JP3641067B2 - Oxide superconducting conductor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention provides a REBa ₂ Cu ₃ O _x oxide superconducting conductor having an electrode portion with a metal conductor that is resistant to thermal cycling and having a high critical current density, and is a material for current leads, permanent current switches, and the like. Used as
[0002]
[Prior art]
The REBa ₂ Cu ₃ O _x- based superconductor is a substance that has a strong pinning force at liquid nitrogen temperature compared to Bi-based and Tl-based superconductors, and is inherently capable of obtaining a high critical current density. However, since the crystal grain boundary acts as a weak bond and a large superconducting current cannot flow across the grain boundary, it must be highly oriented to obtain a high critical current density.
[0003]
Among these, the REBa ₂ Cu ₃ O _x system having large crystal grains with a volume of 50 cm ³ or more is obtained by a melting method, which is a method for producing an oxide superconducting material represented by the QMG method (Japanese Patent Publication No. 4-40289). Production of superconducting bulk materials is possible (M. Morita et al .: Advances in superconductivity III, Springer-Verlag, Tokyo, 1990. p733-736). In the melting method, basically, the raw material of the REBa ₂ Cu ₃ O _x- based superconductor is heated to a semi-molten state in which the RE ₂ BaCuO ₅ phase and the liquid phase mainly composed of Ba, Cu and O coexist, In this method, the REBa ₂ Cu ₃ O _x phase is crystal-grown by slow cooling from directly above the peritectic temperature. In the large crystal grains produced by this method, there are small-angle grain boundaries whose crystal orientations are deviated by several degrees, but there are no grain boundaries having such a large tilt angle that causes weak coupling. For this reason, the critical current density in the grain of this material is as high as 10000 A / cm ² or more at 77 K and 1 T, and it is considered to be used as a material for bulk magnets, magnetic shields, current leads and the like.
[0004]
When this material is used as a current lead, a rod-shaped conductor is cut out from a bulk body and used, but is often cut out linearly with a diamond cutter or the like and used as a polygonal columnar conductor. In particular, cutting into a square pole / bar shape is easier and has a better yield than cutting into a cylindrical conductor, and is advantageous in terms of cost.
[0005]
When cutting into a quadrangular prism shape, the length direction, that is, the direction of current flow, is often parallel to the ab plane. This is because the critical current density when a current flows in the ab plane direction is larger than that when a current flows in the c-axis direction. In many cases, one surface in the length direction is parallel to the ab surface and this surface is wide. In particular, in the case of a thin conductor, this is overwhelmingly common. This is because this material is easy to cause cleavage cracks on the ab surface and has a low strength, so that if the material is not thick in a direction perpendicular to the ab surface, it is easily broken with a small force. That is, in many cases, by providing a plane parallel to the ab plane with a certain width, means for preventing cleavage cracks on the ab plane at the time of sample processing or use are provided.
[0006]
These materials are used by connecting to the metal wire at the end, but for applications where a large current such as a current lead is applied, it is necessary to connect with a large area in order to reduce the connection resistance with the metal wire. . Therefore, as a result, the connection is often made on the ab surface having a large area. In addition, in order to flow a large current, when connecting on the four surrounding surfaces parallel to the current direction, that is, the length direction, connection on the ab surface is always accompanied. For these reasons, a conductor composed of a bulk of a polygonal column represented by REBa ₂ Cu ₃ O _x oriented so far and whose energization direction is parallel to the ab plane is other than the ab plane or a plane orthogonal thereto. There is no application example connected with metal.
[0007]
In addition, the oxide superconducting bulk is inferior in strength to ceramics such as alumina. Therefore, when it is used for a current lead or the like that requires a relatively long length, it is often used by reinforcing it with glass fiber or a metal material. For this reason, when these materials and REBa ₂ Cu ₃ O _x are used in a fixed manner, there is a difference in thermal expansion coefficient between these materials and REBa ₂ Cu ₃ O _x , so that REBa ₂ Cu ₃ O _{x is} caused by cooling. Thermal stress is generated.
[0008]
The thermal shrinkage of REBa ₂ Cu ₃ O _x single crystal from room temperature to about 5K is 0.41% in the c-axis direction and 0.14% in the a · b axis. Glass fiber reinforced plastic (fiber direction) is 1.5%, stainless steel (SUS304) is 0.26%, and conductive steel is 0.29%. Therefore, a stress is generated due to a difference in thermal shrinkage from copper and the reinforcing material due to cooling. Even when materials having the same thermal expansion coefficient are selected, stress is generated when a temperature gradient occurs during cooling due to different heat capacities. For this reason, when it is connected and fixed to the metal on the ab surface as described later, when used in an environment with a severe thermal cycle, the conductor often breaks at the electrode part due to the above-described causes.
