JP3218947B2 - Manufacturing method of oxide superconducting wire - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The oxide superconductor is Nb ₃ S
Compared to metallic superconductors such as n, they have practical advantages such as a higher superconducting transition temperature (hereinafter referred to as Tc) and an upper critical magnetic field (hereinafter referred to as Hc ₂ ). Have been. Among the oxide superconductors, Bi
-Based oxide superconductors have a low Tc phase (Bi
-2122 phase) and high Tc phase of about 110K (Bi-22
23), both of which are expected to have Hc ₂ above 100T.

Therefore, the following applications have been studied using these crystal phases. First, since the Bi-based oxide superconductor has a high Tc in both phases, the superconducting magnet (oxide) is formed by using liquid nitrogen instead of liquid helium which is indispensable for cooling the conventional metal-based superconductor. It is possible to manufacture a device including a superconducting wire and a cryostat for cooling the superconducting wire. Cryogenic liquid helium requires skill in handling and is expensive. Further, there is a disadvantage that the cooling system becomes complicated in order to keep the heat insulating state with the room temperature portion in a good state. In a cooling system using liquid nitrogen using a Bi-based oxide superconductor, such a large restriction is released, and the superconducting magnet can be easily cooled and held. Furthermore, when cooling with a refrigerator or the like, the running cost can be greatly reduced.

As a second application, by utilizing the fact that Hc ₂ is high, it is possible to provide a superconducting magnet that generates a magnetic field stronger than a conventional metallic superconducting magnet. For example, in an NMR analyzer that plays a very important role in determining the molecular structure of a complex high-molecular-weight protein, the stronger the magnetic field, the more information is obtained, and the more detailed the molecular structure can be determined. Becomes possible. Bi-based oxide superconductors have high Hc ₂ , and if a superconducting magnet using this is obtained, the development of a high-performance NMR analyzer that generates a stronger magnetic field can be expected.

The present invention relates to an oxide superconducting wire using the above-described oxide superconductor and a method for producing the same, and more particularly, to a production method for preventing swelling of a Bi-2212 oxide superconducting wire during heat treatment. It is about the method.

[0005]

2. Description of the Related Art A typical method for producing a Bi-2212 oxide superconducting wire is as follows. First, Bi ₂ O ₃ ,
Raw material powders such as SrCO ₃ , CaCO ₃ , and CuO are weighed and mixed, and heat-treated to produce Bi-2212 calcined powder. After the Bi-2212 calcined powder is filled in a sheath material such as a silver pipe or a silver billet, it is drawn and rolled to produce a silver sheath wire material, which is wound into a coil and subjected to heat treatment (Wind & React method). ) Or by performing a heat treatment first and then performing a winding (React & Wind method) to obtain a superconducting wire. The heat treatment is generally performed in the air, and the temperature is raised to a partial melting temperature where the solid phase and the liquid phase of the non-superconducting oxide coexist, and then the Bi-
It is known that 2212 oxide superconductor can be crystallized with good orientation.

[0006] The oxide superconducting wire produced in this manner is classified into a tape wire, a rectangular wire, and a round wire based on its shape. In the above tape wire, the crystallinity and orientation of the oxide portion are improved by rolling, and the critical current density (hereinafter referred to as Jc) obtained by converting the critical current (hereinafter referred to as Ic) into a value per unit area is high. Which is used for producing a pancake coil. On the other hand, the flat wire and the round wire are generally used for producing a solenoid coil which is advantageous as a superconducting magnet in that although it has a lower Jc than a tape wire, there are few restrictions on magnetic field uniformity and superconducting connection. .

When the above-mentioned tape wire is subjected to heat treatment, a phenomenon that the tape wire swells (hereinafter referred to as wire swelling) has occurred as shown in FIG. In the portion where this wire swelling has occurred, the production of normal conducting oxide increases, and Bi-22
Since the orientation of 12 crystals is impaired, a serious problem that Jc is remarkably reduced occurs. It is considered that the cause of the wire swelling is a gas released from the calcined powder. That is, the Bi-2212 calcined powder adsorbs moisture and oxygen gas before being filled in the sheath material, and when heat treatment is performed after filling in the sheath material, gas components are desorbed in a temperature rising process. Since the volume is increased inside the wire, the pressure inside the wire increases and the wire swells.

