JP2593480B2 - Manufacturing method of oxide superconductor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a method for producing a La-Ba-Cu-O-based orthorhombic oxygen-deficient perovskite-type superconductor.

(Prior Art) In recent years, since it was announced that La-Ba-Cu-O-based oxygen-deficient perovskite-type oxides (hereinafter abbreviated as ODPs) may have a high critical temperature, various places have been reported. And La-
Research on Ba-Cu-O-based ODP and Y-Ba-Cu-O-based ODP has been actively conducted (Z. Phys. B Condensed Matter 64, 189-19).
3 (1986)).

La-Ba-Cu-O-based oxide is a crystalline became beginning of these groundbreaking studies, that the critical temperature has a K ₂ NiF ₄ structure (Tc) is a superconductor 30K Known (HT
akagi, S.Uchida, K.Kitazawa and S.Tanaka: Jpn.J.Appl.
Phys. 26 (1987) L123.).

Some research groups have a composition of LaBa ₂ Cu ₃ O _y
La-Ba-Cu-O-based superconductor has a higher Tc of 60K to 90K.
(T. Iwazumi, R.
Yoshizaki, M.Inoue, H.Sawada, H.Hayashi, H.Ikeda and
E.Matsuura: Jpn.J.Appl.Phys.26 (1987) L621 et al.), To date, those that have zero resistance above liquid nitrogen temperature have not been realized.

Recent studies have reported that the 90K superconductor phase in the La-Ba-Cu-O system is tetragonal ODP La _3-x Ba _{3 + x} Cu ₆ O _{14 + δ} (DBMitzi, AF Marshall, JZ Sun, D .
J.Webb, MRBeasley, THGeballe, and A.Kapitulnik: Ph
ys.Rev.B.). Also, other research groups have reported that other tetragonal ODs having the same structure as tetragonal YBa ₂ Cu ₃ O _8-δ
Claims to be P.

In contrast, all other 90K superconductors LnBa ₂ Cu ₃ O
_7-δ (Ln = Y, Yb, Tm, Er, Ho, Dy, Gd, Eu, Sm, N
Regarding d), it is well established that the orthorhombic ODP phase having a Cu-O chain on one plane plays an important role in this high-temperature superconductivity, and furthermore, the superconductivity is determined by this one.
It is often thought to be along a one-dimensional chain on one plane.

Thus, LaBa ₂ Cu ₃ O _y occupies a special position in the view that tetragonal ODP is a 90K superconductor phase.

Thus, the structural differences between LaBa ₂ Cu ₃ O _y and YBa ₂ Cu ₃ O _y raise important fundamental questions about the mechanism of high-temperature superconductivity. In particular, the aforementioned book by Phys. Rev. B. points out that the theory that the conduction mechanism along the one-dimensional chain is very important for materials with high Tc may be rejected.

(Problems to be Solved by the Invention) However, LaBa ₂ Cu ₃ O _y (or La _3-x Ba _{3 + x} Cu ₆ O
Data from previous studies on _{14 + δ} ) were 90K
It was insufficient to assign a specific structure to the superconductor.

As is generally known, at most 16% of the total volume in the sample in these studies exhibits a negative effect, and the temperature at which the diamagnetism begins to increase is much lower than ^Tconset , determined by resistance measurements It is.

Therefore, the phases identified in these findings are true
The possibility of inconsistency with the 90K superconductor phase cannot be ruled out.

Furthermore, the preparation of a 90K superconductor LaBa ₂ Cu ₃ O _y by an improved method would provide important information in the field of high temperature superconductors.

The present inventors have conducted intensive studies to elucidate the above points in consideration of such points, and found that the orthorhombic perovskite type
LaBa ₂ Cu ₃ O _y was found to form a true 90K superconductor phase.

The present invention has been made based on such findings, and La-
An object of the present invention is to provide a method for producing a superconductor composed of a Ba-Cu-O-based orthorhombic ODP.

[Structure of the Invention] (Means for Solving the Problems) In the method for manufacturing a superconductor of the present invention, La, Ba, and Cu are obtained by mixing La: Ba: Cu = 3-x: 3 + x: 6 (atomic ratio) ( 0.4 <x <1.2) The firing step of reacting a mixture of starting materials at a temperature lower than the melting point, and the step of heat-treating the fired product obtained in the firing step in an oxygen-containing atmosphere at 380 ° C. or lower. And producing an oxide superconductor having an orthorhombic oxygen deficient perovskite structure containing La, Ba and Cu substantially at a ratio of 1: 2: 3 (atomic ratio). I have.

The superconductor obtained by the production method of the present invention is generally composed of an orthorhombic single phase represented by LaBa ₂ Cu ₃ O _8-δ (δ is 0 to 1.5).

In the superconductor of the present invention, a tetragonal crystal having the same composition, a semiconductor, and other components may coexist in practice. However, it is desirable that at least 50% of the entire superconductor is an orthorhombic crystal having the above composition.

Note that it is also possible to replace a part of La with another rare earth element, for example, Y or the like.

 The superconductor is manufactured by the following method.

First, the constituent elements of the perovskite-type oxide superconductor such as La, Ba, and Cu are mixed so as to have a stoichiometric composition with respect to the above-described general formula, pulverized, dried, and then dried as powder. Mix well at a molar ratio of In this case, as each raw material, an oxide such as La ₂ O ₃ , BaO, or CuO is generally used. In addition to these oxides, it is also possible to use compounds such as carbonates, nitrates, oxalates and hydroxides which are converted into oxides after firing. The elements constituting the perovskite-type oxide superconductor are basically mixed so as to have a composition with a stoichiometric ratio, but may be slightly shifted depending on the production conditions and the like. For example, if La, Ba and Cu are contained in the range of La: Ba: Cu = 3-x: 3 + x: 6 (atomic ratio) (0.4 <x <1.2), no practical problem occurs.

After mixing the above raw materials, calcined and crushed into pellets,
After being formed into a desired shape, firing is performed. Note that calcining before firing is not always necessary.

In the firing of the present invention, La is hardly reacted to Ba and ionic radii are close, for example, at a relatively high temperature, such as the case 1030 ° C. C. to 1100 ° C. for using BaCO ₃ as Ba starting material Baking is required. Since BaO has higher reactivity than BaCO ₃ , when BaO is used as a starting material, the reaction can be performed at a lower temperature, for example, 800 ° C. or higher.

The firing is performed in an oxygen-containing atmosphere where sufficient oxygen can be supplied, where the melting point of the mixture of the starting materials is M _temp (° C.) and the firing temperature is S _temp (° C.). M _temp > S _temp > M _temp It is preferred to work at a temperature in the temperature range of -100, which is as close as possible to the melting point of the mixture. At this time, besides pure oxygen, a mixed gas of oxygen and an inert gas can be used,
It may be made to bake under pressure as needed. If the sintering temperature exceeds the above temperature range, the sintered body partially melts during sintering, and other phases such as semiconductors are formed. It is not preferable because it is difficult to form an orthorhombic ODP.

The baking time is several hours in consideration of a longer time at a high temperature than at a low temperature depending on the baking temperature.
Set within a range of tens of hours. The mixture of starting materials is crystallized by sintering by heating at this high temperature, and a tetragonal OD
Form P.

Next, the sintered body is cooled and pulverized as it is or by a ball mill or other known means, and annealed in an oxygen-containing atmosphere at a relatively low temperature of 380 ° C. or less, for several hours to several days depending on the annealing temperature. Let it.

Oxygen is introduced into the crystal structure of the tetragonal ODP by this annealing, the value of δ decreases, phase transition occurs, and the orthorhombic OD
P, and the superconductor of the present invention is obtained.

Since the phase transition is considered to occur at about 300 to 350 ° C., high-temperature heating at an annealing temperature exceeding 380 ° C. has an adverse effect on superconductivity. On the other hand, if the temperature is lower than 100 ° C., the reaction rates of the above phase transition and oxygen introduction become extremely slow, which is not preferable. Therefore, the annealing temperature is 20
A range from 0 to 350 ° C, more preferably from 300 to 350 ° C, is suitable.

The atmosphere is usually pure oxygen at normal pressure,
A mixed gas of oxygen and an inert gas may be used, and if necessary, annealing may be performed under pressure.

The above-described production method of the present invention can be performed in various forms of the starting material or the sintered body.

That is, the starting material or the powder of the sintered body is paste-printed on a substrate and screen printed, and the material formed in a pattern on the substrate is fired and annealed, or the starting material or the sintered material is fired. It is also possible to enclose the powder of the compact in an Ag or other metal pipe, draw a wire, and perform the above-described firing and annealing.

(Operation) The superconductor obtained by the production method of the present invention exhibits superconductor characteristics at a temperature equal to or higher than the temperature of liquid nitrogen, and according to the present invention, contains La, which is difficult to react due to a large atomic radius. LaBa ₂ Cu ₃ O _8-δ (δ is 0 to 1.5) can be easily produced.

(Example) Next, an example of the present invention will be described.

Example La ₂ O ₃ powder, BaCO ₃ powder and CuO powder having a particle size of 1 to 5 μm were sufficiently mixed in a mortar at a molar ratio of 1: 2: 3. After calcining the mixture at 1000 ° C. for 10 hours, the mixture is formed into a pellet by pressing, and the pellet is heated at 1050 ° C. in an O ₂ atmosphere.
For 10 hours for sintering.

Next, the sintered body was annealed in an O ₂ stream at 300 ° C. for 20 hours to obtain a superconductor of the present invention.

For the obtained superconductor, X-ray diffraction and DC resistance (4
Probe method) and susceptibility (SHE SQUID temperature-variable susceptibility meter) were measured.

The results are shown in FIGS. FIG. 4 shows the main part of FIG. 3 measured in more detail. In these figures, the comparative example is a sample manufactured under the same conditions as those of the example except that the annealing process was not performed in the oxygen gas flow.

As shown in FIG. 1, in the sample of the comparative example, all peaks coincide with the tetragonal DOP suggested by Michel et al. And the lattice constant is a = b = 3.920 (8)
Å and c = 11.73 (3) Å ≒ 3a.

In contrast, in the examples, most of the peaks are the same as those of the comparative example, but some of them are clearly split or asymmetric. It can be seen that the splitting of the peaks at 46.6 °, 57.6 ° and 67.5 ° indicated by arrows is particularly significant.

Taking the intensity ratio of these split peaks, it is easy to see that one of the parameters a, b, c / 3 is smaller than the others.

In fact, according to the least squares method for determining the parameters, as shown in FIG.
a = 3.901 (8) Å, b = 3.928 (7) Å and c = 11.7
Very good agreement with the orthorhombic lattice constant of 83 (18) Å. The following table shows the diffraction angles observed from the examples and the angles calculated from the orthorhombic lattice constants determined by the least squares method.

In the comparative example, the (020) and (123) peaks are slightly asymmetric, which is due to the presence of a small amount of the orthorhombic phase or the slight a
It is considered that this is an orthorhombic system having only -b split.

From the results of the above tests, it is found that the orthorhombic structure in this example has the same crystal form as the YBa ₂ Cu ₃ O _y 90K superconductor.
₂ Cu ₃ O _8-δ (δ is 0 to 1.5).

FIG. 2 shows the temperature dependence of resistance in Examples and Comparative Examples. These are all Tc ^onset ≒ 9
Superconductivity appears between 0K and 100K, and the clear difference is Tc
^Appears in ^{zero resistance} . The finite resistance drops to 40K for the comparative example, while the zero resistance for the example is 80K
(Center 90K) is achieved. This is the highest zero resistance temperature obtained with LaBa ₂ Cu ₃ O _y investigated so far. It is interesting to note that the highest zero resistance temperature was obtained for orthorhombic crystals, as in YBa ₂ Cu ₃ O _y , in LaBa ₂ Cu ₃ O _y , the 90K superconductor is
This indicates that it is not a tetragonal ODP but an orthorhombic ODP.

In fact, as shown in FIG.
The example samples have been found to produce a shielding current magnetization of about 60% of full diamagnetism and a Meissner signal of 30% of full diamagnetism. As shown in FIG.
This diamagnetic signal was observed up to 80K.

On the other hand, in the comparative example, the resistance decreases from 90K, but only 10% of the total volume shows the Meissner effect, and the diamagnetic signal disappears near 55K.

[Effects of the Invention] As is clear from the above examples, the oxide superconductor obtained by the manufacturing method of the present invention has a Tc higher than the temperature of liquid nitrogen, and can be widely applied to the use of this type of superconductor. It is.

[Brief description of the drawings]

1 to 4 are graphs showing the effects of the embodiment of the present invention.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A 64-72920 (JP, A) JP-A 64-9849 (JP, A) JP-A 64-14148 (JP, A)

Claims (2)

(57) [Claims] 1. A mixture of starting materials containing La, Ba and Cu in a ratio of La: Ba: Cu = 3-x: 3 + x: 6 (atomic ratio) (0.4 <x <1.2) A firing step of reacting at a temperature, and a step of heat-treating the fired product obtained in the firing step in an oxygen-containing atmosphere of 380 ° C. or less, wherein the La, Ba and Cu are substantially 1: 2: 3 A method for producing an oxide superconductor, which comprises producing an oxide superconductor having an orthorhombic oxygen deficient perovskite structure containing at a ratio of (atomic ratio). 2. A mixture of starting materials containing La, Ba and Cu in a ratio of La: Ba: Cu = 3-x: 3 + x: 6 (atomic ratio) (0.4 <x <1.2), When the melting point of the mixture is M _temp (° C.) and the sintering temperature is S _temp (° C.), a sintering step of reacting in an oxygen-containing atmosphere having a temperature range of M _temp > S _temp > M _temp− 100; Heat-treating the fired product obtained in the process in an oxygen-containing atmosphere at 380 ° C. or lower to introduce oxygen into the crystal structure and to cause a phase transition to orthorhombic crystals. A method for manufacturing an oxide superconductor according to claim 1.