[0009]
[Problems to be solved by the invention]
The present invention has been made in view of the current state of the prior art described above, and an object thereof is to provide a REBa ₂ Cu ₃ O _x oxide superconducting conductor having an electrode portion with a metal conductor that is resistant to thermal cycling and having a high critical current density. To do.
[0010]
[Means for Solving the Problems]
In order to solve the above problems, the present invention is provided with means for connecting to a metal on a plane other than the ab plane. Specifically, metal electrodes are connected to both ends of a crystal-oriented polygonal column superconducting bulk body represented by REBa ₂ Cu ₃ O _x , and the energization direction is parallel to the ab surface of the superconducting bulk body. In addition, the oxide superconducting conductor is characterized in that it is connected to the metal electrode at a surface that forms 20 degrees or more and 70 degrees or less with the ab surface at the polygonal column peripheral portion, preferably at 45 degrees.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The ab surface of the REBa ₂ Cu ₃ O _x superconducting bulk body is prone to cleave cracking, and its strength is significantly lower than the other surfaces, and when the force acting on this surface is increased, it is easily broken. Therefore, when connecting and fixing to a metal, it is possible to make an electrode portion that is resistant to thermal cycling by connecting on a surface where the force acting on this surface is small. In particular, when connecting copper or the like on all surfaces in order to pass a large current, peeling failure on the ab surface is difficult to occur by connecting at an intermediate angle other than the ab surface and its perpendicular basic surface. A strong current lead can be made. At this time, the angle at which the force acting on the ab plane is the smallest is 45 degrees, and in the case of the most realistic quadrangular prism conductor, the surrounding four faces are cut out to form a plane that forms 45 degrees with the ab plane. It is desirable to connect with.
[0012]
The specific form of the oxide superconductor of the present invention is described below.
A current lead was produced using a YBa ₂ Cu ₃ O _x -based superconducting bulk produced by the melting method. FIG. 1 shows a prototype current lead. In the bulk material 1 used, the Y ₂ BaCuO ₅ phase has an average size of 2 μm or less and a volume ratio of about 30%, but the matrix is an oriented YBa ₂ Cu ₃ O _x superconductivity without a large grain boundary. It is a single crystal material in phase. The YBa ₂ Cu ₃ O _x bulk was prepared in five types of materials having a length (current direction) of 45 mm, a width of 4 mm, and a thickness of 2 mm. In all five types, the crystal orientation is parallel to the ab plane in the length direction. However, the surfaces in contact with the copper electrode 2 having both ends of 10 mm are different from each other, and are parallel to the ab surface (the angle θ in the figure is 0 degree) and the angle θ formed with the ab surface is 15 degrees, 25 degrees, 30 A sample was prepared by changing the angle to 45 degrees and 45 degrees. In the figure, 4 indicates the direction of the c-axis, that is, the direction perpendicular to the ab plane. The current leads composed of these five samples are designated as current leads A, B, C, D, and E from those connected at 0 degrees.
[0013]
These materials are coated with about 1 μm of silver by sputtering, and are connected to a copper electrode by solder. Moreover, as shown in FIG. 1, it is reinforced with a glass fiber reinforced plastic 3 called G10. This reinforcing plastic is not in close contact with the superconducting bulk, but is fixed and integrated with the copper electrode.
[0014]
These current leads were able to maintain a superconductor state even when energized with 200 A at a liquid nitrogen temperature (77 K). The operation of returning the current lead to room temperature and cooling it again to the liquid nitrogen temperature was repeated 100 times. Among them, the current lead A was peeled and broken from the ab surface near the electrode portions at both ends as shown in FIG. 2 when 10 thermal cycles were applied. The other current leads had no problem in appearance even after 100 thermal cycles. However, when the current lead B was energized at 60 A at the liquid nitrogen temperature, the electrical resistance at both ends increased and the current could not be energized. When this was observed with an optical microscope, a large crack was observed in the vicinity of the electrode portion in parallel with the ab surface of the sample. It seems that the current flow was hindered by this crack, and the critical current was lowered. For the current leads C, D, and E, no abnormality was observed even when a current of 200 A was applied.
