JPH02199056A - Bi-sr-ca-cu multiple oxide superconductor - Google Patents

Abstract

PURPOSE:To improve superconducting critical current density while keeping high critical temperature by sintering a raw material powder mixture containing Bi, Sr, Ca and Cu. CONSTITUTION:A sintered material of a Bi-Sr-Ca-Cu multiple oxide of formula (d is quantity of excess Bi and 0<d<=1.2; 6<=m<=10; 4<=n<=8; P=6+m+n; 0<x<1; -2<=y<=+2) is produced by sintering a powdery mixture of oxides and/or carbonates of Bi, Sr, Ca and Cu. A film is deposited on a substrate in an RF sputtering apparatus at a substrate temperature of 670-750 deg.C in a sputtering gas atmosphere consisting of a mixture of Ar and O2 under a sputtering pressure of 1X10<-2>-1X10<-1>Torr at a high-frequency power of 0.064-1.27W/cm<2> and a film-forming speed of 0.05-1A/min using a target produced by crushing the above sintered material. The deposited film is annealed in an oxygen-containing atmosphere at 870-910 deg.C for 10min-5hr.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to composite oxide superconductors, particularly Bi-3 having a high superconducting critical current density CJc).
The present invention relates to an r-Ca-Cu-based composite oxide superconductor.

The superconducting critical temperature Tc of the conventional superconducting material is Nb over a long period of time.
Although it was not possible to exceed 23K of 3Ge, 198
In 1960, Bednotes and Müller discovered that sintered oxides represented by (La, Sr) 2 COs were superconducting materials with a Tc as high as 30, and the possibility of realizing non-low temperature superconductivity began to emerge. is increasing significantly. After that, Chu et al. announced that the composite oxide represented by Y1Ba2Cus07-x, called YBCO, is a 90-grade superconductor, and subsequently, Bi-3r-Ca-Cu which has a Tc of 100 or higher A complex oxide superconductor has been discovered.

Prior to this, it was known that complex oxides containing bismuth exhibited superconducting properties, and U.S. Patent No. 3.93
No. 2.315 describes that a Ba-Pb-B1 complex oxide exhibits superconducting properties. Furthermore, Japanese Patent Publication No. 173.885/1985 describes that Ba-B1-based composite oxides exhibit superconducting properties. However, since the superconducting transition temperature of these superconductors was about 10°C, liquid helium with a boiling point of 4.2 had to be used as a coolant.

In addition to having a high critical temperature, Bl-based composite oxides have the advantage of being chemically stable and exhibiting less deterioration of superconducting properties over time unlike YBCO, etc., as well as lower production costs as they do not contain rare earth elements as constituent elements. It has the advantage of lowering the

Initially, these composite oxide superconductors were made as bulk sintered bodies by sintering mixed powders of oxides and/or carbonates of the constituent elements that make up the composite oxide, but now they are made in the form of thin films. It is considered practical to form a film in This thin film can be formed by a physical vapor deposition method such as a sputtering method. In this case, the thin film can be formed by an RF sputtering method using a sintered body of a composite oxide obtained by sintering as a target. It is common to have a film. Further, it is common to heat-treat the sintered body and thin film obtained by sintering or film formation in an oxygen atmosphere to adjust oxygen defects in the crystals constituting the sintered body or thin film.

However, until now Bi-3rCa obtained by film-forming method
-Cu-based superconductor thin films have a small critical current density Jc, and therefore cannot be used, for example, as a material for thin film devices. That is, the Bi film formed by the conventional method
-5r-Ca-Cu based composite oxide superconductor thin film is 1
Although it exhibits a high critical temperature Tc of 00K or higher, the critical current density Jc is at most about 10.000 A/cut, making it less practical.

The present inventors conducted various studies in order to improve the characteristics of the superconducting thin film of this system, especially the superconducting critical current density, and found that by adding Bi in excess of the stoichiometric composition to the thin film, the critical current density could be improved. The present invention was completed based on the discovery that the method is effective for the following purposes.

Problems to be Solved by the Invention An object of the present invention is to solve the problems of the prior art described above, and to provide a bismuth-containing composite oxide having a high critical current density Jc, particularly a Bi-3r-Ca-Cu composite oxide superconductor. It's about offering your body.

Means for Solving the Problems The superconductor provided by the present invention satisfies the following formula: % formula % where d is excess and the amount of i satisfies 0<d≦1.2, preferably Q, l<d≦1. 2, m satisfies 6≦m≦10, preferably 7≦m≦9, n satisfies 4≦n≦8, preferably 5≦n≦7, p=5+m+n, Mainly composed of a Bi-3r-Ca-Cu-based composite oxide having a composition represented by X represents a number satisfying Q<x<l, and y represents a number satisfying -2≦y≦+2. It is characterized by

The characteristics of the composite oxide superconductor according to the present invention are the following formula, which was conventionally considered to be the optimal value in terms of stoichiometry: % formula % (where Xs mz n5T) and y are the same as those defined above. Compared to the Bi-3r-Ca-Cu-based composite oxide superconducting material represented by
The point lies in the fact that Q<d≦1.2), rather than Q.

