Elongate superconductor elements comprising oxide superconductors and superconducting coils

Abstract: An elongate superconductor wiring element having, as seen in section, oxide superconductor material region (1) in each of which the c-axes of the oxide superconductor crystals are aligned with each other and are transverse to the longitudinal axis of the element. To reduce the dependence of critical current density on angular position of the element relative to a magnetic field, there are a plurality of said regions (1) whose alignment directions (3) of the c-axes are different as between different ones of said regions, so that the wiring element comprises a plurality of said regions having respectively different c-axis alignment directions.

Description: This invention relates to elongate superconductor wires comprising oxide superconductor materials, and to superconducting coils comprising such wires. Oxide superconductor material herein means material which is superconductive in bulk, i.e. in which the crystal grains are joined, e.g. by sintering or other processes, to permit passage of superconducting current. Since the discovery of oxide superconductors, efforts have continued to be made in many fields to develop technology for their use in industrial applications. Because oxide superconductors have higher critical temperatures and critical magnetic field intensities than conventional metallic alloy superconductors, their application to heavy electric fields, once realized, will unlock an unpredictably wide range of possibilities. To employ oxide superconductors in the heavy electric field requires transforming them into suitable wire products and forming these products into coils or other forms of conductors. For superconducting oxide products to be used as coils, it is an essential requirement that the conductor is able to pass a sufficient amount of superconducting current in a magnetic field to generate magnetic flux. When forming superconductive oxides into conductive products which have large critical current densities in a magnetic field, it has been considered effective to orient the c-axes of the crystals of the superconductor material in certain directions. There is therefore a need for superconducting oxide wire products with oriented crystals which are suited for drawing and coiling operations and also to provide a method of manufacturing such oxide superconductor products. It has been found that wire products that are suited for such purposes can be produced by using yttrium oxide based superconductor materials and forming metal-clad superconductors into wires of flat cross-section, as mentioned in the Japanese Journal of Applied Physics 27 (1988), pages 1715-1717, and in JP-A-1-251515. After the discovery of bismuth and thallium oxide materials, attempts were made to make wires using these oxide materials. One example of fabrication of bismuth (Bi) oxide wire is described in Low-Temperature Engineering, 25 (1990), No. 2, pages 88-93, and another example of a thallium (Tl) oxide wire is discussed in the Japanese Journal of Applied Physics, 27 (1988), pages 2345-2347. The superconductivity of an oxide superconductor in a magnetic field, as pointed out in Physica C, 159 (1989), pages 433-438, is known to be closely related to the crystal orientation relative to the direction of magnetic field. If the crystal c-axes are oriented perpendicularly to an external magnetic field, there is little performance deterioration even when the superconductors are subjected to relatively high magnetic field. On the other hand, when the crystal c-axes are parallel to the external magnetic field, the superconductivity is lost to a large extent even in a small external field. JP-A-1-246801 (corresponding to EP-A-292436) discusses this problem and proposes orienting wire products differently at different parts of a superconducting coil. The expert in this field will know the definition of the c-axis and a-b plane of oxide superconductor crystals, but reference may be made to Hitachi Review, 39 (1990), No. 1, pages 55-62 "Development of high-Tc superconducting wire with high critical current density". In the above-mentioned prior art, with the exception of JP-A-1-246801 mentioned above, there is a problem that consideration is not given to the relationship between the direction of magnetic lines of force and the direction of the conductor. In other words, when subjected to a magnetic field parallel to, rather than perpendicular to, the crystal c-axes or to a magnetic field extending in