JP2002326817A - Oxide superconductor and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Bi-based oxide superconductor having Bi 2212 structure whose superconductive property at 20 K is more excellent than the conventional one. SOLUTION: The subject conductor has Bi 2212 structure and its composition formula is Bi2-x Sr2 Ca1 Cu2+x O8+y . (however, 0.1<=x<=0.3 and 0.5<=y<=0.3) or Bi2+x Sr2-x Caz Cu2 O8+y . (however 0.1<=x<=0.3, 0.05<=y<=0.3 and 0.85<=z<=0.95).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide superconductor and a method for producing the same, and more particularly, to Bi22 having a novel composition.
The present invention relates to a 12-structure oxide superconductor and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, Bi of oxide superconductors has been used.
As the SrCaCu oxide (Bi-based oxide), the composition ratio (element ratio) Bi: Sr: Ca: Cu is 2: 2: 2: 3.
A type called a Bi2223 structure, and a type called a Bi2212 structure having a composition ratio of 2: 2: 1: 2 are known.

[0003] general composition of Bi-based oxide of the Bi2212 _{structure, Bi 2.1 Sr 1 .9 Ca 1} Cu 2 O 8 + y
It is. Bi-based oxide having a Bi2212 structure has a temperature of 20K.
(Kelvin) It is a material that is superior in superconducting properties at lower temperatures and is expected to be used for high-field magnets, magnetic levitation trains, NMR devices, and the like.

[0004]

[Problems to be Solved by the Invention] B of Bi2212 structure
In order to use the i-based oxide for the applications as described above,
It is desirable to have higher critical current densities and irreversible magnetic fields.

[0005] In a general Bi-based oxide having a Bi2212 structure, a 0.2-K.
5T value 0.1 × 10 ⁶ A / cm ² of the critical current density in a state in which the magnetic field has been applied in (Tesla) (J _c) (A: ampere) and not enough, also irreversible magnetic field 1 The value was as low as about 0.5T (tesla).

[0006] Therefore, the appearance of a Bi-based oxide having a Bi2212 structure, in which the critical current characteristic in a temperature range of 20 K or higher is superior to that of the prior art, is desired.

[0007]

In general, in order to improve the irreversible magnetic field, it is effective to reduce the anisotropy of superconducting characteristics and electromagnetic characteristics. Therefore, the inventors of the present invention have focused on reducing the high electrical resistance between BiO layers that brings high electromagnetic anisotropy to a Bi-based oxide having a Bi2212 structure, and conducted research. As a result, Bi
By substituting some Bi atoms of the O layer with Cu,
By introducing a conductive carrier into the BiO layer, the electric conductivity was improved, that is, the electric resistance was reduced. A material in which some of the Bi atoms are replaced with Cu is a material having a new composition of Bi2212 structure.
It was an i-based oxide.

[0008] The first oxide superconductor of the present invention is Bi oxide superconductor.
2212 structure, the element ratio of which is represented by the composition formula Bi _2-x
Characterized by being represented by _{Sr 2 Ca 1 Cu 2 + x} O 8 + y.
X in the above composition formula is a numerical value in the range of 0.1 ≦ x ≦ 0.3, and y is a numerical value in the range of 0.05 ≦ y ≦ 0.3.

In order to increase the electric conductivity of the BiO layer required to reduce the electromagnetic anisotropy of the superconductor having the Bi2212 structure, more oxygen ions are introduced into the BiO layer to increase the electrical conductivity of the BiO layer. There are ways to strengthen the bond.
When the amount of the Bi element is increased so that the trivalent Bi ion replaces the divalent Sr ion site, the positive charge amount increases,
An increase in the amount of oxygen ions between the BiO layers is expected. Furthermore, if the amount of the Ca element can be reduced below the normal value 2 in the composition formula, an increase in the number of conductive carriers in the entire crystal is expected to maintain electrical neutrality.

The second invention of the invention born from such a viewpoint
The oxide superconductor contains a large amount of Bi element and contains Sr and C
It has a composition with a small amount of element a. That is,
The second oxide superconductor has a Bi2212 structure.
And the element ratio is represented by the composition formula Bi _{2 + x}Sr_2-xCa_zCu_TwoO
_{8 + y}It is characterized by being represented by And the composition formula
X is a numerical value in the range of 0.1 ≦ x ≦ 0.3, and z
Is a numerical value in the range of 0.85 ≦ z ≦ 0.95, and
And y are numerical values in the range of 0.05 ≦ y ≦ 0.3.
You.