Preferred specific examples of the composite oxide superconductor according to the present invention include the following composite oxide system: B14-asr<Ca<Cus O2o+y (here,
d represents a number that satisfies 0 and d≦1.2, and y represents a number that satisfies -2≦y≦
(represents a number satisfying +2) This composite oxide is made up of two layers with the same chemical composition stacked one above the other with a certain lattice constant shift, so the superconductor of the present invention can be used instead of the above formula. It can also be represented by the following formula: B12+d/2Sr2Ca2Cu30+o,y/2, where d and y have the same definitions as above.

Although the composite oxide superconductor according to the present invention can be made as a bulk material, it is preferably formed as a thin film on a substrate. The substrate on which the composite oxide thin film of the present invention is formed is MgO1SrTiO+, LaAlO3, LaGa0
It is preferable to use a single crystal of 0 grade. Especially Mf! , the (100) or (110) plane of the O single crystal is preferably used as the film-forming surface.

Method for manufacturing the material of the present invention The composite oxide superconductor according to the present invention can be manufactured by a known method.

When producing a bulk superconductor, a raw material powder mixture adjusted to have the composition of the constituent elements defined in the present invention, such as a mixture of oxides and/or carbonates of the constituent elements of the superconductor of the present invention, is sintered. All you have to do is tie it.

When forming a thin film on a substrate, a known physical vapor deposition (PVD) method or chemical vapor deposition (CVD) method is used. As the physical vapor deposition method, an RF magnetron sputtering method, a vacuum evaporation method, and an ion blating method can be used. The evaporation source or target used in this case is a powder raw material such as the constituent metal element itself, its alloy and/or its oxide, fluoride or carbonate, a sintered body obtained by sintering this powder raw material by a powder sintering method, or a sintered body of this powder raw material. It is preferable to grind the sintered body into a sintered powder. This evaporation source or target can also be divided into multiple parts. When using the molecular beam epitaxy (MBE) method, the constituent metal elements or their oxides are evaporated using a cell or an electron gun. In this case, oxygen is separately supplied into the deposition atmosphere as required.

In any case, the composition ratio of the raw material mixed powder must be adjusted so that the composition ratio of the constituent elements of the bulk or thin film of the composite oxide finally obtained is the composition ratio defined in the present invention. In particular, when using a physical vapor deposition method, the atomic ratio of metal elements in the evaporation source or target is adjusted depending on the evaporation rate or sputtering rate of each element and the difference in bonding force to the substrate. In order to add excessive Bi, which is a feature of the present invention, the ratio of Bi in the evaporation source or target may be made larger than the optimal value conventionally considered.

In a preferred embodiment of the invention, the composite oxide superconductor according to the invention is deposited as a thin film by sputtering. The pressure during sputtering in this case is I Xl0-
'~I
It is preferable to use a mixed gas of Ar and 02%.

The substrate temperature on which the above thin film is formed is as follows: The substrate on which the composite oxide thin film is formed is MgO, Zn0z, SrTiO3, LiN.
b0. , LiTa0. , LaAl0. , LaGa
0. It is preferable to make it into a single crystal such as. In addition to this, LiA1
0a, LxCaO, , KTaO3, CaF, , BeF
, ,C3Z.

YSZ or even a silicon substrate can also be used.

When using a silicon single crystal substrate, it is preferable to provide a buffer layer such as MgO1ZnO between the composite oxide and the substrate. The superconductor of the present invention exhibits anisotropy in electrical resistance, and the critical current density in the direction parallel to the plane defined by the a-axis and b-axis of the crystal is high, but the current flowing in the C-axis direction is small. Therefore, it is preferable to give the thin film a desired orientation. For example, MgO or SrTiO3 single crystal substrate (0
Since the a-axis of the thin film formed on the substrate surface is oriented perpendicularly or substantially perpendicularly to the substrate surface, the critical current density Jc along the substrate surface becomes large. In special cases, the critical current density Jc in the depth direction can be increased by forming a film on the (110) plane of an MgO or 5rTIOz single crystal substrate. The above MgO and SrTiO.

A single crystal substrate such as the above is preferable because it has a coefficient of thermal expansion close to that of the composite oxide of the present invention, so that the thermal stress imparted to the composite oxide when the substrate is heated and cooled is reduced.

During sputtering, the substrate is heated to 670 to 750°C, preferably 670 to 720°C, more preferably 680 to 71°C.
Preferably, heating is carried out to a temperature in the range of 0°C. In addition, the high frequency power applied to the target is 0.064 to 1.27W.
/cnf, and the film forming rate is 0.
It is preferable to set the speed within the range of 0.05 to 1 person/minute.

Furthermore, it is preferable to perform an annealing treatment within a temperature range of 870 to 910° C. after film formation.

