JP3180856B2 - Superconducting critical current measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring a current-carrying characteristic of a superconducting material at a low temperature.

[0002]

2. Description of the Related Art In the development of practical materials for superconductors,
It is essential to improve the critical current density and the critical current. Needless to say, accurate evaluation test methods are important for this purpose, and a wide variety of evaluation test methods have been devised.

[0003] Superconductors are used after being cooled to below their critical temperature by a cryogenic liquefied gas such as liquefied helium or liquefied nitrogen. Therefore, in the evaluation tests of the critical current density and the critical current, it is almost always required to evaluate by applying a current while being immersed in liquefied helium or liquefied nitrogen.

FIG. 1 is a sectional view showing an example of a conventional superconducting critical current measuring device. It has a multilayer structure including a liquefied nitrogen reservoir 22 for heat shield inside a cryostat 21 vacuum-insulated by a vacuum pump 28 and a liquefied helium container 23 inside. Liquid nitrogen reservoir for heat shield 2
In 2, the liquefied nitrogen is replenished as the nitrogen evaporates to prevent heat from entering the internal liquefied helium container 23. The test piece 29 is immersed in liquefied helium and energized through the gas cooling current lead 24. The evaporated helium is discharged through the gas cooling current lead 24 while cooling the current lead. As the helium evaporates, liquefied helium is supplied from the liquefied helium storage container 27 to the liquefied helium container 23 through the transfer tube 26 and the injection tube 25.

[0005] As described above, the conventional superconducting critical current measuring device uses a vacuum heat insulating device to cut off heat conduction with the outside, and obtains a stable cryogenic temperature by utilizing the heat of vaporization of the liquefied gas.
Liquefied gas such as liquefied helium and liquefied nitrogen is packed in a high-pressure container and involves danger. Therefore, it is handled under safety management in accordance with the High Pressure Gas Control Law.

At temperatures other than the liquefied nitrogen temperature (77K) and the liquefied helium temperature (4.2K), measurement at 4K or less using superfluid helium under reduced pressure, and measurement in combustible liquefied gas such as liquefied hydrogen and liquefied oxygen Or in a low-temperature gas atmosphere.

[0007]

As described above, conventionally, in an evaluation test of a critical current density and a critical current of a superconductor, a liquefied gas is used to obtain an extremely low temperature state. However, liquefied gases such as liquefied helium and liquefied nitrogen, which are widely used in general for superconductors, require a high-pressure container or a vacuum heat insulating device, and their handling is complicated and sometimes dangerous. In addition, it is necessary to perform safety management in accordance with the High Pressure Gas Control Law.

In addition to liquefied nitrogen and helium, measurements under 4K or less using superfluid helium, measurements in combustible liquefied gases such as liquefied hydrogen and liquefied oxygen, and measurements in a low-temperature gas atmosphere are performed. I have. These methods are more complicated to handle than liquid nitrogen or liquid helium, require complicated equipment, and in some cases involve greater danger. In addition, since these methods are inferior in constant temperature performance, when a large current is applied to a cooled test piece, the temperature rises, and it is extremely difficult to obtain stable measurement conditions.

[0009] It is an object of the present invention to provide an apparatus which eliminates the complexity of handling a liquefied gas in a superconducting critical current measurement, and which can easily and stably measure a large current carrying characteristic at an arbitrary temperature. .

[0010]

In order to solve the above-mentioned problems, a superconducting critical current measuring apparatus of the present invention uses a GM refrigerator or the like without using a liquefied gas such as liquefied nitrogen or liquefied helium. Is cooled to a desired temperature in a vacuum, and a critical current is measured by energization.

A GM refrigerator constituting an example of the superconducting critical current measuring device of the present invention will be described with reference to the drawings. FIG.
Is a schematic sectional view showing the structure of a general GM refrigerator, which is a regenerative small refrigerator using helium gas as a refrigerant. The low-pressure helium gas is compressed by the compressor 20 to become high-pressure helium gas.
Into the gas passage 30. The valve plate 41 rotates in response to the drive of the motor 35 from the crankshaft 16, and repeatedly connects and disconnects the gas passage 30 and the gas passage 40. The helium gas that has entered the gas passage 40 is sent into the cylinder upper chamber 18.

