JPH05251761A - Current limiting conductor using oxide superconductive film - Google Patents

Abstract

PURPOSE:To raise limit voltage for application to an element without lowering the current limiting performance by forming a lower metallic film on an oxide superconductive film, and forming an upper metallic film high in resistivity than the lower metallic film on this. CONSTITUTION:An oxide superconductive film 6 is made on a substrate 1, and a metallic film 2 is made hereon, and further an upper metallic 3 is made. This upper metallic 3 is a metallic film higher than the lower metallic film 2. Hereby, the stable operation of a current limiting conductor becomes possible without causing the remarkable drop in normal conductive condition. That is, to begin with, the decrease of the resistance in normal conductivity state is minimized by laying most of the role of a shunt current on the upper metallic film 3 high in resistivity. And, for the lightening of the partial heat generation of the superconductive film and the smooth shunting to the metallic film 3 high in resistivity, a lower metallic film 2 to serve as a well electric conductive thermal conductor is made between them.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a current limiting conductor capable of limiting an excessive current such as a short circuit current by using an oxide superconductor.

[0002]

2. Description of the Related Art A current limiting device using metallic sodium has been put into practical use as an element for suppressing an excessive current due to a short circuit in a power circuit. However, this fault current limiter has problems in terms of characteristics, size, life, etc., and for the purpose of miniaturization and high performance, the change in resistance due to superconducting-normal conducting transition above the critical current of the superconductor is required. The current limiting device used is also being researched. In the case of a fault current limiter using this superconductor, the current that can flow in the superconducting state (critical current)
Is required to be high and the resistance in the normal conducting state is high. In particular, when a film-shaped superconductor is used, it is easy to obtain a material having relatively good current limiting characteristics because the critical current and the resistance in the normal conduction state are large. For the technology of this field in general, see, for example, literature (low temperature engineering, Vol.
36 (1991)).

Generally, a current limiting conductor using an oxide superconducting film is used by forming an oxide superconducting film in a desired shape on a ceramic substrate, attaching electrodes and then impregnating it in liquid nitrogen. However, in this case, since the oxide superconductor has poor thermal conductivity, the heat generation due to the transition to the normal conduction accompanying the current limiting operation is likely to be localized, and this heat generation may cause the current limiting conductor to melt. In order to solve this problem, it is possible to form a metal film on the superconducting film and simultaneously divide the current at the time of transition to normal conduction and improve the heat dissipation property. This technique is called stabilization of superconductivity, and examples of stabilization in metal-based superconducting tape wire rods are described in, for example, the literature ("Superconductivity" edited by the Physical Society of Japan, p. 276 (1979)).
However, when such stabilization is carried out, the resistance in the normal conducting state is remarkably lowered, so that the continuous current flowing after the current limiting operation increases, which is not preferable for use as a current limiting conductor.

[0004]

As described above, in order to stably use an oxide superconducting film as a current limiting conductor without fusing, stabilization with a metal is effective.
There is a problem that the current limiting performance is deteriorated because the resistance in the normal conduction state is significantly reduced. In addition, if the film thickness is reduced in order to suppress the decrease in resistance, the current capacity that can be shunted is reduced. On the other hand, when a metal film having a high specific resistance is used, the connection resistance at the interface with the superconducting film is likely to be high, and shunting cannot be performed properly. As a result, good stabilization is not performed, so that the element application limit voltage, which is one of the indexes showing the performance of the current limiting conductor, becomes low, and a current limiting conductor with good characteristics cannot be obtained.

The present invention has been made to solve the above problems, and an object thereof is to obtain an oxide superconducting film current limiting conductor having a high element application limit voltage without deteriorating the current limiting performance. .

[0006]

A current limiting conductor using an oxide superconducting film according to the present invention is a lower metal film formed on an oxide superconducting film, and a lower metal film formed on this lower metal film. The upper metal film having a higher specific resistance is provided.

[0007]

In the present invention, a metal film is formed on the superconducting film to form a lower metal film, and a metal film having a higher specific resistance than the above metal film is formed on the upper metal film as an upper metal film. Stable operation of the current limiting conductor is possible without causing a significant decrease in resistance in the normal conducting state.
That is, first, most of the role of shunting the current is imposed on the upper metal film having a high specific resistance, so that the decrease in the resistance in the normal conducting state is minimized. However, this alone cannot alleviate the local heat generation of the superconducting film and smooth the shunting to the metal film having a high specific resistance, so that a lower metal film serving as a good electrically and thermally conductive medium is formed therebetween.

