JPH09157097A - Production of superconducting fibrous crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently obtain fibrous crystals having Bi2 Sr2 CaCu2 O8 structure by rapidly cooling a melt consisting of Bi, Sr, Ca, Cu, Al and O, heat-treating and firing the resultant glass precursor under specified conditions. SOLUTION: Starting materials are mixed so as to give BiSrCaCu2 Ox and this mixture is put in an alumina crucible and heated to a high temp. By this heating, the mixture is melted and a proper amt. of Al is leached from the crucible to prepare a melt consisting of Bi, Sr, Ca, Cu, Al and O. This melt is rapidly cooled and the resultant glass precursor is heat-treated at 850-870 deg.C for 20-60hr and then fired at 840-880 deg.C for 10-500hr to obtain the objective superconducting fibrous crystals having Bi2 Sr2 CaCu2 O8 structure (2212 phase). When the crystals are used, a superconducting material having high critical current density can be produced.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high quality and large size Bi.
The present invention relates to a method for efficiently producing a superconducting fibrous crystal.

[0002]

2. Description of the Related Art The fact that the critical temperature (Tc) of a superconductor exceeds the temperature of liquid nitrogen means that cooling costs are reduced, which is of great significance. Examples of the oxide superconductor having a critical temperature higher than the temperature of liquid nitrogen include superconductors such as Y-based, Bi-based, and Tl-based superconductors. Bi-based superconductors include Bi
₂ Sr ₂ CuO ₆ phase (2201 phase), Bi ₂ Sr ₂ CaCu ₂ O ₈ phase (221 phase)
There are three different crystalline phases, two phases) and Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ phase (2223 phase), which are also called 20K, 80K and 110K phases from their respective critical temperatures. ing. Among these, the 2212 phase has a superconducting wire and a superconducting wire because of its relatively easy synthesis, less problem of weak bonding at grain boundaries as compared with Y-based, and lower toxicity as compared with Tl-based It is considered to be the most promising material for practical use as a magnetic shielding material.

With respect to this Bi-based 2212 phase, it has been reported that a fibrous crystal can be obtained by a method of heat-treating a glass precursor in an oxygen gas stream (Jpn. J. Appl.
Phys., Vol 28, L1121 (1989)). Since the fibrous crystal is a single crystal, it does not include a grain boundary and exhibits excellent critical current characteristics. However, in obtaining a sintered body of the oxide superconductor, simply sintering the oxide superconductor powder results in a polycrystalline body, and the crystal orientation direction is also random, so the critical current density is insufficient. . However, recently, it has been reported that when the 2212 phase superconducting fibrous crystal and the 2212 phase powder are compounded, the critical current is improved about 10 times as compared with a bulk body composed of only the 2212 powder containing no fibrous crystal. (J. Appl. Phys., Vol 7
6, 4891 (1994)). Thus, the 2212 phase fibrous crystal is
It is a very promising material for bulk applications such as wire rods, but for that purpose, establishment of mass synthesis technology of fibrous crystals, that is, establishment of an efficient method of producing fibrous crystals is urgently required. .

At present, Bi-based 2212 fibrous crystals are synthesized by firing a glass precursor obtained by quenching a melt at an appropriate temperature in an oxygen gas stream, but its efficiency is not satisfactory. I can't say.

[0005]

SUMMARY OF THE INVENTION Accordingly, it is a main object of the present invention to provide a method capable of producing high-quality and large-sized fibrous crystals with higher efficiency, high yield and high yield.

[0006]

Means for Solving the Problems The present inventor has conducted various studies in order to achieve the above object, and as a result, has found a method for producing 2212-phase fibrous crystals with high efficiency, and has completed the present invention.

That is, the present invention provides the following method for producing a 2212-phase fibrous crystal.

[0008] 1. A glass precursor obtained by rapidly cooling a melt composed of Bi, Sr, Ca, Cu, Al and O is heated at 850 to 870 ° C. for 20 times.
After heat treatment 60 hours, and firing 10 to 500 hours at _{840~880 ℃, Bi 2 Sr 2 CaCu} 2 O 8 structure (2212
A method for producing a superconducting fibrous crystal having

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Bi, Sr, Ca, C used in the present invention
A glass precursor obtained by quenching a melt composed of u, Al and O (hereinafter simply referred to as melt quenched glass precursor or precursor) is described in Matsubara et al., Physica C, 167, 503 (1990). Can be manufactured.

