JP4876643B2 - Superconducting wire - Google Patents

Description

  The present invention relates to a superconducting wire, for example, a Bi2223 superconducting wire including a superconducting crystal having a large particle size.

  The Bi2223 superconductor is known to be a material with very strong anisotropy, and it is easy to flow current in the ab plane (CuO crystal plane) direction, and it is difficult to flow current in the c-axis direction. . Therefore, in the case where there are a plurality of Bi2223 crystals joined in the ab plane direction, it is difficult for current to flow if there is a deviation in the joining between the crystals. In addition, when a non-superconducting substance is present between crystals in the ab plane direction and the c-axis direction in a plurality of Bi2223 superconductors, the current path is hindered. Therefore, the ease of current flow in the Bi2223 superconductor is affected by the junction deviation angle, the non-superconducting material, and the like.

In the Bi2223 superconducting wire disclosed in Non-Patent Document 1, it is disclosed that the Bi2223 phase crystal parallel to the ab plane has a size of 10 μm × 10 μm × 1 μm. Note that the size of this crystal is the size of a figure that is generated when incident electrons are subjected to inelastic scattering due to thermal vibration of atoms and then reflected black.
TT Tan, S Li, JT Oh, W Gao, HK Liu and SX Dou, "Crystallographic orientation mapping with an electron backscattered diffraction technique in (Bi, Pb) 2Sr2Ca2Cu3O10superconductor tapes", Superconductor Science and Technology, vol.14, (2001) , pp79.

  However, since the Bi2223 superconducting wire disclosed in Non-Patent Document 1 has a small Bi2223 phase crystal, there is a problem that the number of junctions between crystals in the ab plane direction increases and the crystal junction deviation angle increases. . In addition, when the current path is decreased due to the non-superconducting material, there is a problem that current is less likely to flow if the crystal junction deviation angle is large.

  Accordingly, an object of the present invention is to provide a Bi2223 superconducting wire that allows current to flow more easily, and to provide a superconducting wire that improves the critical current value.

A superconducting wire according to one aspect of the present invention is a superconducting wire including a filament having a plurality of superconducting crystals made of Bi2223 phase and a sheath portion covering the filament, and a non-superconducting substance exists between the superconducting crystals. The superconducting crystals are laminated in the c-axis direction so that the current flowing through the filament flows through the grain boundary between the superconducting crystals laminated in the c-axis direction while avoiding the non-superconducting material , The average particle diameter of the superconducting crystal composed of the Bi2223 phase in the parallel cross section is 20 μm or more.

A superconducting wire in another aspect according to the present invention is a superconducting wire including a filament having a plurality of superconducting crystals made of Bi2223 phase and a sheath portion covering the filament, and a non-superconducting substance exists between the superconducting crystals. The superconducting crystals are laminated in the c-axis direction so that the current flowing through the filament flows through the grain boundary between the superconducting crystals laminated in the c-axis direction while avoiding the non-superconducting material , The average particle diameter of the superconducting crystal composed of the Bi2223 phase in the parallel cross section is 21.1 μm or more.

  According to the superconducting wire of the present invention, in the Bi2223 superconducting wire, the average particle size of the Bi2223 phase superconducting crystal in the cross section parallel to the ab plane through which current easily flows is larger than the average particle size of the conventional Bi2223 phase superconducting crystal. Is also big. Therefore, the number of junctions between crystals in the direction parallel to the ab plane can be reduced, and the crystal junction deviation angle can be reduced. Further, even when the current path due to the non-superconducting material is reduced, the crystal junction deviation angle can be reduced, so that the grain boundaries through which the current passes can be reduced, and the current can be prevented from becoming difficult to flow. Therefore, a Bi2223 superconducting wire that allows current to flow easily can be obtained, and the critical current value can be improved.

  In addition, the above-mentioned “average particle diameter” was measured by measuring the maximum length of a grain boundary that can be visually observed in each crystal grain in a 200 μm square field parallel to the ab plane and taking 25 fields. It means the average length when.

  In the superconducting wire, the FWHM of the (0.0.24) peak measured by the XRD rocking curve of the superconducting crystal composed of the Bi2223 phase is preferably 18 ° or less. By setting the FWHM to 18 ° or less, the in-plane orientation, which is an array of superconducting crystals composed of Bi2223 phase in the longitudinal direction of the superconducting wire (plane parallel to the ab direction), is improved. Therefore, the critical current value can be further improved.