[0015]
The reason why the electrodes of the current leads A and B are damaged is that the difference in thermal expansion coefficient between the copper of the electrode and the YBa ₂ Cu ₃ O _x superconducting bulk, and between the glass fiber reinforced plastic and the YBa ₂ Cu ₃ O _x superconducting bulk This is thought to be because stress was generated in the vicinity of the electrode portion, and cleavage cracks were generated and expanded on the ab surface having low strength. On the other hand, when the electrode bonding surface is larger than 25 degrees with respect to the ab surface, it is considered that the stress applied to the ab surface is small, so that it can withstand repeated stress due to the thermal cycle.
[0016]
Using a YBa ₂ Cu ₃ O _x -based superconducting bulk produced by the melting method, a current lead having a form different from that described above was produced. FIG. 3 shows a prototype current lead. In the bulk material 5 used, the Y ₂ BaCuO ₅ phase has an average size of 2 μm or less and a volume ratio of about 30%, but the matrix has an oriented YBa ₂ Cu ₃ O _x superconductivity without a large grain boundary. It is a single crystal material in phase. The YBa ₂ Cu ₃ O _x bulk was prepared in two types of materials having a length (current direction) of 45 mm, a width of 4 mm, and a thickness of 3 mm. In both types, the crystal orientation is the ab plane in the length direction. This material is connected to the copper electrode 6 by using all the four faces around the end face perpendicular to the length direction and the length of 5 mm, but the surrounding four crystal faces are different from each other. The current lead F has a 4 mm width side on the ab surface and the 3 mm width portion is a surface orthogonal to the ab surface, while the current lead G has a surface that forms 45 degrees with the ab surface and is copper. It is connected to the electrode. These materials are coated with about 1 μm of silver by sputtering, and are connected to a copper electrode by solder. Moreover, as shown in FIG. 3, it is fixed and reinforced from the periphery by a glass fiber reinforced plastic pipe 7 called G10.
[0017]
These current leads were able to maintain a superconducting state even when a current of 200 A was applied at a liquid nitrogen temperature (77 K). The operation of returning the current lead to room temperature and cooling it again to the liquid nitrogen temperature was repeated. At this time, the electric resistance generated at both ends of the current lead was on the order of 10 ⁻⁴ ohm. This is almost the electrical resistance of copper. Of these, when the current lead F was subjected to 10 thermal cycles, the electrical resistance at both ends became greater than kilohms, making it impossible to energize. When the inside was observed, peeling breakage occurred from the ab surface near the electrode part on one side. On the other hand, the current lead G was able to pass a current of 200 A even after 100 thermal cycles, and no abnormality in electrical resistance was observed.
[0018]
The reason why the electrode part of the current lead F is damaged is due to the difference in thermal expansion coefficient between the copper of the electrode part and the YBa ₂ Cu ₃ O _x superconducting bulk and between the glass fiber reinforced plastic and the YBa ₂ Cu ₃ O _x superconducting bulk. This is probably because stress was generated in the vicinity of the electrode portion, and cleavage cracks were generated and expanded on the ab surface having low strength. On the other hand, when the electrode bonding surface is cut out at 45 degrees from the ab surface, it is considered that the stress applied to the ab surface is small, so that it can withstand repeated stress due to the thermal cycle.
[0019]
【The invention's effect】
When a REBa ₂ Cu ₃ O _x bulk is used as a conductor, the surface to which the metal is connected is limited to a surface close to a plane that preferably forms 45 degrees other than the ab surface of the REBa ₂ Cu ₃ O _x bulk according to the present invention. This makes it possible to produce an oxide current lead that is resistant to thermal cycling. This material can be applied to current leads and permanent current switches.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an example of a current lead by an oxide superconductor. FIG. 2 is a diagram showing a YBa ₂ Cu ₃ O _x bulk that has undergone delamination. FIG. 3 is an example of a current lead by an oxide superconductor. Schematic shown 【Explanation of symbols】
1 YBa ₂ Cu ₃ O _x bulk 2 Copper electrode 3 Glass fiber reinforced plastic reinforcement 4 c-axis direction 5 YBa ₂ Cu ₃ O _x bulk 6 Copper electrode 7 Glass fiber reinforced plastic pipe

Claims (2)

REBa ₂ Cu ₃ O _x (RE is one or more elements selected from Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The metal electrodes are connected to both ends of the superconducting bulk body of the crystal-oriented polygonal column represented by), the energization direction is parallel to the ab surface of the superconducting bulk body, and An oxide superconducting conductor characterized in that it is connected to a metal electrode at a surface that forms 20 ° to 70 °. 2. The oxide superconducting conductor according to claim 1, wherein the polygonal column is a quadrangular column and is connected to the metal electrode at four sides around the quadrangular column forming 45 degrees with the ab plane.

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