[0008]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a current limiting conductor using an oxide superconducting film according to an embodiment of the present invention. In the figure, 1 is a substrate, 2 is a lower metal film, 3 is an upper metal film, 4 is a current lead, and 5 is indium solder. Further, FIG. 2 is an enlarged cross section near the electrode portion in FIG. 1, and 6 is an oxide superconducting film. It should be noted that the use of only the lower metal film 2 for connecting the current leads 4 enables better electrode connection with a small connection resistance at the interface with the superconducting film 6, which is better, but the upper metal film 3 is present. Good. As a result, a current limiting conductor having a high element application limit voltage can be realized without increasing the follow current after the current limiting operation.

The current limiting conductor using the above oxide superconducting film is manufactured, for example, as follows. A single crystal strontium titanate having a thickness of 1 mm and a size of 10 mm was used as a substrate 1, and a sputtering method was applied to the substrate 1 at a substrate temperature of 650 ° C. and a pressure of 50 mTorr to form a c-axis-oriented Y-based oxidation film having a film thickness of 0.3 μm. The superconducting film 6 was formed. Further, a platinum mask was used during film formation, and the superconducting film 6 was formed into a pattern having a width of 1 mm and a length of 30 mm. The critical temperature was 91 K, the critical current was 8 A, and the resistance of only the superconducting film 6 immediately after transition to normal conduction was 60Ω. Next, using the same mask, the substrate 1 was not heated, and silver having a film thickness of 100 angstrom was formed as the lower metal film 2 by sputtering in high-purity argon. Subsequently, a mask covering the electrode connection portion was added, the sputtering atmosphere gas was changed to an argon gas containing 20% oxygen and 50% nitrogen, and silver having a film thickness of 400 Å was formed as the upper metal film 3. This film becomes a metal film having a high specific resistance. The resistance of these metal films 2 and 3 in liquid nitrogen was 200Ω for the lower metal film 2 and 18 for the upper metal film 3 based on the measurement results of single film formation with the same shape.
It was 0Ω. The combined resistance of the stabilizing metal film is 95Ω. When the current limiting conductor thus produced was impregnated in liquid nitrogen and a current limiting operation was performed, a stable current limiting operation was possible up to a voltage between elements of 60 V (element application limit voltage = 6).
0V). At this time, the follow current was 1.6 A, which was a sufficiently low value. For comparison, only the lower metal film 2 and the upper metal film 3
The device application limit voltages were compared for those formed with only the stabilizing film as the stabilizing film so that the resistance of the stabilizing film portion was 95Ω. The results are shown in Table 1. From this table,
It can be seen that the current limiting conductor of the present invention has the highest element application limit voltage and is obviously excellent.

[0010]

[Table 1]

The lower metal film 2 used in the current limiting conductor of the present invention can be made of many metals such as gold, platinum and copper in addition to the silver described in the above embodiment. Any material such as a vacuum vapor deposition method or a CVD method can be used as long as a general metal film can be formed, but it is desirable that the specific resistance be 4 × 10 ⁻⁶ Ωcm or less.

Further, in addition to silver described in the embodiment as a metal film having a high specific resistance used for the upper metal film 3, another metal may be used as long as the specific resistance is lower than the lower metal film 2. As the film forming method, various methods like the lower metal film 2 are possible.

As described in the examples, the specific resistance of the sputtered film formed in a mixed gas of oxygen, nitrogen and argon is increased by proper oxidation of the film and segregation of impurities at the grain boundaries of the film. It seems to be due to. The resistance value after forming these metal films as a stabilizing film is not limited to the values shown in the examples,
It is adjusted by appropriately changing the film thickness according to each specific resistance, but the resistance of the stabilizing film as a whole may be the same as or close to the resistance of the superconducting film 6 after transition to normal conduction. desirable.

[0014]

As described above, according to the present invention, the lower metal film formed on the oxide superconducting film and the upper metal film formed on the lower metal film and having a higher specific resistance than the lower metal film are formed. Since it is provided, there is an effect that an oxide superconducting film current limiting conductor with a high element application limit voltage can be realized without increasing the follow current and lowering the current limiting performance.

[Brief description of drawings]

FIG. 1 is a perspective view showing a current limiting conductor using an oxide superconducting film according to an embodiment of the present invention.

2 is an enlarged sectional view showing an electrode portion in FIG. 1. FIG.

[Explanation of symbols]

 1 Substrate 2 Lower Metal Film 3 Upper Metal Film 4 Current Lead 5 Indium Solder 6 Oxide Superconducting Film

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Sadajiro Mori 8-1-1 Tsukaguchihonmachi, Amagasaki City Mitsubishi Electric Corporation Central Research Institute (72) Inventor Tatsuya Hayashi 8-1-1 Tsukaguchihonmachi, Amagasaki Mitsubishi Electric Central Research Institute Co., Ltd.

Claims (1)

[Claims] 1. A current limiting conductor using an oxide superconducting film comprising a lower metal film formed on the oxide superconducting film and an upper metal film formed on the lower metal film and having a higher specific resistance than the lower metal film. .