JPH02275790A - Oxide superconductor - Google Patents

Abstract

PURPOSE:To prepare a bismuth oxide superconductor having a high critical temperature in good regenerativity by forming an oxide superconductor on a bismuth oxide crystal. CONSTITUTION:An oxide superconductor formed on bismuth laminar oxide polycrystals or single crystal. The bismuth laminar oxide polycrystals or single crystal giving a simple integer ratio between the C axis length of the bismuth laminar oxide polycrystals or single crystal and the C axis length of the oxide superconductor includes Bi3TiNbO9, Bi3TiTaO9, CaBi2Nb2O9, CaBi2Ta2O9, SrBi2 Nb2O9, Srbi2Ta2O9, BaBi2Nb2O9 (the C axis lengths of the above compounds are approximately 25Angstrom ), Sr2Bi4Ti5O18, Ba2Bi4Ti5O18 and Pb2Bi4Ti5O16 (the C axis lengths of the above compounds are approximately 49 to 50Angstrom ). The epitaxial growth using the material as the substrate permits to give an oxide superconductor comprising only approximately high Tc phase. The oxide superconductor can be applied to superconductor micro devices, superconductor magnets, superconductor power transmission and power storage, etc.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to oxide superconductors.

[Prior Art] Oxide superconductors consisting of bismuth, strontium, calcium, copper, and oxygen (hereinafter referred to as bismuth-based oxide superconductors) are known as high-temperature superconductors and have a critical temperature (Tc
) ~ 80 low Tc phase (chemical formula is B i ZS r z
Ca Cuzoa+x-, C axis length approximately 30 angstroms) and high Tc phase (B tzs rz
cazcuto+ox, C-axis length of about 36 angstroms).

Furthermore, although it is not a superconductor, it is known that there is a semiconductor phase with a C-axis length of approximately 24 angstroms (chemical formula: Bi25rzCuOa□). Currently, research is being conducted to extract only this high Tc product. On the other hand, the one for high Tc has three layers of CuO plane in the unit cell, but according to recent research, for this high Tc, one layer of CuO (Sr
, Ca) 0 plane was added, and a structure with a longer C axis (structure with a C axis length of 42 angstroms and 48 angstroms) was revealed by high-resolution electron microscopy. It is believed that this structure is the reason why the resistivity of the bismuth-based oxide superconductor starts to decrease above 150.

Therefore, if this structure can be made reproducibly, Tc can reach 150 or more.

[Conventional problems]

However, with normal sintering methods, only 50% of the low Tc phase can be obtained at most for high Tc, and the reaction takes 3 to 7 days, the reaction temperature is very sensitive, and reproducibility is poor (, It wasn't practical.

Numerous experiments have also suggested that the high Tc phase tends to grow from the liquid phase, but if the melt is simply slowly cooled, the low Tc phase will also precipitate at the same time.

Furthermore, in normal sintering methods, such long-period structures are created by chance inside the crystal.
However, its volume was small and its reproducibility was poor, making it impractical.

In view of the above points, the present invention aims to obtain a bismuth-based oxide superconductor for high Tc with good reproducibility.

[Means for Solving the Problem] The inventors of the present invention conducted various experiments in order to achieve the above object, and as a result, they came to the following findings.

The inventor thought that in the presence of a crystal with a C-axis length close to that for high Tc, only the high Tc crystal would be selectively obtained by epitaxial growth.

(Bi20□) (A, -IB-Ch, 5-1) The so-called bismuth layered oxide has a structure similar to that of bismuth-based oxide superconductors. It can serve as a mother crystal for epitaxial growth. The present inventors have proposed that among these bismuth layered oxides, one satisfying the above C-axis length condition is used as a substrate material, and a bismuth-based oxide superconductor is formed in a film form from a liquid phase thereon. Among bismuth-based oxide superconductors, high T
We found that it is possible to selectively grow only C.

C-axis lengths close to those for high Tc include Bi, T
i, Q. (C-axis length 32.8 Angstrom), CaB
fsTi40+s, S r B I aT ] 40
+s, B a B j4T i 4015, P b B
i aT i ao+s, (C-axis length approximately 41 to 42
Angstrom), Biz (S r+-xLax)
ncu30. (x=0.05~0°4, y-s~12,
C-axis length of approximately 36 angstroms).

