JPH01252571A - Production of oxide superconductor - Google Patents

Abstract

PURPOSE:To obtain the title superconductor improved in critical current density, by strongly grinding, through a mechanical alloying technique, the constituent metallic simple substances or alloys therefrom. CONSTITUTION:The objective superconductor consisting of L-M-Cu-O-based oxides (where, L is at least one metallic element selected from lanthanoids and Y; M is Sr or Ba or the both). The raw materials to be used are constituent metallic simple substances or alloys therefrom; for example, in the case of La-Sr-Cu-O-based superconductor, high-purity metals La, Sr and Cu are used. Said raw materials are strongly ground using a ball mill or planet-type grinder to cause solid phase diffusion to effect homogenization followed by press forming and then carrying out sintering in an oxygen atmosphere, thus obtaining the objective oxide superconductor. The above-mentioned process will eliminate the moisture absorption for the raw material, reduce the characteristics deterioration of the final superconductor due to moisture and variation in the manufacturing, also preventing the contamination of impurities.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention relates to a method for producing a high-temperature superconducting oxide, which has a high content of high-temperature superconducting phase and a more homogeneous structure. It is used for body manufacturing.

(Prior art) High temperature superconducting oxides include, for example, Y-Ba-Cu-0+
There are perovskite ceramics such as La-5r-Cu-0, and their superconducting transition temperatures (hereinafter referred to as Tc) are about 90 for the former and about 40 for the latter.

These manufacturing methods use Ittria as the raw material.

Prepare powders of metal oxides such as strontium carbonate, barium carbonate, copper oxide, etc., and use a balance etc. to
In the u-0 system, Y:Ba:Cu is weighed and mixed in a molar ratio of 1:2:3.

Next, this is press-molded and then sintered in the atmosphere using a firing furnace. After taking it out of the furnace, it is ground sufficiently finely and then press-formed and fired again. This operation is repeated several times to obtain a sintered body, and this oxide sintered body exhibits superconducting properties.

The high-temperature superconducting oxide produced in this way contains a certain volume fraction of a phase that causes superconductivity at high temperatures.
When measuring electrical resistance, zero resistance appears because they connect together to form a path for superconducting current.

However, when measuring the critical current density (hereinafter referred to as Tc) and the Meissner effect, the higher the volume fraction of the superconducting phase, the higher the Tc.
c and a large Meissner effect can be obtained.

(Problem to be solved by the invention) Currently, the biggest problem with practical application of oxide superconductors is that Jc is small, and the problem is that the superconducting state is destroyed when the necessary current is passed for practical use. be.

It is believed that Jc increases as the volume fraction of the superconducting phase increases and as the density of the oxide improves, and efforts are being made to improve the density by press-molding at higher pressures. Despite this, the highest JC currently available is approximately 10.
It remains at a relatively low value of 008/CrA.

The extent to which superconducting phases are contained in oxides is measured by measuring the Meissner effect, that is, magnetic susceptibility; for example, Japanese Journal of Appl.
iedPhysics(JJAP) VOL, 2
6 N114. April, 19B? .

pp, L345-L346, it is stated that the superconducting phase obtained by the above method accounts for only 9% of the total, and Jc can be improved by increasing this content.

The present invention provides a method for increasing the volume fraction of the high temperature superconducting phase contained in the oxide superconductor.

In the conventional manufacturing process, it is attempted to obtain a homogeneous mixture by mixing oxide powders such as ittria, so it is thought that the diffusion of atoms is restricted by the particle size, etc., and the original mixing at the atomic level is not possible. I can't imagine it being done. In other words, since a superconducting phase is generated by diffusion in a portion where particles of multiple types of oxide powder come into contact, there is a limit to the volume fraction of the superconducting phase.

Focusing on this point, the present inventors abandoned the method of mixing powders in the oxide state and instead mixed single metal elements and alloyed them to form an ideal mixture of multiple metals in solid solution. They attempted to produce a uniform superconductor by oxidizing the material and later oxidizing it.

Japanese Journal is an example of using liquid quenching method as one of the methods to uniformly mix metal elements by solid solution alloying.
al of Applied Physics (J
JAP) VOL, 26゜11h4 April
, 1987. pp. L334-L336.