These first and second oxide superconductors have an irreversible magnetic field at a temperature of 20 K and a temperature of 2 T in the above-mentioned composition range, as will be apparent from the embodiment described later.
Or more. The critical current density at a temperature of 20 K and a magnetic field of 0.5 T is set to 0.2 × 10 ⁶ A
/ Cm ² it can be higher.

In the oxide superconductor of the first invention, the composition of the general Bi2212 structure is more
Since the amount of Cu having a smaller ionic radius than that of Bi ions is larger, the distance between CuO ₂ layers becomes shorter. Therefore, three-dimensionality can be increased, that is, anisotropy can be reduced. The first oxide superconductor is Bi
Since part of the trivalent Bi in the O layer is replaced by divalent Cu, the carriers in the BiO layer can be increased.
This increase in carriers also contributes to reducing anisotropy. Therefore, the oxide superconductor of the present invention has B
Despite having the i2212 structure, it has a high irreversible magnetic field because the anisotropy is smaller than before.

Similarly, the reduction of the anisotropy of Bi2212 is as follows.
This also occurs in the oxide superconductor of the second invention. That is, if the amount of oxygen ions between the BiO layers is increased by increasing the amount of the Bi element, the distance between the BiO layers is reduced,
The bonding between the layers increases. Further, when the amount of the divalent Ca element is too small, the amount of hole carriers in the crystal including the BiO layer increases, so that the electric conductivity increases and the anisotropy decreases.

In the BiO layers of the first and second oxide superconductors according to the present invention, the former is composed of Bi-O
Because some of the bonds are Cu—O bonds, and in the latter, the amount of oxygen between the BiO layers is large,
Three-dimensionality becomes stronger. This is because the lattice constant L of the crystal of the oxide superconductor in the c-axis direction is 3.02 nm ≦ L ≦ 3.06 n
This is apparent from the fact that the value is m. General Bi
The lattice constant of the material having the 2212 structure is about 3.07 to 3.08. The fact that the length of this connection is shorter means that the connection of the magnetic flux lines is stronger. As a result, the pinning effect of the magnetic flux lines increases, and the critical current density becomes higher than before.

Further, the inventors of the present invention provide the above-mentioned first embodiment.
Among the above composition ranges of the oxide superconductor of
_1.8Sr_TwoCa₁Cu_2.2O_w(8.0 ≦ w ≦ 8.25)
Of composition or above the second oxide superconductor
Among the above composition ranges, in particular, Bi_2.2Sr_1.8Ca_0.9C
u_2.0O_wThe composition of (8.1 ≦ w ≦ 8.3) has 20
At a temperature of K, the value of the irreversible magnetic field in the c-axis direction of the crystal becomes
4T to 7T, at a temperature of 20K and at 0.
The critical current density in a magnetic field of 5T is 1.0 × 10 ⁶A / c
m^TwoIt has been found that this is the case.

In producing the first or second oxide superconductor excellent in such superconducting properties, Bi
It is formed to include a step of mixing and calcining 2212 phase powder or a superconductor precursor powder to be Bi2212 phase by heat treatment, a step of molding a calcined powder, and a step of firing the molded article. Using the raw material rod and the solvent substance rod, a crystal of the oxide superconductor is grown by a solvent transfer floating zone melting method. In the manufacturing method of the present invention,
The invention is characterized in that a heat treatment is performed on the grown crystal at a temperature of 450 to 600 ° C. in an inert gas atmosphere in order to provide an optimal amount of oxygen.

In the manufacturing method of the present invention, it is preferable that the composition of the raw material rod and the composition of the solvent substance rod are different.

Further, in the manufacturing method of the present invention, preferably, the composition of the feed rod _{Bi 2 + x Sr 2-x} Ca z Cu 2 + y
O _q (However, -0.1 ≦ x ≦ 0.3, 1.0 ≦ z ≦
1.1, 0.1 ≦ y ≦ 0.3).

In the manufacturing method of the present invention, preferably, the composition of the solvent substance rod is Bi _{2 + x} Sr _1.9 C _az C
u _{2 +} y O _p (although, 0.4 ≦ x ≦ 0.6,0.9 ≦ z ≦
1.1, 0.4 ≦ y ≦ 0.6).

Further, in the manufacturing method of the present invention,
Preferably, the ratio of Bi in the raw material rod to Bi in the solvent substance rod is set to a value within the range of 0.7 to 0.9 in atomic ratio.

Further, in the manufacturing method of the present invention, preferably, Cu in the raw material rod and C in the solvent substance rod are used.
The ratio with u is preferably a value in the range of 0.8 to 0.95 in atomic ratio.

As a result, an under-doped oxide superconductor having slightly less carriers than usual can be obtained. In the underdoped material, the value of the irreversible magnetic field in the c-axis direction of the crystal and the value of the critical current density at a temperature of 20 K were able to be made higher than those in the related art.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As an embodiment of the present invention, first, an example of a method for manufacturing an oxide superconductor will be described.

Here, the solvent transfer floating zone melting (TSF)
Z: Traveling Solvant Floating Zone)
A single crystal of an oxide superconductor having a Bi2212 structure is manufactured.

First, material rods and solvent substance rods used in the TSFZ method are formed. For these raw material rods and solvent substance rods, sintered rods of Bi2212 oxide are used. For this reason,
First, Bi ₂ O ₃ , SrCO ₃ , CaCO ₃ , and CuO
Are weighed so as to have a predetermined metal element ratio, mixed, and calcined at a temperature of 810 ° C. for 48 hours.

Then, after the calcined powder is pulverized, the powder that becomes the raw material rod is calcined at a temperature of 840 ° C. and the powder that becomes a solvent substance rod is heated at a temperature of 820 ° C. for 72 hours. . After each powder is pulverized again, each powder is packed in a different rubber tube and pressed under a hydrostatic pressure of 2000 kg / cm ² .

Then, the molded material for the raw material rod and the molded material for the solvent substance rod, which were molded under pressure, were heated at 860 ° C. for the raw material rod and 840 ° C. for the solvent substance rod, respectively, in air. Was fired.

Thereafter, in order to stabilize the melting zone of the TSFZ method to be performed later, the fired raw material rod is melt-passed in the zone at a speed of 25 mm / hour to increase the density of the raw material rod. Thereby, a raw material rod and a solvent substance rod are obtained.

Next, Bi2212 is applied by using the TSFZ method.
A single crystal of oxide superconductor having a structure is grown. In Table 1,
The growth conditions are shown.

[0030]

[Table 1]

First, an infrared condenser heating furnace equipped with two spheroidal mirrors each having a 500 W halogen lamp attached to one focal point is used. The light of the lamp is focused on the other focus of the furnace to melt the material. The raw material rod is fixed by a jig in a furnace such that the longitudinal direction is the crystal growth direction. A growth table for growing a single crystal is fixed by a jig on an extension in the longitudinal direction of the raw material rod. Further, a solvent substance cut from the solvent substance rod is adhered to the tip of the raw material rod on the growth table side. The solvent material is melted by rotating the raw material rod by a lamp while rotating, and the raw material rod in contact with the solvent material is further melted. This forms a melting zone. A single crystal precipitates from the melting zone on the growth table. The precipitated single crystal grows on the upper side of the growth table. In order to stabilize the growth of the crystal, the raw material rod and the growth table are rotated in opposite directions to uniformly heat the melting zone.

As shown in Table 1, the crystal growth conditions in this example are such that the diameter of each of the two raw material rods (1) and (2) is 7.4 mm, and the diameter of the single crystal on the growth stage is 6 mm. .2
mm. The rotation speed of the raw material rod is set to 30 rotations / minute, and the rotation speed of the growth stage is set to 24 rotations / minute. The ratio of the feed rate of the raw material rod side with respect to the feed rate of the growth stage-side (V _s) (V _f) to (V _f / V _s) 0.7.

Further, the composition of the solvent substance rod was Bi _2.5 Sr _1.9
And Ca _1.0 Cu _2.5 O _p. Also, the first raw material rod (1)
Is Bi _1.9 Sr _1.9 Ca _1.0 Cu _2.2 O _q , and the composition of the second raw material rod (2) is Bi _2.1 Sr _1.9 Ca _1.1 Cu
_2.3 was O _q.

As a result of growing the crystal under such growth conditions, the first raw material rod (1) was Bi _1.78 Sr _1.99
Crystal composition of Ca _1.03 Cu _2.2 O _r can be obtained and,
For the second raw material rod (2), Bi _2.2 Sr _1.8 Ca
_0.9 Cu _2.0 crystal composition that O _r was obtained (EDX
(Confirmed by energy dispersive X-ray analysis.)

Incidentally, p, q and r in each of the composition formulas of the above-mentioned solvent substance rod, raw material rod and obtained crystal all indicate the amount of oxygen. Oxygen content is usually difficult to measure. Here, it is not necessary to indicate the range of this value. Therefore, the ranges of p, q, and r are not described here. However, if the amounts of other elements in the composition of the stable phase are determined, the values of p, q, and r can be calculated by equilibrium of the charge of the ions.

Since the above crystals generally contain an excessive amount of oxygen, it is necessary to control the amount of oxygen to obtain optimum critical current characteristics. Therefore, in order to control the amount of oxygen, the crystal is further subjected to a heat treatment at 450 to 600 ° C. in an inert gas or reducing gas atmosphere.

