JPH05251762A - Oxide superconductive current limiting conductor - Google Patents

Abstract

PURPOSE:To prevent the current limiting effect and prevent the drop on standing of stabilized performance extending for a longer period by providing a silver layer on the oxide superconductive film provided on a base material, and providing a gold layer hereon. CONSTITUTION:A Y (YBa2Cu3O7-x) oxide superconductive film in is made, using an MGO substrate as a base material and next, using a metal of each ingredient as a starting material. A silver layer, hereon, and next, a gold layer are stacked hereon. Hereby, at current application of the superconductive current limiting conductor, a current is shunt for the silver layer and the gold layer, and besides the heat is conducted, and also the metal prevents the deterioration by oxidation with the passage of time, especially, of the silver layer and performs the function of suppressing the drop on standing of the stabilized performance. The property to show the change on standing of the voltage V between critical terminals is practically constant with the one 1 where the gold layer is stacked on the silver layer, as against dropping to approximately 80% of the first after six months with the one which has the stabilized film of only silver.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide superconducting current limiter for use in a conductor which conducts a current using an oxide superconductor and a current limiting element capable of limiting an excessive current such as a short circuit current. It is about conductors.

[0002]

2. Description of the Related Art In a superconductor, when any one of the critical temperature, the critical current, and the critical magnetic field exceeds the value, the superconducting state is destroyed, and the superconducting state is established and the electric resistance is increased. In recent years, research and development on superconducting current limiting conductors (elements) that utilize this phenomenon to limit an excessive current have become active. The technology of this entire field is described in, for example, a publication {Journal of the Institute of Electrical Engineers of Japan, Volume 110, No. 9, p. 705 (1990)}. Among these, superconducting current limiting conductors utilizing oxide superconductors have recently received attention. The structure of the element used for this is a simple structure in which a pair of electrodes are formed by depositing a metal such as silver, gold or indium on a part of the superconductor by using vapor deposition or paste on the rod-shaped or plate-shaped superconductor. It is something. As a typical example, for example, a publication {Advances in Sup
erconductivity, page 691 (198
8)}. This uses a sintered body of Y-Ba-Cu-based oxide superconducting powder to form a zigzag-like (meaner structure) sheet by the doctor blade method, in a normal conducting state when a current exceeding a critical current flows. The current is limited by utilizing the high resistance. However, such a sintered body (one having a linear shape, a rod shape, or a plate shape) generally has a drawback that the critical current cannot be increased, and thus the short-circuit current can be limited only in a small proportion.

Therefore, a current limiting conductor to which a film-shaped superconductor formed on a substrate is applied has been considered. Due to the progress of film forming technology, it has recently become possible to obtain an oxide superconducting thin film capable of passing a very large critical current. Further, the film-shaped one has advantages as a current limiting conductor such that it has a better cooling characteristic than the above-mentioned one such as a linear one, and can easily have a high resistance. However, the oxide superconducting thin film has a large critical current over a large area because the composition shift tends to occur when the film thickness is increased and the composition becomes inhomogeneous when formed in a large area. It was unavoidable to give rise to the major drawback of difficulty in obtaining things. In particular, the occurrence of inhomogeneity in film quality such as composition when a film is formed in a relatively large area is an unavoidable problem in the current superconducting thin film manufacturing technology, and therefore a weak portion is melted during energization, It was not possible to increase the limit terminal voltage of the current limiting conductor (equivalent to the input power or circuit voltage until the fuse is cut). In order to solve this drawback, it is easy to use the conventional technique as described in, for example, a publication {edited by the Physical Society of Japan, Superconductivity, p. 276, 1979 (Maruzen Co., Ltd.)} on the superconducting film of the element part. It is considered that a kind of thermal and electrical stabilization is performed by coating a metal thin film by a method such as vapor deposition.

[0004]

When an oxide superconducting film is used for stabilizing by applying the above-mentioned conventional technique, silver is most suitable as a stabilizing metal film, and thereby, the limit terminal voltage is reduced. We have found that it can be dramatically increased. However, it has been found that when silver is used as the stabilizing film, the terminal voltage to the device decreases over time after long-term use. As a result of investigating the cause, it was found that the surface stabilization of the silver stabilizing film occurred over time and the stabilization performance was deteriorated by this.

The present invention has been made in order to solve the above problems and has a large current limiting effect, and in particular, an oxide superconducting current limiting conductor in which the deterioration of the stabilizing performance over time is prevented. The purpose is to get.

[0006]

The oxide superconducting current limiting conductor of the present invention is a substrate, an oxide superconducting film provided on the substrate,
The oxide superconducting film has a silver layer and a gold layer provided on the silver layer.

