substrate (1); making a cylindrical substrate (1) by joining said members (8); and forming said layer containing superconductive oxide as a unitary layer in integral form on said intermediate medium (2,3) on said cylindrical substrate (1). A method for manufacturing a superconductive tube for magnetic shielding which has a substrate and a layer containing superconductive oxide thereon, which method comprises: forming an adhesive layer (2) upon the inner and/or the outer surface of a plurality of members (8); making a cylindrical substrate (1) by connecting said members (8); preparing a cylindrical shell of a noble metal; placing said cylindrical shell upon said cylindrical substrate for inhibiting reaction between the substrate and the superconductive oxide, to make a cylindrical green tube; and forming a layer containing superconductive oxide as a unitary layer in integral form upon the surface of said cylindrical shell thus made.

Inventors: SHIMIZU HIDEKI, TOGASHI TAKEYOSHI, SHIMIZU, HIDEKI, TOGASHI, TAKEYOSHI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1991
Main classification: H05K9
IPC codes: H05K9, H01L39