The first stage displacer 32 and the second stage displacer 33 move up and down by the driving force of a motor 35 via a crankshaft 16 and a scotch yoke mechanism 34. The helium gas in the cylinder upper chamber 18 passes through the cold storage material 36 in the first-stage displacer 32, adiabatically expands in the first-stage cold generation section 42, and further expands in the second-stage displacer 33.
Cold storage is stored in the cold storage materials 36 and 37 by passing through the cold storage material 37 inside and performing adiabatic expansion in the second stage cold generation zone 43.
The helium gas then returns from the gas passage 40 to the compressor 20 through the Scotch yoke room 44 and the motor room 45, and circulates.

Hereinafter, the apparatus of the present invention will be described in detail with reference to the drawings. FIG. 3 is a cross-sectional view showing one example of the superconducting critical current measuring device of the present invention using a GM refrigerator. As shown in the figure, a test piece 9 is attached to a second cooling stage 3 of the GM refrigerator 1, which is sealed with a vacuum vessel 4 and a heat radiation shield plate 5, and the test piece 9 is transmitted by the second cooling stage 3. Is cooled so that an electricity test can be performed.
The copper terminal 8 attached to the test piece 9 and the second cooling stage 3 obtain electrical insulation while maintaining thermal contact with the insulating thermal anchor 6 therebetween. Energization of the test piece 9 is performed through the oxide superconducting bulk current lead 7 through the copper terminal 8.
This is performed using Copper lead wire 10 from the room temperature portion to the copper terminal 8 of the oxide superconducting bulk current lead 7, and copper lead wire 11 from the oxide superconducting bulk current lead 7 to the copper terminal of the test piece.
Is supplied with electricity.

At this time, the oxide superconducting bulk current lead 7
Is cooled to a temperature lower than its critical temperature by bringing one end into thermal contact with the first cooling stage 2 and bringing the other end into thermal contact with the second cooling stage 3. An insulating thermal anchor 6 is interposed between the first cooling stage 2, the second cooling stage 3, and the copper terminal 8 of the oxide superconducting bulk current lead 7 to maintain thermal contact and to provide electrical insulation. Have gained.

Usually, the cooling stage of the refrigerator is made of a metal having excellent thermal conductivity such as copper, but is mounted on a copper terminal and a test piece mounted on the oxide superconductor via an insulating heat anchor. It is electrically insulated from the copper terminals and maintains thermal contact. In this way, by supplying a current to the sample through the current lead made of the oxide superconductor that is cooled stably below the critical temperature, the current is supplied without increasing the temperature of the test piece on the cooling stage. I can do it.

Similarly, the test piece can obtain stable cryogenic measurement conditions by interposing an insulating heat anchor between a copper terminal mounted as an electrode at both ends and a metal cooling stage. As the insulating heat anchor, a ceramic plate made of aluminum nitride or the like having high thermal conductivity and excellent electrical insulation is suitable. The apparatus of the present invention can be configured using a conventional refrigerator with a ceramic plate according to the shape and the like of the test piece and the electrode attached to the oxide superconductor.

FIG. 4 is a sectional view showing another example of the superconducting critical current measuring apparatus of the present invention using a GM refrigerator. It is hermetically sealed by a vacuum vessel 4 and a heat radiation shield plate 5, and has copper lead wires 10-copper terminals 8-oxide superconducting bulk current leads 7.
-The point that current is supplied from the room temperature portion via the copper terminal 8 is the same as that of the above-described device.

As shown in this example, between the copper terminal 8 of the oxide superconducting bulk current lead 7 and the copper terminal 8 of the test piece 9,
It is also possible to connect with a high-temperature superconducting lead wire 14.
By using a high-temperature superconducting wire for the lead wire between the low-temperature end of the superconducting current lead and the terminals provided at both ends of the test piece, the temperature rise of the second cooling stage during energization can be further reduced.

A heater 15 for controlling the temperature can be provided below the cooling stage on which the test piece 9 is mounted. If the temperature of the cooling stage is controlled by installing a heater or the like as a heat source below the cooling stage of the refrigerator,
It becomes possible to measure the current-carrying characteristics at any temperature below the critical temperature of the superconducting current lead.

In the example of FIG. 4, the first cooling stage 12 and the second cooling stage 13 of the refrigerator are made of a ceramic material having good heat conductivity as used for the above-mentioned insulating heat anchor. Since there is no need to interpose an insulating heat anchor, electrical insulation and thermal contact can be simultaneously realized with a simple structure, and stable measurement can be performed. Current leads and test pieces can be attached directly to the ceramic cooling stage. Since the thermal anchor can be freely set at an arbitrary position on the cooling stage, the test piece may be mounted on the cooling stage as it is even when the size or shape of the test piece changes.