In particular, when the substrate temperature during film formation is out of the above range, the critical current density sharply decreases. Furthermore, it has been found that it is extremely effective to perform annealing treatment, ie, post-heat treatment, under predetermined conditions after film formation. This annealing treatment is preferably performed in an oxygen-containing atmosphere or an oxygen stream. Note that this annealing treatment temperature is an extremely important control factor, and the treatment temperature is maintained in a temperature range of 870 to 910°C, more preferably in a temperature range of 880 to 905°C for a predetermined period of time (for example, 10 minutes to 5 hours). It is necessary. If the treatment is performed under conditions outside this range, the properties of the thin film may deteriorate depending on the conditions.

The present invention will be described in more detail with reference to specific examples below, but the following examples are merely illustrative of the present invention and do not limit the technical scope of the present invention in any way. .

Example: Based on RF magnetron sputtering method, B1-3r
A -Ca-Cu based composite oxide superconductor thin film was produced.

To make the target used in this RF magnetron sputtering method, we used commercially available 81203 powder and commercially available 5r powder.
COa powder, commercially available CaCO3 powder, and commercially available CuO
The atomic ratio of the mixed powder is Bi:Sr:
Ca diCu has the following ratio: (1) 1.4:
1.0 = 1.0:1.5(2) 1.5:1
．． 0: 1.0: 1.5 (3) 1.6: 1
．． 0: 1.0: 1.5 (4) 1.7: 1.
0: 1. Mouth = 1.5 (5) 1.8: 1.0
Five types of powder mixtures having a ratio of 1.0 to 1.5 were prepared.

Bi-3 obtained by sintering each mixed powder at 800°C for 8 hours
A sintered powder obtained by pulverizing the rCa-Cu-0 composite oxide sintered body was used as a powder target.

Further, an LfgO single crystal substrate was used as the substrate, and the film formation surface was the (110) plane.

A film was formed using an RF sputtering apparatus using the above five types of powder samples with different Bi atomic ratios under the following film forming conditions common to each sample: Substrate temperature: 690°C Sputtering gas: Mixture of Ar and 02 Gas 02/
(Ar+02) =0.2 (volume ratio) Sputtering pressure crab 2 Xl0-”Torr high frequency power
: 50W (0.64W/crd) Each thin film was formed to a film thickness of 2000. The obtained thin film was 900
C. for 1 hour in 1 atm of oxygen gas.

It was confirmed by resistance measurement that each sample thus obtained (sample numbers 1 to 5) was a superconductor.

That is, each sample was immersed in liquid helium at a temperature of 8 in a Tarabiostat, and the temperature at which the sample became a normal conductor (critical temperature Tc) was measured while gradually increasing the temperature.

Further, the superconducting critical current density at 77.3 K was measured using a four-point probe method according to a conventional method. Moreover, the composition ratio of the constituent elements of each thin film obtained was measured by ICP.

These data are summarized in Table 1.

1 to 5 are X-ray resolution charts of each thin film (sample numbers 1 to 5). Figures 2 to 4 show that the thin films of sample numbers 2 to 4 are thin films consisting only of 2-2-2-3 phases with C-axis orientation, and Figures 1 and 4 show that sample numbers 1 and It is shown that the thin film of No. 5 also contains a 2-2-1-2 phase which has a low temperature reduction.

Table 1 (Samples 1 and 5 are comparative examples) Effects of the Invention The Bi-3r-Ca-Cu based composite oxide superconductor thin film according to the present invention is superior to conventionally known composites composed of the same constituent elements as this system. While maintaining the high critical temperature (Tc) originally possessed by oxides, the superconducting critical current density (J
c) is significantly improved. Therefore, this thin film can be used to fabricate various superconducting devices, typified by Josephson devices, which operate at temperatures higher than the liquid nitrogen temperature. That is, in the field of electronics, which is a typical application field of superconducting phenomena, various superconducting elements have been proposed and developed. A typical example is an element that utilizes the Josephson effect, in which a quantum effect appears macroscopically due to an applied current when superconducting materials are weakly bonded together. Further, tunnel junction type Josephson devices are expected to be extremely high-speed switching devices with low power consumption because the energy gap of the superconducting material is small. Furthermore, since the Josephson effect on electromagnetic waves and magnetic fields appears as a precise quantum phenomenon, it is expected that Josephson elements will be used as ultrasensitive sensors for magnetic fields, microwaves, radiation, etc. In ultra-high-speed electronic computers, the power consumption per unit area has reached the limit of cooling capacity, so there is a demand for the development of superconducting elements.Furthermore, as the degree of integration of electronic circuits increases, superconducting materials with no current loss are being developed. There is a demand for using it as a wiring material.

The composite oxide superconducting thin film obtained by the present invention can be used as a material for these applications.

[Brief explanation of the drawing]

1 to 5 are X-ray diffraction charts of thin films of sample numbers 1 to 5 obtained in Examples of the present invention.

Claims (1)

[Claims] (1) The following formula: Bi_4_+_d(Sr_1_-_k, Ca_k)_m
Cu_nO_p_+_y (where d is the amount of excess Bi, satisfying 0<d≦1.2, and m is 6
≦m≦10, n satisfies 4≦n≦8, p=6+m+n, x represents a number that satisfies 0<x<1, y represents a number that satisfies -2≦y≦+2) A superconductor mainly composed of a composite oxide having a composition represented by: