JPH0812422A - Superconductive whisker composite product and its production - Google Patents

Abstract

PURPOSE:To obtain a highly densified superconductive product containing highly oriented superconductive whiskers each comprising a single crystal excellent in the superconductive characteristics and solving the problem of weak bonds on the particle boundary. CONSTITUTION:The oxide superconductive whisker composite product comprises oxide superconductive whiskers and oxide superconductive powdery crystals. The superconductive whiskers and the superconductive powdery crystals both comprise Bi, Sr, Ca, Cu and O, and have a Bi2Sr2CaCu2O8 structure. The content of the superconductive whiskers is 5-40wt.% based on the whole weight of the whisker composite product. The method for producing the superconductive whisker composite product comprises hot-pressing and sintering a mixture comprising (a) 5-40wt.% of oxide superconductive whiskers comprising the Bi, Sr, Ca, Cu and 0 and having the Bi2Sr2CaCu2O8 structure and (b) 95-60wt.% of oxide superconductive powdery crystals comprising the Bi, Sr, Ca, Cu and 0 and having the Bi2Sr2CaCu2O8 structure.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an oxide superconducting whisker composite and a method for producing the same.

[0002]

2. Description of the Related Art Recent advances in oxide superconductors have been made in both basic and applied research fields. In the field of basic research, progress has been made in discovering new composition superconductors, developing new synthetic methods, and elucidating the mechanism of superconductivity. Further, in the applied research field as well, the range of research is expanding from the electric / electronic materials to the medical field, and expectations for development and improvement of functionality of oxide superconducting materials are increasing from various fields.

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

On the other hand, some oxide superconducting materials have a critical temperature higher than the boiling point of liquid nitrogen, and if this is used, the cooling cost can be reduced. Therefore, research and development on electric power storage, electric power transportation, strong magnetic field generation, etc. using oxide superconducting materials have been actively conducted. In order to achieve these objects, it is necessary to convert the superconductor material into a wire rod. As a wire forming method, a method of filling a calcined powder of an oxide superconductor in a silver sheath and re-heat treatment, a sol-gel method,
There is a drawing method from a melt of an oxide superconductor. However, since the wire rods obtained by these methods are polycrystals, in order to obtain good superconducting properties, not only the orientation of crystal grains and the densification of the material but also the weakness at the crystal grain boundaries is required. We also have to overcome the problem of union.

[0005]

Therefore, according to the present invention, the superconducting whiskers, which are single crystals having excellent superconducting properties, are highly oriented, the problem of weak coupling at grain boundaries is solved, and the density is increased. The main purpose is to obtain a superconducting material.

[0006]

The present inventor has conducted various experiments and studies in view of the current state of the art as described above.
As a result, Bi ₂ Sr ₂ CaCu ₂ O ₈ structure by mixing a superconducting powder crystals having a superconducting whisker and Bi ₂ Sr ₂ CaCu ₂ O ₈ structure having, by hot press sintering, only the above powdery crystals We succeeded in obtaining an oxide superconducting material with an improved critical current density as compared with a hot-press sintered body.

That is, the present invention provides the following oxide superconducting whisker composite and a method for producing the same: It is composed of oxide superconducting whiskers and oxide superconducting powder crystals, and the superconducting whiskers and superconducting powder crystals both consist of Bi, Sr, Ca, Cu and O and have a Bi ₂ Sr ₂ CaCu ₂ O ₈ structure. An oxide superconducting whisker composite, characterized in that the content of the superconducting whiskers is 5 to 40% by weight based on the total weight.

2. (A) 5-40% by weight of oxide superconducting whiskers composed of Bi, Sr, Ca, Cu and O, and having a Bi ₂ Sr ₂ CaCu ₂ O ₈ structure, and (b) Bi, S
consisting of r, Ca, Cu and O, Bi ₂ Sr ₂ CaCu
_A method for producing an oxide superconducting whisker composite, which comprises hot-press sintering a mixture of 95 to 60% by weight of oxide superconducting powder crystals having a ₂ O ₈ structure.

The superconducting powder crystal used in the production of the superconducting whisker composite of the present invention has an atomic composition ratio of Bi
= 1.00, Sr = 1.00, Ca = 0.50,
It is obtained by mixing the raw materials so that Cu = 1.00, firing and pulverizing. The firing conditions at this time are usually a temperature of 800 to 860 ° C. and a time of 20 to 100.
In about a time, as a more specific example, after calcination at 800 ° C. for 12 hours, calcination is further performed at 860 ° C. for 60 hours. The particle size of the powder crystal is not particularly limited, but is usually about 1 to 50 μm, and more preferably about 1 to 10 μm.

