JP2523928B2 - Oxide superconductor and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to an oxide superconductor having a high superconducting transition temperature.

2. Description of the Related Art Superconductors have a) zero electrical resistance, b) safe diamagnetism, and c) Josephson effect.
It has properties not found in other materials, such as power transportation, generators,
A wide range of applications such as fusion plasma confinement, magnetic levitation trains, magnetic shields, and high-speed computers are expected.

However, in the conventional metal-based superconductor, even the highest superconducting transition temperature is about 23K, and in actual use it must be cooled using expensive liquid helium and a large-scale heat insulation device, It was a big problem.

Therefore, a search for a material that becomes a superconductor at higher temperatures has been made.

In 1986, IBM's Bednorz and Muller found an oxide-based superconducting material (La _1-z Sr _z ) ₂ CuO _x with a high superconducting transition temperature of about 40 K, and thereafter. YBa ₂ Cu ₃ O _x , Bi-Sr-Ca-Cu-O, Tl-Ba-Ca
It has been reported that superconducting transition at higher temperature is followed by —Cu—O and the like.

It is expected that the higher the superconducting transition temperature, the easier the cooling becomes, and the larger the critical current density and the critical magnetic field when used at the same temperature, the wider the range of applications is expected.

For this reason, many studies are currently being conducted on the manufacturing methods, physical properties, and applications of these materials.

Problems to be Solved by the Invention However, among these materials, (La _1-z Sr _z ) ₂ CuO _x has a superconducting transition temperature of 40 K or less, and thus inexpensive liquid nitrogen cannot be used for cooling. .

YBa ₂ Cu ₃ O _x has a transition temperature of about 90 K, which is above the liquid nitrogen temperature, but the difference is small, and the atmosphere must be controlled during firing.

Bi-Sr-Ca-Cu-O has a transition temperature of more than 100K, but there is a problem that long-term firing near the melting point is necessary to form a single phase.

On the other hand, Tl-Ba-Ca-Cu-O has a maximum transition temperature of about 120 K, and is the best in terms of transition temperature. However, since it contains a large amount of highly toxic Tl and its melting point is low at about 920 ° C, it is necessary to bake near the melting point, so that Tl easily evaporates and has a different transition temperature. There are many compounds with different Tl content and CaCu content (molar ratio of TlBaCaCu, 2212,2223,2234,2245,1212,1223,1234, etc.). There was a problem that it was difficult to control such as.

On the other hand, Tl-Ba-Ca-Cu-O was prepared by substituting Sr for Ba in Tl-Ba-Ca-Cu-O.
Since the melting point of Sr-Ca-Cu-O ceramics is 1000 ° C or more, there is a margin in the firing temperature range, and the types of compounds that can be produced are limited to two types (the molar ratio of TlSrCaCu is 1212, 1223) It is also considered relatively easy to create a single phase.

In this system, however, it is difficult to prepare a sample that undergoes a superconducting transition above the liquid nitrogen temperature, and even if a sample with an onset temperature of about 100K was obtained, the zero resistance temperature was often below 50K.

Moreover, these Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-
A (Tl, Bi) Sr ₂ CaCu ₂ O _x phase in which O was combined was also found, but its superconducting transition temperature was as low as about 80K.

It is an object of the present invention to provide an oxide superconductor having a high zero resistance temperature, a method for producing the oxide superconductor which can be easily produced and has extremely low toxicity.

Means for Solving the Problems The present invention has, as a composition, at least Tl, Bi, Sr, Ca and Cu, and has a crystal structure belonging to a tetragonal system with lattice constants of a = 0.38 nm and c = 1.53 nm, An oxide superconducting material eliminates the above-mentioned drawbacks of the oxide superconductor, and a method for producing a single-phase ceramic.

Action The inventors of the present invention, as a result of diligent search and research on the composition ratio of a conventionally unknown oxide high-temperature superconductor, found a superconducting transition close to 120 K at maximum in the substance having the above composition and structure.