On the other hand, in the case of a flat wire or a round wire, the silver sheath portion is thicker than the tape wire, so that the wire can withstand gas pressure from the inside of the wire and the swelling of the wire is less likely to occur. However, it is the same as the case of the tape wire in that the gas is desorbed from the Bi-2212 calcined powder, and when the inside of the wire is observed, the desorbed gas is the Bi-2212 oxide superconductor layer. It can be seen that pores are formed remaining inside. Even in the case of a rectangular wire or a round wire, the point that the presence of the holes significantly reduces Jc was similar to that of the tape wire.

Therefore, in order to solve the above-mentioned problem, from the viewpoint of preventing moisture and oxygen gas from adhering to the calcined powder, a process until filling the sheath material is performed.
A method of keeping the calcined powder in an atmosphere having a humidity of 20% or less (Japanese Patent Application Laid-Open No. 4-292813) and a method of preparing a calcined powder in a low oxygen atmosphere (Japanese Patent Application Laid-Open No. 4-233110) have been proposed.

However, when wire drawing or rolling is performed in the air, the cause of wire swelling is not only the moisture and oxygen gas adsorbed on the calcined powder, but also the penetration of the sheath material in the processing step before heat treatment. Nitrogen gas and oxygen gas, and these gases expanded during the heat treatment, which also caused wire swelling.

[0011] Japanese Patent Application Laid-Open No. Hei 5-159641 discloses that rolling before heat treatment is performed under reduced pressure or in an inert gas atmosphere, and the rolling process is performed along the longitudinal direction of the wire to be rolled.
A method of displacing gas remaining inside the wire is disclosed. However, even with this method, the swelling of the wire rod has not yet been completely prevented.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to provide a method of manufacturing an oxide superconducting wire capable of preventing swelling of the oxide superconducting wire. It is.

[0013]

Means for Solving the Problems The manufacturing method of the present invention which has solved the above-mentioned problems is that a sheath material filled with a calcined powder as a raw material of an oxide superconductor is heated to a partial melting temperature and gradually cooled. In the method for producing an oxide superconducting wire to be crystallized by performing, degassing the inside of the sheath material by raising the temperature in a vacuum, and subsequently performing heating to a partial melting temperature in an oxygen-rich atmosphere. It is an abstract.

In the case where the oxide superconductor is a Bi-2212 oxide superconductor, it is recommended that the temperature attained by raising the temperature in a vacuum be as high as possible 750 ° C. or less. Further, when heating in an oxygen-rich atmosphere, 780
The removal of carbon dioxide gas from the sheath material can be promoted by maintaining the temperature range of ８840 ° C., and thereafter, the temperature is reduced from the temperature range of 780 ° C. to 840 ° C. to the partial melting temperature in an oxygen-rich atmosphere. The oxygen gas can be removed by gradually raising the temperature. With the Bi-2212 oxide superconducting wire thus manufactured, it is possible to achieve a critical current density of 10 ⁵ A / cm ² or more.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of intensive studies, the present inventors have found that the cause of the wire swelling exists in addition to the above.

FIG. 2 shows the result of measuring the amount of gas (in terms of ionic strength) generated when the Bi-2212 calcined powder is heated by mass spectrometry. FIG. 2A is a chart showing the amount of released oxygen gas.
A peak where the amount of released oxygen gas is maximum can be confirmed in the vicinity. The temperature around 840 ° C. is a temperature at which a partial melting reaction starts in the Bi-2212 calcined powder.
Oxygen gas is released from the i-2212 calcined powder, which is another cause of swelling of the Bi-2212 silver sheath tape wire.

FIG. 2B is a chart showing the amount of released carbon dioxide gas. It can be seen that a large amount of carbon dioxide gas was released from the Bi-2212 calcined powder in the temperature range of 780 to 840 ° C. I understand. This carbon dioxide gas is considered to be generated by decomposition of the raw material powder of carbonate such as SrCO ₃ and CaCO ₃ remaining unreacted in the Bi-2212 calcined powder. This release of carbon dioxide gas also contributed to the swelling of the Bi-2212 silver sheath tape wire.

Further, the present inventors have found that carbon remaining inside the tape wire also causes swelling of the wire. That is, during the wire heat treatment, the residual carbon reacts with oxygen to form carbon dioxide, and the carbon dioxide gas expands inside the wire and causes swelling.

The present inventors have found that the above residual carbon is Bi-
It has been found that it can be reduced by extending the calcining time when preparing the calcined powder 2212 (Cryogenics, V
ol. 35, P127, 1995). However, as a result of further research, Bi-22
12 If carbon remains in the calcined powder even in a trace amount (for example, 0.1 mass%), the Bi-2212 silver sheath tape wire cannot be prevented from bulging by simply performing ordinary heat treatment. found.

