JPH01157420A - Oxide superconductor and production thereof - Google Patents

Abstract

PURPOSE:To obtain an oxide superconductor having a high critical temp. and exhibiting superconductivity by cooling with low-cost liq. nitrogen by forming a thin film of La-Sr-Nb-O type oxide by sputtering. CONSTITUTION:This oxide superconductor has a chemical compsn. represented by a formula LaxSryNbOz (where 0<x<1, 0<y<1 and 1<z<4) in at least the surface layer and has >=100K critical temp. The superconductor is obtd. as follows: one of Nb and La-Sr-Cu-O type oxide is used as a substrate, the other as a target, sputtering is carried out in an Ar atmosphere at a temp. at which the substitution of Nb for Cu is caused, and a formed thin film is rapidly cooled.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is applicable to bulk or thin film materials used, for example, in magnetic coil parts of magnetic levitation trains and particle accelerators, electronic devices, and circuit boards of Josephson computers. The present invention relates to an oxide superconductor consisting of.

[Prior art]

Currently, metal-based superconductors such as Nb3Ge and Nb:ISn are in practical use, but their critical temperature (Tc) is at most 23.2.

However, it has recently been announced that oxide-based superconductors made of a mixture of rare earth elements, alkaline earth elements, and copper oxide have significantly higher critical temperatures than metal-based superconductors (for example, a group from the University of Tokyo Faculty of Engineering It was announced at the American Physical Society that it had achieved a temperature of 90 K), and it became possible to use inexpensive liquid nitrogen as a refrigerant instead of the expensive cryogenic liquid helium (4,2 = -268,8 degrees Celsius). Therefore,
Significant progress has been made in the practical application of this oxide-based superconductor in various fields of application. Along with these announcements, research has been actively conducted to further raise the critical temperature (Tc) of bulk or thin film oxide superconductors in the above application fields to room temperature, but there is still insufficient research. An oxide superconductor with a high critical temperature (Tc) has not yet been obtained.

[Purpose of the invention]

Therefore, an object of the present invention is to provide a novel oxide superconductor that has a high critical temperature (Tc) and exhibits superconductivity when cooled with inexpensive liquid nitrogen, and a method for producing the same.

Another object of the present invention is to provide a method for manufacturing the oxide superconductor using a thin film forming technique using sputtering.

[Summary of the invention]

According to the present invention, one of Nb and La-3r-Cu-0-based oxides is used as a substrate, the other is used as a target,
Sputtering was performed in an argon atmosphere and under temperature conditions that caused the substitution of Cu with Nb.
-0-based oxide and then rapidly cooled the formed thin film, so that at least the surface layer has the formula % formula % where 0<x<1. The present invention aims to obtain an oxide superconductor having a chemical composition expressed by Q<y<l, l<z<4 and a critical temperature of 100 or higher.

In the present invention, when a La-5r-Cu-0 based oxide is sputtered on a Nb metal or a Nb metal is sputtered onto a La-3r-Cu-0 based oxide, Nb becomes a La-5r-Cu-0 based oxide.
Cu is diffused into the 0-based oxide and replaced with Nb, and La-5
It is made of r-Nb-0 based oxide and has a high critical temperature (
This is based on the knowledge that a superconductor having a high 0FF-3ET temperature can be obtained.

In FIG. 1 showing a magnetron sputtering apparatus used in the method of the present invention, there is a support 11 containing a heater (not shown) inside a pressure chamber 10, and a substrate 12 is placed on this support 11. ing.

A planar-magnetron type cathode 13 is provided to face the support 11, and a sputtering target 14 is placed thereon. The chamber 10 has a valve 15
It is connected to an air supply/exhaust port 16 having an air supply/exhaust port.

In a preferred embodiment of the invention, chamber 10 is cooled with liquid nitrogen or the like.

According to the present invention, Nb metal is used as the substrate 12 and La-5r-Cu-0 based oxide is used as the target 14, or alternatively, a La-5r-Cu-0 based oxide is used as the substrate 12 and Nb metal is used as the target 14. use

As the La-5r-Cu-0-based oxide, a sintered body containing these elements is used. However, La-5r-C
Although the composition of the u-0 series oxide varies during sputtering, it is necessary to supply each element in sufficient amount to form a superconductor. Specifically, Lak Sr+, lCu OFI' ” ' (1
), where k is 0.1 to 10. m is 0.1 to 10.
n is 1.25 to 26.5, most preferably the formula La5r
A material having a composition of CuO< is used. FIG. 3 shows the compositional analysis pattern of this product using EPM8. The above La-
The 3r-Cu-0 based oxide sintered body may generally be electrically insulating.

Is the thickness of the substrate 12 enough NbJ for the final superconductor? 3L
a, Sr and 0 components are supplied, 20 to 50 μm in the case of Nb metal, La-5r-Cu-0
The thickness can be varied from 100 to 3000 μm in the case of based oxides.