This superconductor has a high transition temperature, but the highly toxic Tl
The maximum content of 12-13 mol% is Tl-
About 22 of Tl ₂ Ba ₂ Ca ₂ Cu ₃ O _x which is a high Tc phase of Ba-Ca-Cu-O system
It is about 1/2 of the mole percentage and Tl ₂ Ba ₂ Ca ₂ Cu ₃ O _x
It has a higher melting point and is more stable at the same temperature. In addition, we examined the manufacturing method and developed a method for producing ceramics consisting of a single phase.

This preparation method is also effective for the TlSr ₂ Ca ₂ Cu ₃ O _x phase that does not contain Bi, has the effect of facilitating the synthesis of a single phase, and improving its transition temperature.

Example Hereinafter, the composition ratio and the crystal structure of this superconductor will be described in Examples 1 and 2, and the method for producing a ceramic having a single phase will be described in Examples 3 and 4.

Example 1 As a starting material, Tl ₂ O ₃ , Bi ₂ O ₃ , SrCO ₃ ,
Powders of CaCO ₃ and CuO were used.

Of these powders, SrCO ₃ and CuO were weighed so that the ratio was Sr: Cu = 2: 1 and the total amount was 30 g, and 40 ml of ethanol was dispersed using a ZrO ₂ ball with a diameter of 2 mm in a vibration mill. The medium was pulverized and mixed for 1 hour.

After the completion of the mixing, the entire amount of the dispersion medium and the whole was dried at 120 ° C. in a dryer. The obtained powder was calcined in air at 1000 ° C. for 5 hours, pulverized in a vibration mill for 30 minutes in the same manner as described above, and dried at 120 ° C.

When this calcined powder was analyzed by X-ray diffraction, Sr ₂ C
It was confirmed that uO ₃ was generated.

Then, in the same way, from CaCO ₃ and CuO, C by calcination at 950 ° C
A powder of a ₂ CuO ₃ was synthesized.

The composition ratio of these Sr ₂ CuO ₃ and Ca ₂ CuO ₃ powders was analyzed, and it was confirmed that there was no deviation from the target composition.

This Sr ₂ CuO ₃ and Ca ₂ CuO ₃ powder and Tl ₂ O ₃ ,, Bi ₂ O ₃ , CuO powder have a molar ratio of Tl: Bi: Sr: Ca: Cu = 2: 0.5: 2: 3: 4,
Moreover, each was weighed so that the total weight was 20 g.

The weighed powder was pulverized and mixed for 1 hour by a raker. After mixing, 0.4g of this powder is 500kg in a 15mm × 5mm mold.
Uniaxial pressure molding was performed at a pressure of / cm ² .

This molded body was wrapped in Au foil, further sealed in a quartz tube under reduced pressure (about 10 ^-2 Torr), and heated in an electric furnace for 900 ~ 9
It was baked at 50 ° C. for 1 to 4 hours. The temperature raising and lowering rate is 400
C / hr.

A silver electrode is attached to the sintered body, and the temperature change of the electric resistance is measured from 300K to 5K at a measuring current of 10mA by the usual 4-terminal method.
The temperature (T1) at which the electric resistance started to rapidly decrease due to the superconducting transition and the temperature (T2) at which the resistance became 0 were determined. Also,
The temperature change of the magnetic susceptibility of the sintered body was measured, and the temperature (T3) at which the magnetic susceptibility started to change rapidly due to the Meissner effect was obtained.
The results are shown in Table 1.

As is clear from Table 1, a Tc ^onset exceeding 100K was observed at a baking temperature of 900 ° C or higher, and the zero resistance temperature also exceeded 100K when baked at 925 ° C for 40 hours.

Next, these sinters were crushed and then analyzed by X-ray diffraction. As a result, in the sample fired at 850 ° C., a = 0.35 nm, c
The phase belonging to the tetragonal system of = 1.21 nm was the main phase.

This phase is (Tl, Bi) Sr ₂ CaCu as previously reported.
_It is considered to be a ₂ O _x phase and its Tc is about 80K.

On the other hand, in the case of the sample fired at a temperature of 900 ° C or higher, a = 0.3
Formation of a phase belonging to the tetragonal system of 8 nm, c = 1.53 nm was observed.

Sometimes in the samples fired at 925 ° C for 40 hours, this c = 1.53 nm phase became the main phase. As impurities, Ca ₂ CuO ₃ , CuO or c =
A small amount of 1.21 nm phases coexisted. The highest Tc is the sample fired at 925 ° C for 40 hours, so c = 1.53 nm
The phase is the superconducting phase in this system.

Example 2 In the same manner as in Example 1, Sr ₂ CuO ₃ and Ca ₂ CuO ₃ powder, and Tl ₂ O ₃ , Bi ₂ O ₃ were used so that the composition ratio shown in Table 2 and the total weight became 20 g. Then, CuO powder was weighed. However, Tl: Sr: Ca: Cu
The compounding ratios of (2): 2: 3: 4 and 2: 2: 2: 3 are referred to as compounding composition ratios (2234) and (2223).

The weighed powder was mixed, molded and fired in the same manner as in Example 1, and the superconducting transition temperature of the sintered body was measured. The results are shown in Table 3.

As is clear from Table 3, in the case of firing at 975 ° C for 5 hours,
In the composition ratios (2234) and (2223) without addition of Bi, the onset of the superconducting transition was 90 K or less and the zero resistance temperature was 15 K or less in the resistance measurement.

On the other hand, when 0.1 is added to Bi, zero resistance becomes 15K or more, and it is observed, and in the composition in which Bi is added to 0.2 or more, the onset exceeds 110K and zero at the composition ratio (2234). The resistance was over 100K.

In addition, even in the blending composition ratio (2223), a transition temperature close to 100 K was obtained by adding Bi.

Next, if the firing conditions are 975 ° C. for 5 hours and 850 ° C. for 100 hours, the Bi-free sample has a high resistance in the normal conduction state, and onset of resistance change is observed, but zero resistance and Meissner The effect onset is no longer observed.

When the composition ratio of this sample was analyzed, Tl was scarcely contained. It is considered that this is because Tl was evaporated by the long-term firing.

On the other hand, with the Bi-added composition, the composition ratio (223
4) when the highest onset temperature 117K, zero resistance temperature 114K
was gotten. Further, in the composition analysis, about half of the compounded Tl remained.

Next, of these sintered bodies, a sample having a high transition temperature was crushed, a powder X-ray diffraction measurement was performed, and a composition analysis by EPMA of the crystal grains on the fracture surface of the sintered body was performed. The generative phase was identified.

In the Tl-Sr-Ca-Cu-O system, the Tl: Sr: Ca: Cu composition ratio is
The existence of a phase that is approximately 1: 2: 1: 2 and a phase that is 1: 2: 2: 3 has been reported. These are “1212” phase and “1223” respectively
I will call it Ai.

In the Bi-free sample of this example, the compounding composition ratio (2234)
For both (2223) and (2223), the "1212" phase and the unknown phase were calcined at 975 ° C for 5 hrs by X-ray diffraction, and 750 ° C for 5 hrs and then 850 ° C for 100 hrs.
Only the unknown phase was formed in the calcined sample.

This unknown phase was confirmed by EPMA to be a phase containing almost no Tl.

On the other hand, in the system containing 0.2 or more of Bi, the T
Diffraction patterns similar to those of the "1212" phase and "1223" phase of the 1-Sr-Ca-Cu-O system were obtained. The respective lattice constants are a = 0.35 nm, c = 1.21 nm (tetragonal) and a = 0.
It was 38 nm, c = 1.53 nm (tetragonal).

The former has already been reported as a (Tl, Bi) Sr ₂ CaCu ₂ O _x phase, which has the same crystal structure as the “1212” phase and Tl is partially replaced by Bi, and its Tc is assumed to be around 80K. There is.

On the other hand, the latter is similar to the "1223" phase, so T
A (Tl, Bi) Sr ₂ Ca ₂ Cu ₃ O _x phase in which a part of l is replaced by Bi is considered. In EPMA analysis, almost (Tl + Bi): Sr: Ca: Cu =
Crystals of 1: 2: 2: 3 had formed. The phases containing these Bis will be referred to as the “1212” similar phase and the “1223” similar phase.

Comparing the composition ratios (2234) and (2223) in the Bi-added system, in (2234), the “1223” similar phase is the main phase and “1212”
While there are very few similarities, in (2223),
There were considerably more "1212" similarities.