A metal mixture of La-5r-Cu is melted at a high temperature, and once it is uniformly melted, it is rapidly solidified by a liquid quenching method such as a single roll method to form a fine structure, a solid solution, and an amorphous phase in a ribbon shape.

By treating this with oxygen to obtain a superconducting phase, a superconductor can be produced without the step of sintering the powder, and an effect equivalent to the purpose of the present inventors can be expected. However, this method always requires melting at high temperatures, so yttrium (Y) and barium (Y) and barium (Y) do not have solubility in the liquid phase.
In Ba), phase separation occurs in the liquid phase and mixing is impossible.

In this sense, there is a problem in that mixing by the liquid quenching method does not make sense in systems that separate in the liquid phase (for example, Y and Ba systems).

The technical problem of the present invention is to solve the above-mentioned problems and to find out how uniformly single metal elements can be mixed.

[Structure of the invention]

(Means for solving the problem) The technical means taken to solve the above problem are as follows. That is, an oxide superconductor of the LM-Cu-0 system, where L is a lanthanide, one metal element among yttrium, 2M is one metal element among strontium or barium, and the manufacturing process thereof The raw materials are metal elements or alloys selected from them, and are strongly crushed by the mechanical alloying method described below to cause solid phase diffusion and homogenization, and then press-formed in oxygen. It is manufactured by sintering.

(Function) Mechanical alloying method (or mechanical grinding method, hereinafter referred to as MA method) uses a ball mill or planetary crusher to grind multiple metals and alloys into their solid state using strong impact during crushing. This method uses diffusion reactions to mix, alloy, and amorphize metal elements, and allows metal elements to be mixed almost completely and uniformly.

FIG. 1 shows a schematic diagram of a planetary crusher used in the MA method.

In the figure, 1 is a case used for a small ball mill, and a powder 2 to be a sample and several balls 3 are stored inside.

This case rotates at high speed around an axis 4 in the direction of rotation shown at 5. Furthermore, this case is fixed on the disk 6,
The disk 6 itself rotates violently around the axis 7 in the direction of rotation of the disk 8, and as a result, the case 1 of the ball mill is subjected to strong planetary rotation, and the balls inside the case are rotated around the sample inside the case. A strong impact is applied to the paddy.

This impact causes the powder to become more finely pulverized, and finally solid-phase diffusion occurs, resulting in alloying of different metals and amorphous formation.

Since the alloys and amorphous materials produced in this way do not pass through the molten state, they are completely unrelated to segregation or precipitated phases and are ideally homogenized, and furthermore, the branch fibers of the powder can be extremely fine as 1 μm or less.

The metal elements constituting the superconducting oxide, such as Y, Ba, and La, were alloyed using this MA method to achieve an ideal mixture.

When ordinary ittria and barium carbonate are crushed and mixed in a mortar etc., the grains are still ittria and barium carbonate grains, and the diffusion of atoms during sintering occurs only from the contact area between grains, which is natural. For large-diameter grains, the formation of a Y-Ba-Cu-0-based superconducting phase caused by a diffusion reaction is limited to the vicinity of grain boundaries.

This means that there is a big restriction on increasing the volume fraction of the superconducting phase.

If you analyze the powder by X-ray diffraction after mixing itria, barium carbonate, and copper oxide in a mortar or the like, peaks indicating the structure of each grain will naturally appear, but Y, Ba, and Cu are uniformly alloyed by the MA method. The normal Y, Ba, and Cu peaks did not appear in the X-ray diffraction test of the powder.
A halo pattern with a low intensity peak and a large tail was observed, and a homogeneous structure close to an amorphous state was obtained.

The alloy powder produced in this way is put into a mold, press-formed, and fired in an oxygen atmosphere. For example, the Y-Ba-Cu alloy powder obtains oxygen and becomes
-0 series perovskite, and an oxide superconductor with a high content of superconducting phase can be produced.

(Example) Examples will be described below.

In the case of a La-5r-Cu-0 based superconductor, high-purity metal lanthanum, metal strontium and copper are prepared in the form of particles or plates, and the molar ratio of these is set to La using a top balance.
:Sr:Cu=9:1:5.

This was melted in a vacuum arc melting furnace to obtain an alloy. Next, this alloy is coarsely ground, 200 grams of the alloy is collected, and the alloy is sealed in a case for a planetary grinder together with balls and atmospheric gas for pulverization.

