JPH02217320A - Production of bi-based oxide superconductor - Google Patents

Abstract

PURPOSE:To obtain a Bi-based oxide superconductor containing dense oxide superconductors, having a homogeneous composition and continuously formed in the shape of layers by subjecting a double-layer material consisting of the first layer composed of Sr-Ca-O elements and the second layer composed of Bi-Cu-O elements to diffusion heat treatment. CONSTITUTION:A composite material consisting of the first layer (1) composed of at least Sr-Ca-O elements and the second layer (2) composed of at least Bi-Cu-O elements is subjected to diffusion heat treatment to form a Bi-based oxide superconductor layer (3) on the surface of the first layer (1). In the above- mentioned method for production, the first layer is oxides composed of the Sr-Ca-O elements and the atomic ratio thereof is preferably such that Ca is within the range of 0.5-2.0 based on 1 Sr. The second layer is oxides composed of the Bi-Cu-O elements and the atomic ratio thereof is preferably such that Cu is within the range of 0.5-2.0 based on 1 Bi. The aforementioned heat treatment is preferably composed of primary diffusion heat treatment carried out at 600-800 deg.C and secondary heat treatment performed at 800-900 deg.C thereafter.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention is expected to be used as a superconducting magnet wire for magnetic resonance imaging bags (MRI-CT), etc., and as a conductive material for superconducting power transmission, etc. , relates to a method for producing a Bi-based high critical temperature oxide superconducting material, which is currently under development.

(Prior art) Recently, the critical temperature T at which the normal conductive state transitions to the superconducting state
Y (yttrium) with c exceeding the liquid nitrogen temperature
Oxide superconductors such as Bi, Bi (bismuth), and TfI (thallium) have been discovered. Bi-based oxide superconductor is B i 2 S r 2 Ca Cu 20
It is known that a phase with the composition indicated by x has a Te of about 80, and a layer with the composition B i2 S r2 Ca2 Cu3 oY has a T of about 105. These phases with different Tc are usually produced in a mixed state, but recently, when part of Bi is replaced with pb, 105
It is known that phases with high Tc are more likely to be generated. These oxide-based superconductors are different from conventional Nb-Ti and Nb3, which required cooling with liquid helium.
Since it can be used under much more advantageous cooling conditions than metal-based superconductors such as Sn, research and development is progressing as a superconducting material that is extremely promising for practical use. In particular, it does not contain highly toxic elements like the Bi-based oxide superconductor TI.
It is attracting attention because it can obtain Tc higher than OOK.

Since oxide superconductors have extremely fragile mechanical properties, the following method is used as an example of a method for processing them into wire rods. That is, after calcining multiple raw material powders containing elements constituting the oxide superconductor and removing unnecessary components,
This calcined powder is filled into a metal tube made of Ag, etc., and processed into a wire of a desired diameter or tape of a desired thickness by methods such as swaging, wire drawing, and rolling. This is a method for manufacturing superconducting wire by causing a solid phase reaction in a compressed mixed powder of

(Problem to be solved by the invention) In conventional manufacturing methods, it is difficult to mix raw material powders completely uniformly, so even if heat treatment is performed, there is a problem that the entire superconductor does not have a completely uniform composition. Ta. In particular, in the case of long wires, it is not possible to produce a superconductor with a uniform composition over the entire length of the wire. This results in the formation of localized areas with inadequate superconducting properties due to inappropriate composition, resulting in the problem that the properties of the entire wire are limited. Further, the oxide superconductor formed inside the above-mentioned wire is a compact formed by compressing powder and sintered by solid phase reaction, and many fine pores exist inside the oxide superconductor.

For this reason, compared to conventional alloy bodies and intermetallic compound bodies, they lack denseness and have a problem in that it is difficult to increase the practically important critical current density Jc.

Furthermore, in oxide superconductors, the superconducting properties are significantly different between the C-axis direction and the AB-axis direction of the crystal, so it is necessary to align the orientation of each crystal and apply a magnetic field in the direction with the best properties.
Current needs to flow. Conventional powder processing methods require difficult work such as extremely strong processing to align the crystal orientation.

The present invention enables the formation of a dense oxide superconductor with a uniform composition in a continuous layered manner, and also allows the thickness of the superconducting layer to be controlled to a desired size, making it easy to form a structure with uniform crystal orientation. The present invention provides a method for producing a Bi-based acetic acid superconductor that can be obtained in the following manner.

(Means for solving problems) The present inventor has focused on the production of superconductors using a diffusion method.

That is, the production of a superconductor by such a diffusion reaction is difficult for Nb3Sn, which, like oxide superconductors, has hard and brittle mechanical properties and is difficult to directly process.

Applied to wire rods of metal compound superconductors such as V3Ga,
They achieved great success in industrializing them (e.g. K, T
achlkava, Pilamentary A15S
upperconductors, Plenua Pr
ess, (198G), p. 1). According to this diffusion method, a dense superconducting layer with a uniform composition can be continuously produced and excellent superconducting properties can be obtained.

The present invention applies this diffusion method to a method for manufacturing a Bi-based oxide superconductor, which includes the following steps:
As the first layer, Sr-Ca-Cu-0, Sr-Ca-0
Bi-C is used as the second layer.
An oxide composed of a-Cu-01Bi-Cu-0 or the like is used. Next, a second layer is applied to the first layer by VLMi to produce a composite, and then a diffusion heat treatment is performed.

The Bi-based oxide superconductor produced in the present invention has the above-mentioned B i2 S r2 CaCu20X% Bi2 Sr2 C
a2 Contains CLI30Y% and a portion of Bi replaced with pb.

The first layer functions as a base for diffusing the composite, and desirably has a melting point as high as possible. This first layer is produced by mixing raw material powders such as S r CO3, Ca CO3, and CuO added if necessary in a predetermined composition ratio, and performing processes such as calcination, mold pressing, and final firing. on the other hand,
The second layer coated thereon is one in which elements are diffused during the diffusion process, and it is desirable to have a melting point as low as possible. This second layer is produced by mixing raw material powders such as B i20B , CLIO%, CaCO3, Pb3O4, etc. added as necessary in a predetermined composition ratio, and performing processes such as calcination and final firing. 1st
The reason why the first layer has a high melting point and the '1s2 layer has a low melting point is that the first layer with a high melting point serves as a mechanically strong base, and the second layer (low melting point element) has a low melting point. This is because the diffusion reaction proceeds more quickly and it is possible to easily move through a thick diffusion layer.

The inventor's research using thermal analysis methods etc. has revealed that in Bi-based oxide superconductors, Bi is used as the first layer of these oxides.
This was obtained by discovering that a Sr--Ca based oxide containing no Sr is suitable, while a Bi--Cu based oxide containing no Sr is suitable for the second layer. here,
The first layer with the above structure has a melting point of about 950°C to 1100°C, and the second layer has a melting point of about 600°C to 80°C.
It is about 0'C.

In the case of the S r-Ca-Cu-0 system, the composition ratio (atomic ratio) of the first layer is 1 for Sr and 0.25 to 1.0 for Ca.
Cu in the range of 0.25 to 1.5, Sr-Ca-
In the case of 0 series, the composition ratio (atomic ratio) of the first layer is preferably in the range of 0.5 to 2.0 Ca to 1 Sr.

In the case of B 1-Ca-Cu-0, the composition ratio (atomic ratio) of the second layer is 0.25 to 1.0.
Cu is in the range of 0.25 to 1.5, Bi-Cu
In the case of −0, the composition ratio (atomic ratio) of the second layer is desirably in the range of Cu0.5 to 2.0 with respect to Bfl. Here, since the first layer and the second layer are in the form of oxides, the content of 0 is theoretically calculated based on the amount of the other elements.

In the present invention, if the composition ratio deviates from these ranges, it becomes difficult to maintain good superconducting properties. In addition, in the second layer, Bi is added at a composition ratio (atomic ratio) of 0.1 to 0.5.
There is no problem in replacing it with b.

Next, as shown in FIG. 1, a second layer 2 is coated on the first layer 1. The specific method is to continuously apply the first layer 1 onto a base tape (not shown) using a spray method, printing method, etc., and then heat treatment to increase the adhesion to the base tape. Then, the second layer 2 is continuously coated on the surface using the same method, or the second layer is coated around the columnar core made by firing the first layer. included.

Next, a diffusion heat treatment is performed. This diffusion heat treatment can provide a material with better performance if the primary diffusion heat treatment is performed at a low temperature and then the secondary diffusion heat treatment is performed at a higher temperature.

- The secondary diffusion heat treatment temperature is in the range of 600°C to 800°C, and the secondary heat treatment temperature is in the range of 800°C to 900°C. - The heat-forming treatment temperature is near the melting point of the second layer, and a Bi-based acid oxide having a composition that provides a high critical temperature Tc is produced by a diffusion reaction.

The secondary heat treatment temperature is near the temperature for forming a Bi-based bulky critical temperature superconductor, forming a crystal structure with a high Tc.

Although it is possible to generate a superconducting layer even if the second diffusion heat treatment is omitted, the component system of the second layer may rapidly melt and expand, causing cracks to occur in the superconducting layer. Therefore, when the secondary diffusion heat treatment is omitted, it is necessary to raise the temperature during the secondary diffusion heat treatment at a rate of less than 1° C./min in a temperature range of 600° C. or higher.

When coating the second layer around the core of the first layer, the composite is subjected to primary diffusion treatment to increase the adhesion between the two, and then inserted into a case made of Ag or the like to form a long wire. By performing a secondary diffusion treatment after processing, it is possible to produce a high Te Bi-based oxide superconducting wire. and,
According to the present invention, by performing diffusion heat treatment on the reference body, the components of the second layer diffuse into the first layer and react, and the components of the second layer react with each other.
A high Te superconducting layer 3 is generated on the surface of the .

〔Effect of the invention〕

As explained above, according to the diffusion method based on the present invention,
It has the effect of producing an i-layer high Tc oxide superconductor that is dense, has no pores, and has a uniform composition. Therefore, when this manufacturing method is applied to wire manufacturing, it is possible to manufacture an i-layer high Tc oxide wire with a large Jc and uniform characteristics in the length direction. In addition, unlike the normal powder sintering method, the components of the second layer diffuse from one direction to form an oxide superconductor, so crystal grains grow in the direction of diffusion, resulting in high Tc oxidation with excellent crystal orientation. There is also the effect that a physical superconductor can be obtained. Furthermore, the thickness of the high Tc superconducting layer (diffusion layer) can be easily controlled by adjusting the thickness of the second layer. Therefore, this manufacturing method solves the problems of conventional manufacturing methods, and can provide a Bi-based high-Tc oxide superconductor with excellent compactness and uniformity, and with uniform crystal orientation.

Example I Sr2 raw material powders of SrcO3, CaCO3, and CuO
It was blended to have the composition of CaCu04, calcined at 900℃ for 6 hours to remove unnecessary components such as CO2, and then
After calcining at 25°C for 12 hours, pulverization and mixing were repeated. This powder was pressed with a load of 2 tons to form a tape with a width of 4 mm, a length of 30 mm, and a thickness of approximately 2 mm.
Main firing was performed at 30° C. for 12 hours to produce a first layer serving as a base. On the other hand, raw material powders of B i2 O3, CaCO3, and CuO were blended to have a composition of B i2 CaCu20B, and after calcining at 700°C for 6 hours, pulverization and mixing were repeated, and main firing at 730°C for 8 hours. A second layer was produced.

According to the results of thermal analysis, the melting point of the first layer is approximately 1070°C.
The melting point of the second layer was approximately 750°C.

Next, a slurry in which powders of the components forming the second layer were suspended in polyvinyl alcohol was applied to a thickness of about 100 μm on the first layer as a base to prepare a composite. This composite was subjected to secondary diffusion heat treatment at 750° C. for 5 hours, and then subjected to secondary diffusion heat treatment at 0° C. for 50 hours to prepare a sample. Note that all the heat treatments in this example were performed in the atmosphere. The components of the second layer were diffused into the second layer by the second diffusion heat treatment, and a diffusion layer with a thickness of about 100 μm was generated, but superconductivity was not yet exhibited.

FIG. 2 shows a cross-sectional photograph of the sample after the secondary diffusion heat treatment.

From this photograph, it can be seen that a diffusion layer 3 with a thickness of about 100 μm is formed on the first layer base 1, and that the crystal grains in the diffusion layer grow in an orientation almost perpendicular to the high melting point element base. Inside the diffusion layer, there are no pores that are found in ordinary powder sintered bodies, and a dense structure is exhibited. Further, FIG. 3 shows the surface analysis results of Bi after the secondary diffusion heat treatment of the sample, and FIG. 4 shows the surface analysis results of Sr. In FIGS. 3 and 4, 1 is a first layer which is a base, and 3 is a diffusion layer. The white dots indicate the distribution of elements, and it can be seen that Sr constituting the first layer and Bi constituting the second layer are each uniformly diffused into the diffusion layer. In addition, when the X-ray diffraction pattern of this diffusion layer was photographed, it was found that mainly Tc8G)[B i2 S r
2 Consists of the diffraction peak of the CaCu20x phase, and a part of the B [2S r2 Ca2 Cu30y
A phase diffraction peak was observed. Figures 5 and 5 show the temperature change in the resistance of the sample measured by the DC 4-terminal method. Zero resistance temperature. Although the degree is 88, superconducting transitions also appear at 80K and 103K.

According to this example, it was found that a dense and thick Bi-based Tc oxide layer having a uniform composition can be produced by the diffusion method.

Example 2 The first layer had a composition of Sr2CaCu205 and Bi was replaced with 20 at% PB (B i 01IP
b 0.2) 2 A second layer having a composition of CaCu208 was prepared in the same manner as in Example 1, and the second layer with a thickness of about 100 μm was coated on the first layer in the same manner as in Example 1. . This composite was subjected to secondary diffusion heat treatment at 150° C. for 5 hours in the air, and then to secondary diffusion heat treatment at 840° C. for 50 hours. As in Example 1, this diffusion heat treatment produced a diffusion layer having a thickness of about 100 μm and having a uniform composition. FIG. 6 shows the change in resistance of this sample with temperature. About to
The resistance was changed to zero at OK, indicating that the generation of a high Te phase was facilitated by substituting part of Bi with pb.

In addition, when the critical current of this sample was measured in liquid nitrogen (77K) using the DC four-probe method, the superconducting state was not broken even when a current of 5 A was applied. The thickness of the superconducting layer is approximately 10
Since it is 0 μm, converting it to critical current density Jc is 12
This is a value of 50 A/cj or more, which is sufficient for practical use.

Example 3 Raw material powders of SrCO3 and CaCO3 were blended and mixed to have a composition of Sr2Ca204, and 9
After calcining at 0°C for 6 hours and at 1025°C for 12 hours, pulverization and mixing were repeated. This powder is spread in 4 layers,
Length: 30-1 Thickness: Approx. 2mm Press molded at 1030℃
The first layer, which is the base layer, was prepared by firing for 12 hours. In addition, B i203 , Pb304 and CuO (Bit, 7
. In the same manner as in Example 1, powders of the components of the second layer were applied onto the base of the first layer to prepare a composite.

This composite was subjected to diffusion heat treatment at 740°C for 5 hours and then secondary diffusion heat treatment at 840°C for 5 hours to prepare a sample. These diffusion heat treatments produced a diffusion layer with a thickness of approximately 50 μm.

When the Tc of this sample was measured by the DC four-terminal method, a zero resistance temperature of 98 was obtained.

[Brief explanation of the drawing]

Figure 1 is a diagram for explaining the principle of the present manufacturing method, Figures 2 to 6 are detailed explanations of the present invention, and Figure 2 shows the particle structure of a cross section of the sample of Example 1. A scanning electron micrograph of the structure; Figure 3 shows a surface analysis image of Bi, and a microphotograph showing the grain structure of a cross section of the sample; Figure '1s4 shows a surface analysis image of Sr, a microphotograph showing the grain structure of a cross section of the sample. , FIG. 5, and FIG. 6 are diagrams showing temperature changes in electrical resistance of samples according to Examples 1 and 2, respectively. 1... First layer (high melting point element), 2... Second layer (low melting point element), 3... Diffusion layer (high Tc superconducting layer) Applicant's representative Patent attorney Takehiko Suzue 1st Figure m ,-,/””-
−”” ”””” ”” −',, *, /-9″
-11\Electric pit (mΩ) Electric gun tower (mΩ)

Claims (5)

[Claims] (1) A composite consisting of a first layer composed of at least the element Sr-Ca-O and a second layer composed of at least the element Bi-Cu-O is subjected to a diffusion heat treatment to form a Bi-based oxide. A method for producing a Bi-based oxide superconductor, which comprises forming a superconductor layer. (2) The first layer is an oxide composed of the elements Sr-Ca-Cu-O, and the atomic ratio is Sr: 1 and Ca:
0.25-1.0, Cu0.25-1.5, and the second layer is an oxide composed of the elements Bi-Ca-Cu-O, with an atomic ratio of Bi 1 as Ca0.
25-1.0 and Cu0.25-1.5, the method for producing a Bi-based oxide superconductor according to claim 1. (3) The first layer is an oxide composed of the elements Sr-Ca-O, and the atomic ratio thereof is 1 for Sr and 0.5 for Ca.
~2.0, and the second layer is Bi-Cu-O
An oxide composed of the elements whose atomic ratio is Bi to 1
2. The method for producing a Bi-based oxide superconductor according to claim 1, wherein Cu is within a range of 0.5 to 3.0. (4) A portion of Bi in the second layer is set at 0.1 with respect to Bi1.
4. The method for producing a Bi-based oxide superconductor according to claim 2 or 3, characterized in that Pb is substituted at a composition ratio of ~0.5 atomic %. (5) Primary diffusion heat treatment in which the heat treatment is in the range of 600°C to 800°C, and 800°C to
5. A method for producing a Bi-based oxide superconductor according to any one of claims 1 to 4, which comprises a secondary diffusion heat treatment at a temperature of 900°C.