JPH02258665A - Production of superconductive material - Google Patents

Abstract

PURPOSE:To obtain superconductive material having high critical electric current density and high strength by heat-treating oxide of Bi-Pb-Sr-Ca-Cu system in controlling atmosphere and temperature, molding resultant superconductive material, heat-treating, further molding under pressure and heat-treating at fixed temperature. CONSTITUTION:An oxide of Bi-Pb-Sr-Ca-Cu system is heat-treated at 800-860 deg.C temperature in an atmosphere of 2-22% oxygen-containing volume fraction to obtain crystal structure of superconductor. Then, the resultant material is molded to a superconductive molded material and heat-treated at 820-860 deg.C temperature, thus sintered superconductor is subjected to pressure-molding treatment without crushing process. Next, resultant material is subjected to heat treatment at 820-860 deg.C temperature to afford a superconductive material of oxide of Bi-Pb-Sr-Ca-Cu system. The resultant superconductive material has a critical electric current density larger than 5-10 times comparing to conventional production method and also has a sintered density larger than 80% of theoretical density.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing superconducting materials, and more specifically to manufacturing oxide superconducting materials that have a high critical current density and are used for power transmission cables, magnets, etc. Regarding the method.

(Prior art) It has been reported that the Bi-Sr-Ca-Cu based oxide superconductor discovered in January last year has two types of superconducting phases, one at a critical temperature of 110 degrees and one at a critical temperature of 80 degrees. It has a critical temperature 20 degrees higher than that of conventional Y-based superconductors and has excellent chemical stability, so its commercialization in industrial applications is urgently needed. This ll0K phase is 87
It is said that it is synthesized only in a narrow temperature range of 5 to 885°C and cannot be synthesized at other temperatures. Recently, it has been reported that by adding pb, the synthesis temperature of the ll0K phase can be lowered to 850°C and the synthesis time can also be shortened (M
, Takano et at; Japan J
our own of Applied Physics
s, vol 27. L1041.1988), and further,
It has been reported that the synthesis conditions for the ll0K phase are influenced by the oxygen partial pressure in the heat treatment atmosphere, and that the sum synthesis temperature range expands the most to 110 in a low nitrogen atmosphere (1/13 Torr) (U, Endo et al;Japan
s Journal of Applied Physi
cs, vol 27. L1476.1988).

(Problem to be solved by the invention) Bi-Pb-Sr-Ca-Cu-based oxide superconductors are usually produced by heat-treating various raw material compounds to precipitate the crystal structure of the superconductor, and then crushing them into a molded body. is formed, and then heat treated and sintered. The superconducting sintered body produced by this method has a low density of about 50% of the theoretical density, and the critical current density at liquid nitrogen temperature remains below 300 A/cm'', making it superconducting. This is far from the minimum required 100,000 A/c rr+' for practical use in power transmission cables, and recently a method of adding pressure forming treatment during heat treatment has been developed to improve the critical current density of sintered bodies. Although it was proposed (Y, Tanaka et al; Japan
es Journal of Applied Phys.
ics, vol 27. L1655.1988) The critical current density achieved in this method is 700 A/
cm''.

(Means for Solving the Problems) As a result of intensive research to solve the above problems, the inventor of the present invention set the heat treatment temperature of Bi-Pb-Sr-Ca-Cu based oxide superconductor to a specific temperature. However, by performing pressure molding treatment between heat treatments, the present invention was completed by discovering that a superconducting sintered material with a critical current density exceeding 5000 A/crn' at liquid nitrogen temperature could be produced. reached.

That is, the present invention suppresses the evaporation of volatile components, which cause a decrease in the density of a superconducting sintered body, by heat treatment under specific temperature conditions, and also suppresses the generation of grain boundary precipitated phases, which causes a decrease in critical current density. This article relates to a method for producing superconducting materials that not only suppresses the heat treatment, but also improves the sintered density and controls the crystal orientation of the superconducting phase by performing pressure forming between heat treatments. However, B1-Pb-3
In the method for producing an r-Ca-Cu-based oxide superconducting material, in an atmosphere with an oxygen-containing volume fraction of 2% or more and 22% or less,
The oxide was heat-treated at a temperature of 800°C or more and less than 860°C to form a superconductor crystal structure, and then molded into a superconducting molded body, heat-treated at a temperature of 820°C or more and less than 860°C, and sintered. The oxide superconductor is subjected to pressure molding treatment without adding a pulverization process, and is further heated to a temperature of 820°C or higher at 860°C.
The present invention provides a method for producing a superconducting material having a high critical current density, characterized by performing heat treatment at a temperature of less than °C.

The present invention will be explained in detail below.

The raw materials in the present invention are Bi, Sr, Ca, C
Oxides, hydroxides, inorganic acid salts, and organic acid salts of u and pb can be used. Each of the raw materials is used as a mixture in powder form, or prepared by coprecipitation, sol-gel method, spray drying method, etc. The mixing ratio of raw materials is Bi in elemental ratio.
:Pb:Sr:Ca:Cu=1.6:0.4:2:
There is no limitation as to whether 2:3 is preferable or not.

In the present invention, these raw materials are heat-treated at a temperature of 800° C. or more and less than 860° C. for a certain period of time in an atmosphere with an oxygen-containing volume fraction of 2% or more and 22% or less to obtain a superconductor crystal structure. This material mainly has a crystal structure of 80-phase, with some 1-phase crystal structure.
10 also includes Japanese crystals. In air, at temperatures exceeding 860° C., superconductor materials begin to melt and decompose into a mixture of a crystal structure phase called a semiconductor phase and an oxide insulator. The melting start temperature varies depending on the oxygen partial pressure in the heat treatment atmosphere, and specifically, it is 840° C. in an atmosphere with an oxygen content of 4%, and 850° C. in an atmosphere with an oxygen content of 8%. Heat treatment is performed below these temperatures and in any atmosphere.

At temperatures below 800°C, almost no superconductor crystals precipitate.

Next, in the present invention, this sintered body is crushed, mixed, and shaped. In the present invention, molding includes filling, pattern formation, coating,
The concept includes forming a film into a predetermined form.9 For example, by mixing powder with an organic acid binder, it can be made into ceramics such as alumina, zirconia, magnesia, silicon, or silver, copper, nickel, stainless steel, etc. These methods include forming a pattern on a metal substrate and the commonly used method of press molding powder. The molded body is then heated at 820°C or more and 86°C in an atmosphere with an oxygen content of 2% or more and 22% or less.
Heat treatment is performed at a temperature below 0"C to crystallize the crystal structure of the 110 phase.The heat treatment temperature varies depending on the treatment atmosphere, but is set at a temperature within 20 °C below the melting start temperature. The superconducting phase inside the compact is 80
The conversion from the phase to the phase 110 proceeds rapidly, but at the same time, gas is generated from the compact due to the evaporation of low melting point elements, mainly Bi and Pb, or the decomposition of the raw material compounds.
A large number of pores are generated inside the molded body, and the density of the molded body is reduced. In particular, at temperatures near the melting start temperature, Bi, Pb
The amount of evaporation of low-melting-point elements mainly consisting of
As a result of the decrease in the amount of b, the ll0K phase, which had been stabilized by the addition of pb, began to decompose into an 80° phase and a non-superconducting oxide, deteriorating the superconducting properties of the molded article. Therefore, it is necessary to select a specific temperature for heat treatment, and as a result of intensive research, the optimal heat treatment temperature in the present invention is - in an atmosphere with an oxygen content of 2%≦P≦22%. o, osp”
+3. 4P+816≦T(℃)≦-o, osp2
+3.4P+820, specifically, 830 in an atmosphere with an oxygen content of 4±0.5%.
840 in an atmosphere of ±2℃ and oxygen content of 8±0.5%
85 at ±2℃ and atmospheric oxygen content of 21±0.5%
It was found that the temperature was 5±2℃. The heat treatment is preferably carried out for 50 hours or more under the optimum temperature in each atmosphere.

The superconducting molded body after heat treatment is subjected to pressure molding treatment in order to improve density and impart crystal orientation of the superconducting phase. The pressurizing method is not particularly specified as long as it is a method that compresses the sample volume, such as press molding or roll molding.This compression molding process collapses the pores existing inside the molded body and improves the density of the molded body. . At the same time, a-b of the superconducting phase crystal
The surfaces are selectively oriented in a direction perpendicular to the pressing direction of compression molding. This is because the crystal shape of the superconducting phase is plate-like or needle-like, and as a result, it is easy to align in the direction perpendicular to the pressurizing direction. The crystal orientation can be improved by devising a method of pressurizing the compact. Specifically, it is preferable to alternately repeat pressurization and non-loading, thereby advancing crystal alignment and improving crystal orientation. It is known that the critical current density is highest in the a-b plane direction of Bi-based superconductor crystals, and the pressure forming process used in the present invention imparts selective crystal orientation to the superconductor compact. is expected to improve the critical current density.

