JP2554658B2 - How to connect complex oxide superconductors - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Field of Industrial Application) The present invention relates to a method for connecting a composite oxide superconductor.

(Prior Art) In recent years, since it was announced that Ba-La-Cu-O-based layered perovskite type oxides may have a high critical temperature, research on oxide superconductors has been conducted at various places. (Z.Phys.B Condensed Matter 64,189-193 (198
6)). Among them, a defect perovskite type ((LnBa ₂ Cu ₃ O having an oxygen defect represented by a Y-Ba-Cu-O system.
_7-δ type) (δ represents an oxygen defect and is usually 1 or less, and Ln is
At least one element selected from Y, La, Sc, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, and part of Ba is Sr.
, Etc.)) has a critical temperature T _C of 90
It has attracted attention as a very promising material because of its high temperature of over k and over liquid nitrogen (Phys. Rev. Lett. vol. 58 N
o.9,908-910).

However, since this superconductor is a crystalline oxide and is obtained as a sintered body or a powder obtained by crushing this, it is difficult to process it into a long product. For this reason,
It has been proposed that the oxide superconductor powder be housed in a metal coating to be thinned. However, when the metal and the oxide superconductor are made into a complex thin wire as described above, the connecting end surfaces are mutually separated. There is a problem that connection work is difficult because matching is difficult.

(Problems to be Solved by the Invention) In an oxide superconducting wire in which a metal and an oxide superconductor are composited as described above, there is a problem that it is difficult to make the connection end faces coincide with each other.

Further, in the above perovskite type oxide superconductor, a superconducting current flows along the C-plane of the crystal. Therefore, if they are simply connected, the arrangement direction of the crystals becomes random at the connecting portion, and a desired current density can be obtained. There is also the problem of not having it.

The present invention has been made in response to such a situation,
It is an object of the present invention to provide a method for connecting a composite oxide superconductor in which oxide superconducting wires can be easily connected to each other and a desired current density can be obtained at a connecting portion.

[Structure of the Invention] (Means for Solving the Problems) The method for connecting a composite oxide superconducting wire according to the present invention is a method for connecting composite oxide superconductors including an oxide superconductor and a normal conductor as described above. Through the mixed powder of the oxide superconductor powder and the normal conductor powder, contact each end face of the composite oxide superconductor, at least the oxide superconductor powder and the normal conductor powder After heating at a temperature equal to or higher than the melting point of either of the above, at least the connecting portion is cooled while giving a temperature gradient in one direction.

Although many oxide superconductors are known, the practical effect is particularly large when applied to the connection of a rare earth element-containing perovskite type oxide superconductor having a high critical temperature.

The oxide superconductor containing a rare earth element and having a perovskite type structure as long as it can realize a superconducting state is referred to as LnBa ₂ Cu ₃ O _7-δ system (δ represents an oxygen defect and is usually 1 or less. Number, Ln is Y, La, Sc, Nd, Sm, Eu,
At least one element selected from Gd, Dy, Ho, Er, Tm, Yb and Lu: a part of Ba can be replaced by Sr etc.) and other defect perovskite type having oxygen defects, Sr-La-Cu- An oxide having a perovskite type in a broad sense such as an O-based layered perovskite type is exemplified. Rare earth elements are also defined in a broad sense and include Sc, Y and La systems. As a typical system, in addition to the Y-Ba-Cu-O system, Y is Eu, Dy, H
Systems substituted with rare earths such as o, Er, Tm, Yb, Lu, Sc-Ba-C
Examples thereof include u-O system, Sr-La-Cu-O system, and a system in which Sr is replaced with Ba and Ca.

The oxide superconductor in the present invention is manufactured, for example, as shown below.

First, the constituent elements of the perovskite type oxide superconductor such as Y, Ba and Cu are sufficiently mixed. When mixing, Y ₂ , O ₃ ,
An oxide such as Eu ₂ O ₃ , BaO or CuO can be used as a raw material. In addition to these oxides, compounds such as carbonates, nitrates, and hydroxides that are converted into oxides after firing may be used. Further, an oxalate obtained by a coprecipitation method or the like may be used. 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, in the Y-Ba-Cu-O system, Ba2 is added to Y1 mol.
The standard compositions are mol and Cu3mol, but in practice there is no problem with deviations of Ba2 ± 0.6mol and Cu3 ± 0.2mol from Y1mol.