  The “FWHM (half wave height full width value)” means the half width of the (0.0.24) peak measured by the XRD rocking curve, and serves as an index indicating in-plane orientation. The smaller the FWHM value, the better the in-plane orientation.

  Preferably, the superconducting wire further includes a superconducting crystal composed of a Bi2212 phase, and the Bi2223 phase and Bi2212 phase peak intensities measured by X / 2 diffraction by the θ / 2θ scanning method are Bi2223 (0.0.14) / ( Bi2223 (0.0.14) + Bi2212 (0.0.12)) is 0.95 or more. By setting the value to 0.95 or more, the Bi2223 phase in the superconducting wire is sufficiently contained, and more current flows.

  According to the superconducting wire of the present invention, since the average particle size of the superconducting crystal composed of the Bi2223 phase in the cross section parallel to the ab plane is large, it can be a Bi2223 superconducting wire that easily flows current, and the critical current value can be improved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

  FIG. 1 is a schematic view showing a superconducting wire in an embodiment of the present invention. 2 is a cross-sectional view taken along line II-II in FIG. FIG. 3 is an enlarged schematic view of a certain visual field in FIG. FIG. 4 is a schematic diagram of a superconducting crystal composed of a Bi2223 phase in a cross section parallel to the ab plane according to the embodiment of the present invention. FIG. 5 is a diagram for explaining a current flow in a certain region of the superconducting wire according to the embodiment of the present invention. FIG. 6 is a diagram for explaining a current flow in a certain region of a conventional superconducting wire. With reference to FIGS. 1-6, the superconducting wire in embodiment of this invention is demonstrated. Superconducting wire 10 in the embodiment is a superconducting wire containing a superconducting crystal made of Bi2223 phase, and the average particle size of the superconducting crystal made of Bi2223 phase in a cross section parallel to the ab plane is 20 μm or more.

  In the embodiment, as shown in FIG. 1, the superconducting wire 10 includes a filament 1 and a sheath portion 2. The sheath portion 2 is made of, for example, silver and covers the filament 1.

  The crystal constituting the filament 1 includes an ab plane extending in the same direction as the width direction of the superconducting wire 10 and a c-axis extending in the same direction as the thickness direction of the superconducting wire 10 (vertical direction in FIG. 1). And have. In the superconducting wire 10, the current mainly flows in a direction parallel to the ab plane.

  The filament 1 has a composition of Bi—Pb—Sr—Ca—Cu—O system, and the element ratio of (bismuth and lead): strontium: calcium: copper is approximated at a ratio of 2: 2: 2: 3. The superconducting crystal which consists of Bi2223 phase expressed as above is included.

  Further, since the filament 1 cannot be completely changed to the Bi2223 phase, it may include a superconducting crystal composed of a Bi2212 phase in which the element ratio of (bismuth and lead): strontium: calcium: copper is approximately 2: 2: 1: 2. . In this case, Bi2223 (0.0.14) / (Bi2223 (0.0.14) + Bi2212 (0...) In the peak intensities of the Bi2223 phase and Bi2212 phase measured by X / 2 diffraction by the θ / 2θ scan method. The value obtained by 0.12)) is preferably 0.95 or more, more preferably 0.96 or more, and most preferably 0.98 or more. By setting it to 0.95 or more, the number of superconducting crystals composed of the Bi2223 phase increases, so that the current flows more easily. By setting it to 0.96 or more, it becomes easier for the current to flow. By setting it to 0.98 or more, the current can flow very easily.

  The FWHM of the (0.0.24) peak measured by the XRD rocking curve of the superconducting crystal composed of the Bi2223 phase is preferably 18 ° or less, more preferably 10 ° or more and 16 ° or less. FWHM is an inclination of the direction in which the superconducting crystal composed of the Bi2223 phase extends with respect to the direction in which the superconducting wire 10 extends (the direction in which current flows in the superconducting wire 10). By setting the FWHM to 18 ° or less, the in-plane orientation is excellent, and the current flows more easily. By setting the FWHM to 16 ° or less, the current can flow more easily. On the other hand, the superconducting wire 10 can be easily manufactured by setting the angle to 10 ° or more.

  Next, the crystals constituting the filament 1 will be described in detail. As shown in FIG. 2, in the filament 1, a cross section in a direction parallel to the ab plane direction, and a 1 mm square sample is taken out for observation with a microscope. In addition, as a microscope, a scanning electron microscope (SEM: scanning electron microscope) can be used, for example. Then, in the visual field 20 having a 200 μm square of the sample shown in FIG. 2, as shown in FIG. 3, the lengths x1 to x9 of grain boundaries that can be visually observed in each crystal grain of the superconducting crystal composed of Bi2223 phase are measured. Among these lengths x1 to x9, the maximum length (for example, length x1 in FIG. 3) is the length in a certain visual field 20. The superconducting crystal 30 made of the Bi2223 phase existing in the visual field 20 preferably has a length x in the ab plane direction of 20 μm or more, as shown in FIG. And when the average length of the superconducting crystal made of Bi2223 phase is the average length when measured by taking 25 fields of view 20, the average particle diameter is 20 μm or more, preferably 21.1 μm or more, and more preferably Is 27 μm or more and 50 μm or less. If the average grain size is less than 20 μm, the number of junctions between crystals in the ab plane direction cannot be reduced and the crystal junction deviation angle cannot be reduced when a current is passed. By setting the average grain size to 21.1 μm or more, the number of junctions between crystals in the ab plane direction can be reduced and the crystal junction deviation angle can be reduced when a current is passed. On the other hand, when the average particle size is 50 μm or less, a crystal that can be produced practically can be obtained.

  Further, as shown in FIG. 5, when a certain region 40 in the superconducting wire 10 is viewed, if a non-superconducting substance 41 exists between superconducting crystals composed of Bi2223 phases in the ab plane direction and the c-axis direction, current ( The arrows in FIG. 5 flow through the three crystals 42 to 44, that is, the two grain boundaries while avoiding the non-superconducting material 41. On the other hand, as shown in FIG. 6, when the average particle size of the conventional superconducting crystal composed of the Bi2223 phase is small, when the region 50 in the same range as the region 40 in the conventional superconducting wire is viewed, the non-superconducting substance 51 is When present, the current (arrow in FIG. 6) flows through the nine crystals 52, 53, 55 to 58, 60, 62, 63, that is, eight grain boundaries to avoid the non-superconducting material 51. As described above, when a current is passed through the superconducting wire of the embodiment, the number across the grain boundaries can be reduced as compared with the conventional superconducting wire. Therefore, even when a non-superconducting substance is present in the superconducting wire 10, it is possible to prevent the current from becoming difficult to flow.

Next, the manufacturing method of the superconducting wire 10 in the embodiment of the present invention will be described. First, raw material powder (precursor) of an oxide superconductor is prepared, and this is filled in a metal tube. Specifically, for example, the raw material powder is mixed so that the composition ratio is Bi: Pb: Sr: Ca: Cu = 1.7: 0.4: 1.9: 2.0: 3.0. This was subjected to heat treatment at about 700 to 860 ° C. several times, and a large amount of (BiPb) ₂ Sr ₂ Ca ₁ Cu ₂ O _Z (Bi-2212 phase) and a small amount of (BiPb) ₂ Sr ₂ Ca ₂ Cu ₃ O _Z ( Bi-2223 phase) and a non-superconducting phase are prepared. In the process of preparing this powder, sprayed droplets are introduced into a heating furnace, and after evaporation of the solvent and nucleation / growth of fine particles by chemical reaction, sintering is performed to form and refine the structure and shape of the spray. The method (CSP method) is preferably used. Further, from the viewpoint of quickly removing heat generated by the quench phenomenon, it is preferable to use silver or a silver alloy having a high thermal conductivity as the metal tube.

  Next, the metal tube filled with the powder is drawn to produce a single core wire coated with a metal such as silver using the raw material powder as a core material. Next, many single core wires are bundled and fitted into a metal tube made of a metal such as silver. Thereby, the wire of multi-core structure which has many raw material powders as a core material is obtained.

  Next, a wire having a multi-core structure is drawn to a desired diameter to produce a multi-core wire in which the raw material powder is embedded in a sheath portion 2 such as silver. As a result, a long multifilamentary wire having a form in which the raw material powder of the superconducting wire 10 is coated with metal is obtained. And the density of raw material powder is raised by rolling this multifilament wire, and a tape-shaped wire is obtained.

  Next, the multifilamentary wire is heat-treated at a temperature of about 760 ° C. for 2 hours under an atmosphere of 1 atm using an atmosphere gas composed of, for example, 99.9% nitrogen gas and 0.1% oxygen gas. To do. By performing heat treatment at a high temperature of 760 ° C., the Bi2212 phase grows from the raw material powder and becomes the filament 1. By performing the above manufacturing process, the superconducting wire 10 shown in FIG. 1 is obtained.

  As described above, according to superconducting wire 10 according to the embodiment of the present invention, a superconducting wire including a superconducting crystal composed of Bi2223 phase, which is a superconducting crystal composed of Bi2223 phase in a cross section parallel to the ab plane. The average particle size is 20 μm or more. Thus, the average particle size of the Bi2223 phase superconducting crystal in the superconducting wire 10 is sufficiently larger than the average particle size of the conventional Bi2223 phase superconducting crystal. Therefore, when current flows, the number of junctions between crystals in the ab plane direction can be reduced, and the number of crystal grain boundaries through which current flows can be reduced. Further, the number of junctions between crystals in the ab plane direction can be reduced, and the crystal junction deviation angle can be reduced. Furthermore, even when the current path by the non-superconducting material 41 is reduced, the junction deviation angle can be reduced, so that the grain boundary through which the current passes can be reduced and the current can be prevented from becoming difficult to flow. Therefore, the superconducting wire 10 can be a Bi2223 superconducting wire in which current flows more easily, and the critical current value can be improved.

  In the Bi2223 superconducting wire disclosed in Non-prior art document 1, the Bi2223 phase crystal parallel to the ab plane is a measurement comparable to the average grain size of the superconducting crystal composed of the Bi2223 phase of the present invention. It is measured by the method. Therefore, the average particle size of the Bi2223 phase superconducting crystal in the embodiment of the present invention is about twice or more the particle size of the Bi2223 phase superconducting crystal of Non-prior art document 1.

[Example]
In the examples of the present invention, the effect of the average particle diameter of the superconducting crystal composed of the Bi2223 phase in the cross section parallel to the ab plane being 20 μm or more was examined.

Example 1
The superconducting wire of Example 1 of the present invention was manufactured according to the manufacturing method of Embodiment 1 of the present invention. The heat treatment was performed at 828 ° C. for 50 hours at an oxygen concentration of 8%. And the superconducting wire with the average particle diameter of the superconducting crystal which consists of Bi2223 phase in a cross section parallel to ab surface was 21.1 micrometers was obtained.

(Example 2)
The superconducting wire of Example 2 is basically the same as the superconducting wire of Example 1, except that the average particle size of the superconducting crystal composed of Bi2223 phase in the cross section parallel to the ab plane is 27.3 μm. It differs only in. Note that the heat treatment was performed at 828 ° C. for 70 hours at an oxygen concentration of 8%.

(Example 3)
The superconducting wire of Example 3 is basically the same as the superconducting wire of Example 1, except that the average particle size of the superconducting crystal composed of the Bi2223 phase in the cross section parallel to the ab plane is 20.0 μm. It differs only in. The heat treatment was performed at 828 ° C. for 150 hours at an oxygen concentration of 8%.

(Comparative Example 1)
The superconducting wire of Comparative Example 1 is basically the same as the superconducting wire of Example 1, but the average particle size of the superconducting crystal composed of Bi2223 phase in the cross section parallel to the ab plane is outside the scope of the present invention. It differs only in that it is 5.1 μm. The heat treatment was performed at 822 ° C. for 50 hours at an oxygen concentration of 8%.

(Comparative Example 2)
The superconducting wire of Comparative Example 2 is basically the same as the superconducting wire of Example 1, but the average particle size of the superconducting crystal composed of Bi2223 phase in the cross section parallel to the ab plane is outside the scope of the present invention. It differs only in that it was 8.6 μm. The heat treatment was performed at 822 ° C. for 70 hours at an oxygen concentration of 8%.

(Comparative Example 3)
The superconducting wire of Comparative Example 3 is basically the same as the superconducting wire of Example 1, but the average particle size of the superconducting crystal composed of Bi2223 phase in the cross section parallel to the ab plane is outside the scope of the present invention. It differs only in that it was 15.5 μm. The heat treatment was performed at 822 ° C. for 200 hours at an oxygen concentration of 8%.

(Comparative Example 4)
The superconducting wire of Comparative Example 4 is basically the same as the superconducting wire of Example 1, except that the average particle size of the superconducting crystal composed of Bi2223 phase in the cross section parallel to the ab plane is 19.5 μm. It differs only in. The heat treatment was performed at 828 ° C. for 35 hours at an oxygen concentration of 8%.

(Measuring method)
For the superconducting wires of Examples 1 and 2 and Comparative Examples 1 to 3, the critical current value was measured in a self-magnetic field at a temperature of 77K. The critical current value was defined as an energization current value when an electric field of 10 ⁻⁶ V / cm was generated. The results are shown in Table 1 and FIG. FIG. 7 is a diagram showing the relationship between the average grain size of the superconducting crystal composed of the Bi2223 phase and the critical current value in the cross section parallel to the ab plane in the embodiment of the present invention. In FIG. 7, the vertical axis represents the critical current value (unit: A), and the horizontal axis represents the average particle size (unit: μm) of the superconducting crystal composed of the Bi2223 phase in a cross section parallel to the ab plane.

(Measurement result)
As shown in Table 1 and FIG. 7, Examples 1 and 2 in which the average particle size of the superconducting crystal composed of the Bi2223 phase in the cross section parallel to the ab plane was 20 μm or more had very high critical current values. Further, as apparent from FIG. 7, the larger the average particle size, the higher the critical current value, but the critical current value is dramatically improved at the point where the average particle size is 20 μm or more. all right. On the other hand, Comparative Examples 1 to 3 in which the average particle size of the superconducting crystal composed of the Bi2223 phase in the cross section parallel to the ab plane was less than 20 μm had a lower critical current value than Examples 1 and 2.

  As described above, according to the embodiment of the present invention, the superconducting wire in which the average particle diameter of the superconducting crystal composed of the Bi2223 phase in the cross section parallel to the ab plane is 20 μm or more can improve the critical current value. Was confirmed.

  The embodiments and examples disclosed above are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is shown not by the above embodiments and examples but by the scope of claims, and is intended to include all modifications and variations within the meaning and scope equivalent to the scope of claims. .

It is the schematic which shows the superconducting wire in embodiment of this invention. It is sectional drawing in line segment II-II in FIG. It is an expansion schematic diagram in a certain visual field in FIG. It is a schematic diagram of the superconducting crystal which consists of Bi2223 phase in the cross section parallel to the ab surface of embodiment of this invention. It is a figure for demonstrating the flow of the electric current in the area | region with the superconducting wire of embodiment of this invention. It is a figure for demonstrating the flow of the electric current in the area | region with the conventional superconducting wire. It is a figure which shows the relationship between the average particle diameter of the superconducting crystal which consists of Bi2223 phase in the cross section parallel to the ab surface in the Example of this invention, and its critical current value.

Explanation of symbols

  1 Filament, 2 sheath part, 10 superconducting wire, 20 fields of view, 30 superconducting crystal, 40, 50 region, 41, 51 non-superconducting material, 42-44, 52-63 crystal, x1-x9 length.

Claims (4)

A superconducting wire comprising a filament having a plurality of superconducting crystals composed of a Bi2223 phase and a sheath portion covering the filament ,
A non-superconducting material exists between the superconducting crystals,
The superconducting crystal is laminated in the c-axis direction so that a current flowing through the filament flows through a grain boundary between the superconducting crystals laminated in the c-axis direction while avoiding the non-superconducting material;
A superconducting wire in which an average particle diameter of a superconducting crystal composed of the Bi2223 phase in a cross section parallel to the ab plane is 20 μm or more. A superconducting wire comprising a filament having a plurality of superconducting crystals composed of a Bi2223 phase and a sheath portion covering the filament ,
A non-superconducting material exists between the superconducting crystals,
The superconducting crystal is laminated in the c-axis direction so that a current flowing through the filament flows through a grain boundary between the superconducting crystals laminated in the c-axis direction while avoiding the non-superconducting material;
A superconducting wire in which an average particle diameter of a superconducting crystal composed of the Bi2223 phase in a cross section parallel to the ab plane is 21.1 μm or more.   The superconducting wire according to claim 1 or 2, wherein a (0.0.24) peak FWHM measured by an XRD rocking curve of the superconducting crystal composed of the Bi2223 phase is 18 ° or less. A superconducting crystal composed of a Bi2212 phase;
In the peak intensities of the Bi2223 phase and the Bi2212 phase measured by the θ / 2θ scan method by X-ray diffraction, Bi2223 (0.0.14) / (Bi2223 (0.0.14) + Bi2212 (0.0.12). The superconducting wire according to any one of claims 1 to 3, wherein the value obtained by)) is 0.95 or more.

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