The temperature for pressure forming treatment is usually room temperature, but when pressure treating a superconductor formed on a metal substrate, the temperature is preferably about 200 °C or higher to facilitate rolling of the metal substrate. is preferred.

Due to the rearrangement of crystal grains during the pressure molding process, many cracks occur inside the molded body, which subsequently requires heat treatment to sinter the crystal grains. The heat treatment is performed for 50 hours or more under the optimum temperature in each atmosphere as described above. However, during this heat treatment, gas generation occurs as ll0K phase superconductor is synthesized from the 80 phase superconductor remaining inside the compact, and low melting point elements mainly consisting of Bi and Pb evaporate. , the density of the molded product may decrease. In this case, perform the above-mentioned pressure molding process again,
Subsequently, in addition to the above-mentioned method of performing sintering heat treatment, the heat treatment time of the 5 compact was set to 50 hours or less, and the heat treatment and pressure forming treatment were alternately repeated to mainly
A molded body made of a K-phase superconductor may be provided with high density and high orientation. Even in this method, the heat treatment time in the final stage requires 50 hours or more. If the heat treatment time in the final stage is, for example, about 20 hours, sintering of individual crystal grains does not proceed, which causes a decrease in the critical current density at the grain boundaries. Therefore, the critical current density of this material is as low as several hundred A/cm' since it forms a weakly coupled portion.

Further, the successive steps of heat treatment, pressure molding, and heat treatment of the superconducting molded body may be performed one time or two or more times in succession.

This is to increase the sintering area between crystal grains, reduce the proportion of weakly bonded portions at grain boundaries, and improve critical density. The heat treatment atmosphere before and after pressure molding does not necessarily have to be the same, but if changed, it is desirable that the heat treatment atmosphere after pressure molding be an atmosphere with a higher oxygen partial pressure than the atmosphere before molding. For this, an atmosphere with an oxygen content of 4%8
After heat treatment at 30℃, pressure molding and further heating at 85℃ in air.
This method involves heat treatment at 5°C. By increasing the processing temperature, it is possible to increase the crystal grains of the superconducting phase in the compact and reduce the proportion of weakly bonded parts at grain boundaries, making it possible to further improve the critical current density. Become.

By the method described above, a superconducting molded body having a critical current density exceeding 2000 A/cm' at liquid nitrogen temperature can be easily obtained. In particular, the continuous steps of heat treatment, pressure molding, and heat treatment of the superconducting molded body are performed five or more times,
If the heat treatment time between steps is set to 50 hours or less, and only the final heat treatment time is set to 50 hours or more, the sintered density and orientation will further improve, so the critical current density will be 5000.
Easily exceeds A/c rn'. Hereinafter, the present invention will be explained in more detail using Examples.

(Example 1) Bi(NO,)3-5H10, Sr(NO2, Ca(
NOi)r'4H20.

Cu(NO+)z"llH*0, Pb(NO3)z
The element ratio is Bi:Pb:Sr:Ca:Cu=1.6:0
．． Weigh and mix with 2001 pure water in the ratio of 4 + 1: 1: 1.5. Next, add appropriate amounts of tartaric acid and ethylene glycol to this solution and stir and mix at 90°C for 3 hours to form a bluish-white gel-like substance. Obtained 0, then at 400 °C for 2 hours, 8
A phase superconductor was pulverized and mixed with 80 obtained by heat treatment in the air at 00° C. for 15 hours, and a load of 7 tons/crn' was applied to obtain a molded body of lOs+sφX1 mm. After this molded body was heat-treated at 830° C. for 66 hours in an atmosphere with an oxygen content of 4%, it was taken out of the furnace and cooled to room temperature within 5 minutes. Next, a load of 7 ton/am' was repeatedly applied to this sintered body 10 times in succession to form a molded body of 10 V + iφ x 0.8 mm, which was further heat-treated at 830° C. for 60 hours in an atmosphere with an oxygen content of 4%. . When the critical current density of the obtained superconducting molded body was measured by the maximum transport current method at liquid nitrogen temperature and in zero magnetic field, a value of 2300 A/c was obtained.

(Example 2) The 10 mmφ x 1 mm molded body produced in Example 1 was heat treated at 840°C for 100 hours in an atmosphere with an oxygen content of 8%,
Next, a load of 7 ton/crn' was repeatedly applied to this sintered body 10 times in a row to form a molded body of 10 mmφ x 0.75 mm.
Heat treated for 00 hours. When the critical current density of the obtained superconducting molded body was measured by the maximum transport current method at liquid nitrogen temperature and in zero magnetic field, a value of 2500 A/am' was obtained.