a direction that is detrimental to the superconductivity, the wires with the oriented c-axes sharply deteriorate in performance as current conductors. Consider an example, in which the previously proposed wire product is wound into a solenoid coil and a current is applied to generate a magnetic field. In this case, if the wire is coiled so that the c-axes are perpendicular to the coil axis, the wire close to the coil centre will exhibit the best performance for that particular magnetic field direction. At the end of the coil, however, since the magnetic lines of force curve in, the angles between the magnetic lines of force and the c-axes are not necessarily optimized and the coil may, depending on the location, be used with magnetic lines of force running parallel with the c-axes. Such a magnetic flux distribution changes according to the magnetic field intensity generated by the coil, so that this problem cannot be solved by simply changing the coiling angle at the end of the coil. The maximum current that can be passed through the entire coil is limited by the portion of the wire where the critical current is the lowest. That is, if there is even a single part of the wire where the magnetic flux is parallel to the c-axes or passes through the wire in a direction detrimental to the superconductivity, the magnetic field generated by the entire coil is limited. EP-A-380115 discloses superconductor wires having a plurality of superconductor bodies disposed within a matrix, as set out in the first part of claim 1. The object of this invention is to provide an oxide superconductor wires whose critical current density will not greatly fall when subjected to a magnetic field in any particular direction and which is suitable for making a coil. According to the present invention in one aspect there is provided an elongate superconductor wire as set out in claim 1. In another aspect there is provided an elongate superconductor wire as set out in claim 8. The statement in this description and claims that "the c-axes of most of the oxide superconductor crystals are transverse to the longitudinal axis of the element" means that more than 50% of the crystals have their c-axes oriented in the range 60 to 90° to the longitudinal axis. Preferably at least 80% of crystals have their c-axes in the range 60 to 90° to the longitudinal axis and more preferably 80% of crystals have their c-axes in the range 80 to 90° to the longitudinal axis. Likewise, the statement herein that the c-axes of the crystals in a region are "generally aligned with each other" means that more than 50% of the crystal c-axes in the region (preferably at least 80%) are oriented within 30° of the average c-axis direction of the region. Preferably at least 80% of the crystal c-axes are within 10° of the average c-axis direction of the region. Such requirements, including the most preferred requirements, are easily met by the embodiments of this invention described below. In order to achieve especially good performance in magnetic fields of any direction, preferably in the element of the invention the different c-axis alignment directions of the regions are such that any notional direction perpendicular to the longitudinal direction of the element is at an angle of not more than 30° to at least one of the c-axis alignment directions. Furthermore, preferably the c-axis alignment direction of at least one region is at an angle of at least 45°, e.g. 90°, to the c-axis alignment direction of at least one other such region. Preferably the cladding material of the matrix is an electrically conductive metal, e.g. with the metal in direct contact with said oxide superconductor material. Alternatively ceramic materials such as MgO and SrTiO 3 may be used. The invention further consists in superconducting coils comprising coiled elongate wires of the invention. When applied to a solenoid coil, the superconductor wire should preferably be formed into a circular (cf. Claim 1 or 8) or regular polygonal (cf. Claim 8) shape in transverse cross-section. In this case, the superconductor body or bodies are preferably arranged about the centre of the transverse cross-section of the element and the angle between the c-axis crystal alignment of each body and a diametric line connecting the axis of the element to the body is set to a specified angle. One superconductor body may be provided at the centre of the transverse cross-section of the element. When applied to a pancake coil, the element should desirably be flat in cross-section. In a solenoid or