JPH02248321A - Oxide superconductor - Google Patents

Abstract

PURPOSE:To increase the critical temp. of the low critical temp. phase of an oxide superconductor and rapidity at the time of transition and to further improve the superconducting characteristics of the high critical temp. phase by substituting Pb for a specified amt. of Bi in the Bi-Sr-Ca-Cu-O type oxide superconductor. CONSTITUTION:This oxide superconductor contains Bi, Pb, Sr, Ca and Cu in (2-a):a:b:c:d atomic ratio (0.1<=a<=1, 1<=b<=2.5, 0.5<=c<=1.5 and 1.6<=d<=2.5) and is produced as follows: Bi, Pb, Sr, Ca and Cu are mixed in a prescribed molar ratio in the form of powders of simple substances or compds. such as carbonates or oxides, this powdery mixture is calcined and brought into a reaction in an oxygen-contg. atmosphere and the resulting calcined product is pulverized, molded and sintered in an inert atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a B1-9r-Ca-Cu-0 based oxide superconductor.

(Prior art) In 1986, La-
Since the Ba-Cu-0 layered perovskite type oxide superconductor was announced, oxide superconducting materials have attracted attention. In 1987, it was confirmed that the critical temperature of a defective perovskite oxide superconductor with oxygen defects, represented by the Y-Ba-Cu-0 system, is about 90°C, higher than the liquid nitrogen temperature (-77K). It has become possible to apply superconductors using cheaper liquid nitrogen instead of expensive liquid helium as a refrigerant, and research is being actively conducted in various places.

In addition, in 1988, the critical temperature of B was as high as around 105.
A 1-8r-Ca-Cu-0 based oxide superconductor was discovered. This B1-based oxide superconductor is the above-mentioned Y
It has a higher critical temperature than other oxide superconductors, which not only provides a practically sufficient thermal margin when cooling with liquid nitrogen, but also eliminates the need for expensive rare earth elements and is chemically stable against moisture. It is attracting attention as a superior superconductor due to its advantages such as high conductivity and resistance to oxygen escape.

By the way, the second B1-8r-Ca-Cu-0 based oxide superconductor has a critical temperature (hereinafter referred to as Tcend) at which it exhibits zero resistance.
It is written as There are two types of superconducting phases: a low critical temperature phase with a chemical formula of approximately 80 (1) and a high critical temperature phase with a chemical formula of approximately 110. It has been confirmed that

However, when a Bi-based oxide superconductor is produced by a normal sintering method like other oxide superconductors, the resulting B1-8r-Ca-Cu-0-based oxide superconductor has the above-mentioned low critical temperature phase. At present, only those with a high volume fraction of the low critical temperature phase can be obtained because it is not easy to synthesize the high critical temperature phase.

For such problems, B1-8r-Ca-Cu-0
There are reports that the volume ratio of the high critical temperature phase can be significantly increased by adding PB to the raw material powder of oxide superconductors, but this method has poor reproducibility;
It has not yet been possible to obtain a single high-critical-temperature phase with good reproducibility.

On the other hand, in the low critical temperature phase, Teend is 80 as mentioned above.
Since the temperature is close to that of liquid nitrogen and the transition from normal to superconducting state is gradual, it has been virtually impossible to stably obtain superconducting properties using liquid nitrogen as a coolant.

(Problems to be Solved by the Invention) As described above, the B1-8r-Ca-Cu-0 based oxide superconductor is obtained as a mixture of a low critical temperature phase and a high critical temperature phase, and the Tcend is Since such critical properties are almost determined by the low critical temperature phase, there is a problem in that the properties of the high critical temperature phase cannot be fully utilized. Therefore, it is strongly desired to improve the superconducting properties of the low critical temperature phase itself, which can be stably obtained.

Furthermore, it is desired that the superconducting properties of the high critical temperature phase itself be further improved in view of the thermal margin during actual use.

The present invention was made to deal with such problems, and the first purpose is to provide B5 that can be stably obtained.
-8r-Ca-Cu-0 based oxide superconductor's superconducting properties such as the critical temperature of the low critical temperature phase and the steepness of the transition, and the second purpose is to improve the superconducting properties of the high critical temperature phase of the oxide superconductor. The aim is to further improve the characteristics.

[Structure of the invention]

(Means for solving the problem) That is, the oxide superconductor of the invention of tiAl has B1, P
b, Srs Catp-J- and Cu in atomic ratio 2-a
: a : b : c : d (However, asbsc
Each sd is a number that satisfies the following formula. same as below.

O6l≦a≦1゜ l≦b≦2.5. 0.5≦c≦1.5. 1.6≦d≦2.5) It is characterized by

Further, the oxide superconductor of the second invention is old, pb, 5r
SCa and Cu in atomic ratio 2-e:e:r:g
: Contained by h (however, e s r and g s h are numbers that satisfy the following formulas. The same applies hereinafter.

0.15 e≦1゜ 1.5≦f≦2.5. 1.5≦g≦2.5. 2.5≦h≦3.5) It is characterized by

The first oxide superconductor of the present invention is basically represented by the chemical formula % (in the formula, δ represents an oxygen defect and is usually a number from 0 to l, 0). The superconducting properties are improved by substituting part of the Bi in the so-called low critical temperature layer with PB, but if the ratio of each element is within the above range, the desired superconducting properties cannot be obtained. can.

Further, the second oxide superconductor of the present invention is basically expressed by the chemical formula % (in the formula, δ represents an oxygen defect and is usually a number from 0 to 1.0). The superconducting properties are further improved by substituting a part of Bl in the so-called high critical temperature layer with PB, but similarly, if the ratio of each element is within the range mentioned above, the desired superconductivity can be achieved. characteristics can be obtained.

The reason why the amount of substitution of Bl by pb in the first and second inventions is specified in the range of 0.1-1 is because when the amount of substitution by Pb is less than 0.1, a sufficient substitution effect by pb can be obtained. Moreover, when l exceeds 1, although the steepness of the transition from the normal conductive state to the superconducting state improves, the transition temperature itself tends to decrease, and if the amount of substitution is further increased, the superconducting state cannot be obtained. The preferred range of the amount of substitution by pb is 0.2 to 0.8.

The oxide superconductor of the first invention can be produced, for example, by the method shown below.

First, Bl, Pb%Sr, and Ca5Cu alone or as a compound are mixed at a predetermined molar ratio. As compounds of these constituent elements, carbonates and oxides can be used, as well as
Other than carbonates, nitrates, hydroxides, etc. which are converted into oxides by heating, organic acid salts, organic metals, etc. may also be used. In addition, as a starting material for Pb, relatively stable C
It is also possible to use az pbo 4 or the like. These constituent elements are basically mixed so as to satisfy the atomic ratio of formula (m) above, but it is sufficient as long as it is within the above-mentioned range, and even if there is a deviation of about 10% due to manufacturing conditions etc. No problem.

Next, this mixed powder was heated to 80% in an oxygen-containing atmosphere.
The calcined product is reacted by calcining at a temperature of about 0°C, and the calcined product is pulverized using a ball mill, sand grinder, or other known method, and the calcined powder is molded into a desired shape using a press molding method or various known molding methods. Body. Note that the above calcination step is not necessarily necessary.

Thereafter, this molded body is fired and sintered in an inert atmosphere such as a nitrogen atmosphere at a temperature in the range of 700°C to 800°C. If the atmosphere during sintering is an oxidizing atmosphere, a phase change will occur and the oxide superconductor of formula (m) cannot be stably obtained. Note that this inert atmosphere includes N2, A
The atmosphere contains about 0 to 3% oxygen in an inert gas such as r.

Further, the oxide superconductor of the second invention, like the oxide superconductor of the first invention, is obtained by first adjusting the starting materials of each constituent element so that the atomic ratio of the above formula (IV) is basically satisfied. Mix with Also in the oxide superconductor of this second invention,
It suffices as long as it is within the above-mentioned range, and there is no problem even if it deviates further by about 10% depending on manufacturing conditions and the like. Then, this mixed powder is calcined and then molded and sintered.

The sintering step of the oxide superconductor of the second invention is also preferably carried out in an inert atmosphere, and the sintering temperature is 700°C.
It is preferable to set it as ℃~880℃. Sintering temperature is 700
If the temperature is below 10°C, the replacement of B1 by pb will not proceed sufficiently, and if the temperature exceeds 10°C, the high critical temperature phase will decompose. The atmosphere contains about 0 to 3% oxygen in the gas.

(Function) It is known that the crystal structure of a Bl-based oxide superconductor has a unique modulation structure. This modulation structure is not observed in the TI-based oxide superconductor (Tl-13a-Ca-Cu-0 system), which has a crystal structure similar to that of the Bl-based material and whose critical temperature is slightly higher than that of the Bl-based material. The difference in critical temperature between the Tl system and the Tl system is considered to be due to this modulation structure. In the first invention, the modulation structure is relaxed or eliminated by replacing a part of the old low critical temperature phase of the 81-based oxide superconductor with pb. This improves the critical temperature of the low critical temperature phase and also improves the steepness of the transition from the normal conductor state to the superconductor state.

Further, pb in the second invention provides the same effect as pb in the first invention, and the critical temperature of the high critical temperature phase and the steepness of the transition from the normal conductive state to the superconducting state are further improved.

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

Example 1 First, B120 m , PbO, 5rC as starting materials
Ox, CaC01, and CLIO powders are prepared, and the cation atomic ratio is B1:Pb:Sr:Ca:C.
u-IJ: 0.2: 2: l: After weighing a predetermined amount and mixing it thoroughly, this mixed powder was calcined in the air at 700°C for 24 hours. The calcined product was thoroughly ground using a ball mill.

Next, the above-mentioned calcined powder was press-molded into a pellet shape, and then the compact was fired at 740° C. for 24 hours in a nitrogen stream to produce a Bl-based oxide superconductor.

For the Bl-based oxide superconductor thus obtained,
When i-diffraction was performed, it was confirmed that the material consisted of a low critical temperature phase and did not contain a high critical temperature phase. Then, it was subjected to evaluation of superconducting properties, which will be described later.

Example 2.3 The mixing ratio of the starting materials in Example 1 above was changed to B in atomic ratio.
i:Pb:Sr:Ca:Cu-1,6:0.4:2:1
:2 and B1:Pb:Sr:Ca:Cu=1.4:0
．． Bi-based oxide superconductors were produced under the same conditions as in Example 1 except that the ratio was 8:2:1:2. When X-ray diffraction was applied to each of the old yarn oxide superconductors obtained in this way, it was confirmed that each had a single phase with a low critical temperature phase.

Comparative Example 1 B1203.5rC(h, CaCO3
, CuO powders were prepared, and a predetermined amount was weighed so that the atomic ratio of cations was B1:Sr:Ca:Cu=2:2:1:2, and after thoroughly mixing, the Calcination was carried out under the same conditions as in Example 1, and the calcined product was sufficiently ground in a ball mill. Next, after press-molding this calcined powder into a pellet shape, this molded body was placed in an oxygen stream at 840°C
A Bl-based oxide superconductor was produced by firing under the following conditions.

It was confirmed by X-ray diffraction that the B1-based oxide superconductor of this comparative example was also a single phase with a low critical temperature phase.

The superconducting properties of the B1-based oxide superconductors of each of these Examples and Comparative Examples were evaluated using a 5QtllD magnetometer. The results are shown in FIG.

As is clear from Figure 1111, the 81-series oxide superconductor of the comparative example not only has a low transition temperature but also changes very slowly to the superconducting state, whereas the old system of each of the above examples In oxide superconductors, the transition to the superconducting state becomes sharper, and an improvement in the critical temperature is also observed.

Examples 4 to 6 Each starting material used in Example 1 had an atomic ratio of old:Pb:
Sr:Ca:Cu-111:0.2:2:2:3 (Example 4), Bi:Pb:Sr:Ca:Cu-1,6:0
．． 4:2:2:3 (Example 5) and Bi:Pb:S
The mixture was then mixed so that r:Ca:Cu=1.4:0.6:2:2:3 (Example 6), and this mixed powder was calcined in air at 700°C for 48 hours. Then, this calcined product was thoroughly ground in a ball mill.

Next, after press-molding the above calcined powder into a pellet shape, this molded body was fired at 780°C for 24 hours in a mixed gas of 02:Ar=0.5:99.5 to form a Bi-based oxide. Each superconductor was fabricated.

When the Bi-based oxide superconductors thus obtained were subjected to X-ray diffraction, it was confirmed that each of them consisted of a high critical temperature phase. Then, as in Example 1 above, the magnetic susceptibility was set at Δ-1 and the superconducting properties were evaluated.

Comparative Example 2 81203, SrCO3, CaCO3 as starting materials
, CuO powders are prepared, and a predetermined amount of these are weighed so that the atomic ratio of cations is B1:Sr:Ca:Cu=2:2:2:3, and after thoroughly mixing, Calcination was carried out under the same conditions as in Example 4, and the calcined product was thoroughly ground in a ball mill. Next, after press-molding this calcined powder into a pellet shape, this molded body was heated to 02: Ar-0,5: 99.
A Bi-based oxide superconductor was produced by firing in a mixed gas of No. 5 at 780° C. for 48 hours. It was confirmed by X-ray diffraction that the Bi-based oxide superconductor of this comparative example was also a single phase with a high critical temperature phase.

The magnetic susceptibility of the Bi-based oxide superconductor of Comparative Example 2 was 111 J.
The results are shown in Figure 2.

As is clear from FIG. 2, by substituting a part of Bl with Pb, the critical temperature of the high critical temperature phase and the steepness of the transition to the superconducting state are also improved.

[Effect of the invention] As explained above, according to the present invention, a part of Bl is converted into pb
By substituting with liquid nitrogen, the critical temperature of the low critical temperature phase that can be stably obtained and the steepness of the transition to the normal conducting state can be improved. It becomes possible to obtain superconducting properties.

Furthermore, since the low critical temperature phase can be sintered in a nitrogen atmosphere, there is an advantage that a metal that is easily oxidized can be used as a sheath material, for example.

Furthermore, in the high critical temperature phase, the superconducting properties are further improved, the thermal margin etc. are further increased, and the stability when used as various superconducting members is improved.

[Brief explanation of drawings]

FIG. 1 is a graph showing the temperature dependence of diamagnetic magnetic susceptibility in the low critical temperature phase of the Bi-based oxide superconductor according to the present invention and the conventional low critical temperature phase, and FIG. 2 is a graph showing the temperature dependence of the diamagnetic susceptibility in the Bi-based oxide superconductor according to the present invention. 1 is a graph showing the temperature dependence of diamagnetic magnetic susceptibility in a high critical temperature phase of a physical superconductor and a conventional high critical temperature phase. Applicant: Toshiba Corporation

Claims (1)

[Claims] (1) Bi, Pb, Sr, Ca and Cu in an atomic ratio of 2
-a: a: b: c: d (including a, b, c, d
are numbers that satisfy the following formulas. 0.1≦a≦1, 1≦b≦2.5, 0.5≦c≦1.5, 1.6≦d≦2.5). (2) Bi, Pb, Sr, Ca and Cu in an atomic ratio of 2
-e:e:f:g:h (including e, f, g, h
are numbers that satisfy the following formulas. 0.1≦e≦1, 1.5≦f≦2.5, 1.5≦g≦2.5, 2.5≦h≦3.5).