[Example 3] The molded body of 10s + mφ x 1mm produced in Example 1 was
Heat treated at 830°C for 90 hours in an atmosphere with an oxygen content of 4%,
Next, a load of 7 ton/cm' was repeatedly applied to this sintered body 10 times in succession to form a molded body having a size of 1 ossφ×0.8 cm, and then heat-treated at 840° C. for an additional 120 hours in an atmosphere with an oxygen content of 8%. The critical current density of the obtained superconducting molded body was measured by the maximum transport current method at liquid nitrogen temperature in zero magnetic field, and a value of 3500 A/crn' was obtained.

(Example 4) The 10 m IIφ x 1 mm molded body produced in Example 1 was
Heat treated at 840°C for 20 hours in an atmosphere with an oxygen content of 8%,
Then, a load of 7 ton/crn' was repeatedly applied to this sintered body 10 times in succession, and heat treatment was again applied under rotating conditions for 20 hours. This heat treatment and pressure molding treatment were performed alternately five times in total, and finally a molded product of 10 mmφxO, '7aa+ was obtained, and then heat treated at 840° C. for an additional 100 hours in an atmosphere with an oxygen content of 8%. The critical current density of the obtained superconducting compact was 5700 A/c rn'.

(Example 5) The superconducting powder produced in Example 1 was mixed with 20 vol 1% polyvinyl alcohol to form a paste. Next, a film with a thickness of 0.1 mm was formed using the paste on an Ag plate with a thickness of 1 cm. After heating at 200°C to remove the binder, heat treatment was performed at 825°C for 15 hours in an atmosphere with an oxygen content of 4%.

Then, 1.
After performing pressure molding treatment with a load of ton/c rn', heat treatment was performed at 830° C. for 15 hours in an atmosphere with an oxygen content of 4%.

The obtained superconductor thick film had a thickness of about 0.06 mm and a critical current density of 2100 A/crn'.

(Effects of the Invention) According to the method for producing a superconducting material according to the present invention, a superconducting material whose critical current density is 5 to 10 times higher than that of the conventional production method can be produced. In addition, since the crystals of the superconducting phase are oriented in the rotation direction and the sintered density is 80% or more of the theoretical density, it is easy to manufacture a superconducting molded body with high mechanical strength. The superconducting molded body produced by the method of the present invention can be used for superconductor structures for magnetic shielding, magnet wires, power transmission wires, and the like.

Claims (7)

[Claims] (1) In the method for producing a Bi-Pb-Sr-Ca-Cu based oxide superconductor, the oxide is heated at 800°C or more and less than 860°C in an atmosphere with an oxygen-containing volume fraction of 2% or more and 22% or less. The oxide superconductor is heat-treated at a temperature of 820°C or higher and lower than 860°C, and then subjected to a crushing process. Pressure molding treatment is performed without adding, and furthermore, 82
A method for producing a superconducting material having a high critical current density, the method comprising performing heat treatment at a temperature of 0°C or more and less than 860°C. (2) The superconducting molded body was heated to −0.08P^2+3.4P+816 in an atmosphere with an oxygen content of 2%≦P≦22%.
Claim 1 characterized in that the heat treatment is carried out at a temperature within the range expressed by ≦T (℃)≦-0.08P^2+3.4P+820.
A method for manufacturing the superconducting material described. (3) The method for producing a superconducting material according to claim 1 or 2, characterized in that the successive steps of heat treatment, pressure molding, and heat treatment of the superconducting molded body are performed once or twice or more consecutively. . (4) The heat treatment temperature of the superconducting molded body is the oxygen content of 4±0.
4. The method for producing a superconducting material according to claim 1, wherein the temperature is 830±2° C. in a 5% atmosphere. (5) The heat treatment temperature of the superconducting molded body is the oxygen content of 8±0.
5. The method for producing a superconducting material according to claim 1, wherein the temperature is 840±2° C. in a 5% atmosphere. (6) Heat treatment temperature of superconducting molded body is oxygen content 21±0
．． 855±2℃ in 5% atmosphere, generally air
Claim 1, 2, 3, 4 or 5 characterized in that
2. Method for manufacturing the superconducting material described in Section 1. (7) Claims 1, 2, 3, 4, and 5, characterized in that the method for pressure forming a superconducting molded body imparts crystal orientation to the molded body by repeating pressurization and no load. 6. A method for producing a superconducting material according to item 6.