JPH0717714A - Production of superconducting oxide - Google Patents

Abstract

PURPOSE:To suppress growth of an impurity phase and to obtain high critical current density (Jc) characteristics by partially melting a specified source material in an oxidative atmosphere. CONSTITUTION:Starting powders of the source material Bi2O3, SrCO3, CaCO3 and CuO are compounded and mixed to obtain the molar ratio of Bi:Sr:Ca:Cu=2:2:1:2. The mixture is calcined at about 800 deg.C for about 12 hours, pulverized, mixed, and further calcined. The calcined material is partially molten in an oxidative atmosphere at >=0.8atm oxygen partial pressure to obtain a Bi-based superconducting oxide essentially comprising Bi2Sr2Ca1Cu2Oy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention mainly comprises Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _y (this compound is hereinafter referred to as Bi (2212)) having excellent superconducting critical current density (Jc) characteristics. And a method for producing a Bi-based oxide superconductor.

[0002]

2. Description of the Related Art Various studies have been conducted in various fields on the practical use of oxide superconductors discovered in recent years. Oxide superconductors, especially at liquid helium temperature (4.2 K)
It has been confirmed that the magnetic field characteristics of Jc are superior to those of conventional metal-based superconductors, and it is expected to be applied to high-field generating coils of 30 Tesla class.

As a technique for forming a wire into an oxide superconductor, there are a powder in tube method, a doctor blade method, a Dick coat method and the like. The powder-in-tube method is a method in which raw material powder calcined in a silver tube is packed into a wire material by die drawing, roll rolling, etc.The doctor blade method and the Dick coat method apply the raw material powder on a silver plate. This is a method in which a thick film is produced and the length is increased by roll rolling or the like. The Bi-based superconductor can be made into a wire rod by any method, but the so-called powder-in-tube method is widely adopted as a wire rod forming technique.

Among the Bi-based superconductors, the Bi (2212) wire is
By partially melting at around 890 ° C and then slowly cooling, a highly oriented structure with uniform crystallographic orientation can be obtained, and a wire with little deterioration in Jc characteristics due to a magnetic field can be obtained.

For example, Japanese Journal of Applied Ph
ysics, Vol.29, No.7, the L1066-L1068 (Document 1), Bi ₂ O ₃ as a raw material _{powder, SrCO 3, CaCO 3, CuO} , each powder of _{Ag 2 O Bi: Sr: Ca} : Cu The atomic ratio of: Ag is 2: 2: 1: 2 :.
The mixture was mixed to 0.8 and calcined at 880 ℃ to fill the powder into a silver pipe, which was drawn and rolled to form a tape-shaped wire, and then a mixed gas of Ar and O ₂ A maximum of 8.88 × 10 ⁴ A after heat treatment of partial melting at 880 ℃ and slow cooling
It is reported that Jc of / cm ² (temperature 4.2K, magnetic field 1 Tesla) was achieved.

What is common in the heat treatment of the Bi (2212) series wire is that the temperature is raised to 875 ° C. or higher for partial melting, and then gradually cooled for further grain growth. Note that partial melting means that the Bi (2212) phase is wholly melted, but the other C
Impurity phases such as aO and Sr-Ca-Cu-O phases do not melt.

However, if the partial melting temperature is too high or the time is too long, grain growth of the Sr-Ca-Cu-O phase, which is an impurity phase, is promoted and Jc is lowered. So generally 8
After partially heating by raising the temperature from 75 ℃ to 895 ℃, Bi
800 ° C so that the (2212) phase crystallizes and further grain growth occurs
Heat treatment is performed at a slow cooling rate of 5 to 20 ° C / hr to the surrounding temperature (eg Japanese Journal
l of Applied Physics, Vol.30, No.12A, pp.3371-3376 ...
Reference 2,).

On the other hand, the heat treatment atmosphere used for these partial melting is atmospheric air in the above Document 2 and argon gas in the above Document 1.

[0009]

The problems of the above-mentioned heat treatment methods known so far are as follows. That is, as described above, the Bi (2212) phase melts at the partial melting temperature, but the impurity phase such as CaO and Sr-Ca-Cu-O phase exists in the wire as it is in the form of solid particles. Therefore, a solid-liquid coexisting state occurs above the crystallization temperature of the Bi (2212) phase, and the impurity phase causes grain growth.

In particular, the grain growth rate of the Sr-Ca-Cu-O phase is high, and the coarse Sr-Ca-phase grown even at a low temperature below the crystallization temperature.
Cu-O grains remain.

When the solid impurity phase causes grain growth in the solid-liquid coexistence state described above, the composition of the liquid phase changes, and a deviation from the intended composition of the Bi (2212) phase occurs. As a result, in addition to the Sr-Ca-Cu-O grains, the normal phase Bi ₂ Sr
₂ Cu ₁ O _y , that is, the (2201) phase is generated and prevents the superconducting current path. In addition, in the vicinity of the grain-grown Sr-Ca-Cu-O grains,
The orientation of the Bi (2212) phase is disturbed, which also causes deterioration of Jc characteristics.