Grinding continues for 10 to 20 hours, during which time mechanical alloying progresses and the powder is homogeneously alloyed by solid phase diffusion. In the X-ray diffraction pattern at this time, no peaks due to the usual powder method appeared, and a halo-like pattern close to an amorphous state was observed, proving alloying by solid phase diffusion.

As a result of SEM observation, the particle size of the powder at this time was very fine, 1 μm or less.

Next, this powder was applied to a mold, and the pressure was 200 kg/cj.
After press forming, the temperature was 1000℃1 in an oxygen atmosphere.
A heat treatment was performed for 2 hours, and then the material was cooled in a furnace to obtain a sintered body.

- The temperature change in resistance of this superconducting oxide is shown in Figure 2 (a). Tc was 31. When this superconducting phase was identified by X-ray diffraction, it was found to be a perovskite-type oxide with a KJiF4 structure.

Table 1 shows the properties of this sintered body.

Table 1 The volume fraction occupied by the superconducting phase was determined to be 35% by magnetic susceptibility measurement.
It was confirmed that the conventional La-5r-Cu-0
It was found that this was improved compared to the system oxide content which was 18%.

Next, an example of manufacturing a Y-Ba-Cu-0 based oxide superconductor will be described.

In the Y-Ba-Cu-0 system, unlike the La system, Y and Ba do not mix in the liquid phase, so first the Y-Cu alloy and Ba are mixed.
-Cu alloys are each produced by arc melting. At this time, an alloy of Y:Cu-1:LBa:Cu=l:l was used.

Next, the overall molar ratio of Y:Ba:Cu is 1:2:
Weigh the Y-Cu and Ba-Cu alloys so that
Approximately 20 grams of the mixture is placed in a planetary crusher together with balls and argon gas.

Grinding was carried out for about 20 hours at 780r, p, m, and
A fine powder of less than μm was obtained. The X-ray diffraction pattern of this powder also exhibits a fine structure close to amorphous, similar to the La-based powder, indicating that solid phase diffusion is progressing.

Next, set it in this press mold and press 200 kg/-
The material was press-molded at a pressure of 200° C., heat treated at 890° C. for 12 hours in an oxygen atmosphere, and cooled in a furnace.

The resistance characteristics of this sample are listed in Table 1 above, and the change in resistance with temperature is shown in Figure 2 (b).

The Tc in this case was as high as 89, making it an excellent superconductor. Furthermore, the volume fraction of the superconducting phase obtained by measuring the magnetic susceptibility was as high as 50.9%, which was superior to the value of 9% obtained by the conventional sintering method.

FIG. 3 shows the measurement results of magnetic susceptibility in this case.

The generation of demagnetizing field when the magnetic field is increased is the applied magnetic field of 20,000
The superconducting phase in the sample was calculated to be 50.9% from this slope.

In this way, the present invention is revolutionary in improving the properties of superconducting oxides.

〔Effect of the invention〕

The present invention provides the following effects.

By homogenizing the oxide superconductor using the MA method, we can: (1) Since the starting material is a metal element, the raw material has no hygroscopicity, and there is no possibility of deterioration of properties due to moisture or manufacturing variations during the press forming and firing processes; Less is.

(2) When carbonate compounds or oxalate compounds such as barium carbonate are used, performance deteriorates due to the inclusion of carbon and oxalic acid as impurity elements, but according to the MA method, this point is completely eliminated. It can be prevented.

[Brief explanation of the drawing]

Figure 1 is a simplified explanatory diagram of the device using the MA method, Figure 2 is a diagram showing temperature changes in the resistance of an oxide superconductor, and Figure 3 is the measurement result of the magnetic susceptibility of the Y-Ba-Cu-0 system. FIG. 1...Small ball mill. 2...sample, 3...ball.

Claims (1)

[Claims] LM-Cu-O system oxide superconductor, where L
is a metal element among lanthanoids and yttrium,
M is one of the elements strontium or barium, and its raw material is a metal element or an alloy selected from these when it is manufactured, and it is crushed by a ball mill or a planetary crusher. By pulverizing it, solid phase diffusion is caused and homogenized, and after press molding,
A method for manufacturing an oxide superconductor manufactured by sintering in oxygen.