JP2905862B2 - Method for producing superconducting whisker composite - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a superconducting whisker composite.

[0002]

2. Description of the Related Art In recent years, remarkable development has been recognized in oxide superconductors in both basic and applied fields. In the basic fields, progress has been made in discovering new composition superconductors, developing new synthetic methods, and elucidating the mechanism of superconductivity. Also, in application fields,
The range of research from electrical materials to applications in the medical field is expanding, and expectations for the development and functional improvement of oxide superconducting materials are increasing in various fields.

At present, non-oxide superconducting materials such as niobium-titanium alloy are widely used as superconducting materials. However, since the critical temperature of these non-oxide superconducting materials is low, liquid helium must be used for cooling, which increases the cooling cost.

On the other hand, some oxide superconducting materials have a critical temperature equal to or higher than that of liquid nitrogen, and if used, the cooling cost can be reduced. Therefore, research and development on power storage, power transport, generation of a strong magnetic field, and the like using an oxide superconducting material have been actively conducted. In these applications, it is necessary to convert the superconductor material into a wire. Examples of the method include a method in which calcined powder of an oxide superconductor is packed in a silver sheath and reheat-treated, a sol-gel method, and a method of drawing an oxide superconductor from a melt. However, since the wires obtained by these methods are polycrystalline, they have drawbacks such as a decrease in superconducting properties and mechanical strength due to crystal grain boundaries, poor workability, and are widely put to practical use. Has not been reached.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an oxide superconducting material having excellent superconducting properties and mechanical strength.

The present inventor has conducted various studies in view of the state of the art as described above, and as a result, has found that Bi ₂ Sr ₂ having a certain size is obtained.
Superconducting whisker having CaCu ₂ O ₈ structure and Bi ₂
When mixed with a superconducting powder crystal having an Sr ₂ CaCu ₂ O ₈ structure, pressed and then sintered, the superconducting properties and mechanical strength are higher than those of a sintered body obtained from the above powder crystal alone. It has been found that an excellent oxide superconducting composite material can be obtained.

That is, the present invention provides a method for producing a superconducting whisker composite comprising a sintered body of the following superconducting whiskers and superconducting powder crystals: Bi, S
consisting of r, Ca, Cu and O, Bi ₂ Sr ₂ CaC
a superconducting powder crystal having a u ₂ O ₈ structure and a particle size of 1 to 50 μm, and Bi, Sr, Ca, Cu and O;
It has a Bi ₂ Sr ₂ CaCu ₂ O ₈ structure and a length of 1 to 10
mm, a width of 10 to 150 μm, and a thickness of 1 to 10 μm with a superconducting whisker: [powder crystal]: [whisker] =
1.00: A method for producing a superconducting whisker composite, comprising mixing at a weight ratio of 0.01 to 0.50, forming the mixture under pressure, and sintering at 820 to 875 ° C.

The superconducting powder crystal used for producing the superconducting whisker composite of the present invention has an atomic composition ratio of Bi =
1.00, Sr = 1.00, Ca = 0.50, C
After mixing the raw materials so that u = 1.00, the mixture is calcined and pulverized. The firing conditions at this time are usually about 820 to 860 ° C. for about 20 to 100 hours.
As a more specific example, the heat treatment is performed at 820 ° C. for about 60 hours, and further at 860 ° C. for about 20 hours. The particle size of the powder is usually about 1 to 50 μm, more preferably 1 to 10 μm.
m.

The superconducting whiskers used in the production of the superconducting whisker composite of the present invention are prepared by quenching the melt of the raw material as disclosed in Japanese Patent Application Laid-Open No. 2-252621. Assuming that the atomic composition ratio is Bi = 1.00, Sr = 1.00, Ca
= 1.00, Cu = 2.00, Al = 0.50-1.0
The glass plate which is 0 in an oxygen stream, for example,
It is obtained by heat treatment for a time. The dimensions of the superconducting whisker are usually about 1 to 10 mm in length and 10 to 15 in width.
The thickness is about 0 μm and the thickness is about 1 to 10 μm, more preferably about 5 mm in length, about 100 μm in width, and about 5 μm in thickness.

[0010] In any of the above-mentioned powder crystals and whiskers, the raw material is not particularly limited as long as it can form an oxide by firing, and may be a simple metal, an oxide, various compounds (carbonate). Etc.) can be used. As the raw material, a compound containing two or more of the above atoms may be used. In the firing step, any known apparatus such as an electric heating furnace or a gas heating furnace may be used.

Next, the superconducting powder crystal and the superconducting whisker obtained by the above method are sufficiently mixed at a weight ratio of [powder crystal]: [whisker] = 1.00: y (0.01 ≦ y ≦ 0.50). 820 to 875 ° C.
Firing is performed for about 15 to 30 hours (for example, at 860 ° C. for 20 hours). The pressure during molding is not particularly limited,
Usually, it is about 10 to 1000 kg / cm ² , more preferably about 100 to 500 kg / cm ² . The firing means at this time is not particularly limited, and any known apparatus such as an electric heating furnace or a gas heating furnace can be adopted.

In the present invention, in a sintered body in which the amount of superconducting whiskers is less than 0.01 part per 1 part by weight of superconducting powder crystal, superconducting properties (critical current density, magnetic shielding properties) and mechanical strength are within the composite. , The effect of mixing whiskers is hardly recognized. On the other hand, when the amount of the latter is more than 0.50 parts with respect to the former, even if firing at a high temperature of 860 ° C. or more, the sintering is not sufficiently performed, so that the mechanical strength of the composite is: It becomes lower than the powder crystal sintered body.

The superconducting whisker composite (powder crystal: whisker weight mixing ratio = 1.0) obtained in the present invention.
FIG. 1 shows a powder X-ray diffraction pattern of one example (0: 0.05) (this is No. 1 in Example 1 described later, and the production method and the like refer to Example 1). Similar X-ray diffraction patterns were observed in all the production examples of the superconducting whisker composite and the superconducting powder crystal sintered body. These diffraction patterns are unique to the Bi ₂ Sr ₂ CaCu ₂ O ₈ structure.

The critical temperature (Tc) of the superconducting whisker composite and the superconducting powder crystal sintered body obtained in Example 1 of the present invention.
_zero ) was 70 to 75K (see Table 1 below).

Superconducting whisker composite (weight ratio of powder crystal: whisker = 1.0
0: 0.05) (No. 1 in Example 1 described later).
2 is shown in FIG. 2. The temperature at which the electrical resistance became zero was 72K.

In order to further improve the sinterability, the above-mentioned pressed body was fired at 875 ° C. for a long time (for example, 10 hours), and further fired at 860 ° C. for 20 hours, followed by furnace cooling. The critical temperature of the superconducting whisker composite is higher than the critical temperature of the composite obtained by baking the same pressed molded body at 875 ° C. for a short time (for example, 3 hours) and then baking at 860 ° C. for 20 hours. Got lower. In order to raise the critical temperature of the composite obtained by firing at 875 ° C. for a long time to the critical temperature of the composite obtained by firing at 875 ° C. for a short time, the temperature after firing is 8
It is necessary to rapidly cool the composite from a high temperature of 00 ° C. or higher. In addition, when the superconducting whisker composite is manufactured, 875
If baked at a high temperature of not less than ° C., the surface of the superconducting whisker or the superconducting powder crystal is partially melted and the crystal structure is broken, so that the superconducting properties of the composite are deteriorated (for example, the critical temperature is lowered). Therefore, the firing temperature of the pressed body is
The range is 820 to 875 ° C.

FIG. 3 shows No. 1 of Example 1 of the present invention in which the weight mixing ratio of powder crystal: whisker was 1.00: 0.05.
1 and superconducting whisker composite (a) according to Example 1
No. 2 shows an X-ray diffraction pattern of a pressing plane of a superconducting powder crystal sintered body (b) according to No. 2 in a pressing step in a production process thereof. In the superconducting whisker composite (a) according to the present invention, the peak intensity due to the (00n) plane is larger than the other peaks. This means
Ab of superconducting whisker in superconducting whisker complex
This shows the fact that the plane is oriented parallel to the pressing plane. It is considered that the orientation of the ab plane occurs during pressure molding. On the other hand, in the case of the superconducting powder crystal sintered body (b), a characteristic large peak due to the orientation of the ab plane as in the case of (a) is not recognized.

The powder crystal: whisker weight mixing ratio is 1.
No. 00 of Example 1 of the present invention, that is, No. 00: 0.05. 6 and the superconducting whisker composite (a) according to No. 6 of Example 1.
FIG. 4 shows the temperature dependency of the magnetic susceptibility when a magnetic field of 0.01 G is applied perpendicularly to the pressing plane at the time of pressing the superconducting powder crystal sintered body (b) according to No. 2. In this case, as described above, the whisker in the superconducting whisker composite (a) has a magnetic field perpendicular to the ab plane because the ab plane of the superconducting whisker is oriented parallel to the pressing plane. It is hung. The diamagnetic susceptibility at the same weight and the same shape at a temperature of 70 K or less is determined by the superconducting whisker composite (curve A).
Was larger than the superconducting powder crystal sintered body (curve B). From this result, it is clear that the superconducting whisker composite according to the present invention is superior to the superconducting powder crystal sintered body in the magnetic shielding effect in a low magnetic field of about 1 G or less. This is because the ab plane of the whisker in the superconducting whisker complex is oriented parallel to the pressing plane as described above. That is, in the superconducting powder crystal sintered body, the magnetic flux penetrates into the sintered body through the crystal grain boundary, whereas in the superconducting whisker composite,
Since the ab plane of the superconducting whisker is oriented perpendicular to the magnetic field, the whisker prevents the magnetic flux from entering the inside of the composite.

Critical current density Jc measured by energization method
Is 33.8 A / cm ² (measuring temperature 65.8 K) for a superconducting powder crystal sintered body, and 252 A for a superconducting whisker composite of the present invention having a powder crystal: whisker weight mixing ratio of 1.00: 0.05. / Cm ² (measurement temperature 71.1K), and the Jc value of the whisker composite was increased by almost one digit, even though the temperature difference between the measurement temperature and _Tc was smaller.

In order to compare and evaluate the mechanical strength between the superconducting whisker composite according to the present invention and the sintered superconducting powder crystal, the length is 25 mm × thickness 4.0 to 4.6 mm × width 3.0.
Using a test piece whose surface was polished with a # 800 lap plate at a distance of 6.5 mm, the three-point bending strength was measured with a span of 20 mm. From this result, it was found that there was no significant difference in bending strength between the superconducting whisker composite and the superconducting powder crystal sintered body (the weight mixing ratio of powder crystal: whisker was 1.00: 0.05. No. 1 of the first embodiment.
Average value of superconducting whisker composite of No. 1 = 6.73MP
a: No. 1 of Example 1 consisting of only superconducting powder crystals Average value of the sintered body of No. 2 = 8.52 MPa).

The bending strength was calculated from the following equation.

Bending strength = (3 × load × span) / (2 ×
Width x (thickness) ² ) However, the span is the distance between two fulcrums on the tensile surface side during three-point bending measurement.

However, comparing the state of fracture with the load-mutation curve (FIG. 5) of the superconducting whisker composite and the powder crystal sintered body, the powder crystal sintered body (b) shows that the stress is instantaneous after the maximum load. In contrast, the superconducting whisker composite (a) showed metastable fracture in which the stress gradually decreased. This result shows that when the sintered body is cracked,
While the cracks spread instantaneously throughout the sintered body and break at once, the superconducting whisker composite cracks more slowly than the powder crystal sintered body, No destruction will occur. By the way, when the fracture surface of both sintered body test pieces was observed with an electron microscope (SEM), in the superconducting powder crystal sintered body, several thick cracks were mainly formed along the load line. In the whisker complex, thin cracks were observed circumventing the rim to avoid the whiskers. From the above results,
When an excessive load was applied to the superconducting whisker composite and the composite was cracked, it was found that the whiskers in the composite delayed the crack growth and prevented instantaneous fracture.

The superconducting properties and mechanical strength of the superconducting whisker composite are affected by the sintering conditions during production and the mixing ratio between the powder crystal and the whiskers. In order to improve the superconductivity, the mixing ratio of the whiskers may be increased. However, when the mixing ratio of the whiskers is 0.5 or more, the sinterability deteriorates and the mechanical strength decreases. On the other hand, in order to improve the mechanical strength, it is preferable to fire at a high temperature (for example, 875 ° C. or more) for a long time, but in the case of a sintered body fired at a high temperature for a long time, the whisker and the The surface of the tsu crystal is melted, and the superconductivity is reduced. In order to harmonize both the superconducting properties and the mechanical strength while considering the above conflicting events, the weight ratio of powder crystal: whisker in the raw material of the superconducting whisker composite is set to 1.00: 0.05. Further, the sintered body obtained when the firing temperature of the press-formed body is 860 ° C. is the most excellent as a high-temperature superconducting material.

[0025]

According to the present invention, Bi, Sr, Ca, C
and a superconducting powder crystal having a structure of Bi ₂ Sr ₂ CaCu ₂ O ₈ , and Bi, Sr, Ca,
Made of Cu and O, were mixed and the superconducting whiskers having a Bi ₂ Sr ₂ CaCu ₂ O ₈ structure, after pressure molding, by sintering at eight hundred and twenty to eight hundred and seventy-five ° C., can be obtained superconducting whisker composites .

The superconducting whisker composite of the present invention thus obtained has a junction between the superconducting whiskers inside, and the whisker ab
Since the planes are oriented in parallel, the superconductivity is superior to that of a sintered body composed of only superconducting powder crystals. Further, since the superconducting whisker delays the growth of cracks generated in the composite, the superconducting whisker is also superior in mechanical strength to the superconducting powder crystal sintered body.