The object of the present invention is to solve the above problems,
It is an object of the present invention to provide a method for producing a superconductor mainly composed of Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _y having excellent Jc characteristics.

[0013]

[Means for Solving the Problems] The present invention provides "Bi, Sr, Ca
And the molar ratio of Cu (Bi: Sr: Ca: Cu) is 2: 2: 1: 2.
The method for producing an oxide superconductor is characterized in that the raw material mixed as described above is subjected to partial melting treatment in an oxygen atmosphere having an oxygen partial pressure of 0.8 atm or more ”.

[0014]

[Operation] As shown in the above-mentioned references 1 and 2,
The partial melting heat treatment of the Bi-based oxide superconductor is usually performed in the atmosphere or argon gas. It is considered that this is because, when the superconducting material of this system was discovered, the heat treatment for making the superconducting single phase was performed in a low oxygen atmosphere. However, the partial melting heat treatment in such an atmosphere has the above-mentioned disadvantages.

Contrary to conventional wisdom, the present inventor tried an experiment in which the partial melting heat treatment was carried out in an oxygen atmosphere.

Samples (cut out from a tape-shaped wire) produced under the conditions shown in the following Examples were heated to 885 ° C. in an atmosphere of various oxygen partial pressures (PO ₂ ) shown in Table 1 and held for 10 minutes. The structure was observed for the sample rapidly cooled to room temperature at a cooling rate of 5000 ° C./min.

The atmosphere is adjusted with Ar gas (purity 99.9%).
O ₂ gas (purity 99%) was performed by changing each flow rate.

Table 1 shows the relationship between the oxygen partial pressure and the average particle size of Sr-Ca-Cu-O particles. The higher the oxygen partial pressure, the more Sr-Ca-
The average particle size of Cu-O grains show a smaller tendency but, PO ₂ is
There is no big change up to 0.7 atm, and the particle size is about 40 μm. However, when PO ₂ exceeds 0.8 atm, Sr-Ca-
Cu-O grain growth was remarkably suppressed, and when PO ₂ = 0.99 atm, 10
It becomes as fine as μm or less. The average particle size is Sr-Ca.
The average area of -Cu-O grains was calculated, and the grains were regarded as spheres for calculation.

[0019]

[Table 1]

FIG. 1A is an observation view of the cross-sectional structure of sample No. 1 in Table 1 and FIG. 1B is the same as sample No. 8 (magnification: 1000
Times). The black phase designated by the symbols D and E is Po ₂ = 0.
In (a), which was heat-treated in an atmosphere of 2 atm (atmosphere), the large one grew to 100 μm or more, but PO ₂ = 0.99
10 μm for the sample (b) heat-treated in an oxygen atmosphere of atm
It can be seen that the particles are finely dispersed in the size of the degree. (a)
D in the figure is (Sr _0.5 Ca _0.5 ) ₁ Cu ₁ O _y phase grains, (b) E in the figure is (S
r _0.5 Ca _0.5 ) ₃ Cu ₅ O _y phase grains, which are generally collectively referred to as Sr-Ca-Cu-O phase.

FIGS. 1 (c) and 1 (d) show the same atmospheres as (a) and (b), respectively, and after partially melting at 885 ° C. for 10 minutes, 5 ° C./840° C. slowly cool with hr
FIG. 6 is an observation view of a cross-sectional structure of a sample that was rapidly cooled to room temperature after crystallizing a Bi (2212) phase. Also in this case, heat treatment was performed in the atmosphere
The black phase of (c) [D, that is, (Sr _0.5 Ca _0.5 ) ₁ Cu ₁ O _y phase] is coarse, while the black phase of (d) [E, that is (Sr _0.5 Ca _{0 . 5) 3} grains of Cu ₅ O _y phase] is small. In addition, F of the figure (b) is a grain of Bi-Sr-Ca-O phase. This F
The phase was not observed in the structure that was rapidly cooled after rapid cooling, and it is considered that it does not affect Jc.

As is apparent from FIG. 1, if an oxygen atmosphere is used, the material is maintained at the partial melting temperature and then immediately quenched, or is gradually cooled once to crystallize Bi (2212) and then rapidly cooled. However, the impurity phase in the finally obtained superconductor becomes fine as shown in (b) and (d). As shown in Examples described later, the same result is obtained when the partial melting temperature is gradually cooled to room temperature. This means that if the heat treatment is performed in an oxygen atmosphere, a superconducting material having a fine impurity phase and excellent Jc characteristics can be obtained regardless of the cooling heat pattern.

From the above test results, in order to suppress the growth of Sr-Ca-Cu-O grains, it is preferable that the oxygen partial pressure (PO ₂ ) in the atmosphere during the partial melting heat treatment be 0.8 atm or more. Furthermore, it can be said that a pure oxygen atmosphere with PO ₂ ≧ 0.99 atm is effective.