Here, the cause of the swelling of the Bi-2212 silver sheath tape wire is summarized as follows. That is, moisture and oxygen gas adsorbed on the calcined powder, nitrogen gas and oxygen gas in the air remaining inside the sheath material, oxygen gas generated when the calcined powder is partially melted, SrCO ₃ , Carbon dioxide gas generated by decomposition of a raw material powder of a carbonate such as CaCO ₃ , and carbon dioxide gas generated by the reaction of residual carbon and oxygen in the calcined powder.

The gas present inside the Bi-2212 silver sheathed tape wire may be recrystallized silver grain boundaries or silver during heat treatment if the partial pressure of the gas outside the wire is lower than the internal partial pressure. It diffuses outside through the grains. Therefore, the moisture, oxygen gas, and nitrogen gas due to the above-mentioned causes can be sufficiently expelled by raising the temperature of the Bi-2212 silver sheath tape wire in a vacuum.

Therefore, in the manufacturing method of the present invention, when performing heat treatment on the sheath material, first, the temperature is raised in a vacuum. Specifically, for example, up to a temperature range of 600 to 750 ° C., 1
What is necessary is just to raise a temperature at the rate of -10 degreeC / h. It should be noted that the lower the pressure in the heat treatment furnace, the higher the gas release rate to the outside of the Bi-2212 silver sheath tape wire. The degree of vacuum is preferably 10 ⁻² Torr or less, and more preferably 10 ⁻³ Torr or less. The oxygen gas is described later.

From the results obtained in FIG. 2B, it can be seen that the carbon dioxide gas is released in a temperature range of about 780 to 840 ° C. Therefore, by holding in this temperature range for a predetermined time, the wire is gradually released outside the wire,
The swelling of the wire can be prevented. The holding time is preferably 20 hours or more, and more preferably 40 hours or more.

The carbon dioxide gas due to the residual carbon is frequently generated in a temperature range where a large amount of oxygen gas is released from the calcined powder during partial melting. Here, it is necessary to sufficiently react with the residual carbon while the generated oxygen is not removed to the outside of the Bi-2212 silver sheath tape wire, by introducing an oxygen gas into a furnace in a vacuum and performing an atmosphere for the heat treatment. Is desirably changed from a vacuum to an oxygen-rich atmosphere. The temperature at which oxygen gas is introduced into the furnace in a vacuum is preferably 750 ° C. or less. The reason is that at a high temperature exceeding 750 ° C., the oxygen partial pressure is 0.1 mm.
If it is 1 atm or less, the amount of oxygen is deficient and Bi-221
This is because the two-oxide superconductor is decomposed into a non-superconducting oxide, and good crystallization cannot always be expected in the subsequent heat treatment.

From FIG. 2A, it can be seen that the temperature range of the peak of the oxygen gas release is about 830 to 840 ° C., and this temperature range is included in the aforementioned temperature range of 780 to 840 ° C. That is, any of the carbon dioxide generated due to the above can be excluded from the sheath material by maintaining the carbon dioxide at 780 to 840 ° C. in an oxygen-rich atmosphere for a predetermined time.

Further, comparing the charts of FIGS. 2A and 2B, it can be seen that the oxygen gas emission temperature range is wider than the carbon dioxide gas emission temperature range, and the oxygen gas is also emitted on the high temperature side. Therefore, it is recommended that oxygen gas be removed while gradually raising the temperature of the oxygen gas to a partial melting temperature of about 890 ° C. That is, from the viewpoint of sufficiently removing the oxygen gas due to the above-described causes, it is desirable that the rate of temperature rise be low. The heating rate is Bi
It is desirable to set according to the thickness of the sheath portion of the -2122 silver sheath tape wire. For example, when the thickness of the silver sheath is 70 μm, it is desirable to increase the temperature at a rate of 60 ° C./h or less.

In the present invention, these heat treatment steps need to be performed continuously (ie, continuously) without being performed in the air. The reason is that Bi-221
This is because, when the silver sheath tape wire is taken out to the atmosphere, nitrogen gas and oxygen gas in the atmosphere enter the tape wire and cause swelling of the wire.