Therefore, the superconducting whisker composite according to the present invention is subjected to pressure molding to a required shape in a pressure molding step before firing, and the obtained molded body is sintered to be used for power transport and power storage. It is also expected to be used as a high-temperature superconducting material such as a wire for a magnetic field generating magnet and a magnetic shielding material.

[0028]

EXAMPLES Examples are shown below to further clarify the features of the present invention. Example 1 Bi = 1.00, Sr = 1.00, Ca = 0.5
0, and after sufficiently mixing the following starting materials so that Cu = 1.00, the mixture was placed in an alumina crucible and placed in an electric furnace at 820 ° C.
And then calcined at 860 ° C. for 20 hours after molding. Next, the obtained fired body is sufficiently pulverized,
A superconducting powder crystal (average particle size: about 10 μm) having a structure of Bi ₂ Sr ₂ CaCu ₂ O ₈ was obtained.

Then, 1.00 g of the superconducting powder crystal and 0.05 g of superconducting whiskers (average length 5 mm, average width 100 μm, average thickness 5 μm) having a structure of Bi ₂ Sr ₂ CaCu ₂ O ₈ were sufficiently mixed. , 100kg / c
After pressing under m ² , place on an alumina plate and place in an electric furnace for 8 minutes.
It was baked at 60 ° C. for 20 hours and cooled in a furnace at a cooling rate of about 50 ° C./min. The obtained superconducting whisker composite No. 1 is
It was a disk having a diameter of 16 mm and a thickness of 1.5 mm.

Further, the superconducting powder crystal and the superconducting whisker are mixed at a predetermined ratio shown in Table 1 and then press-molded, or the above-mentioned superconducting powder crystal is press-molded alone, and then, as shown in Table 1. And sintered under the conditions shown in
o. 2-13 were obtained.

Table 1 shows the critical temperature of each of the obtained sintered bodies.

[0032]

[Table 1] The raw materials for producing superconducting whiskers and superconducting powder crystals used in this example were as follows. -Bi source ... bismuth oxide (Bi ₂ O ₃₎ -Sr source ... strontium carbonate (SrCO ₃₎ -Ca source ... calcium carbonate (CaCO ₃₎ -Cu source ... copper oxide (CuO) Further, the superconducting whisker composites 6 No. 6 (a) and superconducting powder crystal sintered body no. The SEM photograph of 2 (b) is shown. It is clear that the superconducting whisker composite according to the invention has a unique structure in which the superconducting whiskers are completely embedded in the powder crystals.

[Brief description of the drawings]

FIG. 1 shows a superconducting whisker composite N obtained in Example 1.
o. 1 is an X-ray powder diffraction pattern of Example 1.

FIG. 2 shows a superconducting whisker composite N obtained in Example 1.
o. 3 is a graph showing the temperature dependence of the electrical resistance of No. 1.

FIG. 3 shows a superconducting whisker composite N obtained in Example 1.
o. X-ray diffraction pattern (a) of the pressing plane of No. 1 and superconducting powder crystal sintered body No. 2 is an X-ray diffraction pattern (b) of a pressurized plane at the time of press-molding No. 2;

FIG. 4 shows a superconducting whisker composite N obtained in Example 1.
o. 6 (curve A) and superconducting powder crystal sintered body no. 2 is a graph showing the temperature dependence of the magnetic susceptibility at 0.01 G after the zero magnetic field cooling of Curve 2 (curve B).

FIG. 5 shows a superconducting whisker composite N obtained in Example 1.
o. 6 (a) and superconducting powder crystal sintered body no. 2B is a load-displacement curve of FIG.

FIG. 6 shows a superconducting whisker composite N obtained in Example 1.
o. 6 (a) and superconducting powder crystal sintered body no. 2 (b) is a photograph of the fracture surface.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hiroshi Yamashita 18-8 Doinouchi, Sasabe, Kawanishi-shi, Hyogo (72) Inventor Minoru Kinoshita 1-2-17-106 Midorioka, Ikeda-shi, Osaka (56) References JP-A-63-285159 (JP, A) JP-A-3-150225 (JP, A) JP-A-2-252621 (JP, A)

Claims (1)

(57) [Claims] The present invention comprises Bi, Sr, Ca, Cu and O, has a Bi ₂ Sr ₂ CaCu ₂ O ₈ structure, and has a particle size of 1 to 2.
50 μm superconducting powder crystal, Bi, Sr, Ca,
A superconducting whisker made of Cu and O, having a Bi ₂ Sr ₂ CaCu ₂ O ₈ structure, a length of 1 to 10 mm, a width of 10 to 150 μm, and a thickness of 1 to 10 μm is referred to as [powder crystal]: [whisker] = 1.00: 0.01-0.50
After mixing at a weight ratio and press-molding, 820-875 ° C.
A method for producing a superconducting whisker composite, characterized by sintering.