JPH03236606A - Waveguide and resonator - Google Patents

Abstract

PURPOSE:To obtain a resonator with a large Q by using a thin film or bulk made of an oxide superconducting material as a wall member and coating the surface of the superconducting material with a metallic film of a specific thickness. CONSTITUTION:A thin film or bulk made of an oxide superconducting material having a critical temperature of a liquid nitrogen temperature or over is used as a wall member and the surface of the oxide superconducting material of the inner face of the wall is coated by a metallic film whose thickness is 10-1000nm to form a waveguide and a resonator. Moreover, as the oxide superconducting material, it is desired to be selected from an oxide superconducting material of Y (or lanthanoide group element), Ba, Cu and O and an oxide superconducting material mainly made of Bi, Sr, Ca, Cu and O. Moreover, as a metal coating the surface of the oxide superconducting material, it is desired to selected from Ag, Au, Cu, Pb or their alloys.

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD OF THE INVENTION The present invention relates to waveguides and resonators in the microwave region using oxide superconductors.

[Detailed description of the invention]

``Conventional technology/problems'' Low-loss waveguides and high-Q resonators that take advantage of the extremely low surface resistance of superconductors to high-frequency electromagnetic waves in the microwave region are industrially viable. It is predicted that there will be a large demand for it, and it is being actively researched.

Superconductors have a lower surface resistance to microwaves than ordinary metals, so if you use them to create a waveguide, you can create a waveguide with extremely low loss, and if you create a resonator using it, you can create a waveguide with very little loss. A resonator with a very large Q value can be created.The present invention aims to further improve the characteristics of superconducting waveguides and resonators.If the waveguide according to the present invention is used in an acceleration cavity, micro In addition to wave communications, for example, highly efficient electron, ion, and heavy ion accelerators can be created, which can be widely used in industry, agriculture, medicine, etc. Particularly in recent years, since the discovery of high-temperature oxide superconductors that exhibit superconductivity above liquid nitrogen temperatures, one of their applications is the creation of waveguides and resonators using oxide superconductors. The research mentioned is currently being carried out. For example, JP-A-63-26
9805. In addition, many basic experiments have been carried out to date. According to this study, it has been revealed that when high-quality thin films and bulk materials are cooled with liquid nitrogen, the surface resistance is more than an order of magnitude lower than that of copper at the same temperature. This value does not reach the level of conventional metallic superconductors.
However, considering that metallic superconductors require expensive liquid helium during operation, but cheap liquid nitrogen is sufficient for oxide superconductors, this is a sufficiently practical value in terms of operating costs.

Furthermore, in experiments to date, polycrystalline samples of oxide superconducting materials have properties that are about an order of magnitude worse than single-crystalline samples. When actually fabricating waveguides and resonators, it is more reliable to use polycrystalline samples, which are easy to fabricate and can produce large-area, homogeneous samples, than single-crystal samples, which are difficult to fabricate. Desirable in terms of performance and cost. Therefore, it has been a challenge to obtain high-frequency characteristics comparable to single-crystal samples in polycrystalline thin films or bulk oxide superconductors.

Structure of the Invention The present invention provides a resonator and a waveguide using an oxide superconductor that achieves sufficiently low surface resistance in order to solve the above-mentioned problems.

That is, a thin film or bulk of an oxide superconductor having a critical temperature higher than the liquid nitrogen temperature is used as a wall material, and the surface of the oxide superconductor on the inner surface of the wall is covered with a metal film with a thickness of 10 to 1000 nm. It is a waveguide and a resonator characterized by the following.

When using oxide superconducting materials for microwave waveguides and resonators, the cause of the inferior properties of polycrystalline samples compared to single-crystalline samples has not been much of an issue. It has been thought that this is simply because the critical current density is small in polycrystalline samples. However, as a result of detailed research by the inventors, it was discovered that polycrystalline samples have grain boundaries, and these grain boundaries weaken the bonds between individual superconductor crystals, resulting in the deterioration of the properties. It was revealed. As a result of more detailed research, the inventors of the present invention found that properties deteriorate not only due to the presence of grain boundaries themselves, but also due to chemical reactions caused by exposure of grain boundaries to moisture and carbon dioxide in the atmosphere. Therefore, we fabricated an oxide superconductor thin film without any exposure to the atmosphere, formed a thin film of a metal with low chemical reactivity with the oxide superconductor as a protective film on its surface, and measured its high-frequency characteristics. As a result, although the properties were not as good as those of single crystal, it was found that the properties were improved by more than an order of magnitude compared to conventional polycrystalline thin films. This is thought to be because the metal film functioned as a protective film and also because the metal film strengthened the bonds between the superconductor crystal grains. Further, after producing this protective film, it was left in a constant temperature and humidity chamber at 80° C. and 70% i.degree. for 1000 hours, but the characteristics deteriorated by only 5%. A test without a protective film was conducted under the same conditions, but it was found to have deteriorated by about 30%.

Further, if the metal film protecting the surface has a thickness of 10 nm or more, it becomes a film and can cover the surface of the oxide superconductor. If the thickness is less than that, it will not form a film and will not serve as a protective film.If a metal film is formed over 11,000 nm, the adhesion between it and the underlying oxide superconducting material will deteriorate, resulting in changes in temperature. When I applied it several times, it peeled off.

