JPH08725B2 - Manufacturing method of bismuth superconductor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a bismuth-based superconductor, more specifically, a first step of firing, annealing, crystallization and cooling and heat treatment under an inert gas atmosphere. And a second step for producing a bismuth-based superconductor.

[Conventional technology]

In recent years, using a superconducting magnet made of a superconducting material having superconducting properties, a nuclear magnetic resonance (NMR) analyzer, a nuclear magnetic resonance computer tomography diagnostic apparatus (MRI: Magnetic Reson
ance Imaging), magnetic levitation trains, etc. are being developed,
Also, the application of the strong magnetic field of superconducting magnets to new energy development technologies such as fusion reactors and new energy conversion technologies such as MHD power generation is under consideration.

As such superconducting materials, various oxide superconductors have been researched and developed. Among these oxide superconductors, Bi-Sr-Ca-Cu-O-based bismuth-based (hereinafter referred to as Bi-based) superconducting ceramics have a high critical temperature (Tc) and have been particularly attracting attention.

High critical current density (J
In order to obtain c), it is necessary to partially melt during firing and then crystallize, but also in this crystallization, a high Tc phase and a low Tc phase are simultaneously precipitated, or a heterophase other than the superconducting phase is generated. There is a problem such as.

Therefore, various proposals have hitherto been made regarding control in the partially molten state and heat treatment for crystallization. For example, in JP-A-1-203257, as a method for producing a Bi-based superconductor, after heat treatment sintering at 750 to 880 ° C., the sintered body is gradually cooled to 500 to 700 ° C., or 500 to 740. It is proposed to hold at 0 ° C and then quench.

[Problems to be Solved by the Invention]

However, in the production of the Bi-based superconductor of the above-mentioned proposal, it will be rapidly cooled from 500 to 740 ° C., cracks and cracks are likely to occur, and particularly in large-sized products, the occurrence of such cracks is remarkable and large-sized products having good superconducting properties. It is difficult to obtain a product, and a device for quenching is required, which is economically problematic industrially.

Furthermore, in other conventional proposals, Bi-based superconductors having high Jc and having sufficient superconducting properties have not yet been obtained.

The present invention has a high Jc, and for the purpose of providing a Bi-based superconductor having good superconducting properties regardless of the size of the product, Bi-based superconductor precursor bismuth (Bi) and copper (Cu As a result of various studies on the steps such as firing and crystallization from a superconducting oxide containing), a Bi-based superconducting fired body is obtained, and then a heat treatment for imparting high Jc is performed.
The present invention has been completed by finding that a Bi-based superconductor free from defects such as cracks can be stably produced by treating a superconducting oxide containing Cu in two steps.

[Means for solving the problem]

According to the present invention, in the method for producing a bismuth-based superconductor, a superconducting oxide containing bismuth and copper is gradually cooled to a crystallization temperature after firing in an oxygen atmosphere, and further cooled to 200 ° C. or less after crystallization. A method for producing a bismuth-based superconductor is provided, which comprises the first step and a second step in which the cooled fired body of the first step is heat-treated in an inert gas atmosphere.

 Hereinafter, the present invention will be described in more detail.

The Bi-based superconductor of the present invention is composed of bismuth (Bi) and copper (C
Bi ₂ Sr obtained by firing a superconducting oxide containing u)
_A typical composition is ₂ CaCu ₂ O _y , BiSrCaCu ₂ O _y, or the like, but any composition may be used as long as it exhibits superconducting properties in the composition / crystallization step. For example, lead (P
b), composition with addition of antimony (Sb), non-stoichiometric composition deviated from stoichiometric composition, or composition in which main element of Bi-based superconductor composition is partially or completely replaced with other element Etc. may have any composition.

The raw material of the Bi-based superconductor is not particularly limited, an oxide known as a normal Bi-based superconductor raw material, for example, a superconducting oxide containing Bi and Cu, and superconducting by firing or the like. Bi so that the characteristics are expressed
_{1 type of} compound powder, which is a mixture of ₂ O ₃ , SrCO ₃ , CaCO ₃ , CuO, etc. in a predetermined amount, or its calcined powder, or a glassy powder that is melted and rapidly cooled above its melting point to become amorphous. Alternatively, two or more kinds can be used.

In the first step of the present invention, the above superconducting oxide is fired in an oxidizing atmosphere at 880 to 920 ° C for 1 minute or more, preferably 15 minutes to 2 hours, and then slowly cooled to 750 to 870 ° C. Crystallize, and after crystallization, further cool to 200 ° C or lower.

The firing temperature in the oxidizing atmosphere in the first step is 880 to 920.
If the temperature is outside the range of ° C, it becomes difficult to obtain a single phase of the Bi-based superconducting crystal such as Bi ₂ Sr ₂ CaCu ₂ O _{y, and} a Bi-based superconductor having a high Jc cannot be obtained.

In the slow cooling after firing, it is preferable to lower the temperature of the fired body to 750 to 870 ° C at a cooling rate of 2 ° C / min or less, preferably 1 ° C / min or less. When the cooling rate exceeds 2 ° C./min, the superconducting crystal phase is not a single phase and a heterogeneous phase occurs, crystal grains are not sufficiently grown, and high Jc cannot be obtained. In this case, normally, after firing, the material is gradually cooled in the firing furnace as it is.

In the first step of the present invention, the fired body is gradually cooled and crystallized, and then cooled to 200 ° C. or lower. In this case, the cooling rate is preferably 5 ° C./min or less, and the cooling can be performed in the firing furnace as it is, similarly to the slow cooling. If this cooling rate exceeds 5 ° C./min, defects such as cracks may occur in the fired body, which is not preferable. 200 ℃ after cooling
The following is to prevent defects such as cracks due to thermal shock.

In addition, cooling may be performed by gradually cooling from the firing temperature to the crystallization temperature and then immediately cooling as described above, or gradually cooling and lowering at the crystallization temperature, that is, at 750 to 870 ° C.,
Cooling may be started after holding for a certain period of time, for example, 1 to 50 hours. To maintain the crystallization temperature for a certain period of time,
It is preferable because the crystal grains of the superconducting phase can be further grown.

The cooled fired body obtained in the first step of the present invention as described above is good in appearance without causing defects such as cracks, but as it is, Jc is not high and the superconducting property is not good.

In the present invention, the cooled fired body obtained in the first step is further heat-treated in an inert gas atmosphere in the second step.

The present invention improves the Bi-based superconductor, which is a cooled and fired body in the first step, which has a relatively low Jc, by performing the heat treatment in the second step.
It can be converted to Bi superconductor of Jc. The reason why it can be converted to a Bi-based superconductor with a high Jc is not clear. However, the calcination in the first step, followed by slow cooling under an oxygen atmosphere and cooling process up to 200 ° C. Although it increases the oxygen content and deteriorates the superconducting property, the heat treatment in the inert gas atmosphere in the second step causes excess oxygen to be released from the Bi-based superconductor composition containing excess oxygen element. Bi that has been removed and has excellent superconducting properties
It is presumed that it will be converted into the system superconductor composition.

As the inert gas atmosphere in the heat treatment of the second step of the present invention, for example, a nitrogen atmosphere, a rare gas atmosphere such as an argon atmosphere, or the like can be used. Of these, the nitrogen atmosphere is industrially useful, but the oxygen partial pressure is There is no particular limitation as long as it is under an atmosphere of 0.01 atm or less.

The heat treatment in the second step of the present invention is preferably 300 to 600 ° C.
It is better to carry out at a temperature in the range. If the heat treatment temperature is lower than 300 ° C, the excess oxygen contained in the cooled fired body obtained in the first step cannot be sufficiently released / removed, and if the temperature is higher than 600 ° C, the oxygen release rate is high. The Bi-based superconductor composition cannot be controlled to an appropriate oxygen content, and the superconducting properties are further deteriorated, which is not preferable.

Further, the heat treatment time of the second step may be appropriately selected depending on the heat treatment temperature and the composition of the cooled fired body obtained in the first step so that the superconducting characteristics are excellent and the Bi-based superconductor has an appropriate component composition. it can. 5 to 20 at 300 ° C
The time is 1 to 5 hours at 600 ° C.

Furthermore, in the present invention, when a Bi-based superconductor is obtained as a molded body, any known method such as a press molding method, a slurry coating method, a doctor blade method, etc. may be used as a molding method, depending on the starting raw material. Just select it. During firing in the first step, the superconducting oxide or molded body of the above Bi-based superconductor is placed on the substrate or is directly applied to the substrate having a desired shape by a spray coating method or the like, and then in the first step. Generally, it is baked. For example, it is preferable to use a silver substrate as the metal substrate at the time of firing, and the silver substrate is composed of a simple substance of silver, or a silver thin film of about 1 μm to 1 mm on another metal plate such as stainless steel or a ceramic plate. There is no particular limitation as long as the starting material or molded body of the Bi-based superconductor can be placed and partially melted on the silver component such as those formed with.

In the present invention, the first step and the second step may be processed in the same furnace, such as an electric furnace, or may be processed in different furnaces. If you use a different furnace,
The atmosphere in each process is easy to control, the product production efficiency is high, and it is industrially useful.

〔Example〕

Examples of the present invention will be described in detail below. However, the present invention is not limited to this embodiment.

Examples 1 to 9 Raw materials Bi ₂ O ₃ , SrCO ₃ , CaCO ₃ , and CuO were added to Bi: Sr: Ca: Cu = 2: 2:
The mixture was prepared to be 1: 2 (molar ratio), distilled water was added, and the mixture was pulverized and mixed in a ball mill for 16 hours. The water content of the obtained mixture was evaporated, and the mixed powder granulated to a particle size of about 100 μm was calcined in the air at 800 ° C. for 10 hours. Then, the calcined product was pulverized with a ball mill to a particle size of 10 μm or less using toluene as a solvent to obtain a superconducting oxide pulverized powder.

Next, the obtained superconducting oxide pulverized powder was press-molded with a die to prepare a rectangular parallelepiped superconducting oxide compact having a thickness of 2 mm and 20 × 30 (mm).

The obtained molded body was fired in an electric furnace at 910 ° C. for 10 minutes in an oxygen atmosphere, then gradually cooled to 800 ° C. in the furnace at a cooling rate of 0.5 ° C./minute, and then kept at 800 ° C. for 10 hours. afterwards,
Cooled to 200 ° C at 3 ° C / min.

The cooled fired body was taken out of the electric furnace and the appearance was observed, but no cracks or cracks were found.

Next, the cooled fired body obtained above was heat-treated in the inert gas atmosphere, temperature and time shown in Table 1 to obtain a Bi-based superconductor. 1mm thick, 2 x 20 from the obtained Bi-based superconductor
A (mm) test piece was cut out, and the critical temperature (Tc) and the critical current density (Jc) at the liquid nitrogen temperature were measured. The measurement results are shown in Table 1.

Comparative Examples 1 to 5 The cooled fired bodies obtained in the same manner as in Examples 1 to 9 and the cooled fired bodies thereof were heated in the atmosphere as shown in Table 1 and in an inert gas nitrogen at high and low temperatures, respectively. Heat treatment was performed to obtain a Bi-based superconductor.

 The results are shown in Table 1.

From the above Examples and Comparative Examples, it can be seen that there is not much difference between the present invention and Comparative Examples in terms of Tc, but Jc exhibits remarkably good superconducting properties.

Example 10 A superconducting oxide pulverized powder was obtained in the same manner as in Example 1, and the pulverized powder was used to form a sheet having a thickness of 1 m.
A plate-shaped molded body of m, 300 × 300 (mm) was produced.

Then, the plate-shaped molded body is baked in an electric furnace in an oxygen atmosphere at 900 ° C. for 30 minutes, and then in the furnace at a cooling rate of 1.0.
The mixture was gradually cooled to 800 ° C at a rate of ° C / min, and then kept at 800 ° C for 10 hours. Then, it cooled to room temperature at 2 degree-C / min.

The cooled fired body was taken out of the electric furnace and the appearance was observed, but no cracks or cracks were found.

Then, the cooled fired body obtained above, a nitrogen gas atmosphere, 50
It heat-processed at 0 degreeC for 6 hours, and obtained the plate-shaped Bi type | system | group superconductor. No defects such as cracks were observed in the plate-shaped Bi-based superconductor obtained here.

0.7 mm thick from the obtained plate-shaped Bi-based superconductor, 2
A × 20 (mm) test piece was cut out and Tc and Jc at liquid nitrogen temperature were measured. The measurement result is that Tc is 87K and Jc is 960.
A / cm ² showed good superconducting properties.

Comparative Example 6 A plate-shaped compact similar to that of Example 10 was fired in an electric furnace in an oxygen atmosphere at 900 ° C. for 30 minutes, and then gradually cooled to 800 ° C. in the furnace at a cooling rate of 1.0 ° C./minute, and subsequently 800 Hold at 10 ° C for 10 hours. Immediately thereafter, when the sample was taken out of the furnace and rapidly cooled, three large cracks occurred.

〔The invention's effect〕

The present invention, in the production of Bi-based superconductor, from the first step of firing Bi-based superconducting oxide in an oxygen atmosphere, slow cooling, crystallization and cooling, and a second step of heat treatment in an inert gas atmosphere Since it is composed, it does not cause defects such as cracks and cracks regardless of the size of the Bi-based superconducting compact, and the conversion control to the Bi-based superconductor composition that has excellent superconducting properties in the second step is accurately controlled. In particular, it is possible to obtain a large-sized Bi-based superconductor such as a superconducting magnetic shield, which is extremely difficult by the conventional method, and is industrially useful.

Claims (2)

[Claims] 1. A method for producing a bismuth-based superconductor, wherein a superconducting compound containing bismuth and copper is calcined in an oxygen atmosphere and then gradually cooled to a crystallization temperature, and further 200 ° C. after crystallization.
A method for producing a bismuth-based superconductor characterized by comprising a first step of cooling to the following and a second step of heat-treating the cooled fired body of the first step in an inert gas atmosphere. 2. The heat treatment in the second step is 300 to 600 ° C.
The method for producing a bismuth-based superconductor according to claim 1, wherein