After mixing the above-mentioned raw materials, they are calcined and pulverized into a desired shape, and then fired at about 850 to 980 ° C. Calcination is not always necessary. The calcination and firing are preferably performed in an oxygen-containing atmosphere so that sufficient oxygen can be supplied. After firing into a desired shape, heat treatment in oxygen, and especially slow cooling to about 300 ° C. can improve superconducting properties. This heat treatment is usually performed at about 300 to 700 ° C.

The thus obtained oxide superconductor has an oxygen-defective perovskite structure (LnBa ₂ Cu ₃ O
_7-δ (δ is usually 1 or less)). In addition, Ba is Sr, Ca
It is also possible to substitute at least one of the above, and it is also possible to substitute a part of Cu with Ti, V, Cr, Mn, Fe, Co, Ni, Zn or the like. Substitution elements of these Cu element and Ba element enter the site in the form of substitution. This substitution amount can be appropriately set within a range that does not deteriorate the superconducting property, but since a too large amount of substitution deteriorates the superconducting property, it is set to 80 mol% or less, and practically to about 20 mol% or less.

Examples of the composite oxide superconductor in the present invention include those in which a coating of a normal conductor is provided on the outer circumference of a strip made of an oxide superconductor, and those in which an oxide superconductor is embedded in the normal conductor. , For example, used as a wire rod.

Such oxide superconductor wire rod in which a coating of a normal conductor is provided on the outer periphery of a strip made of such an oxide superconductor is, for example, a ball mill of a fired product obtained by firing the above oxide superconductor and crystallizing the fired product. It is crushed by a means, and this powder is filled into a metal tube at a filling rate of about 50 to 70% and drawn to form a wire, or the surface of an oxide superconductor element wire is sprayed to form a normal conductor. Obtained by forming a coating.

The perovskite-type oxide superconductor powder is a plate-like crystal and is divided into fine powders by cleavage from the cleavage plane, and the ratio of the diameter in the C-plane direction to the thickness in the C-axis direction is 3 ~ 5 and its diameter (long axis on C plane) is 1 to 5 μm
Something is preferable. Further, it is desirable that the C-plane of the crystal is oriented in the direction of current flow at an orientation rate of at least 70%, preferably 80-90%.

Such a composite oxide superconductor is processed to the final shape and then annealed at 800 to 940 ° C. for several hours in an oxygen or oxygen-containing atmosphere. By this annealing in oxygen or an oxygen-containing atmosphere, oxygen is introduced into oxygen vacancies of the perovskite type superconductor, the value of δ is reduced, and the current density of the complex oxide superconductor is further improved.

As the normal conductor used in the complex oxide superconductor of the present invention, Ag or Cu is particularly preferable. This is because these serve as oxygen carriers in the oxygen introduction step into the oxygen vacancy described above, and sufficient oxygen introduction is possible.

The connection method of the present invention will be described in more detail. First, between each end face of the complex oxide superconductor to be connected, the same powder of the oxide superconductor as the object to be joined and the normal electric current used for coating the oxide superconductor are used. Arrange the mixed powder with the conductor powder. The mixing ratio of the mixed powder is preferably approximately the same as the oxide superconductor / normal conductor ratio in the oxide superconducting wire which is the object to be joined. Further, the thickness of the mixed powder to be interposed is preferably about 500 μm to 3 mm.

Next, the oxide superconductor powder and the normal conductor powder are heated to a melting point of at least one of them to melt the mixed powder at least partially, and at least the connecting portion is gradually heated while giving a temperature gradient in one direction. Cool and solidify by cooling.
As a result, the oxide superconductor at the connecting portion is solidified and precipitated in a fibrous form in one direction from the joint end surface of one of the composite oxide superconductors, and is well connected as a superconductor. Further, this prevents the current density from being impaired by the connection portion in which the crystal orientations are uniform.

After this heating connection, heat treatment is performed in oxygen or an oxygen atmosphere at the connection portion, if necessary.

(Operation) In the connection method of the present invention, since the end faces of the composite oxide superconductor are brought into contact with each other via the mixed powder of the oxide superconductor powder and the normal conductor powder to perform heat treatment, The time alignment is easy, and the normal conductor of the composite oxide superconductor and the oxide superconductor can be simultaneously connected. Furthermore, since it is cooled and solidified while giving a temperature gradient in one direction, the crystal of the oxide superconductor in the connection part grows in a fibrous shape in the connection direction, and it is possible to satisfactorily maintain the characteristics as an oxide superconductor. Connected.