Furthermore, after producing the underlying superconducting material, it is important to form a metal film without exposing it to the outside air and to ensure that there is no moisture or carbon dioxide between the superconducting material and the metal film. be. For this reason, we devised ways to avoid exposure to the outside air by forming the superconducting material and metal film in the same reaction chamber and using a multi-chamber type film forming apparatus.

On the other hand, if a superconducting material is formed and then exposed to the outside air, it must be kept in a high vacuum state while applying a temperature of about 100 to 300°C to remove moisture and carbon dioxide gas when forming a metal film. It was important to form a metal film after removal.

EXAMPLES The present invention will be explained in more detail with reference to Examples below.

“Example 1” In this example, yttrium,
An oxide superconductor (hereinafter referred to as YBCO) consisting of barium, copper, and oxygen was used.

All reactions were carried out in a composite chamber as shown in FIG. First, a film consisting of indium, barium, and copper was deposited in a 2-inch radius in a high vacuum (10-'torr or less) by electron beam evaporation and resistance heating evaporation using yttrium, barium, and copper as evaporation sources. The film was formed on a strontium titanate substrate. No substrate heating was performed. The ratio of yttrium, barium, and copper was 1:2=3, and the film thickness was about 2 μm. Next, 1 atm of high-purity oxygen was introduced into the chamber through the oxygen introduction tube (12), and the substrate was heated with the heater (1) to grow crystals of the oxide superconductor. Heating is at first 70
30 minutes at 0°C, then 30 minutes at 800°C, then 900°C.
The test was carried out in four steps: 10 minutes at ℃ and finally 10 hours at 400 ℃. This heating causes the oxide superconductor (
YBCO) crystals are formed. The film becomes polycrystalline with a grain size of several μm. The formed film had irregularities of about 0.1 μm.

After crystal growth is completed, the chamber is evacuated again using a turbo molecular pump and placed in a high vacuum (10-'tor).
After removing the oxygen remaining on the oxide superconducting surface, silver was deposited using the evaporation source (8) in the reaction chamber. Approximately 20 mm thick on the oxide superconductor film
A nanometer-thin silver film was formed. During this time, the substrate was not heated.

The surface resistance of the YBCO film with a diameter of 2 inches thus prepared in the high frequency range was measured at 77°C. As a result, characteristics as shown by the black triangles in FIG. 2 were obtained.

This property is due to the superconducting state of niobium (N) at 4.2K.
Although it was worse than the surface resistance value of ``b''), it was one to two orders of magnitude better than the surface resistance value of silver in 77.
1 higher than the surface resistance of conventionally reported polycrystalline thin films.
An order of magnitude improvement can be seen.

Using the thin film created in this way, 1 cm x 1
A rectangular waveguide cavity resonator of c+a x 2C■ was fabricated. Its Q value was 77 and about 3xlO'. The Q value of a conventional copper or silver cavity resonator is 4 to ~10.
-105,77 is ~104, and those made of superconductor niobium are ~10' at 4. It has been revealed that cavity resonators fabricated using oxide superconductors have better characteristics than cavity resonators made of copper or silver, although they are inferior to those made of niobium.

In this example, since a turbo molecular pump was used for high vacuum evacuation, there was no back diffusion from the pump, and a thin film free of impurities such as oil mist could be formed.

r effect J As shown in the examples, the present invention made it possible to fabricate a resonator with an extremely large Q value. In the examples of the present invention, only the resonator was described, but of course ordinary waveguides and Naturally, the gist of the present invention also includes application of the present invention to high-frequency three-dimensional circuits such as microtrip lines.

Furthermore, since a metal protective film was formed on the surface of the superconducting material, long-term reliability was increased, and the material was close to the level where it could be used as a gas for industrial applications.

[Brief explanation of drawings]

FIG. 1 schematically shows an apparatus for producing an oxide superconductor thin film and forming a surface film. FIG. 2 shows the frequency dependence of the surface resistance at 77 of the superconducting thin film produced according to the present invention. chamber 2, heater 3, substrate (strontium titanate) and substrate holder 4, yttrium evaporation source 5, 6, 7, 8, 9, 10, 11, 12, electron gun, barium evaporation source, copper evaporation source, silver evaporation source, turbo molecule Pump rotary pump high pressure power supply oxygen introduction tube

Claims (4)

[Claims] 1. A thin film or bulk of an oxide superconductor having a critical temperature higher than liquid nitrogen temperature is used as a wall material, and the surface of the oxide superconductor on the inner surface of the wall is covered with a metal film with a thickness of 10 to 1000 nm. Waveguides and resonators featuring: 2. In claim 1, the oxide superconductor is an oxide superconductor consisting of yttrium (or lanthanide group element), barium, copper, and oxygen;
A waveguide and a resonator characterized in that they are selected from oxide superconductors mainly consisting of bismuth, strontium, calcium, copper and oxygen; or oxide superconductors mainly consisting of thallium, barium, calcium, copper and oxygen. . 3. A waveguide and a resonator according to claim 1, wherein the metal covering the surface of the oxide superconductor is selected from silver, gold, copper, lead, or an alloy thereof. 4. In claim 1, both the production of the oxide superconductor and the process of forming a metal film on its surface are performed in a reaction system with a completely controlled atmosphere without any exposure to outside air. Waveguides and resonators characterized in that they are carried out continuously.