Here, in an argon atmosphere, at 600 ° C.
At a temperature of

As described above, Bi221 of this embodiment
First oxide superconductor Bi having two structures _1.8Sr_TwoCa₁Cu
_2.2O_w(W is 8.0 ≦ w ≦ 8.25) or the second
Oxide superconductor Bi_2.2Sr_1.8Ca_0.9Cu_2.0O
_w(W is 8.1 ≦ w ≦ 8.3) crystals were obtained.

The irreversible magnetic field in the c-axis direction at a temperature of 20 K of the obtained first and second oxide superconductors is 7 T, and the conventional irreversible magnetic field of Bi2212 at 20 K is about 1.5 T. Very high values were obtained compared to those that were. The value of the critical current density (J _c) in a state in which the magnetic field is applied in the temperature in and c flux density parallel to the axis 0.5T of 20K was ^{1.0 × 10 6 A / cm 2} . The value of the critical current density is the same as that of the conventional Bi2 under the same conditions.
About 10 times the value of 212 was obtained. Therefore, it was found that these obtained oxide superconductors have superconducting properties at a temperature of 20 K which are much better than those of the conventional Bi2212 structure material. The values of the irreversible magnetic field and the critical current density are determined by the superconducting quantum interference device (SQUI).
D: From a magnetization curve measured using a magnetometer (SQUID magnetometer) using a superconducting quantum interface device, a critical current density was calculated using a bean model, and the critical current density was 100 A / cm ^2. The magnetic field at the time of was defined as an irreversible magnetic field.

The c-axis lattice constant of each crystal measured using an X-ray diffractometer was 3.04 nm. Since the lattice constant of a general Bi2212 material is 3.07 to 3.08, it can be seen that the composition of the present invention has a shorter lattice constant. This means that the bonding force between the BiO layers has increased, and some of the Bi
Is replaced by Cu. When the Bi ³⁺ site is replaced by Cu ²⁺ , carriers (holes) increase to maintain electrical neutrality. For this reason, the coupling force between the magnetic flux pancakes in the c-axis direction increases, and the hardness of the magnetic flux lines increases. Thereby, it is considered that the pinning effect of the magnetic flux lines is increased, and the value of the critical current density is higher than before.

When a part of Bi is replaced by Cu, or when oxygen is introduced between BiO layers, C
Since the distance between the uO ₂ layers is reduced, a structure with low anisotropy can be realized. It is a well-known fact that the value of the irreversible magnetic field increases as the anisotropy decreases.

Further, for each of the obtained crystals,
XRD (X-ray diffraction), HRTEM (High-resolution transmission electron microscope) and LRTEM (Low-resolution transmission electron microscope) showed that the impurity phase (heterophase) and B
No i2223 phase was found.

Further, the FWHM (full width at half maximum) of the X-ray diffraction line of each of the obtained crystals was 0.05 degree (degree), indicating that the crystals were high quality crystals.

Further, the temperature change of the magnetization of each of the obtained crystals was measured using a SQUID magnetometer. Similar results were obtained for both crystals.

FIG. 1 shows the results for the crystal of the second oxide superconductor. The temperature T (K) is plotted on the horizontal axis and the magnetic susceptibility (SI) is plotted on the vertical axis in FIG. The curve with a white square in FIG. 1 shows the temperature dependence of magnetization in ZFC (zero field cooling) in which the temperature is increased in a magnetic field after cooling in the absence of a magnetic field. In the figure, a curve with a black square indicates magnetization in FCW (field cool warming) in which a temperature is increased after cooling under a magnetic field of 3 Oe (Oe: Oersted) (237 A / m) applied in parallel to the c-axis. Shows the temperature dependence of.

According to FIG. 1, the critical temperature (T _c ) of this crystal was 93.4K. Also, the difference in magnetization between the FCW curve and the ZFC curve below the critical temperature is only 0.04-0.06 (SI). General Bi2
Since the difference is about 0.4 (SI) in the crystal having the 212 structure, it was found that the crystal was uniform and had very few structural defects.

[0047]

EXAMPLES Hereinafter, as a first example of the present invention, an example in which a heat treatment is performed on a crystal of an oxide superconductor under the same or different conditions as those described in the above embodiment will be described.

<Example 1> Crystals of the first and second oxide superconductors (hereinafter, also simply referred to as first and second crystals) are formed in the same manner as in the method described in the above embodiment. Let it grow. Next, four samples of crystals grown in the same manner are prepared. These samples for each of the first and second oxide superconductor crystals are referred to as Sample 1 to Sample 4.