In the superconducting critical current measuring apparatus of the present invention, a refrigerator used for cooling the test piece may be a GM refrigerator, a Stirling refrigerator, or other types of refrigerators. The material of the oxide superconducting current lead used in the present invention may be a material other than the well-known Bi-based superconductor and Y-based superconductor. The electrodes at both ends of the test piece and the current lead may be made of a material other than copper as long as the metal has high electric conductivity.

[0022]

As described above, in the present invention, a test piece is brought into thermal contact with a cooling stage of a refrigerator to obtain a cryogenic state in a measurement test of a critical current value of a superconducting conductor. The test piece is placed in thermal contact with the cooling stage of the refrigerator by interposing a good insulator with high thermal conductivity.
The cooling stage of the refrigerator is directly installed by using a good insulator with high thermal conductivity. A high-temperature superconducting wire is used for a current lead in a low-temperature part for energizing the test piece, thereby suppressing Joule heat and intrusion heat during energization, and keeping the temperature of the cooling stage constant. A refrigerant such as liquefied helium or liquefied nitrogen is not required, and a vacuum heat insulating device having a complicated structure is not required. The complexity of safety management and handling is eliminated, and in particular, the use of combustible liquefied gas can be avoided, thereby reducing the risk associated with measurement. When the cooling stage is made of ceramics, the test piece can be easily installed and the thermal anchor can be set at any position.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a conventional superconducting critical current measuring device.

FIG. 2 is a schematic sectional view showing the structure of a general GM refrigerator.

FIG. 3 is a cross-sectional view showing one example of a superconducting critical current measuring device of the present invention.

FIG. 4 is a sectional view showing another example of the superconducting critical current measuring device of the present invention.

[Explanation of symbols]

 Reference Signs List 1 GM refrigerator 2 First cooling stage 3 Second cooling stage 4 Vacuum container 5 Heat radiation shield plate 6 Insulating heat anchor 7 Oxide superconducting bulk current lead 8 Copper terminal 9 Test piece 10 Copper lead 11 Copper lead 12 First cooling stage 13 Second cooling stage 14 High-temperature superconducting lead wire 15 Heater 16 Crankshaft 17 Housing 18 Upper cylinder chamber 20 Compressor 21 Cryostat 22 Liquid nitrogen reservoir for heat shield 23 Liquid helium container 24 Gas cooling current lead 25 Injection Tube 26 transfer tube 27 liquefied helium storage container 28 vacuum pump 29 test piece 30 gas passage 31 valve 32 first stage displacer 33 second stage displacer 34 scotch yoke mechanism 35 motor 36 cold storage material 37 cold storage material 40 gas passage 41 valve Plate 42 first-stage cold generating Ward 43 second stage cold generating Ward 44 scotch yoke room 45 motor room

 ──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-258103 (JP, A) JP-A-57-86062 (JP, A) JP-A-2-256206 (JP, A) JP-A-1- 50483 (JP, A) JP-A-4-298246 (JP, A) JP-A-4-107805 (JP, A)

Claims (2)

(57) [Claims] An apparatus for measuring a critical current value by placing a superconducting conductor test piece on a cooling stage, cooling the same, and applying a current to the cooling stage, comprising: (1) a refrigerator having at least two stages of cooling stages; (2) an oxide superconductor lead that guides current to the superconducting conductor test piece, and (3) an end electrode of the superconducting test piece and an end electrode of the oxide superconductor lead, between the cooling stage. A superconducting critical current measuring device, comprising: an electrically insulating member having good thermal conductivity under cryogenic temperature for interposing thermal contact. 2. An apparatus for measuring a critical current value by placing a superconducting conductor test piece on a cooling stage for cooling and applying a current, comprising: (1) a refrigerator having at least two stages of cooling stages; (2) an oxide superconductor lead that guides current to the superconducting conductor test piece, and (3) the cooling stage is formed of a good heat conductive electrical insulating member at a very low temperature, and an end electrode of the superconducting test piece. A superconducting critical current measuring device, comprising: a structure in which an end electrode of the oxide superconductor lead is in thermal contact with the cooling stage.