In addition, a series of so-called bismuth layered oxides (BizO□) (A-+B-03-1°) developed as dielectrics are a
There are many materials whose axis and b-axis lengths are close to those of bismuth-based oxide superconductors and whose C-axis length is long (for example, 5rz
B! aT 1sO1s (C-axis length 48°8 angstroms), BazBi4TisO+a (C-axis length 50.3
angstrom), PbzB 14Ti50+a(
C-axis length: 49.7 angstroms).

On the other hand, among bismuth-based oxide superconductors, B12 (Sr
, Ca)i,cusoz,x has a C-axis length of approximately 48 angstroms. The present inventor uses a polycrystalline or single crystalline bismuth layered oxide that satisfies the above-mentioned C-axis length conditions, that is, has a high C-axis length close to that of the Tc phase, as a substrate material. If a bismuth-based oxide superconductor film is formed in a film form rather than a liquid phase on top of the substrate, these substrates will serve as host crystals for epitaxial growth. We have found that it is possible to selectively grow bismuth-based oxide superconductors with a structure. By this method, it was possible to obtain a 150-grade bismuth-based oxide superconductor of several percent by volume.

Furthermore, numerous experiments have suggested that high Tc materials tend to grow from the liquid phase. Moreover, based on the fact that a high TC phase can be obtained together with a semiconductor phase with a C-axis length of 24 angstroms, the inventors have concluded that in the presence of a crystal with a C-axis length of 24 angstroms, a high T phase with a C-axis length of 36 angstroms is obtained.
I found that the c type is easy to grow. The reason for this is that the ratio of the C-axis length is a simple integer ratio (in this case, 24:36=2
:3).

By the way, (BizO□)(AI, 1-IBI, lO3
The so-called bismuth layered oxide formed by □, ) is
Because it has a structure very similar to bismuth-based oxide superconductors,
It can serve as a host crystal for epitaxial growth of bismuth-based oxide superconductors.

The present inventors have proposed that among these bismuth layered oxides, one satisfying the above C-axis length condition is used as a substrate material, and a bismuth-based oxide superconductor is formed in a film form from a liquid phase thereon. Among bismuth-based oxide superconductors, high Tc
We have found that it is possible to selectively grow only the plants.

BizTiNbC19, BizTiTa07, C
a B i2N bzOq, Ca B i 2T
a zOq, S r B! zN b zOq,S
rB! zTazoq, BaBi2Nb, 09, (more than C axis length about 25 angstroms) SrzBiaTi,
O+a, Ba2Bi4T! 5Ova, PbzB 14T
isO+a, (C-axis length approximately 49-50 angstroms), B12(Sr, -XL aX)4Cu30
yCX=0.05~0.4, y-8~12, C-axis length -3
6 angstrom], etc. As a result of attempting epitaxial growth using such materials as a substrate,
It was possible to obtain an oxide superconductor consisting only of extremely high Tc.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

“Example IJ This example is B 14Ti: +o+z (C axis length 32.8
This figure shows a case in which a film of bismuth-based oxide superconductor is formed using a screen printing method using a bismuth-based oxide superconductor (Angstrom) as a substrate material.

B14Tiz01□Substrate is bismuth dioxide (Bi20,
), titanium dioxide (T i O,) powder (all purity 99.9%) was mixed in a stoichiometric composition ratio and calcined in air at 800°C for 112 hours to form a powder with a diameter of 2.
Formed into a disk shape of 0 mm and 1 mm thick, and placed in air again for 1
It was obtained by firing at 000°C for 12 hours. Obtained Bi<T
The izO+z substrate is a single-phase 3i4T by X-ray diffraction method.
It was confirmed that isO+z. This B i aT
The C-axis length of i 30 I□ is 32°8 angstroms.

Bismuth-based oxide superconductor powders used for screen printing include bismuth trioxide (BizOs), strontium carbonate (SrC03), and calcium carbonate (CaCO).
, copper oxide (CuO) powder (both 99.9% pure). That is, these raw material powders were prepared in the following manner: Bi:Sr:Ca:Cu=2:2:2:
3 (molar ratio), calcined in air at 800°C for 12 hours, and then pulverized. The particle size of the obtained powder is 1
~5 microns. The powder was mixed in the appropriate amount of ethanol and spread onto a B i aT i 3012 beret.