Sputtering is performed in an argon atmosphere and under temperature conditions that cause the substitution of Cu with Nb. For this purpose, prior to sputtering, the inside of the chamber 10 is heated from 10-8 to 10-
'Reducing the pressure to about Torr, and increasing the Ar pressure to 0.05 to Q, 3
Torr, especially 0.1 to 0.7 Torr, and the substrate 12 is maintained at 600 to 1250°C, especially 700 to 12
Sputtering is performed by heating to 00°C. Sputtering uses a voltage of 200 to 300V and a current value of 300 to 40V.
It is best to perform magnetron sputtering at 0 mA, and the oxygen partial pressure during sputtering is 10-'To
It is better to maintain it within the range of rr or less.

When using Nb as a substrate, the film forming temperature is generally 50 to 50
00 people/min, especially 1IIin for 100 to 200 people
In the case of La-5r-Cu-0 based oxide, the production rate of La-5r-Nb-0 based oxide film (hereinafter referred to as sputtering rate) is preferably within the range of 100 to 50.
It is preferable to set it in the range of 00 people/l1lin.

In the present invention, it is preferable to rapidly cool the formed thin film after sputtering is completed in order to stabilize the superconductor phase. It is preferable to perform the operation while the sputtering chamber 10 is cooled with liquid nitrogen because the thin film is rapidly cooled as soon as the power to the heater is cut off after sputtering is completed. The rapid cooling is preferably performed at a cooling rate of 400 to 3000°C/hr to a temperature of 400°C or less.

In order to control the amount of oxygen in the final superconductor, the formed thin film may be brought into contact with an oxygen atmosphere of 1 to 5 Torr at a temperature of 400 to 700° C. during an intermediate stage of cooling.

According to the invention, at least the surface layer is thus made of La-5r.
An ultra-thick electric body made of a -Nb-0 series oxide and having a critical temperature (Tc) of 100 or higher, particularly within the range of 200 to 300, can be obtained. Comparison between FIG. 3 and FIG. 4 shows that in the superconductor of the present invention, the characteristic peak of Cu disappears and the characteristic peak of Nb appears instead, indicating that Cu is replaced by Nb. You can see that

It is preferable that at least the surface layer of this superconductor has a composition represented by the formula % (2) where O<x<1.0<y<1.1<z<4, most preferably the formula % It has a composition expressed by the formula %.

As shown in FIG. 7, the superconductor of the present invention is made entirely of La-
3r-Nb-0 based oxide 1, and as shown in FIG. It may also consist of a transition layer 3 in which the Nb content increases toward . Furthermore, in the super 1 conductor according to another embodiment of the present invention, as shown in FIG. It has become.

In the superconductor of the type shown in FIG. 9, an intermediate layer may be formed between the surface layer 1 and the lower layer 4 in which the Nb content decreases from the surface layer 1 toward the lower layer 4. The composition of these intermediate layers having a gradient in content is expressed by the formula (2)
It is more desirable to have a gradient within the composition range shown in .

By forming a layer having a gradient in the content of constituent elements, a non-equilibrium phase is formed, thereby stabilizing the entire system.

The superconductor of the present invention has a liquid nitrogen temperature of over 100 (7
It has a critical temperature at a temperature higher than 7.3 K). This fact becomes immediately apparent by referring to FIGS. 2 and 5. Further, the superconductor of the present invention has Nb of 9.2
It exhibits a Meissner effect of about -5 to -30χ when the magnetic susceptibility is taken as a standard.

The invention will now be explained with the following examples.

〔Example〕

A thin film was formed by sputtering a La5rCuO compound onto a 0.03 pm thick Nb metal plate. The film was formed by direct current high speed magnetron sputtering in a liquid nitrogen tank at Ar pressure of 0.
．． 1 to 0.4 Torr, voltage 200 to 300V, electric vL
It was carried out at 300-400 mA. Before sputtering, the pressure inside the Pelger is reduced to ~10-'Torr, and the sputtering temperature is 0.
□Performed at a partial pressure of ~10-'Torr. The sputtering target (LaSrCu04) was used after being annealed at 900° C. for 8 hours.

During sputtering, heat to 700-800°C, then 400°C.
It was rapidly cooled down to room temperature at °C/Hr. The properties of the film are affected by this cooling rate, and according to the present invention, the properties can be stabilized by rapid cooling. The film thickness was 20 μm.

The resistance was measured using the four-terminal method. The electrodes were attached with In (indium) using ultrasonic solder. Measurement current is 10A/cm
” and was changed from time to time.The temperature of the sample was measured at Au-0,
It was conducted with a 07XFe-chromel thermocouple, and liquid helium and liquid nitrogen were used as coolants. Warm air was blown to raise the temperature above room temperature. The cooling rate from room temperature to liquid helium was ~300/hr. The inductive bridge method was used for AC magnetic susceptibility measurement.

The sample was heated to 335K with hot air and then cooled. The resistance rapidly decreased upon cooling, and the resistance change curve versus temperature shown in FIG. 2 was obtained. As will be seen, the onset temperature of this sample is at least 2
At 28, the offset temperature is at least 100K. That is, a superconducting thin film whose resistance became zero (0) at 100 K (-173° C.) was obtained.