Although the tape wire has been described,
The same applies to a flat wire or a round wire.
In the case of a -2212 silver sheath rectangular wire or a Bi-2212 silver sheath round wire, the silver sheath portion is thicker than the Bi-2212 silver sheath tape wire, so that the rate of temperature rise in vacuum and partial melting in oxygen-rich atmosphere By alleviating the rate of temperature rise to the temperature, an effect of degassing can be expected as in the case of the Bi-2212 silver sheath tape wire.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention. It is included in the technical scope.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Conventional Example 1 Raw material powders of Bi ₂ O ₃ , SrCO ₃ , CaCO ₃ , CuO, and Ag were mixed at a molar ratio of Bi, Sr, Ca, Cu, and Ag of 2.1: 2: 1: 1.9: The mixture was weighed and mixed in an atmosphere of a humidity of 20% or less in a glove box so as to be 0.1. The obtained mixed powder was heat-treated at 770 ° C. for 8 hours and at 820 ° C. for 60 hours in an atmosphere having an oxygen partial pressure of 1 atm to prepare a Bi-2212 calcined powder. This Bi-
After filling the 2212 calcined powder into a silver pipe, it was drawn and rolled to produce a single-core Bi-2212 silver sheath tape wire having a thickness of 200 μm (silver sheath thickness 70 μm), a width of 8 mm, and a length of 10 m. . After winding this tape wire together with an alumina insulating tape on a ceramic winding frame, it was heat-treated in the pattern shown in FIG. That is, 300 ° C. in the atmosphere
The temperature was raised to 890 ° C. at a rate of / h, maintained at that temperature for 10 minutes, cooled to 840 ° C. at a rate of 2 ° C./h, and then rapidly cooled by turning off the heater. Here, what was rapidly cooled from 840 ° C. was Bi, which is a non-superconducting oxide.
-2201 phase (Bi, Sr, Ca, Cu molar ratio is 2: 2:
0: 1).

The degree of swelling of the obtained tape wire (length 5 mm)
The number of swollen portions described above) and Jc from the end to the end of the tape wire were measured at 4.2 K in an external magnetic field of 0 T using 0.1 μV / cm as a voltage reference. As a result, the degree of swelling was 150 and Jc was 2.5 × 10 ⁴ A / cm ^2.
Met.

Example 1 Bi-22 was prepared in the same manner as in Conventional Example 1 except that the steps of weighing and mixing the raw material powders and performing a heat treatment were performed in the air.
Twelve calcined powders were produced. Next, a Bi-2212 silver sheath tape wire was produced in the same manner as in Conventional Example 1, wound around a bobbin, and heat-treated in the pattern shown in FIG. That is, the temperature is raised to 750 ° C. at a rate of 300 ° C./h in a vacuum (3 × 10 ⁻⁴ Torr), the vacuum pump is stopped, and oxygen gas is introduced to change the inside of the furnace to an atmosphere with an oxygen partial pressure of 1 atm. , 30
The temperature was raised to 890 ° C. at a rate of 0 ° C./h and maintained at the same temperature for 10 minutes. Thereafter, cooling was performed in the same manner as in Conventional Example 1 in an atmosphere having an oxygen partial pressure of 1 atm. The swelling degree of the obtained tape wire was 53, and Jc was 6.8 × 10 ⁴ A / cm ² .

Example 2 In the same manner as in Example 1, a Bi-2212 silver sheath tape wire was produced, wound around a winding frame, and heat-treated in the pattern shown in FIG. That is, the temperature is raised to 750 ° C. at a rate of 300 ° C./h in a vacuum (3 × 10 ⁻⁴ Torr), the vacuum pump is stopped, and oxygen gas is introduced to change the inside of the furnace to an atmosphere with an oxygen partial pressure of 1 atm. The temperature was raised to 830 ° C. at a rate of 300 ° C./h and maintained at that temperature for 20 hours. Then oxygen partial pressure 1
The temperature was raised to 890 ° C. at a rate of 300 ° C./h in an atmosphere of atm and kept at the same temperature for 10 minutes. After that, Example 1
It cooled similarly to. The swelling degree of the obtained tape wire was 21, and Jc was 10.5 × 10 ⁴ A / cm ² .

Example 3 In the same manner as in Example 1, a Bi-2212 silver sheath tape wire was produced, wound around a winding frame, and heat-treated in the pattern shown in FIG. That is, as compared with the heat treatment pattern of Example 2, the rate of temperature increase from 830 ° C. to 890 ° C. was 300 ° C.
The conditions were the same except that the temperature was changed from / h to 60 ° C / h. The swelling degree of the obtained tape wire is 0, and Jc is 22.1.
× 10 ⁴ A / cm ² .