[0007]

In the present invention, since the silver layer and the gold layer are provided on the oxide superconducting film, when the superconducting current limiting conductor is energized, the current is shunted to the silver layer and the gold layer, and heat is transmitted. The gold layer prevents oxidative deterioration of the silver layer over time, and serves to suppress deterioration of the stabilizing performance over time.

[0008]

EXAMPLES Example 1. A rectangular MgO substrate having a width of 5 mm and a length of 20 mm is used as a base material, and then each component metal is used as a starting material, and a Y-based (YBa ₂
Cu ₃ O _7-X) was formed of the oxide superconductor film. At this time, oxygen gas was introduced to adjust the gas pressure to 1.5 × 10 ⁻⁴ Tor.
The temperature was adjusted to r, and after film formation, the film was gradually cooled to room temperature in this atmosphere. Thereafter, a silver layer and then a gold layer were laminated thereon by vacuum evaporation under a vacuum of 1.8 × 10 ⁻⁶ Torr to manufacture an oxide superconducting current limiting conductor according to an embodiment of the present invention. Then, cover the central part of the superconducting film with an insulating tape,
Both ends were electrolessly plated with gold to form terminals, and lead wires were attached thereto. The critical temperature of the superconducting layer is 86K,
The thickness of the superconducting layer, silver layer and gold layer is about 4000 each.
It was Å, 300 Å and 200 Å.

Comparative Example 1. Using the same electron beam evaporation method as in Example 1 and the same kind of substrate, film formation was carried out in an oxygen stream having the same composition and the same gas pressure as in Example 1 above, and the same Y-system of about 4000 Å as above was formed. After forming the superconducting film, a 300 Å silver layer was laminated for stabilization to obtain a conventional superconducting current limiting conductor. The critical temperature of the superconducting film in the conventional superconducting current limiting conductor thus obtained was 86K, which was the same as that of Example 1.

The current limiting elements of Example 1 and Comparative Example 1 were each placed in liquid nitrogen for cooling, and an AC circuit (frequency of 50 Hz) was used to change the terminal voltage applied to the elements to melt the elements. The limit voltage between terminals was investigated. As a result, the one of the embodiment of the present invention was the same at 75V and the conventional one not covering the gold layer was the same at 75V. In the case of the above-mentioned limit terminal voltage, the estimated short circuit current 180A of the used circuit could be greatly limited to 2A which is about 1/9 of both elements. Next, these devices were left in the air and the limit terminal voltage was measured every month to examine the change over time, and the results are shown in FIG. FIG. 1 is a characteristic diagram comparing the present invention with a conventional example and showing a change with time of a limit terminal voltage, wherein 1 is a characteristic of Example 1 and 2 is a characteristic of Comparative Example 1. From FIG. 1, the conventional one having the stabilizing film of only silver dropped to about 80% of the initial limit terminal voltage after 6 months. On the other hand, in Example 1, the value of the limit terminal voltage was almost constant even after 6 months. This revealed the stability of the current limiting conductor of one embodiment of the present invention over time.

Embodiment 2. Using a rectangular SrTiO ₃ substrate having the same size as in Example 1 as a base material, a YBa ₂ Cu ₃ O _7-X oxide superconducting layer was formed by the sputtering method. At this time, oxygen gas was introduced to adjust the gas pressure to 1.1 × 1.
It was adjusted to 0 ⁻⁴ Torr, and after film formation, it was gradually cooled to room temperature in this atmosphere. 1.8 × 10 ^-6 Tor on the superconducting layer
A silver layer and then a gold layer were laminated by sputtering in a vacuum of r to prepare an oxide superconducting current limiting conductor of another example of the present invention. Next, both ends were electrolessly plated with gold to form terminals, and lead wires were attached to the terminals to obtain oxide superconducting current limiting devices. The critical temperature of the superconducting film is 88
The thickness of each of K, superconducting layer, silver layer and gold layer is about 3
They were 000Å, 200Å and 200Å.

Comparative Example 2. Using the same sputtering method and the same kind of base material as in Example 2, film formation was performed in the oxygen gas flow of the same composition and the same gas pressure as in Example 2 above, and about 3
After forming a 000 Å Y-based superconducting film, a 200 Å silver layer was laminated for stabilization to obtain a conventional superconducting current limiting conductor. The critical temperature of the superconducting film in the conventional superconducting current limiting conductor thus obtained was 88K, which was the same as that of Example 2.