On the other hand, the superconducting whiskers used in the production of the superconducting whisker composite of the present invention are disclosed in JP-A-2-252621 by the present inventors.
It is obtained by heat-treating the glass plate obtained by rapidly cooling the melt of the raw material. More specifically,
For example, assuming that the atomic composition ratio is Bi = 1.00, Sr =
It is obtained by heat-treating a glass plate having 1.00, Ca = 1.00 and Cu = 2.00 in an oxygen stream at 865 ° C. for 80 hours. The size of the superconducting whiskers is not particularly limited, but is usually about 1 to 15 mm in length and 10 to 1 in width.
The thickness is about 50 μm and the thickness is about 1 to 10 μm, and more preferably about 10 mm in length, about 100 μm in width, and 5 in thickness.
It is about μm.

In both cases of the above-mentioned superconducting powder crystals and superconducting whiskers, the raw material is not particularly limited as long as it can form an oxide by firing, and a simple metal, an oxide and various compounds ( Carbonate, etc.) can be used. As the raw material, a composite oxide or compound containing two or more of the above atoms in combination may be used. The firing process and heat treatment process described above are not particularly limited, and can be performed by any means such as an electric heating furnace and a gas heating furnace.

In the superconducting whiskers obtained by the above method, a trace amount of Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _x phase (2223 phase) may exist, and the superconducting powder crystal also has a small amount. The phases other than the Bi ₂ Sr ₂ CaCu ₂ O ₈ phase may be contained in trace amounts, but the presence of these trace components does not particularly affect the properties of the composite according to the present invention.

Then, the superconducting whiskers obtained by the above method and the superconducting powder crystals are mixed so that the content of the superconducting whiskers is 5 to 40% by weight, and the resulting mixture is heated at about 815 to 830 ° C. for 2 hours. It is subjected to hot press sintering for about 36 hours.

As a method for mixing both materials, a wet mixing method in which a superconducting whisker and a superconducting powder crystal are stirred and mixed in an organic solvent, or a superconducting powder crystal and a superconducting whisker are mixed without using an organic solvent or the like. Any of the dry mixing methods performed may be employed, and any method can produce a superconducting whisker composite having similar characteristics. As an example of the wet mixing method, both materials are mixed in a magnetic stirrer for 10 minutes in n-hexane as an organic solvent, the solvent is removed, and the mixture is dried to obtain uniform superconducting whiskers and superconducting powder crystals. Different mixtures can be obtained. The organic solvent used in the wet mixing method is not particularly limited, and any organic solvent can be used as long as it does not chemically react with the superconducting whiskers and the superconducting powder crystals.

The superconducting whisker composite of the present invention is produced by molding the mixture of the superconducting whiskers and the superconducting powder crystals obtained as described above into any desired shape and then hot pressing and sintering. To be done. The hot press sintering conditions are normally a temperature of 815 to 830 ° C. and a pressure of 6 to
It is about 2 to 36 hours at about 12 MPa. As an example, the above mixture was molded into a cylindrical shape having a diameter of 15 mm and a thickness of 20 mm, and then fired at 815 ° C. and 12 MPa for 24 hours to obtain the superconducting whisker composite of the present invention. The means for hot press sintering is not particularly limited, and any pressure sintering means such as an electric heating furnace equipped with a pressure device and a gas heating furnace can be adopted.

In the superconducting whisker composite according to the present invention, it is essential that the superconducting whisker component accounts for 5 to 40% by weight and the balance consists of the superconducting powder crystal component. When the content of superconducting whiskers is less than 5% by weight, in the superconducting properties (critical current density, magnetic shielding properties) of the composite, whiskers are hardly added, but when it exceeds 40% by weight. In addition, since the sinterability is lowered, the mechanical strength of the composite is lower than that of the sintered body of powder crystals alone.

[0017]

Since the superconducting whiskers of the present invention function as a current path by the superconducting whiskers, the problem of weak bonding of grain boundaries is alleviated, and the superconducting whiskers are compared with a sintered body composed of only superconducting powder crystals. Since the grain growth of grains is further advanced and the grains are highly oriented, they have a high critical current density. That is, the superconducting whisker composite of the present invention exhibits an extremely high critical current density of more than 3000 A / cm ² at the liquid nitrogen temperature.

Therefore, by forming into a required shape in a forming step before hot press sintering, it can be used as a high temperature superconducting material such as a wire rod for electric power transportation, electric power storage and a magnetic field generating magnet, and a magnetic shield material. There is expected.

[0019]

EXAMPLES Examples and comparative examples will be shown below to further clarify the features of the present invention.