The thickness of the melt quenched glass precursor used in the present invention is about 0.5 to 1 mm. In the prior art, this melt quenched glass precursor was placed under an oxygen gas flow of 100 ml / min or more,
By performing the heat treatment for several tens to several hundred hours, a fibrous crystal was synthesized. The inventor of the present invention has observed in detail the fibrous crystal growth process according to the prior art. Important observations were that both of them continue to grow longer and that (b) the growth rate is almost constant. From these findings, it was conjectured that if the precursor was set to start growing in the later stage of the heat treatment in advance, a long fibrous crystal could be obtained in a short time. Successfully obtained the result.

More specifically, in the present invention, the precursor is subjected to heat treatment in advance under appropriate conditions (hereinafter, this heat treatment is referred to as pretreatment), and then calcined (heat treatment) under an oxygen gas stream in the same manner as in a conventional method. Then, a fibrous crystal is grown (hereinafter, this heat treatment is referred to as growth processing).

The means for heating the precursor is not particularly limited as long as a heating schedule for an arbitrary pretreatment and growth treatment can be achieved, and any means such as an electric heating furnace, a gas heating furnace, and a light heating furnace can be used. Can be adopted.

[0013] The holding temperature and time in the pretreatment vary depending on the composition ratio of the glass precursor to be used.
It is in the range of about 850 to 870 ° C. for about 20 to 60 hours, for example, 865 ° C. for 40 hours. Generally, when the temperature is low, it is necessary to lengthen the holding time, whereas when the temperature is high, the holding time can be shortened.

The holding temperature and time in the growth process are also
Depending on the composition ratio of the glass precursor used, etc., it is usually in the range of about 840 to 880 ° C. and about 10 to 500 hours,
An example is 870 ° C. for 40 hours. Also in this case, when the temperature is low, it is preferable to lengthen the holding time, and when the temperature is high, it is preferable to shorten the holding time.

Since the fibrous crystals grow from the surface of the precursor during the firing process, it is appropriate to evaluate the yield of the fibrous crystals by the weight of the fibrous crystals grown per unit surface area of the precursor. . For example, in the prior art precursor firing schedule without pretreatment (870 ° C. for 40 hours), the yield of fibrous crystals was 2.6 mg / cm ² , whereas the precursor was heated at 865 ° C. according to the present invention. Pretreatment for 40 hours, then the same calcination yield of fibrous crystals, 18.0mg
/ cm ² and the yield increased about 7-fold.

In the method of the present invention, the fibrous crystal grown by firing after performing the pretreatment was confirmed to be a Bi-based 2212 phase by X-ray diffraction measurement. Further, from the measurement of the temperature dependence of the resistivity and the magnetic susceptibility, it was confirmed that the obtained fibrous crystal transitioned to a superconducting state at 75 to 80K. Their crystal structure and superconducting properties are consistent with those of fibrous crystals grown on prior art firing schedules. That is, according to the present invention, by performing the pretreatment of the precursor, the yield can be improved without impairing the superconducting characteristics of the crystal grown by firing. Furthermore, since the pretreatment does not need to flow oxygen gas and can be performed in the atmosphere, a large box-shaped electric furnace can be used, and a large amount of samples can be pretreated at one time. Also in this respect, the present invention is useful for highly efficient synthesis of fibrous crystals.

In the present invention, the pretreatment of the precursor
The cause of the increase in the yield of fibrous crystals has not yet been sufficiently elucidated, but the following possibilities are conceivable.

Generally, crystal growth can be considered in two stages: nucleation, which is the initial stage, and the subsequent growth stage. Nucleation is likened to germination in a plant. A nucleus is formed and grows to become a crystal having a certain size. In the present invention, pretreatment is believed to be effective in achieving sufficient nucleation. That is, the number of nuclei to be generated is increased by the preprocessing as compared with the case where the preprocessing is not performed. And in the firing stage following the pretreatment, each nucleus grows,
As a result, it is considered that the yield of fibrous crystals increases.