The current limiting devices of Example 2 and Comparative Example 2 were each put into liquid nitrogen to be cooled, and an AC circuit (frequency of 50 Hz) was used to change the terminal voltage applied to the devices to melt the devices. The limit voltage between terminals was investigated. As a result, the voltage of Example 2 was 71 V, and the voltage of Comparative Example 2 not covering the gold layer was 71 V, which was the same. In the case of the above-mentioned limit terminal voltage, the estimated short circuit current 180 of the used circuit
It was possible to greatly limit A to 1.5 A, which is about 1/10 or less of both elements. Next, the cycle of soaking these devices in liquid nitrogen for 12 hours a day and leaving them in the air for the next 12 hours is repeated for 6 months, and the limit terminal voltage is measured every month during the cycle. Then, the change with time was investigated. The results are shown in Figure 2. FIG. 2 is a characteristic diagram comparing the present invention with a conventional example and showing a change with time of the limit terminal voltage, in which 3 is the characteristic of Example 2 and 4 is the characteristic of Comparative Example 2. It can be seen from FIG. 2 that the conventional one having the stabilizing film containing only silver dropped to about 70% of the initial limit terminal voltage after 6 months. On the other hand, in Example 2, the value of the limit inter-terminal voltage was almost constant even after 6 months. This has revealed the stability of the current limiting conductor of another embodiment of the present invention over time.

In the method of forming a film-shaped current limiting conductor according to the present invention, so-called physical vapor deposition method, chemical vapor deposition method, electrodeposition method utilizing electrophoresis, etc. are used, and after forming an oxide superconducting film, silver is formed. A layer is formed, and a gold layer is formed on the layer to form a laminated structure. By the way, as the base material according to the present invention, Al ₂ O ₃ , SiO ₂ , MgO, SrTiO ₃ and Zr are used.
Ceramics such as O ₂ and Y ₂ O ₃ and single crystals, metals such as gold, silver, stainless steel, copper and nickel and alloys thereof can be used. In the present invention, after forming the superconducting film, it is necessary to stack a silver layer and a gold layer of arbitrary thickness thereon, but this method is the same as the case of forming the superconducting film as shown in the examples. It is optimal to use the method of. However, the present invention is not limited to this, and the layers may be stacked by a film forming method different from the method of forming the superconducting layer. The thicknesses of the superconducting film, silver layer and gold layer are not limited and can be arbitrarily selected. According to experiments, a suitable thickness of the superconducting film is about 1000 to 8000Å, and a suitable thickness of the silver layer and the gold layer is 100 to 1000Å.

When the oxide superconducting film is formed on the substrate in the present invention, oxygen gas is generally introduced to form the film. An arbitrary value may be used as the oxygen gas pressure because it greatly differs depending on the method used. For example, in the CVD method, it is generally necessary to have a high gas pressure, and in the sputtering method and the vapor deposition method, the gas pressure is usually low (the degree of vacuum is high).

As a method of forming a metal coating to be an electrode terminal after forming the oxide superconductor current-limiting conductor of the present invention, any one of a vapor phase film forming method, an electroless plating method, an electrolytic plating method and a pressure bonding method can be used. You may use. Silver and gold are preferable for the electrode terminals.

By the way, in the embodiment, the Y-based material is used as an example of the oxide-based superconducting material, but the effect of the present invention is Y.
It was confirmed by the same examination as in the example that it is not limited to the system but is expressed in any oxide superconducting material such as Bi type, Tl type, Nd type and the like. Further, in the above embodiment, the case where the current limiting conductor is used for the current limiting element has been described, but the present invention is not limited to this. It goes without saying that there is no power loss and the current can be limited when an excessive current flows.

[0018]

As described above, the present invention comprises a substrate, an oxide superconducting film provided on the substrate, a silver layer provided on the oxide superconducting film, and a gold layer provided on the silver layer. By using such a material, it is possible to obtain an oxide superconducting current limiting conductor having a large current limiting effect, and in particular, a deterioration of the stabilizing performance with time is prevented for a longer period of time.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing a change with time of a limit terminal voltage when comparing the present invention with a conventional example.

FIG. 2 is a characteristic diagram showing a change with time of a limit terminal voltage when comparing the present invention with a conventional example.

[Explanation of symbols]

 1 Characteristics of Example 1 2 Characteristics of Comparative Example 1 3 Characteristics of Example 2 4 Characteristics of Comparative Example 2

 ─────────────────────────────────────────────────── ─── 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. An oxide superconducting current limiting conductor comprising a substrate, an oxide superconducting film provided on the substrate, a silver layer provided on the oxide superconducting film, and a gold layer provided on the silver layer.