JPH0446053A - Oxide superconductor and its production - Google Patents

Abstract

PURPOSE:To improve the critical current density of the superconductor in a strong magnetic field by dispersing a specified multiple oxide in a crystal contg. Bi, Sr, Ca, Cu and O. CONSTITUTION:The superconducting material contg. Bi, Sr, Ca, Cu and O and 0.5-20wt.% of the multiple oxide of >=1 kind of metal selected from group IIa elements such as ABO3 (A is >=1 kind selected from Mg, Ca, Sr and Ba, and B is >=1 kind selected from Zr, Sn, Ce and Ti) and >=1 kind of metal selected from group IVa, group IVb and rare-earth elements are mixed. The mixture is heated above the partial melting temp. (880-900 deg.C) of the superconducting phase, a crystal contg. Bi, Sr, Ca, Cu and O is unidirectionally solidified from the melt at the temp. gradient of >=20 deg.C/cm and at the crystal growth rate of <=20mm/hr, and an oxide superconductor in which the multiple oxide is insularly dispersed in the matrix in which the plate crystals of the oriented superconductor are present in layers in obtained.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides B1zr-Ca-Cu-0 having a novel structure.
System oxide super t,! This article describes the J1 body and its manufacturing method.

[Prior art] Conventionally, B1-5r-Ca-Cu-0 superconductor (hereinafter referred to as B
(also referred to as i-based electromotive conductor), BizSr2Ca2Cum
Oy (hereinafter also referred to as 2223 phase), BiaSri
CaC+gOy (hereinafter also referred to as 2212 phase),
There are three types of structures of Bi*5rtCuOy (hereinafter also referred to as 2201 phase), each with a critical temperature of approximately ll0.
It is known that K 90 K 20. Research and development aimed at practical use at the liquid kiln temperature (77K) is being conducted mainly on the 2223 phase and 2212 phase, which have high critical temperatures. As a method for producing a bulk body of a Bi-based superconductor, there is a method in which a crystal powder having the above-mentioned composition is synthesized, and then this is formed and sintered. In addition, it is known to manufacture by a sol-gel method and a melt solidification method.

[Problems to be solved by the invention] Superconductors manufactured by the sintering method are usually porous polycrystalline bodies with crystal grains arranged in random directions and weak bonds between the grains. . In a Bi-based superconductor, the direction in which current flows easily within the crystal grains is determined, so that it is difficult for current to flow between crystal grains having different orientations.

In addition, a small bonding area between grains means that the effective current path is narrow.For this reason, polycrystalline Bi-based superconductors made by sintering can have a high critical current density. 0 If these polycrystals are pressed or the like to control the direction of the crystal grains, the direction of the crystal grains will be aligned, which will improve the critical current density.Similarly, if manufactured by the melt solidification method, The critical current density improves as the grains become thicker (grow), become denser, and the bonds between grains become stronger.

The critical current density of Bi-based superconductors can be further increased if manufactured by a unidirectional solidification method that combines the control of grain direction and melt solidification, but when manufactured by any of these methods, the The critical current density decreases significantly in a magnetic field. This is thought to be due to the weak binding force of the magnetic flux penetrating the crystal in a magnetic field higher than the lower critical magnetic field of the Bi-based superconductor.

The main field of application for superconductors is considered as electromagnets that create strong magnetic fields by processing wire or tape materials into coils. Therefore, in order to put Bi-based superconductors into practical use, it is necessary to create a dense structure with crystals aligned in the same direction, and to introduce a binding center into the crystal to strengthen the binding force. It is considered necessary to create materials with high critical current densities.

Bi-based superconductors partially melt at temperatures above 880°C,
At the initial stage of solidification, 2201 phase crystals are formed, and when the mixture is allowed to cool slowly, it easily undergoes a phase transformation to 2212 phase. However, at present, a method for solidifying and growing the 2223 phase has not been established in the manufacturing method of Bi-based superconductors by the melt-solidifying method.

Fine precipitates, grain boundaries, and various defects are considered to be the center of bottle fixation. For rare earth superconductors, RE2BaCu
It is known that Oi phase (RE is a rare earth element) and other fine particles of non-superconducting phase (dispersed in the crystal) can become the center of bottle fixation.
There is no non-superconducting phase deposited in the superconducting crystal like a phase,
There are also no reports of introducing a non-superconducting substance completely different from a superconductor into a crystal.