After drying, the screen-printed film of the bismuth-based oxide superconductor thus formed was heated at 900°C in air for 30 minutes.
After being heated for 1 hour, it was cooled to room temperature at a cooling rate of 60°C per hour. The resulting BizSrzCazC
u:+0+o+x The thickness of the film is approximately 30 microns, and
Linear diffraction revealed that the film was composed primarily of high Tc material. The superconductor film formed on the substrate having a C-axis length of 32.8 angstroms had a C-axis length of 36 angstroms. Furthermore, as shown in Figure 1, this film exhibits zero resistance at 10K, and the critical current density at liquid nitrogen temperature is approximately 50,000 A/cm.
It was 2. This value is sufficiently large for a thick film.

In the present invention, the oxide superconductor was formed by a sputtering method, but a vacuum evaporation method, an electron beam evaporation method, a reactive evaporation method, a chemical vapor deposition method (CVD method), a screen printing method, a spray pyrolysis method, etc. , it may be formed by a method normally used for forming an oxide superconductor film.

? Example 2j This example is S r zB I 4T i so +e
(C-axis length is 49 to 50 angstroms) as a substrate material, bismuth-based oxide superconductor (BjzOz) (
A case is shown in which a film of Sr3Ca:+Cu5O+ e-x (C-axis length 40 angstroms) is formed by a screen printing method.

s rzB i4T iso+8 substrate is bismuth trioxide (Bi, 03), titanium dioxide (TiO2), strontium carbonate (SrCO:+) powder (all purity is 99
．． 9%) in a stoichiometric composition ratio was calcined in air at 800°C for 112 hours, formed into a disk shape with a diameter of 20mm and a thickness of 1mm, and then heated again in air for 10
It was obtained by firing at 00°C for 12 hours. Obtained S r 2
From the X-ray diffraction method, the B i 4T i 5oda substrate was
rzB i4T iso+a with a small amount of Sr B l
It was confirmed that aT 14015 was included.

The bismuth-based oxide superconductor powder used for screen printing includes bismuth trioxide (B tzo:+), strontium carbonate (S r C03), and calcium carbonate (C
aCCL+), copper oxide (Cub) powder (both purity 9)
9.9%). That is, these raw material powders were mixed with Bi:Sr:Ca:Cu=2:3.
:3:5 (molar ratio), calcined in air at 800°C for 12 hours, and then ground. The particle size of the powder obtained was 1-5 microns. The powder is mixed in an appropriate amount of ethanol to give S r 2B i 4T 1so
da pellets. After drying, the screen-printed bismuth-based oxide superconductor film thus formed was heated in air at 900°C for 30 minutes, and then cooled to room temperature at a cooling rate of 60″C per hour. The result was (BizOz) (sr2calcusol
The thickness of the film 11- is about 30 microns, and the C-axis length is 4
0 angstrom or more, and it was confirmed by X-ray diffraction that the film contained many high Tc phases.

As shown in Figure 2, which shows the resistance-temperature curve of a bismuth-based oxide superconductor film formed on a 5rzBi, Tis○II+ polycrystalline substrate, the resistance of this film decreased sharply at 150°C. Further, from FIG. 3 showing the results of magnetic susceptibility measurement by 5QtJID, the magnetic susceptibility becomes negative from around 150, confirming the Meissner effect. The volume fraction of the Tc-15QK phase estimated from the magnetic susceptibility was 3 to 8 percent.

In the present invention, the oxide superconductor was formed by a sputtering method, but a vacuum evaporation method, an electron beam evaporation method, a reactive evaporation method, a chemical vapor deposition method (CVD method), a screen printing method, a spray pyrolysis method, etc. As in Example 1, the film may be formed by a method normally used for forming an oxide superconductor film.

r Example 3J This example uses B13TiNb○9 (C-axis length 25.1 angstroms) as the substrate material, and B+zSr2CazCu30+o+x (
The case where a film with a C-axis length of approximately 36 angstroms was formed by screen printing is shown.

The B13TiNbOq substrate is made of bismuth dioxide (B!zc
h), titanium dioxide (TiOz), niobium pentoxide (Nb
zOs) powder (all purity 99.9%) was mixed in a stoichiometric ratio and heated in air for 800'
After pre-firing for 112 hours, it was formed into a disk shape with a diameter of 20 mm and a thickness of 1 mm, and was baked again in air at 1000'C for 12 hours. The obtained Bi:+T1Nb0q substrate is
It was confirmed by the line diffraction method that a single-phase B13TiNb○ was obtained.

The bismuth-based oxide superconductor powders used for screen printing include bismuth trioxide (BizCh), strontium carbonate (SrCO3), and calcium carbonate (CaCO3).
, copper oxide (Cub) powder (both 99.9% pure). That is, these raw material powders were prepared in the following manner: Bi:Sr:Ca:Cu=2:2:2
: 3 (molar ratio) and 1 in air at 800°C.
After baking for 2 hours, the product was crushed. The particle size of the powder obtained was 1-5 microns. The powder was mixed in an appropriate amount of ethanol and spread onto a B13TiNbOq berent. After drying, the screen-printed film of the bismuth-based oxide superconductor thus formed was exposed to air for 90°C.
After heating at 0°C for 30 minutes, it was cooled to room temperature at a cooling rate of 60°C per hour. The resulting BizS
The thickness of the rzCazCu30+o+x film was approximately 30 microns, and X-ray diffraction showed that the film was composed primarily of a high Tc phase. As a result, the ratio of the C-axis length of the substrate material to the C-axis of the obtained oxide superconductor was a simple integer ratio of 2:3.

Furthermore, as shown in Figure 4, which shows the resistance-temperature curve of a bismuth-based oxide superconductor film formed on a B13TiNb09 polycrystalline substrate by the screen printing method, this film showed zero resistance at 110°C, and the liquid The critical current density at nitrogen temperature was about 50,000 A/cm2. This value is sufficiently large for a thick film.

In the present invention, the oxide superconductor was formed by sputtering method, but vacuum evaporation method, electron beam evaporation method, reactive evaporation method, chemical vapor deposition method (CVD method), screen printing method, spray bicolysis method As in Example 1, the film may be formed by a method normally used for forming a film of an oxide superconductor.

? Example 4J Chemical formula B + 2 (S r + - xL ax) 4C
The oxide represented by u30y (x −〇, 05~0.4, y=s~12) has a structure that is a high Tc bismuth-based superconductor.
Although it is the same as the phase, it does not exhibit superconductivity because the carrier concentration is small. This B l z (S r 1-XL
ax) 4Cu, oy was used as the substrate material, and a high Tc phase of bismuth-based superconductor was grown on it. B i, (
S rl-xLaX)4Cu30. The substrate is polycrystalline,
It was manufactured using the usual ceramic solid phase reaction sintering method. Raw materials include Bi2O3,5rCOz, Lazy,...C
Using powder of u0 (all purity 99.9%), Bi:
Sr:La:Cu=2:3.5:0.5:3 (molar ratio)
The bismuth-based superconductor film was obtained by laser sputtering. A schematic diagram of this apparatus is shown in Figure 5. The laser sputtering method uses excimer laser
A film formation method in which high-power laser beams such as Recently, it has been attracting attention as a film forming method. Laser sputtering is performed in an argon-oxygen mixed atmosphere (Ar:O□) = 4: 1, total pressure Q,
1 torr).

The apparent composition of the target is B1zsrzcazcu.
30y oxide was used. The substrate temperature was 650° C., and the deposition rate of the 1 film was about 10 nm/min. The thickness of the obtained film was approximately 500 nm, and a zero resistance temperature of 106 K was obtained from resistivity measurements. From this, the film has a high Tc
It turns out that it is made of phase.

"Example 5" This example shows a case where Bi25r4Fe+O is used as the substrate material.

B iz S r4F e3 o has a structure extremely similar to the Bi-based copper oxide high temperature superconductor B iz S ra F e3 o. That is, it has a structure in which the copper sites and calcium sites of Bi-based copper oxide high temperature superconductors Bi2Sr, Fe, and O are replaced with iron and strontium, respectively. Using this substance as a substrate material, oxide superconductor BizS r2 Ca2Cu:
+o, An example of epitaxially growing a thin film thereon is shown below. BizSraFe=O, was used as a buffer film (buffer layer), and the liquid phase epitaxy method was used as the method for producing it. In addition, superconductor Bi25rz C
az Cu and OyrItX were created using a laser sputtering method. For details, see the above-mentioned Example 2.
It was formed according to the method described in Ill. The method for creating the Biz Sra Fez O film by liquid phase epitaxy is as follows: Biz Sra Fez O, S
Powders of r CO3, F ez O3+ (purity of all 3N) were weighed out to a ratio of Bi:Sr:Fe=2:4:3, put into a crucible, and melted at 1300°C. Next, put the oxide mag screw and RAM single crystal substrate into the crucible,
It was immersed in the B15rFe solution described above and taken out. In this state, BjSr attached to the magnesium oxide single crystal substrate
The Fe film was an amorphous state chamber. This film was then annealed in air at 900°C for 3 hours. Through this heat treatment, Bi25r4Fe whose C-axis is oriented perpendicular to the substrate
, O, crystal growth was confirmed by X-ray diffraction. When observed using a scanning electron microscope, the film had a grain size of 50
Polycrystalline film consisting of ~100 micron grains, film thickness approximately 1
It turned out to be 0 micron. On the Biz Sra Fe30y film created in this way, “Example 4J
A thin film of BizSrzCazCu3o was grown under the same conditions. This buffer layer B iz Srs
Fe5o has a layer structure similar to the layer showing high Tc of BizSrz Caz Cu30y, and BizSrzCaz Cu, oy grown on this has a layer structure showing high Tc over the entire surface, As a result of resistivity measurement, a zero resistance temperature of 106K was obtained.

"Effects" The present invention provides bismuth layered oxide polycrystalline or single crystal C
The axis length is equal to or close to the C-axis length of the oxide superconductor to be formed, or the C-axis length of the bismuth layered oxide polycrystal or single crystal and the C-axis length of the oxide superconductor to be formed. By selecting a bismuth layered oxide polycrystal or single crystal so that the ratio with the axial length is a simple integer ratio, an oxide superconductor having a high Tc layer on the bismuth layered oxide polycrystal or single crystal can be obtained. can be obtained with good reproducibility.

As shown in the examples, a thick film with a practically sufficient critical current density was obtained by a simple method called screen printing. Of course, similar or even better effects can be expected with thin films with a thickness on the order of microns. It is clear that the present invention can be applied to superconducting microdevices, superconducting magnets, superconducting power transport/storage, etc., and the present invention is industrially useful.

[Brief explanation of drawings]

Figure 1 shows Bi, Ti, 01
□A diagram showing the resistance-temperature curve of a bismuth-based oxide superconductor film formed on a polycrystalline substrate. Figures 2 and 3 show the resistance-temperature curve and magnetic susceptibility-temperature curve of a bismuth-based oxide superconductor film formed on a 5rzBiaTis○Il+ polycrystalline substrate by screen printing, respectively. FIG. 3 is a diagram showing a resistance-temperature curve of a bismuth-based oxide superconductor film formed on a B13TiNbOq polycrystalline substrate by a screen printing method. FIG. 5 shows a schematic diagram of an apparatus when using the laser sputtering method. Laser lens empty heater sample chamber target valve turbo pump rotary pump

Claims (1)

[Claims] 1. An oxide superconductor characterized by being formed on a bismuth layered oxide polycrystal or single crystal. 2. In claim 1, the oxide superconductor is:
An oxide superconductor characterized by being a bismuth layered oxide. 3. In claims 1 and 2, the oxide superconductor includes bismuth, strontium, calcium,
An oxide superconductor comprising copper and oxygen and having a critical temperature of 77.3 K or higher, or a c-axis length of 40 angstroms or higher. 4. In claim 1, the bismuth layered oxide polycrystal or single crystal is one whose c-axis length is equal to or close to the c-axis length of the oxide superconductor, or the bismuth layered oxide polycrystal or single crystal An oxide superconductor characterized in that the ratio of the c-axis length of a single crystal to the c-axis length of an oxide superconductor is a simple integer ratio.