Note that the sputtering target (LaSrCuO,
) and the crystal phase after film formation by EPMA (Electro
n-Probe-Micro-Analyzer) and are shown and compared in FIGS. 3 and 4. As is clear from this, the Cu peak disappears after film formation, and a strong Nb peak appears instead. That is, La5rCu0
Cu in No. 4 is almost completely substituted with Nb. This is N
It is thought that Cu at the center of the oxygen octahedron was substituted with Nb because b and O have a strong affinity and a provskite structure represented by KNbOz is easily formed. It was found that although the amounts of La and O did not change significantly, the amount of Sr decreased slightly.

[Example 2] A bulk body made of La-5r-Nb-0 is also produced in a high vacuum apparatus. A La5rCub with a thickness of 2111111 mm, a width of 5 mm, and a length of 10 mm was used as the substrate. The Pelger was depressurized to 10-” Torr and Nb (
(purity 99.9%) on La5rCu04 substrate^r
Sputtering was performed at a pressure of ~0.5 Torr. The temperature of the board is 90
The temperature was 0°C and the sputtering rate was ~100 people/min.

After sputtering, the substrate was held at 700° C. for 30 minutes in a Pelger. The composition of Nb was adjusted by sputtering time. After sputtering, the inner wall of the chamber was covered with a Cu film and was walled. This is considered to be because Nb replaced Cu and Cu was scattered from the La5rCu04 substrate.

Measurements were carried out in the same manner as in Example 1, and the results showed that the resistance began to sharply decrease at least in the room temperature region where the on-cent temperature (Tco) was 200 or higher, and the Meissner effect was also exhibited.

[Example 3] A La5rCubn compound was sputtered onto a 200 μm thick Nb foil. Film formation is performed by direct current high speed magnetron sputtering using a La5rCuOn target with a diameter of 100 mm at a film formation rate of 5000 people/min in a chamber cooled with liquid nitrogen. The film thickness is ~15 μm and the film shape is 4 mm square. Sputtering is performed at high Ar pressure and 0.
5 to 0.7 Torr, and the substrate temperature is 1200''c.

After sputtering, heat at 50°C/1200°C to 400°C.
quenched at 2 Torr and held for 5 minutes in oxygen (02) at 2 Torr.

Measurements were performed in the same manner as in Example 1. Current during measurement is 3 A
/cm”, but to see the current dependence, IOA/cm2
was also used.

Composition analysis was performed using EPMA and IMA (Ion-Micro
-Analyzer). Although the amounts of La and SrO did not change significantly, the presence of Cu was not absorbed. IMA analysis revealed that there is a gradient in Nb content in the thickness direction from the Nb foil to the Nb oxide. The composition of La-5r-Cu-04 on Nb foil is L
The result was an a-3r-Nb-0 thin film. As Nb diffuses from the Nb foil into this thin film, Cu in the film phase is replaced with Nb, and the central OCu! ! 'Nb has been replaced. LazSrlNb50 on the surface. A layer with the composition was formed.

The resistance change and magnetic susceptibility with respect to temperature of the above sample were measured and are shown in FIG. Immediately after stopping heating with hot air,
The resistance started to drop rapidly, and at 300, the resistance became O even at the noise level. This noise continued up to 255K. The negative magnetic susceptibility was 5χ of the magnetic susceptibility of Nb at 9.2, and the existence of the Meissner effect was confirmed.

In addition, as shown in FIG. 6 (,~2BOKTENb(7)20!
,! : Samples with very large magnetization were also obtained.

〔Effect of the invention〕

As detailed above, since the oxide superconductor of the present invention has a high critical temperature, it exhibits superconductivity when cooled with inexpensive liquid nitrogen, and is highly practical and can be used in various fields such as magnetic coil parts and circuit boards. It is something that can be expanded.

[Brief explanation of the drawing]

FIG. 1 is a schematic layout diagram of a magnetron sputtering apparatus used for manufacturing an oxide superconductor, FIG. 2 is a resistance-temperature characteristic diagram of the superconductor obtained in Example 1, and FIG.
The figure shows the el of the sputtering target used in Example 1.
ectron probe micro-analyj
er (EPMA), FIG. 4 is a composition analysis pattern of EPMA of the superconductor obtained in Example 1, and FIG. 5 is a resistance - FIG. 6 is a graph showing temperature characteristics and Meissner effect-temperature characteristics; FIG. 6 is a graph showing Meissner effect-temperature characteristics for other superconductors obtained in Example 3;
8 and 9 are cross-sectional views showing several examples of the superconductor of the present invention. 11...Support 12...Substrate 14...Target

Claims (2)

[Claims] (1) An oxide superconductor in which at least the surface layer has a chemical composition represented by the formula La_xSr_yNbO_z, where 0<x<1, 0<y<1, 1<z<4, and has a critical temperature of 100K or higher. (2) Sputtering is performed using either Nb or La-Sr-Cu-O-based oxide as a substrate and the other as a target in an argon atmosphere and at a temperature that causes the substitution of Cu with Nb, and then forming A method for producing a La-Br-Nb-based oxide superconductor, which comprises rapidly cooling a thin film formed by