However, in the molten state of Bi-based superconductors,
Fine particles of a non-superconducting substance that does not react with any solid phase or liquid phase and do not grow at high temperatures (melting temperature) are homogeneously dispersed in the material before solidification, and are unidirectionally solidified to become homogeneous in the superconducting phase crystal. The method of dispersion is considered to be excellent in strengthening the center of the bottled material.

[Means for Solving the Problems] The present invention includes at least one metal selected from Group 2A elements and Group 4A and Group 4B elements in a crystal containing Bi, Sr, Ca, Cu, and O as constituent elements. The present invention provides an oxide superconductor having a structure in which a composite oxide with at least one metal selected from rare earth elements is dispersed.

In the present invention, a composite oxide of a metal selected from group 2A elements (all metals) and one or more metals selected from group 4A, group 4B, and rare earth elements is ^BO.
, (A is 1 selected from Mg, Ca, Sr, Ba
(B is preferably one or more selected from Zr, Sn, Ce, and Ti). In this case, Bow is a crystal with a perovskite structure. It is a substance that is compositionally stable up to around 1200°C in the atmosphere, and does not react with the Bi-based superconductor melt at temperatures of 880 to 900°C, which is the partial melting temperature of the Bi-based superconductor, and hardly causes grain growth.

The superconductor of the present invention can be produced by mixing, for example, a raw material with a 2212 phase composition with a composite oxide of one or more alkaline earth metals and other metals, and heating this to a temperature equal to or higher than the partial melting temperature of the 2212 phase. It can be suitably manufactured by cooling and solidifying.

When a mixture of the 2212 phase and the composite oxide is heated to a temperature higher than the partial melting temperature of the 2212 phase and then cooled and solidified, the composite oxide crystals form the 2212 phase while maintaining the particle size added at the time of preparation. It is incorporated into the crystal in the form of islands. At this time, if the above-mentioned composite oxide in which only fine particles are selected is used, a non-superconducting substance of the same size as the composite oxide can be dispersed in the 2212 phase crystal, which is desirable from the viewpoint of strengthening the bottle-holding force. In particular, when only particles of 0.5 μm or less are used, the critical current density increases dramatically and does not decrease much even when a magnetic field is applied.

When using ABO, the amount added is 0.5 wt%
It is preferably at least 20 wt%. Added amount is 05wt
%, there is a risk that the effect of the present invention will not be fully expressed, and if the amount added exceeds 20 wt%, there is a risk that the ABO phase will segregate in a part of the material, resulting in discontinuity of the superconductor. I don't like it because there is. More preferable AB
The amount of O3 added is 1 to 10 wt%.

The superconductor of the present invention is preferably produced by unidirectionally producing superconductor phase crystals from a melt under conditions of a temperature gradient of 20° C./cm or more and a crystal growth rate of 20 mm/h or less. As a result, a solidified product having a structure in which the granular crystals of the composite oxide are dispersed in the form of islands in a matrix in which the plate-shaped crystals of the superconductor arranged are superimposed in a layered manner is obtained.

[Example] Example I Bi:Sr:Ca:Cu atomic ratio is 2:2:
A calcined powder of oxide with a ratio of 1:2 was prepared, and 5 wt% of ABO powder (average particle size 0.5 μm) consisting of the combination of A and B shown in Table 1 was added and mixed. The powder is molded into 70mm x 40mm x 2mm using a mold press.
It was molded into a shape, fired at 860°C for 10 hours in an oxygen stream, and after cooling was cut using a diamond cutter to obtain a prismatic sintered body measuring 70 mm x 4 mm x 2 mm.

Next, one end of this prismatic sintered body was fixed, and it was heated in the long axis direction using an electric furnace with a temperature gradient of 50'C/cm and a maximum temperature of 890°C under an oxygen stream. to 3
It was moved at a speed of mm/h. The resulting solidified product was further heated at 50"C in an atmosphere with an oxygen partial pressure of 0001 atm.
The mixture was heated to 100 mL, held for 10 hours, and then rapidly cooled.

Scanning electron microscope and X
When observed using a line elemental analyzer, as shown in Figure 1, plate-shaped 2212-phase crystal grains overlapped in a layered manner, and within these, ABO particles with a grain size of approximately 0.5 μm were found in island-like fractions. It was confirmed that the tissue was present. The above-mentioned good structure was observed throughout the sample.

Table 1 shows the measurement results of superconducting properties. For these measurements, a sample cut into a size of 1 mm x 0.1 mm x 10 mm was used. The critical temperature is the temperature at which zero resistance was measured using the DC four-probe method, and the critical current density was measured using the same DC four-probe method at liquid nitrogen temperature with an external magnetic field of 2 Tesla applied. The magnetic field is an electromotive force J
Bicaro was parallel to the C axis of the crystal.

Table 1 Example 2 A calcined powder of an oxide with an atomic ratio of Bi2Sr:CaCu of 2 was prepared, and A and B shown in Table 2 were added to it.
After adding and mixing 5 wt% of ABO, which consists of a combination of After baking for 10 hours and cooling, cut out using a diamond cutter.
A prismatic sintered body measuring 70 mm x 4 mm x 2 mm was obtained.

Next, one end of this prismatic sintered body was fixed, and it was heated in an electric furnace with a temperature gradient of 50°C/cm and a maximum temperature of 890'C under an oxygen stream in the longitudinal direction. 3mm
It was moved at a speed of /h. The resulting solidified product was further heated to 500°C in an atmosphere with an oxygen partial pressure of 0.001 atm, held for 10 hours, and then rapidly cooled.

Scanning electron microscope and X
Observation using a line elemental analyzer revealed a structure in which plate-shaped 2212-phase crystal grains overlapped in a layered manner as shown in Figure 1, and ABOa particles with a grain size of approximately 0.15 μm were dispersed in islands within the layered crystal grains. It was confirmed that it has. A good structure as described above was observed throughout the sample.

Table 2 shows the superconducting properties measured in the same manner as in Example 1.

Table 2 Comparative Example A calcined powder of an oxide with an atomic ratio of Bi:Sr:Ca:Cu of 2+2:l:2 was prepared, and this was molded into a size of 7[) mm x 40 mm x 2 mm using a mold press, and Fired at 860'C in air flow for 10 hours, cut out using a diamond cutter after cooling, and cut into 70mm
A 4 mm x 2 mm prismatic sintered body was obtained.

Next, one end of this prismatic sintered body was fixed, and the interior was heated in the longitudinal direction using an electric furnace with a temperature gradient of 50'C/cm and a maximum temperature of 890'C under an oxygen stream. to 3
)/h. The resulting solidified product was further heated at 500'C in an atmosphere with an oxygen partial pressure of 0001 atm.
The mixture was heated to 100 mL, held for 10 hours, and then rapidly cooled.

Scanning electron microscope and X
When observed using a line elemental analyzer, it was confirmed that the structure had a structure in which plate-shaped 2212 phase crystal grains overlapped in layers as shown in FIG. A good structure as described above was observed throughout the sample. Also 221
No particular fine precipitates were observed during the two-phase bonding. Measurements were made in the same manner as in Example 1, and the critical temperature was 89, and the critical current density was 20 OA/cm2.

[Effects of the Invention] The superconductor of the present invention has very fine non-superconductor crystal grains dispersed therein, and this acts as a center for good binding of magnetic flux, so that the critical current density is high even in a strong magnetic field.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the structure of a superconductor obtained in an example. FIG. 2 is a schematic diagram showing the structure of a superconductor obtained in a comparative example.

Claims (3)

[Claims] 1. At least one metal selected from Group 2A elements and at least one metal selected from Group 4A, Group 4B, and rare earth elements in a crystal containing Bi, Sr, Ca, Cu, and O as constituent elements. An oxide superconductor with a structure in which a composite oxide of 2. A composite oxide of at least one metal selected from group 2A elements and at least one metal selected from group 4A, group 4B, and rare earth elements is ABO_3 (A is Mg, Ca, Sr, One or more types selected from Ba, B is Z
2. The oxide superconductor according to claim 1, wherein the oxide superconductor is one or more selected from r, Sn, Ce, and Ti. 3. Temperature gradient is 20℃/cm or more, crystal growth rate is 20℃/cm or more
Bi, Sr, Ca, Cu from the melt under conditions of less than mm/h
, O. The method for producing an oxide superconductor according to claim 1 or 2, characterized in that the crystal containing O is unidirectionally solidified.