In Table 2, since sample Nos. 4 and 6 have higher transition temperatures than sample Nos. 8 and 9, it can be concluded that the "1223" -like phase is a superconductor having a transition temperature exceeding 110K.

The composition is almost (Tl, Bi) Sr ₂ Ca ₂ Cu ₃ O _x , but in order to increase the production amount, it was necessary to use the composition ratio of (2234). However, in this case, of course Ca and Cu
Is excessive and these impurities are generated,
It was confirmed by line diffraction and EPMA analysis.

Next, the (Tl, Bi) Sr ₂ Ca ₂ Cu ₃ O _x phase (Tl + Bi) / B
The composition ratio of i was analyzed by EPMA.

As a result, the average values were about 1 / 0.22 and 1 / 0.29 for Bi additions of 0.2 and 0.33, respectively.

However, the composition ratio of (Tl + Bi) / Bi in the (Tl, Bi) Sr ₂ Ca ₂ Cu ₃ O _x phase was 1 / 0.31 even when the Bi content was 0.4, and the Bi content increased much even if the Bi composition ratio was further increased. There wasn't.

On the other hand, when the added amount of Bi was 0.1, the effect of improving the characteristics was obtained from Table 3, but it was not so clear.

Therefore, the addition temperature of Bi improves the transition temperature, but sometimes
It is desirable that the value of Tl / Bi is within the range of 5/1 to 2/1.

Example 3 As described in Examples 1 and 2, in the superconductor of the present invention, the Tl site of the TlSr ₂ Ca ₂ Cu ₃ O _x phase was partially replaced by Bi, and the superconducting material was almost (Tl, Bi) Sr. It is a phase having a crystal structure consisting of a composition of ₂ Ca ₂ Cu ₃ O _x and having a lattice constant of a = 0.38 nm and c = 1.53 nm and belonging to a tetragonal system. However, in the conventional preparation method, only a small amount of this phase was formed unless Ca and Cu started from an excessive composition, and thus a single-phase sample of this phase was not obtained.

In addition, the TlSr ₂ Ca ₂ Cu ₃ O _x phase itself is difficult to form by the usual production method, and even if a sample having an onset temperature of about 90 K was obtained, zero resistance was not often observed.

Therefore, first, the _{(Tl 1-y Bi y)} Sr 2 Ca 2 Cu 3 O x, for the synthesis of TlSr ₂ Ca ₂ Cu ₃ O _x phase is y = 0, was studied in terms of the starting material.

Tl ₂ O ₃ , Bi ₂ O ₃ , CuO, and Sr used in Example 1 as starting materials
_{In addition to the 2} CuO ₃ and Ca ₂ CuO ₃ powders, Tl ₂ O, Cu ₂ O, and Cu powders were used to prepare samples.

Similarly to Example 1, powders of Sr ₂ CuO ₃ and Ca ₂ CuO ₃ were synthesized, and Tl ₂ O, Tl ₂ O ₃ , Bi ₂ O ₃ , CuO, Cu ₂ O and Cu powder were mixed with T
l: Sr: Ca: Cu = (1 + Z): 2: 2: 3 molar ratio and Tl
_It was weighed so that the ratio of ₂ O and Ti ₂ O _{3 and} the ratio of CuO, Cu ₂ O and Cu metal were as shown in Table 4. The total weight was 5 g.

Although objective composition is _{TlSr 2 Ca 2 Cu 3 O x} , since Tl evaporates during firing, Z is conducted experiments as 0-3.

The weighed powder was mixed, molded and fired in the same manner as in Example 1, and the superconducting transition temperature of the sintered body was measured.

 The results of the characteristics of the sintered body are shown in Table 5.

From Table 5, in the case of Sample No. 1 in which only Tl ₂ O ₃ or CuO is used and Z = 1, X = 10. In this case, a slight resistance change is observed around 90K, but zero resistance is It was not observed up to 15K, and the Meissner effect was not confirmed.

When Tl ₂ O ₃ was replaced with Tl ₂ O, the resistance in the normal conduction state of the sintered body decreased with the composition of X = 9.8 or less, and zero resistance and Meissner effect were also confirmed.

The highest onset 99 for sample number 5 with X = 9.5
A K and a zero resistance temperature of 92K were obtained.

When X was further reduced, the superconducting transition was observed until X = 9.0, but below that, the resistance in the normal conduction state was increased and the superconducting transition was not observed.

In the sample No. 7, the Meissner effect is observed, which is due to the nonuniformity of the sample.

Therefore, good superconducting transition was observed in the range of 9.0 ≦ X ≦ 9.8 when Z = 1, that is, in the range of (7.5 + 1.5Z) ≦ X ≦ (8.3 + 1.5Z).

In addition, when the obtained sample was examined by X-ray diffraction,
A TlSr ₂ Ca ₂ Cu ₃ O _x phase was formed.

Cu ₂ O or metal instead of Tl ₂ O to adjust X
As is clear from the results of the sample numbers 10 to 14, the sample number 1 using only Tl ₂ O ₃ and CuO was obtained when Cu was used and when the value of Z was changed from 0 to 3. The superconducting property was improved as compared with the case.

The inventors examined various composition ratios other than the sample numbers 10 to 14, and in any case, (7.5 + 1.5Z) ≦
Good superconducting transition was observed within the range of X ≦ (8.3 + 1.5Z).

Although Sr: Ca: Cu = 2: 2: 3 was set, no significant difference was found in the results even if there was a slight deviation from this value.

On the other hand, there is no limit to the value of Z because Tl evaporates, but if it is too much, it will take time to evaporate, and an excessive Tl
Is likely to remain, so 1 or less is desirable.

Further, in the examples, the firing was carried out under reduced pressure, but when firing in air or oxygen, the effect of improving the characteristics was not remarkable. This is because Tl ₂ O, Cu ₂ O and Cu metal are easily oxidized. Therefore, firing in a non-oxidizing atmosphere is desirable.

Example 4 Next, the synthesis conditions of the (Tl, Bi) Sr ₂ Ca ₂ Cu ₃ O _x phase were examined.

As in Example 3, powders of Sr ₂ CuO ₃ and Ca ₂ CuO ₃ , and
Tl ₂ O, Tl ₂ O ₃ ,, Bi ₂ O ₃ ,, CuO, Cu ₂ O, Cu powder, Tl: Bi: Sr: Ca: Cu
= 2: 0.25: 2: 2: 3 molar ratio, and weighed so that the ratio of Tl ₂ O to Tl ₂ O _{3 and} the ratio of CuO, CuO ₂ to Cu metal were as shown in Table 6. . However, the total weight was 5 g.

The target composition is Tl _0.75 Bi _0.25 Sr ₂ Ca ₂ Cu ₃ O _x.
Was evaporated during firing, so an experiment was conducted with an excessive amount of Tl.

In the same manner as in Example 1, the weighed powder was mixed, molded and fired, and the transition temperature of the sintered body was measured. The results are shown in Table 7.

From Table 7, compared with the sample No. 1 using only Tl ₂ O ₃ or CuO, the sample Nos. 2, 3, 5 and 6 partially using Tl ₂ O, Cu ₂ O or Cu metal are all The transition temperature was improved, and the difference was particularly remarkable in the sample fired at 975 ° C. for 5 hours and then fired at 850 ° C. for 100 hours.

Sample No. 2 had the highest onset temperature of 118K and zero resistance temperature of 115K.

On the other hand, the sample using only Tl ₂ O without using Tl ₂ O ₃ was an insulator.

Next, when the sample was crushed and the generated phase was identified by powder X-ray diffraction, it was found that in the sample of sample No. 1, in addition to the target (Tl, Bi) Sr ₂ Ca ₂ Cu ₃ O _x phase under both firing conditions. , A (Tl, Bi) Sr ₂ CaCu ₂ O _x phase with a low transition temperature and an unknown impurity phase were formed.

On the other hand, in sample numbers 2, 3, 5 and 6, (Tl, Bi) Sr ₂ Ca ₂ Cu ₃ O
_{The x} phase is the main phase, and only a small amount of (Tl, Bi) Sr ₂ CaCu ₂ O _x phase and unknown phase are seen. Especially, the sample No. 2 was fired at 975 ° C for 5 hours and then at 850 ° C for 100 hours. Most of the samples (T
l, Bi) Sr ₂ Ca ₂ Cu ₃ O _x phase became a single phase.

Also, the sample using only Tl ₂ O without using Tl ₂ O ₃ at all
The (Tl, Bi) Sr ₂ CaCu ₂ O _x phase was a small amount, and was also composed of an unknown phase.

The inventors made samples by changing the ratio of Tl ₂ O, Cu ₂ O, or Cu metal in addition to the sample numbers 2 to 6.
Although there is some difference in the transition temperature, good superconducting transition was observed by setting (7.5 + 1.5Z) ≦ X ≦ (8.3 + 1.5Z) in all cases, and (Tl, Bi ) Sr ₂ Ca
_It was easy to obtain a single-phase sample of ₂ Cu ₃ O _x phase.

Therefore, including the case of y = 0 (Tl _1-y Bi _y ) Sr ₂ Ca ₂ Cu ₃ O
Regarding the _x- phase, the effect of production under adjustment of the oxygen amount was confirmed.

EFFECTS OF THE INVENTION The present invention has at least Tl, Bi, Sr, Ca and C as a composition.
A ceramic consisting of an oxide superconductor and a single phase of this superconductor phase characterized by having a crystal structure containing u and having a lattice constant of a = 0.38 nm and c = 1.53 nm belonging to a tetragonal system. Since it is a manufacturing method of, there are the following effects.

This superconductor consists of Bi-Sr-Ca-Cu-O system and Tl-Ba- system.
It has the effect of having the advantages of the Ca-Cu-O-based superconducting oxide.

In other words, it has the effect of realizing a superconducting oxide with a high zero resistance temperature of up to 115 K, and since it is not necessary to control the firing temperature within the range of 5 to 10 ° C just below the melting point, unlike the case of Bi type, superconducting oxidation is possible. Since the content of highly toxic Tl is about half that of Tl-based oxide superconductors, it is extremely toxic.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukihiro Takahashi 1006 Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Hirofumi Hirano 1006 Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Industrial (56) References J. M. Whereley et al. , NATURE, Vol. 333, No. 6169, P.I. 121. (MAY 1988)

Claims (8)

(57) [Claims] 1. An oxidation characterized by comprising at least Tl, Bi, Sr, Ca and Cu as a composition and having a crystal structure belonging to a tetragonal system with lattice constants of a = 0.38 nm and c = 1.53 nm. Thing superconductor. 2. A composition ratio of (Tl + Bi): Sr: Ca: Cu = 1: 2: 2: 3 or close to each other, and a Tl / Bi ratio within a range of 5/1 to 2/1. The oxide superconductor according to claim 1, wherein the oxide superconductor is present. 3. The composition ratio of the metal oxide equivalent component contained in the starting material is (Tl _1-y Bi _y ): Sr: Ca: Cu: O = (1 + Z): 2:
2: 3: X is set (0 ≦ y ≦ 1/3), and the value of X is (7.5 + 1.5Z) ≦
(Tl _1-y Bi _y ) Sr ₂ Ca ₂ Cu, characterized by being in the range of X ≦ (8.3 + 1.5Z) and firing in a non-oxidizing atmosphere
₃ O _y- based oxide superconductor manufacturing method. 4. Tl ₂ O, Cu ₂ O or Cu as a part of the starting material
Characterized in that at least one of the metals is used,
The method for producing an oxide superconductor according to claim 3. 5. The method for producing an oxide superconductor according to claim 3, wherein the value of Z is in the range of 0 to 1. 6. The method for producing an oxide superconductor according to claim 3, wherein the firing is performed after the molded body is closed. 7. The method for producing an oxide superconductor according to claim 3, wherein the firing temperature is 900 ° C. or higher and 1000 ° C. or lower. 8. The method for producing an oxide superconductor according to claim 3, wherein after firing at a temperature of 900 ° C. or more and 1000 ° C. or less, annealing is performed at a temperature of less than 900 ° C.