(Example) Next, the Example of this invention is described.

Example 1 BaCO ₃ powder 2 mol% and Y ₂ O each having a particle size of 1 to 5 μm
₃ powder 0.5 mol%, the atmosphere was sufficiently mixed CuO powder 3 mol% 90
After firing at 0 ° C for 48 hours to react, this powdered raw material was fired in oxygen at 800 ° C for 24 hours to react, introduce oxygen into the oxygen vacant space, then pulverize with a ball mill and classify. A perovskite-type oxide superconductor powder having an average particle diameter of 2 μm and a diameter-to-thickness ratio of 1 to 5 was obtained.

Next, the oxide superconducting powder was applied with an outer diameter of 12 mm and an inner diameter of 10
mm, flow 1000 mm, put one end in a silver tube sealed with silver material, put a silver plug on the other end and weld with leaving a vent hole, then cold draw to an outer diameter of 1 mm, then nitrogen. At 900 ℃ 12
Annealed for a period of time.

When the orientation ratio of the superconductor powder in the length direction of the oxide superconductor wire thus obtained was measured, it was about 80%,
The filling rate was 80%. Furthermore, when the superconducting property was measured, the critical current density was 2400 A / cm ² and the critical temperature was 87K.

Next, two lines of the above oxide superconducting wire are prepared, the end portions are polished and faced each other, and the perovskite type oxide used for manufacturing the oxide superconducting wire is provided between the surfaces to be joined. A weight ratio of superconductor powder to copper powder having the same particle size as 1:
1 interposed allowed to abut such that the mixed powder thickness of 1mm at 30 throughout the 1060 ° C. under a pressure of 5 kg / cm ²
Heat treatment was performed for one minute, and then the whole was cooled while being moved to one of the oxide superconducting wire sides with a temperature gradient of 0.5 ° C./minute·mm.

When microscopic observation was performed by cutting the connection portion of the oxide superconductor wire rods thus connected, the crystals of the oxide superconductor in each oxide superconductor wire rod of the counterpart oxide superconductor wire rod It has been confirmed that the fiber has grown in the direction of fibrous material and has been sufficiently joined as an oxide superconductor wire.

Further, when the superconducting properties of the connected oxide superconductor wire were measured, the critical current density was 2000 A / cm ² and the critical temperature was 87 K, and the effect of the connection was hardly recognized.

[Effects of the Invention] As is clear from the above examples, according to the method for connecting a composite oxide superconductor of the present invention, the oxide of the composite oxide superconductor can be obtained without impairing the continuity as a superconductor. The superconductor part and the normal conductor part can be connected at the same time.

Claims (6)

(57) [Claims] 1. When connecting a composite oxide superconductor composed of an oxide superconductor and a normal conductor, the composite oxide is mixed with a powder of the oxide superconductor and a powder of the normal conductor. The end faces of the oxide superconductor are brought into contact with each other, and after heating at least a melting point of at least one of the oxide superconductor powder and the normal conductor powder, at least the connecting portion is subjected to a temperature gradient in one direction. A method for connecting a composite oxide superconductor, characterized by cooling while applying. 2. The complex oxide superconductor according to claim 1, wherein a coating of the normal conductor is formed on an outer periphery of a strip made of the oxide superconductor. Method for connecting complex oxide superconductors. 3. The complex oxide superconductor according to claim 1 or 2, wherein the oxide superconductor is a perovskite type oxide superconductor containing a rare earth element. How to connect. 4. The oxide superconductor comprises a Ln element (Ln is
At least one element selected from Y, La, Sc, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu), Ba and Cu
Is substantially contained in an atomic ratio of 1: 2: 3.
7. A method for connecting a composite oxide superconductor according to the item. 5. The oxide superconductor is LnBa ₂ Cu ₃ O _7-δ.
The method for connecting a complex oxide superconductor according to any one of claims 1 to 4, which has an oxygen-defective perovskite structure represented by (δ represents an oxygen defect). 6. The method for connecting a complex oxide superconductor according to any one of claims 1 to 5, wherein the normal conductor is Cu or Ag.