The sample 1 is not subjected to a heat treatment. Sample 2 is subjected to a heat treatment at a temperature of 450 ° C. in an oxygen atmosphere. The sample 3 is heat-treated at a temperature of 450 ° C. in an Ar (argon) atmosphere. The sample 4 is subjected to a heat treatment at the same condition as the above embodiment, that is, at a temperature of 600 ° C. in an Ar atmosphere. The heat treatment time was 20 days for each sample. Thereby, the amount of oxygen in the crystal changes, and the amount of carrier changes. Since the sample 1 was not heat-treated, the carrier was slightly overdoped. Sample 2 is an over-doped crystal because heat treatment is performed in oxygen. Sample 3 has the optimum doping amount of carrier exhibiting the highest critical temperature (T _c). Sample 4 is an under-doped crystal.

[0050] For the heat treatment is finished samples, respectively, were measured value of the critical temperature (superconducting transition temperature) T _c.
Substantially the same measurement results were obtained for each sample of the first and second crystals. Table 2 shows the results.

[0051]

[Table 2]

[0052] According to Table 2, T _c of the specimen 1 is 88K, T _c of the sample 2 is 75K, the T _c of the sample 3 94.5K, and T _c of the sample 4 becomes 93.4K.

The temperature dependence of the irreversible magnetic field in the temperature range from 20 K to 95 K was examined for each of the above samples 2 to 4 of the first and second oxide superconductor crystals. Substantially the same measurement results were obtained for each sample of the first and second crystals. The result is shown in FIG. FIG. 2 is a characteristic diagram showing the result. The curve indicated by the diamond in FIG. 2 is the characteristic curve of Sample 2, and the curve indicated by the triangle is the characteristic curve of Sample 3. The curve shown by a white circle is the characteristic curve of Sample 4. According to FIG. 2, the value of the irreversible magnetic field of the sample 4 around 20K is clearly higher than the others. Table 3 shows the values of the irreversible magnetic field of Samples 2 to 4 at a temperature of 20K.

[0054]

[Table 3]

According to Table 3, the value of the irreversible magnetic field of Sample 2 was 4.8 T. The value of the irreversible magnetic field of Sample 3 was 1.5T. Then, the value of the irreversible magnetic field of Sample 4 was 7T.

FIG. 2 shows a conventional general composition (B
_{i 2.1 Sr 1.9 Ca 1 Cu 2} O x x: Bi22 oxygen amount)
Crystals with 12 structures (Reference 1: F. Iga, AKGrover, Y. Yamag)
uchi, Y.Nishihara, N.Goyal, SVBhat, Phys.Rev.B.Vo
l.51 No.13, (1995), pp.8521-8528) also shows the temperature dependence of the irreversible magnetic field. The characteristic curve of this conventional crystal is shown by a curve with a black circle in FIG. According to FIG. 2 and Table 3, the value of the irreversible magnetic field at a temperature of 20 K is about 1.5 T, so that Samples 2 and 4
In the crystal of the above, the value of the irreversible magnetic field could be made much higher than before.

Next, the critical current density (J _c ) of the crystals of Samples 2 and 4 at a temperature of 20 K and a magnetic field parallel to the c-axis direction was measured.
It indicates the value of the J _c in Table 4.

[0058]

[Table 4]

FIG. 3 shows the J _c values of Samples 2 and 4.
FIG. 4 is a characteristic diagram of the magnetic field strength dependence of FIG. This characteristic is the first
And the second crystals were substantially identical for each sample. In FIG. 3, a curve indicated by a white diamond is a characteristic curve of Sample 2, and a curve indicated by a white circle is a characteristic curve of Sample 4. Further, a curve indicated by a downward white triangle indicates the magnetic field dependence of the sample 4 at 25K. FIG. 3 shows, as a comparative example, 15K of the crystal into which Pb was introduced.
J _c in a state in which the magnetic field has been applied in 0.5T at a temperature
Are indicated by black diamonds. The value of the crystal of J _c is shown in terms of black rectangle Ti was introduced under the same conditions. Furthermore, the value of the crystals of J _c of conventional general Bi2212 structure is shown in terms of black circles.

According to FIG. 3 and Table 4, 20K
The J _c of the temperature, even in the field of any intensity, much higher J _c is obtained than other materials.

Here, J at a temperature of 20 K_cThink about
Sympathize. Sample 2 J_cIs 0.3 × 10 ⁶A / cm^TwoSo
Was. In addition, J of sample 4_cIs 1.0 × 10⁶A / cm^TwoIn
Was. In addition, a conventional Bi2212 crystal having a 15K
J at temperature_cIs 0.1 × 10⁶A / cm^TwoIt is. Yo
I mean J at a temperature of 20K_cIs a lower value
It is clear. Therefore, sample 2 and sample 4
Has a higher critical current density than before.
You. For sample 4, J at a temperature of 25K _cAlso
Measured (Table 4 and the song indicated by the downward triangle in FIG. 3)
See line. ). As a result, at a temperature of 25K, 0.24 × 10
⁶A / cm^TwoJ_cwas gotten. Therefore, Sample 4,
For the same crystal as produced in the above embodiment,
Even at a temperature of 25K, the conventional Bi2212 structure
Better than the superconducting properties of crystals at a temperature of 15K
It is.

As a comparative example, reference 2 (reference 2: T.
W.Li, R.J.Drost, P.H.Kes, H.W.Zandbergen, N.T.Hie
n, A.A.Menoysky, J.J.M.Franse, Physica C274 (1997),
p.197), B as the pinning center
The crystal in which Pb was introduced into the i2212 structure was 15K
J at temperature_cIs 0.9 × 10⁶A / cm^TwoSo
Was. However, since the value is at a temperature of 15 K,
0.9 × 10 at temperature ⁶A / cm^TwoAnd even lower values than
Become.

In addition, Ti is introduced into the Bi2212 structure.
J at a temperature of 15 K _cIs 0.3 × 10⁶
A / cm^Two(See Table 4 and FIG. 3).

Therefore, in the Bi2212 structure,
The first and second oxide superconductors having the compositions found by the inventors of the present invention are better than the Bi2212 structure crystals obtained by introducing other materials in order to improve the superconducting properties. It has been found that it has excellent superconducting characteristics.

Example 2 As Example 2, the superconducting characteristics of a crystal having a composition around the composition of the first oxide superconductor obtained in the above embodiment are examined.

First, the composition ratio of A / Bi / Cu (where A is a combination of Sr and Ca) is 3 / about in substantially the same manner as described in the above embodiment.
1.6 / 2.4 crystals (referred to as third crystals),
A crystal that becomes 3 / 1.7 / 2.3 (referred to as a fourth crystal), a crystal that becomes 3 / 1.9 / 2.1 (referred to as a fifth crystal), and 3 / 2.0 / 2.0 (6th crystal)
Crystal. ) Are formed respectively. By changing various crystal growth conditions and the intensity of the halogen lamp, crystals having different compositions can be formed.

Each of the grown crystals is heated at a temperature of 600 ° C. in an Ar atmosphere similarly to the above embodiment. Thereby, third to sixth crystals are obtained.

Next, the superconducting characteristics of these crystals will be examined. Here, the irreversible magnetic field at 20 K and the critical current density in a state where a magnetic field of 0.5 T is applied at a temperature of 15 K and parallel to the c-axis direction are measured.

Table 5 shows the results. In Table 5, the superconductivity of the crystal obtained in the above embodiment is also shown. In the composition of the first crystal obtained in the embodiment, A / Bi / Cu is represented by 3 / 1.8 / 2.2.

[0070]

[Table 5]

According to Table 5, the third crystal has an irreversible magnetic field at 20 K of 2 to 5 T and a critical current density of 0.3 to 5 T.
Was ^{0.7 × 10 6 A / cm 2} . The fourth crystal has an irreversible magnetic field of 3 to 6 T and a critical current density of 0.1 T.
It was 5 to 1.0 × 10 ⁶ A / cm ² . The fifth crystal had an irreversible magnetic field of 2 to 4 T and a critical current density of 0.2 to 0.5 × 10 ⁶ A / cm ² . Also, the sixth
Crystal had an irreversible magnetic field of 1.5 T or less and a critical current density of 0.1 × 10 ⁶ A / cm ² or less. The irreversible magnetic field at 20 K of the crystal obtained in the embodiment is as follows.
4 to 7 T, and the critical current density is 1.0 × 10 ⁶ A / c
The value was larger than m ² .

Therefore, the superconductivity of the crystal obtained in the embodiment is the best. The third to fifth crystals also have improved superconductivity compared to the conventional material having a general composition of the Bi2212 structure.

However, the third crystal has a problem that the crystal is difficult to grow. Therefore, if this crystal can be easily manufactured, the possibility of its use can be expected.

Thus, within the range of the composition of the fourth crystal and the composition of the fifth crystal, it was found that this crystal exhibited better superconductivity than the conventional one.

Therefore, in the Bi2212 structure,
Bi: Sr: Ca: Cu is 1.7 to 1.9: 2:
Crystals having a composition of 1: 2.1 to 2.3 have good characteristics. When this is represented by a composition formula, Bi _2-x Sr ₂ Ca
₁ Cu _{2 + x} O _{8 + y} Then, x in this composition formula is 0.1 ≦ x ≦ 0.3, and y is 0.05 ≦ y ≦ 0.3.

Example 3 As Example 3, the superconducting characteristics of a crystal having a composition around the composition of the second oxide superconductor obtained in the above embodiment are examined.

First, the composition ratio of A / Bi / Cu (where A is a combination of Sr and Ca) is 2.6 in substantially the same manner as in the method described in the above embodiment.
/2.3/2 crystal (referred to as seventh crystal),
A crystal that becomes 2.8 / 2.2 / 2 (referred to as an eighth crystal), a crystal that becomes 2.9 / 2.1 / 2 (referred to as a ninth crystal), and 3.0 / 2. 0/2 crystal (first
It is referred to as a zero crystal. ) Are formed respectively. By changing various crystal growth conditions and the intensity of the halogen lamp, crystals having different compositions can be formed.

Each of the grown crystals is heated at a temperature of 600 ° C. in an Ar atmosphere similarly to the above embodiment. Thus, seventh to tenth crystals are obtained.

Next, the superconducting characteristics of these crystals are examined. Here, the irreversible magnetic field at 20 K and the critical current density in a state where a magnetic field of 0.5 T is applied at a temperature of 15 K and parallel to the c-axis direction are measured.

Table 6 shows the results. In Table 6, the superconducting characteristics of the crystals obtained in the above embodiment are also shown. In the composition of the second crystal obtained in the embodiment, A / Bi / Cu is represented by 2.7 / 2.2 / 2.

[0081]

[Table 6]

According to Table 6, the seventh crystal has an irreversible magnetic field at 20K of 3 to 6 T and a critical current density of 0.5 to 6 T.
Was ^{0.9 × 10 6 A / cm 2} . The eighth crystal has an irreversible magnetic field of 2 to 5 T and a critical current density of 0.1 T.
It was 4 to 0.7 × 10 ⁶ A / cm ² . The ninth crystal had an irreversible magnetic field of 2 to 4 T and a critical current density of 0.3 to 0.6 × 10 ⁶ A / cm ² . Also, the first
Crystal 0 had an irreversible magnetic field of 1.5 T or less and a critical current density of 0.1 × 10 ⁶ A / cm ² or less. Also,
The irreversible magnetic field at 20 K of the crystal obtained in the embodiment is 4 to 7 T, and the critical current density is 1.0 × 10 ⁶ A.
/ Cm ² .

Therefore, the superconductivity of the crystal obtained in the embodiment is the best. The seventh to ninth crystals also have improved superconducting properties as compared with the conventional material having a general composition of the Bi2212 structure.

However, the seventh crystal has a problem that it is difficult to grow the crystal. Therefore, if this crystal can be easily manufactured, the possibility of its use can be expected.

Therefore, in the range of the composition of the seventh to ninth crystals,
This crystal was found to exhibit better superconducting properties than before.

Therefore, the Bi2212 structure has
The ratio of Bi: Sr: Ca: Cu is 2.1 to 2.3: 1.
Crystal having a composition of 9 to 1.7: 0.85 to 0.95: 2
Have good properties. When this is made into the composition formula,
Bi_{2 + x}Sr_2-xCa_zCu_TwoO _{8 + y}Becomes And this
X in the composition is 0.1 ≦ x ≦ 0.3, and z is 0.85 ≦ z
When ≦ 0.95, y becomes 0.05 ≦ y ≦ 0.3.

[0087]

[Effect of the Invention] As apparent from the above description, according to the oxide superconductor of the present invention, a Bi2212 structure, its composition formula _{Bi 2-x Sr 2 Ca 1} Cu 2 + x O
_{8 + y} (However, 0.1 ≦ x ≦ 0.3 and 0.05 ≦
y ≦ 0.3), _{or, Bi 2 + x Sr 2 -x} Ca z Cu 2 O
_{8 + y} (However, 0.1 ≦ x ≦ 0.3, 0.05 ≦ y ≦
0.3, and 0.85 ≦ z ≦ 0.95).

In these oxide superconductors, the lattice constant in the c-axis direction has a value smaller than the lattice constant of a conventional crystal having a Bi2212 structure. Thereby, in the first superconducting material, a part of the Bi-O bond in the crystal structure is changed to Cu-O
It is suggested that it is binding. By introducing Cu into a part of the BiO layer, carriers in the BiO layer increase, and the distance between the CuO ₂ layers also decreases. Therefore, three-dimensionality is improved. That is, since the anisotropy becomes smaller, the value of the irreversible magnetic field becomes higher than before. Also,
Since the magnetic flux lines are strongly pinned, the generation of the resistance of the crystal can be suppressed. As for the second superconducting material, it is estimated that the amount of oxygen ions existing between the BiO layers increased and the lattice constant in the c-axis direction decreased. At the same time, since the amount of Ca is small, the amount of carrier increases,
It is considered that the anisotropy was reduced. Therefore, a crystal having a higher irreversible magnetic field and a higher critical current density than before can be obtained.

[Brief description of the drawings]

FIG. 1 shows the temperature dependence of the magnetization of the oxide superconductor of the present invention.

FIG. 2 shows samples 2 to 4 of Example 1 and a conventional Bi22.
4 shows the temperature dependence of the irreversible magnetic field of a 12-structure crystal.

FIG. 3 shows the magnetic field dependence of the critical current density of Samples 2 and 4 of Example 1.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Huang ▲ Tatsu ▼ 1-10-13 Shinonome, Koto-ku, Tokyo Foundation International Research Institute of Superconductivity Technology, Superconductivity Engineering Laboratory (72) Inventor Naoki Koshizuka 1-10-13 Shinonome, Koto-ku, Tokyo Inside the Superconductivity Engineering Research Center, International Superconducting Technology Research Center (72) Inventor Susumu Shibata Oki Electric Industry Co., Ltd. 1-7-12 Toranomon, Minato-ku, Tokyo F term (reference) 4G047 JA02 JC10 KB04 KC01 KC06 4G048 AA05 AB01 AB06 AC04 AD01 AE05 4G077 AA02 BC58 CE03 EC07 FE05 FE11 HA08 5G321 AA05 DB28 DB46 DB47 DB55

Claims (11)

[Claims] 1. A Bi2212 structure having a composition formula of Bi _2-x Sr ₂ Ca ₁ Cu _{2 + x} O _{8 + y} , where 0.1 ≦ x ≦ 0.3 and 0.1. 05 ≦ y ≦
An oxide superconductor characterized by being 0.3. 2. A Bi2212 structure, its composition formula, a _{Bi 2 + x Sr 2-x} Ca z Cu 2 O 8 + y, however, 0.1 ≦ x ≦ 0.3,0.85 ≦ z ≦ 0.9
5. An oxide superconductor, wherein 0.05 ≦ y ≦ 0.3. 3. The lattice constant L in the c-axis direction is 3.02 nm.
≦ L ≦ 3.06 nm, the irreversible magnetic field at a temperature of 20 K is 4 T or more, and the critical current density at a temperature of 20 K and a magnetic field of 0.5 T is 0.3 × 10 ⁶ A / cm ² or more. The oxide superconductor according to claim 1 or 2, wherein 4. A Bi2212 structure having a composition formula of Bi _1.8 Sr ₂ Ca ₁ Cu _2.2 O _w , wherein _w in the composition formula is 8.0 ≦ w ≦ 8.25. An oxide superconductor characterized by the above-mentioned. 5. A Bi2212 structure having a composition formula of Bi _2.2 Sr _1.8 Ca _0.9 Cu _2.0 O _w , wherein w in the composition is 8.1 ≦ w ≦ 8.3. An oxide superconductor characterized by the above-mentioned. 6. A step of mixing and calcining a Bi2212 phase powder or a precursor powder for a superconductor which becomes a Bi2212 phase by heat treatment, and a step of molding a calcined powder.
Using a raw material rod and a solvent substance rod formed including a step of firing a molded product, wherein a method of growing a crystal of the oxide superconductor by a solvent transfer floating zone melting method is used. A method for producing an oxide superconductor, comprising heating the crystal of the oxide superconductor thus formed at a temperature of 450 to 600 ° C. in an inert gas or reducing gas atmosphere. 7. The method for manufacturing an oxide superconductor according to claim 6, wherein the composition of the raw material rod and the composition of the solvent substance rod are different. Wherein said composition of the raw material rod _{Bi 2 + x Sr 2-x} Ca z Cu 2 + y O q However, -0.1 ≦ x ≦ 0.3,1.0 ≦ z ≦ 1.1,
The method for producing an oxide superconductor according to claim 6, wherein 0.1 ≦ y ≦ 0.3. 9. The composition of the solvent material rod _{Bi 2 + x Sr 1.9 Ca z} Cu 2 + y O p However, 0.4 ≦ x ≦ 0.6,0.9 ≦ z ≦ 1.1,
9. The method for producing an oxide superconductor according to claim 6, wherein 0.4 ≦ y ≦ 0.6. 9. 10. The atomic ratio of Bi in the raw material rod to Bi in the solvent substance rod is in a range of 0.7 to 0.9. Or the method for producing an oxide superconductor according to 7. 11. The atomic ratio of Cu in the raw material rod to Cu in the solvent substance rod is in the range of 0.8 to 0.95. 9. The method for producing an oxide superconductor according to claim 7, 7 or 8.