Thus, the oxygen atmosphere is Sr-Ca-Cu-O.
The reason why it is effective in suppressing grain growth is the usual 875-895 ℃.
In the atmosphere of PO ₂ ≧ 0.8 atm, the grains of (Sr _0.5 Ca _0.5 ) ₃ Cu ₅ O _y [35 phase] grow and the PO ₂
In an atmosphere of <0.8 atm, (Sr _0.5 Ca _0.5 ) ₁ Cu ₁ O _y [11 phase] grows grains, but it is presumed that [35 phase] has a smaller grain growth rate than [11 phase]. It

The temperature at which partial melting is performed is preferably about 875 to 895 ° C., and as described above, there are no particular restrictions on the cooling conditions thereafter. In addition, it is preferable that the oxygen atmosphere is at least up to about 840 to 865 ° C. at which crystallization of the Bi (2212) phase is completed.

The oxide superconducting material can be formed into a wire by a conventional method of filling a silver sheath tube or coating it on a silver plate. That is, the raw material powder is blended in a predetermined ratio and calcined to obtain a powder, which is filled in a silver sheath tube or applied on a silver plate by a doctor blade method or the like.

Although the following heat treatment may be performed immediately after filling the silver sheath tube, it may be further performed after forming the wire by a method such as wire drawing or rolling. Further, even when applied on a silver plate, the heat treatment may be performed as it is, or the heat treatment may be performed after processing.

[0028]

Example: As raw material powders, Bi ₂ O ₃ , SrCO ₃ , CaCO ₃ ,
CuO and Ag ₂ O powders were used and weighed so that the molar ratio of Bi: Sr: Ca: Cu: Ag was 2: 2: 1: 2: 0.8, and mixed in a mortar. The reason for adding AgO is to improve the orientation of the Bi (2212) phase and the Jc characteristics.

The above blended raw materials were calcined in the air at 800 ° C. for 12 hours, pulverized and mixed, and further calcined in the air at 800 ° C. for 12 hours to obtain raw material powder. This raw material powder was filled into a silver pipe with a length of 10 cm, an outer diameter of 12 mm and an inner diameter of 6 mm, and this was drawn by die and rolled to obtain a width of 3 mm and a thickness.
It was processed into a 0.1 mm tape-shaped wire.

A sample having a length of 10 cm was cut out from the above tape-shaped wire and subjected to the following heat treatment continuously.

(1) The temperature was raised to 885 ° C. at a rate of 10 ° C./hr and kept for 10 minutes to partially melt. (2) Slowly cool to 820 ° C at 5 ° C / hr, and then cool to room temperature in a furnace.

Both (1) and (2) above were carried out in an atmosphere of oxygen partial pressure shown in Table 2 which was adjusted by changing the flow rate ratio of argon gas (purity 99.9%) and oxygen gas (purity 99%). . The reason was partially melted at 885 ° C. is, 885 ° C. is in the air (PO ₂ = 0.2atm) and an oxygen stream (PO ₂ = 0.99at
This is because it was separately confirmed that the heat treatment temperature at which the highest Jc value was obtained was obtained in any of m).

Table 2 shows the relationship between Jc in a liquid magnetic helium temperature (4.2 K) in an external magnetic field of 1 Tesla and oxygen partial pressure during heat treatment. The external magnetic field was applied parallel to the sample surface.

As is clear from Table 2, PO ₂ is 0.2 atm.
To 0.7atm, no significant change in Jc characteristics
(Sample Nos. 1 to 4). However, the improvement of Jc was observed when PO ₂ was 0.8 atm or more, and 100% oxygen atmosphere (PO ₂ = 0.99at
The highest Jc is obtained in the sample (No. 8) of m).

The measured Jc values in Table 2 are the Sr-Ca-Cu values in Table 1 above.
It is shown that the growth of Sr-Ca-Cu-O particles is suppressed and dispersed as fine particles in response to the change in -O particle size, leading to improvement of Jc characteristics.

[0036]

[Table 2]

[0037]

According to the method of the present invention in which the partial melting heat treatment, which is an essential step in the production of a superconductor mainly composed of Bi (2212), is performed in an atmosphere of high oxygen partial pressure, the growth of an impurity phase is suppressed. However, it is possible to manufacture a superconductor having a high critical current density (Jc) characteristic.

[Brief description of drawings]

Figure 1: 885 ° C or 84 in air or oxygen
It is a copy of a cross-sectional view of a Bi (2212) wire quenched from 0 ° C., and a black-looking phase is Sr—Ca—Cu—O impurity particles.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location // H01B 12/00 ZAA 7244-5G

Claims (1)

[Claims] 1. A molar ratio of Bi, Sr, Ca and Cu (Bi: Sr: C).
a: Cu) is mixed in a ratio of 2: 2: 1: 2, and a partial melting treatment is performed in an oxygen atmosphere having an oxygen partial pressure of 0.8 atm or more to produce an oxide superconductor. Method.