Example 1 Sr = 1.00 and Ca = 0.5 with Bi = 1.00
0, Cu = 1.00, the starting materials were thoroughly mixed, placed in an alumina crucible, calcined in an electric furnace at 820 ° C for 20 hours, crushed, pressure-molded, and then calcined at 860 ° C for 60 hours. did. The obtained sintered body was sufficiently crushed to obtain Bi ₂
A superconducting powder crystal having an Sr ₂ CaCu ₂ O ₈ structure (average particle size of 3 μm or less) was obtained.

On the other hand, with Bi = 1.00, Sr = 1.
00, Ca = 1.00, Cu = 2.00, the starting materials were sufficiently mixed, placed in an alumina crucible, melted at 1200 ° C. for 30 minutes in an electric furnace, and rapidly cooled to obtain a glass precursor. . This glass precursor was placed in an electric furnace under an oxygen flow of 865
Bi ₂ Sr ₂ CaC by heat treatment at ℃ for 80 hours
Superconducting whiskers with u ₂ O ₈ structure (average length 10 m
m, average width 100 μm, average thickness 5 μm).

Next, 4.50 g of the superconducting powder crystals and 0.50 g of superconducting whiskers were suspended in hexane,
After mixing with a magnetic stirrer, hexane was removed and the mixture was dried.

The resulting mixture was pressure-molded at 100 kg / cm ² , wrapped in a gold sheet, baked in an electric furnace for hot pressing at 815 ° C. and 12 MPa for 24 hours, and then cooled in the furnace. The obtained superconducting whisker composite has a diameter of 20 m.
m and the thickness was 2.0 mm.

The raw materials for producing the superconducting whiskers and superconducting powder crystals used in this example and the following example 2 were as shown below.

[0025] -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) Table 1, the present embodiment The sample according to Example 1 (Sample No. 1) and the sample No. 1 obtained in Example 2 below. About 2-11
The content of superconducting whiskers, hot press sintering conditions (temperature, pressure and time), zero resistance temperature and density of the sintered body are shown.

[0026]

[Table 1] Example 2 No superconducting whiskers were used (Sample No. 2) or the content of superconducting whiskers was changed (Sample No. 3 to No. 3).
A sintered body was obtained according to the method of Example 1 except for 11).

Reference Example 1 Superconducting whisker composite according to the present invention (Sample No. 1)
The powder X-ray diffraction pattern of the above is shown in FIG. This superconducting whisker composite has a superconducting whisker content of 10% by weight. All peaks in this pattern are made of Bi ₂ Sr ₂ Ca.
It could be assigned to the Cu ₂ O ₈ phase, and no diffraction peak due to the impurity phase was observed.

Sample No. The X-ray diffraction pattern of the bulk body of No. 1 is shown in FIG. In the X-ray diffraction pattern (a) parallel to the pressing surface during hot pressing, the (00n) pattern
Although the peaks appeared remarkably, a peak of (100) was observed in the X-ray diffraction pattern (b) on the surface perpendicular to the pressing surface. This indicates that the ab planes of the superconducting powder crystal grains and the superconducting whiskers are oriented parallel to the pressing surface.

Further, the composite of the present invention containing 10% by weight of superconducting whiskers (Sample No. 1) and a sintered body consisting only of superconducting powder crystals (0.0% by weight sample, sample No. 1).
2) Scanning electron microscope (SE
M) Pictures are shown as FIG. 3 and FIG. 4, respectively.

As is apparent from FIG. 3, in the composite of the present invention, the superconducting whiskers and the superconducting powder crystal grains are:
Their ab planes are oriented parallel to the pressure plane. Since the shape of the superconducting whiskers is a ribbon shape having a wide ab plane where crystal growth is well performed, the ab plane is oriented by pressure so that it is parallel to the pressure plane. Also, the superconducting powder crystal grains are B
Since the i ₂ Sr ₂ CaCu ₂ O ₈ phase has a two-dimensional crystal structure, grains grow so that the ab plane becomes parallel to the pressure plane during hot press sintering. It is clear from FIG. 3 that the crystal grains in the vicinity of the superconducting whiskers, particularly in the portion sandwiched between the two whiskers, have a better orientation than the crystal grains at a position distant from the superconducting whiskers and the grain growth is proceeding. From this, it can be seen that the superconducting whiskers promote the orientation and grain growth of the powder crystal grains in the vicinity thereof. The orientation and particle size of the powder crystal grains separated from the superconducting whiskers were almost the same as those of the 0.0 wt% sample (see also FIG. 4). From these results, Bi ₂ Sr ₂ CaCu ₂ O ₈
Superconducting whiskers of the same composition are mixed with the polycrystal,
It was found that by performing hot press sintering, it is possible to obtain a superconducting whisker-superconducting polycrystalline composite sintered body which is more highly oriented and has more grown crystal grains than a sintered body having only powder crystals.

The zero resistance temperature of the superconducting whisker composite according to the present invention and the superconducting powder crystal sintered body as a comparative example was 65 K or less to 84 K (see Table 1).

For the superconducting whisker composite according to the present invention (Sample No. 1, whisker content 10%) and the superconducting powder crystal sintered body as a comparative example (Sample No. 2, whisker content 0.0%), DC four terminals FIG. 4 shows the relationship between the electric resistance measured by the method and the absolute temperature.

For the superconducting whisker composite according to the present invention (Sample No. 1, whisker content 10%) and the superconducting powder crystal sintered body as a comparative example (Sample No. 2, whisker content 0.0%), DC four terminals The voltage-current curve measured by the method in liquid nitrogen temperature (77.4K) and zero magnetic field is shown.

Sample No. Table 2 shows the critical current densities of 1, 2, and 6 when the threshold value of voltage generation is 1.0 μV / cm.

[0035]

[Table 2] Sample No. 2 (0.0% sample) has a critical current density of 38A
/ Cm ² , whereas the sample No. No. 1 (10% composite), 3100 A / cm ² , sample No. 6 (20% composite) was 4800 A / cm ² , and the value of the superconducting whisker composite was two orders of magnitude higher than that of the 0.0% sample. There are two possible causes for this increase in critical current density. The first cause is the progress of grain growth and high orientation of the crystal grains around the superconducting whiskers, as shown in FIG.
The second cause is that the problem of weak coupling of grain boundaries due to the superconducting whiskers acting as a current path is reduced. In other words, it is speculated that the current flowing through the superconducting whiskers reduces the number of times the current passes through the grain boundaries, and as a result, the reduction in the critical current density due to weak coupling is reduced.

[Brief description of drawings]

1 is a sample No. 1 of Example 1. FIG. 2 is a chart showing a powder X-ray diffraction pattern of the superconducting whisker composite obtained from Example 1.

2 is a sample No. 1 of Example 1. FIG. Chart (a) showing the X-ray diffraction pattern of the plane parallel to the pressure plane of the bulk of the superconducting whisker composite obtained from No. 1, and chart showing the X-ray diffraction pattern of the plane perpendicular to the pressure plane. It is (b).

3 is a sample No. 1 of Example 1. FIG. 2 is an SEM photograph showing a crystal structure in a cross section (a plane perpendicular to a pressing surface) of the superconducting whisker composite obtained from Example 1.

4 is a sample No. 2 of Example 2. FIG. 2 is a SEM photograph showing a crystal structure in a cross section (a plane perpendicular to a pressing surface) of a superconducting powder crystal sintered body obtained in No. 2 (comparative product).

5 is a sample No. 1 of Example 1. FIG. No. 1 superconducting whisker composite obtained from Example 1 and the sample No. 2 of Example 2. 3 is a graph showing the temperature dependence of electric resistance of the superconducting powder crystal sintered body obtained from No. 2.

6 is a sample No. 1 of Example 1. FIG. No. 1 superconducting whisker composite obtained from Example 1 and the sample No. 2 of Example 2. It is a graph which shows the voltage-current curve of the superconducting powder crystal sintered compact obtained from No. 2.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Yamashita 1-831 Midorigaoka, Ikeda City, Osaka Prefecture Industrial Technology Institute, Osaka Institute of Technology (72) Inventor Junji Hayakawa 1-8 Midorigaoka, Ikeda City, Osaka Prefecture No. 31 Industrial Technology Institute Osaka Industrial Technology Research Institute (72) Inventor Hiroshi Ishikawa 1-831 Midorigaoka, Ikeda City, Osaka Prefecture Industrial Technology Institute Osaka Industrial Technology Research Institute

Claims (2)

[Claims] 1. An oxide superconducting whisker and an oxide superconducting powder crystal, wherein the superconducting whisker and the superconducting powder crystal both consist of Bi, Sr, Ca, Cu and O and have a Bi ₂ Sr ₂ CaCu ₂ O ₈ structure. Has
An oxide superconducting whisker composite, wherein the content of the superconducting whiskers is 5 to 40% by weight based on the total weight. 2. An oxide superconducting whisker 5a to 40% by weight comprising (a) Bi, Sr, Ca, Cu and O and having a Bi ₂ Sr ₂ CaCu ₂ O ₈ structure, and (b) Bi, Sr, C.
oxide superconducting whisker composite, characterized by hot-press sintering a mixture of a, Cu and O, and 95% to 60% by weight of oxide superconducting powder crystals having a Bi ₂ Sr ₂ CaCu ₂ O ₈ structure. Manufacturing method.