Conventional Example 2 A Bi-2212 calcined powder was produced in the same manner as in Conventional Example 1, and the Bi-2212 calcined powder was filled in a silver pipe, and then drawn and rolled to a thickness of 0.1 mm. 5mm (silver sheath thickness 0.15mm), width 2.5mm, length 10m single core Bi
A -2122 silver sheath tape rectangular wire was produced. The rectangular wire was passed through an alumina insulating sleeve, wound around a ceramic bobbin, and then heat-treated in the pattern shown in FIG. The swelling degree of the obtained rectangular wire is 0.5, and Jc is 1.8 × 10.
^{It was 4} A / cm ² .

Example 4 A Bi-2212 calcined powder was prepared in the same manner as in Example 1, and the Bi-2212 calcined powder was filled in a silver pipe, and then drawn and rolled to obtain the same as Conventional Example 2. Single core Bi of the same size
A -2122 silver sheath rectangular wire was produced. The rectangular wire was passed through an alumina insulating sleeve, wound around a ceramic bobbin, and then heat-treated in the pattern shown in FIG. That is, compared with the heat treatment pattern of Example 3, the temperature was 830 ° C. to 89 ° C.
The conditions were the same except that the rate of temperature rise to 0 ° C was changed from 60 ° C / h to 30 ° C / h. The swelling degree of the obtained rectangular wire was 0, and Jc was 15.1 × 10 ⁴ A / cm ² .
The results are summarized in Table 1.

[0037]

[Table 1]

As is evident from Table 1, the Bi-2212 silver sheath wire of the embodiment satisfying the requirements specified in the present invention has a reduced or prevented wire swelling as compared with the conventional example. In addition, Jc is also a high value of 10 ⁵ A / cm ² or more, which cannot be achieved by the conventional coil material.

[0039]

As described above, the present invention provides a method for producing a Bi-2212 oxide superconducting wire which can prevent swelling of the Bi-2212 oxide superconducting wire and obtain a high Jc. It can be done. Thereby, Bi-2212 of high quality which is extremely advantageous for superconducting applications
It became possible to produce pancake coils and Bi-2212 solenoid coils.

[Brief description of the drawings]

FIG. 1 is a perspective cross-sectional view of a Bi-2212 silver sheath tape wire manufactured by a conventional method, showing a portion where swelling has occurred.

FIGS. 2A and 2B are charts in which the amount of gas (in terms of ionic strength) generated when Bi-2212 calcined powder is heated is measured by mass spectrometry, wherein FIG. 2A is an oxygen gas, and FIG. Is the result of

FIG. 3 is a graph showing heat treatment patterns used in Conventional Example 1 and Conventional Example 2.

FIG. 4 is a graph showing a heat treatment pattern used in Example 1.

FIG. 5 is a graph showing a heat treatment pattern used in Example 2.

FIG. 6 is a graph showing a heat treatment pattern used in Example 3.

FIG. 7 is a graph showing a heat treatment pattern used in Example 4.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Bi-2212 superconductor 2 Silver sheath 3 Bulging part

Continued on the front page (72) Inventor Seiji Hayashi 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Kobe Steel, Ltd. Kobe Research Institute (56) References JP-A-7-17714 (JP, A JP-A-5-159641 (JP, A) JP-A-6-196032 (JP, A) JP-A-2-116661 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01B 12/00-13/00 C01G 1/00 C01G 29/00

Claims (5)

(57) [Claims] 1. A method for producing an oxide superconducting wire, in which a sheath material filled with a calcined powder as a raw material of an oxide superconductor is heated to a partial melting temperature and gradually cooled to crystallize, the method comprising: Degassing the inside of the sheath material by raising the temperature in the above step, and subsequently performing heating to a partial melting temperature in an oxygen-rich atmosphere. 2. The method according to claim 1, wherein the oxide superconductor is a Bi-2212 oxide superconductor. 3. The ultimate temperature in a temperature rise in a vacuum of 75
3. The method according to claim 2, wherein the temperature is 0 ° C. or lower. 4. When heating in an oxygen-rich atmosphere, 780
The production method according to claim 3, wherein removal of carbon dioxide gas from the sheath material is promoted by maintaining the temperature in a temperature range of -840 ° C. 5. The oxygen gas removal treatment according to claim 3, wherein the oxygen gas is removed by gradually raising the temperature from a temperature range of 780 to 840 ° C. to a partial melting temperature in an oxygen-rich atmosphere. Production method.