cylindrical coil, the wire may be so arranged that the crystal c-axis alignment of each superconductor body is directed at a specified angle with respect to the cylinder core axis. It is desired that the oxide bodies be clad with metal because the metal cladding material helps to stabilize the superconductor bodies. In that case, the use of silver, silver alloy, gold or gold alloy is recommended for the portions where the metal cladding contacts the superconductor. This invention thus provides an oxide superconductor element whose current carrying capacity shows little degradation in magnetic fields. Although the invention is not limited in the range of applicable superconductors, it is effectively useful for all oxide superconductors with anisotropic superconductivity. The following are examples of oxide superconductor materials which can be used in the invention. (I) (Tl 1-x1 Pb x1 ) a1 (Ba 1-y1 -Sr y1 ) b1 -Ca c1 -Cu d1 -O e1 where x1 and y1 are 0-0.5, a1, b1 and c1 are 1.8-2.2, d1 is 1.7-3.3, e1 is 8-10 and a1+b1+c1+d1 is 9 or less. (II) (Bi 1-x2 -Pb x2 ) a2 )a 2 -Sr b2 -Ca c2 -Cu d2 -O e2 where x2 is 0-0.5, a2, b2 and c2 are 1.8-2.2, d2 is 1.7-3.3, e2 is 8-10 and a2+b2+c2+d2 is 9 or less. (III) Y a3 -Ba b3 -Cu c3 -O d3-δ where Y is one or more elements chosen from yttrium and the lanthanide series, a3 is 0.9-1.1, b3 is 1.8-2.2, c3 is 2.7-4.4, a3+b3+c3 is 7 or less, 6 is 0-0.5, and d3 is 8 or less. In more detail, various kinds of known oxide superconductive materials may be used. For example: Tl a -Ba b -Ca c -Cu d -O e , where (1) a:b:c:d:e = 1.8-2.2 : 1.8-2.2 : 1.8-2.2 : 2.7-3.3 : 9-10, (2) a:b:c:d:e = 1.8-2.2 : 1.8-2.2 : 0.8-1.1 : 1.8-2.2 : 7.2-8.8, or (3) a:b:c:d:e = 1.8-2.2 : 1.8-2.2 : 0 : 0.9-1.1 : 5.4-6.6. Tl a -(Ba 1-x, Sr x ) b -Ca c -Cu d -O e , where x = 0-0.5 and (1) a:b:c:d:e =1.8-2.2 : 1.8-2.2 : 1.8-2.2 : 2.7-3.3 : 9-11, (2) a:b:c:d:e = 1.8-2.2 : 1.8-2.2 : 0.9-1.1 : 1.8-2.2 : 7-9, or (3) a:b:c:d:e =1.8-2.2 : 1.8-2.2 : 0 : 0.9-1.1 : 5-7. (Tl 1-x , Pb x ) a -Ba b -Ca c -Cu d -O e , where x = 0-0.5 and (1) a:b:c:d:e =1.8-2.2 : 1.8-2.2 : 1.8-2.2 : 2.7-3.3 : 8-12, (2) a:b:c:d:e =1.8-2.2 : 1.8-2.2 : 0.9-1.1 : 1.8-2.2 : 7-9, or (3) a:b:c:d:e = 1.8-2.2 : 1.8-2.2 : 0 : 0.9-1.1 : 5-7. Y a -Ba b -Cu c -O d , where Y is an element chosen from yttrium and the lanthanide series and (1) a:b:c:d = 0.9-1.1 : 1.8-2.2 : 2.8-3.3 : 6.5-7.3, or (2) a:b:c:d = 0.9-1.1 : 1.8-2.2 : 3.8-4.4 : 7.2-8.8. (Bi 1-x , Pb x ) a -Sr b -Ca c -Cu d -O e , where x = 0-0.5 and (1) a:b:c:d:e =1.8-2.2 : 1.8-2.2 : 1.8-2.2 : 2.7-3.3 : 9-11, (2) a:b:c:d:e = 1.8-2.2 : 1.8-2.2 : 0.9-1.1 : 1.8-2.2 : 7-9, or (3) a:b:c:d:e = 1.8-2.2 : 1.8-2.2 : 0 : 0.9-1.1 : 5-7. One example of a method of making an oxide superconductor wire of the invention is as follows. Oxide powder is filled into a metal cladding pipe, which is then drawn by a wire-drawing machine and rolled by a rolling mill to form it into a wire body of a desired transverse cross-section. A plurality of these wire bodies, preferably six or more of them, are assembled together and inserted into a metal cladding pipe and subjected to similar plastic working to form them into a multicore element of a desired transverse cross-sectional structure. In aligning the crystals of each component wire body, it is effective to form them flat in cross-section. The thickness of the flat oxide superconductor wire body is preferably 0.1 µm to 0.2 mm and more preferably 1 µm to 100 µm. While the wire should desirably be isotropic in cross-section for easy plastic working, it may be flat in cross-section. The coiling of the wire requires the use of insulation between the coil layers. This may effectively be accomplished by forming an electrically insulating metallic oxide over the surface of the outermost metal cladding layer of the wire. Metals that may be used include those which will form a dense electrically insulating oxide film in the presence of ambient oxygen, such as nickel, nickel alloy, stainless steel, titanium, titanium alloy and various other heat-resistant, oxidation-resistant alloys. Some further explanation of the operation of the wires of the invention will now be given. There are conflicting requirements to maintaining a high critical current density Jc in an oxide superconductor wire while preventing the current carrying capacity from deteriorating due to magnetic flux in particular directions. That is, as the uniformity of crystal orientation of the wire product is enhanced to increase the critical current density Jc, the sensitivity to the direction of magnetic field increases. Therefore, when the wire product with high critical current density is used in a coil, the coiling operation should be performed in such a way that the magnetic field direction lies parallel to the c-axes of the crystals at all times. This, however, is not practical because the magnetic field distribution itself will change with the intensity of the field generated. To cope with this problem, the products of this invention have the following features. The product is made up of core member(s) (oxide body or bodies) whose crystals are oriented in the direction of longitudinal axis of the wire member, that is to say the a-b plane of oxide crystals making up each body is oriented in the longitudinal direction. In addition, the crystal c-axis alignment is made to vary from one body to another in the transverse cross-sectional plane of the wire. In one preferred transverse cross-sectional structure, a plurality of core bodies are located symmetrically about the centre of cross-section of the product and these core bodies are so disposed that the crystal alignment of each body approximately coincides with a line connecting the cross-sectional centres of the body and the whole wire respectively. The number of core bodies contained in the wire may vary depending on the kind of superconductor used. For instance, the number of core bodies may be four or more for lanthanum and yttrium oxides; for thallium oxide at least six core bodies are preferably used; and for bismuth oxide preferably more than twelve. In any case, the greater the number of bodies, the less the wire is susceptible to performance deterioration due to magnetic field in particular directions. This is because the core bodies have c-axis alignment in various directions in the transverse cross-sectional plane of the wire. If the direction of magnetic flux changes, there are always one or more core bodies that have high current densities, preventing the critical current density of the multicore wire as a whole from falling markedly. The conductor wires of this invention are particularly suited for applications in coils, such as solenoid coils and pancake coils. Performance can further be improved if the angle between the core cylinder axis of the coil and the crystal orientation of each wire is set to a specific angle. The cylindrical solenoid coil, for example, has the strongest magnetic field intensity at the central portion of the winding in the innermost layer, with the intensity reducing towards the outer layer and ends. At the central portion of the winding, the magnetic field is directed along the longitudinal axis of the coil while at the ends the radial components of the field increase. These changes in the magnetic field direction can be dealt with by winding the multicore element of this invention appropriately. By arranging the cross-sectional structure of the wire as explained above, the dependence of the critical current density of wire on the direction of magnetic field can be reduced or eliminated. This is because the wire of this invention maintains a high degree of alignment of a-b crystal planes along the longitudinal direction while at the same time the crystal alignment as a whole is varied in the transverse cross-sectional plane. The dependence of the current carrying performance of the core body varies according to the material used. For strongly anisotropic materials, which have a great dependence on the magnetic field direction, the distribution of crystal alignment on a transverse cross-sectional plane can be optimized. Embodiments of the invention will now be described by way of non-limitative example, with reference to the accompanying drawings in which:- Figures 1 to 3 are transverse cross-sectional views of respective elongate superconductor wires embodying this invention. Figure 1 shows a transverse cross-section of an elongate oxide superconductor wire embodying this invention. Core bodies 5 made of oxide superconductor are disposed so that the c-axes of crystals in the core bodies 5 are perpendicular to the longitudinal direction of the wire product (in Figure 1, this longitudinal direction is perpendicular to the plane of the sheet). The wires are embedded in a silver clad 2. The core bodies 5 in the superconductive oxide element are flat in cross-section. The total number of core bodies 5 in the element is sixteen, with the c-axis alignment direction 3 of each core wire 1 disposed perpendicular to the plane of flatness of the core wire. These flatness planes are disposed along radial directions of the element. The flat core wires 5 are 0.1 µm to 0.1 mm thick. Next, one example of a process of making the element of this invention shown in Figure 1 will be explained. The materials used for fabricating the oxide superconductive wire were Tl 2 O 3 , BaO, SrO, CaO and CuO. These components were mixed at a specific ratio (2:1.6:0.4:2:3) and sintered in an alumina crucible with a lid in the presence of air at 850°C for seven hours. This roasted powder was pulverized by automated mortar for 30 minutes and formed into pellets each measuring 30 mm across and 3 mm thick. The pellets have a critical temperature of 115 K and a critical current density of 700 A/cm 2 . The pellets were pulverized, the powder was loaded into a silver tube 6 mm in outer diameter, 4 mm in inner diameter and 400 mm in length. The loaded silver tube was drawn into a wire rod 1.0 mm across and 8 m long. This wire rod was then rolled by a cold rolling mill whose roll diameter is 110 mm, to form a tape 50 µm thick and 3 mm wide. Lengths of this tape were inserted as tapes 5,5' into flat holes cut in a round silver bar 15 mm in outer diameter whose transverse cross-section is as shown in Figure 1. Then, the silver bar loaded with the superconductive tapes was subjected to repetitive wire-drawing operations to form a wire product 8 mm in outer diameter and 2 m in length. This wire product was further heat-treated at 850°C for 200 hours. As a result, the a-b planes of the oxide crystals were grown parallel to the flat surfaces of the superconductive tapes 5,5'. The tape-shaped core wires 5,5' had a thickness of 15-20 µm. X-ray diffraction examination found that more than 95% of the crystals are oriented with a-b planes within ±2.5° of the flat surfaces of the core. Therefore, the direction of the a-b plane is radial in the cross-section of Figure 1. The core wire thickness can further be reduced to about 2-5 µm by performing additional wire-drawing operations. In this case, the orientation showed a significant improvement, i.e. more than 99% of the crystals in each core 5,5' were oriented in the same direction. The crystal alignment in the same direction resulting from the crystal growth along the surface became more prominent as the core wire thickness decreased. However, since the oxide powder is not easily subjected to plastic working, the superconductor core wire thickness could not easily be reduced below 5-15 µm with this method when oxide powder is present. Further reduction in thickness resulted in a lower core wire thickness, making it impossible to provide a sufficient critical current density Jc. Figure 2 shows another example of the invention in the form of a flat wire product. In Figure 2, twelve flat superconductive component wires 16 are arranged as shown at each side portion of the flattened wire product to reduce the effect of nonuniform magnetic field distribution at the sides of the element. At the central region, flat superconductive wires 17 are arranged parallel to each other. The c-axes of each superconductive core wire 16,17 are normal to the flat surface. An example of a wire of the invention which uses a plurality of the first matrices distributed in a second matrix is shown in Figure 3 which shows a multicore element, in which seven component elements 29 each with eight core bodies are embedded in a metal matrix 28. In each element 29 core bodies made of Tl 2 Ba 2 Ca 2 Cu 3 O 19 are disposed so that the c-axes of crystals in the core body are perpendicular to the longitudinal direction of the wire, and are aligned in the diameter direction of the element 29 at the body (as indicated by arrows in Fig. 3. In this case, the metal cladding material used is silver and the outermost layer may be coated with a nickel oxide insulating layer. The nickel oxide insulating layer on the outer surface of the product may be formed by first coating the outermost layer of the metal pipe with nickel and oxidizing the nickel layer during the final heat treatment. The present invention is concerned with the wire structure and does not place particular limitations on the superconducting oxide material used. Table 1 lists some of the currently known major superconductive oxides. With this invention as claimed it is thus possible to provide superconductive wires and coils thereof which have critical current densities at a liquid nitrogen temperature, let alone at a liquid helium temperature, and which have little dependence on the direction of the magnetic flux in strong magnetic fields.

Claims: An elongate superconductor wire having a plurality of discrete bodies (5,16) of oxide superconductor material extending parallel to the longitudinal axis of the wire and embedded in a matrix (2) of cladding material, wherein in each said body (5,16), as seen in section perpendicular to said longitudinal axis, the c-axes of most of the oxide superconductor crystals are transverse to said longitudinal axis of the wire and are generally aligned with each other, said bodies being of plate-like shape and being arranged so that the average c-axis alignment directions of the respective bodies are different as between different ones of said bodies, characterized in that as seen in said section perpendicular to the longitudinal axis of the wire, said bodies (5,16) are arranged in a circle or a semi-circle with the long direction of the plate-like cross-section of each body lying radially with respect to the centre of said circle or semi-circle and the average c-axis alignment direction of each body lying in the tangential direction with respect to the centre of said circle or semi-circle. A superconductor wire according to claim 1, having a circular periphery and wherein said bodies (5) are arranged in a circle around the centre of said circular periphery. A superconductor wire according to claim 1, having a ribbon shape, with a periphery having semi-circular ends and flat sides, and wherein said plate-like bodies (16) are arranged in two semi-circles respectively within said semi-circular ends, and between said flat sides said wire has, embedded in said matrix (2), a plurality of further plate-like bodies (17) of oxide superconductor material extending parallel to the longitudinal axis of the wire with, in each said further body (17), the c-axes of most of the oxide superconductor crystals transverse to said longitudinal direction of the wire and generally aligned with each other, and the average c-axis alignment directions of said further plate-like bodies (17) being aligned with each other and perpendicular to said flat sides. A superconductor wire according to any one of claims 1 to 3 wherein said different average c-axis alignment directions of said bodies are such that any notional direction perpendicular to said longitudinal axis is at an angle of not more than 30° to at least one of said average c-axis alignment directions. A superconductor wire according to any one of claims 1 to 4 wherein said matrix of cladding material (2,12,20,22,25) is made of metal. A superconductor wire according to any one of claims 1 to 5 wherein said average c-axis alignment direction of each said body is at an angle of at least 45° to the average c-axis alignment direction of at least one other said body. A superconductor wire according to any one of claims 1 to 6 wherein there are at least six of said bodies. An elongate superconductor wire having a plurality of discrete bodies of oxide superconductor material extending parallel to the longitudinal axis of the wire, said bodies being embedded in a plurality of discrete first matrices (29) which are themselves embedded in a second matrix (28), wherein in each said body, as seen in section perpendicular to said longitudinal axis, the c-axes of most of the oxide superconductor crystals are transverse to said longitudinal axis of the wire and are generally aligned with each other, characterized in that each said first matrix (29) has a plurality of said bodies therein, the average c-axis alignment directions of said plurality of bodies within each said first matrix being different as between different ones of said bodies therein so that each said first matrix has a plurality of said bodies having respectively different c-axis alignment directions. A superconductor wire according to claim 8 wherein each said first matrix (29) has a circular periphery as seen in said section perpendicular to the longitudinal axis of the wire and has said plurality of bodies arranged on a circle within it. A superconductor wire according to claim 9 wherein said c-axis alignment direction of each said body in each said first matrix is radial with respect to the centre of the circular periphery of the first matrix. A superconductor wire according to any one of claims 8 to 10 wherein in each said first matrix (29) said different average c-axis alignment directions of said bodies are such that any notional direction perpendicular to said longitudinal axis is at an angle of not more than 30° to at least one of said average c-axis alignment directions of said bodies within that first matrix. A superconductor wire according to any one of claims 8 to 11 wherein said first matrices (29) and said second matrix (28) are made of metal. A superconductor wire according to any one of claims 8 to 12 wherein in each said first matrix said average c-axis alignment direction of each said body is at an angle of at least 45° to the average c-axis alignment direction of at least one other said body within that first matrix. A superconductor wire according to any one of claims 8 to 13 wherein in each said first matrix there are at least six said bodies. A superconductor coil comprising at least one coiled elongate superconductor wire according to any one of the preceding claims.

Inventors: AIHARA KATSUZO, MATSUDA SHINPEI, MATSUMOTO TOSHIMI, OKADA MICHIYA, TANAKA KAZUHIDE, YUASA TOYOTAKA, AIHARA, KATSUZO, MATSUDA, SHINPEI, MATSUMOTO, TOSHIMI, OKADA, MICHIYA, TANAKA, KAZUHIDE, YUASA, TOYOTAKA
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H01L39
IPC codes: C01G1, H01F6, H01L39, C04B35, H01B13, C30B29, H01B12