[0019]

According to the present invention, Bi ₂ Sr ₂ CaCu ₂ O is obtained by preliminarily heat-treating a melt quenched glass precursor comprising Bi, Sr, Ca, Cu, Al and O under appropriate conditions. Superconducting fibrous crystals having _eight structures (2212 phases) can be produced with high efficiency, and a technique for mass-production of superconducting fibrous crystals has been established. As a result, it is expected that a superconducting material having a high critical current density can be produced using the obtained fibrous crystal, which is expected to be useful for improving the properties of a magnetic material for generating a magnetic field and wires for storing and transporting power.

[0020]

EXAMPLES Examples are shown below to further clarify the features of the present invention.

Example 1 After sufficiently mixing the raw materials to be the respective atomic sources so that the atomic composition ratio of BiSrCaCu ₂ O _x was obtained, 15 g of the raw material was placed in an alumina crucible and melted at 1200 ° C. for 30 minutes in an electric furnace. Next, the melt was poured out onto a copper plate at room temperature, and quenched by sandwiching it with another copper plate to obtain a glass precursor. In addition, since a proper amount of Al is eluted from the alumina crucible during melting of the raw materials, the glass precursor has a composition containing proper Al.

Next, the obtained glass precursor was placed in an annular electric furnace and fired under an oxygen gas stream (150 ml / min).
The holding temperature during firing was 870 ° C., and the holding time was 40 hours. After collecting the fibrous crystals generated after firing,
The yield was 2.6 mg / cm ² (Comparative Example).

On the other hand, when the glass precursor is pretreated and then fired under the same conditions as above, the yield is as follows: (a) When the pretreatment conditions are 855 ° C. for 40 hours, 2.9 mg / cm ² , (B) 8
5.1 mg / cm ² for 40 hours at 60 ° C, 40 ° C at 865 ° C
In the case of time, 18.0 mg / cm ² , (d) In the case of 870 ° C for 40 hours, 12.3 mg / cm ² , (e) In the case of 875 ° C for 40 hours,
It was 6.2 mg / cm ² .

From the above results, even if the firing conditions for fibrous crystal growth were the same, the yield of fibrous crystals was increased up to about seven times by appropriate pretreatment.

The raw materials used as atomic sources used in this example and the following examples 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) Example 2 Glass precursor prepared in the same manner as in Example 1 The body was placed in an annular electric furnace and calcined under an oxygen gas flow (150 ml / min). The holding temperature during firing was 870 ° C., and the holding time was 40 hours. When the fibrous crystals formed after the calcination were recovered, the yield was 2.6 mg / cm ² (Comparative Example).

On the other hand, when the glass precursor is pretreated and calcined under the same conditions as above, the yield is as follows: (a) When the pretreatment conditions are 855 ° C. for 20 hours, the yield is 3.1 mg / cm ² , (B) 8
4.6 mg / cm ² at 60 ° C for 20 hours, (c) 20 mg at 865 ° C
In the case of time, 14.5 mg / cm ² , (d) at 870 ° C for 20 hours, 10.3 mg / cm ² , (e) at 875 ° C for 20 hours,
It was 3.3 mg / cm ² .

Example 3 A glass precursor prepared in the same manner as in Example 1 was placed in an annular electric furnace and fired under an oxygen gas stream (150 ml / min). The holding temperature during firing was 870 ° C., and the holding time was 40 hours. When the fibrous crystals formed after the calcination were recovered, the yield was 2.6 mg / cm ² .

On the other hand, when the glass precursor is pretreated and calcined under the same conditions as above, the yield is as follows: (a) When the pretreatment condition is 855 ° C. for 60 hours, 5.2 mg / cm ² , (B) 8
7.1 mg / cm ² at 60 ° C for 60 hours, (c) 60 ° C at 865 ° C
In the case of time, 13.9 mg / cm ² , (d) in the case of 870 ° C for 60 hours, 9.3 mg / cm ² , (e) in the case of 875 ° C for 60 hours,
It was 4.2 mg / cm ² .

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

Claims (1)

[Claims] 1. A glass precursor obtained by quenching a melt comprising Bi, Sr, Ca, Cu, Al and O at 850-870 ° C. for 20-60
After time heat treatment, and firing 10 to 500 hours at _{840~880 ℃, Bi 2 Sr 2 CaCu} 2 O 8 structure (2212)
A method for producing a superconducting fibrous crystal having: