JPH0656426A - Thermal treatment of oxide superconductor wire - Google Patents

Abstract

PURPOSE:To produce the silver sheath wire of Bi2Sr2Ca1Cu2Oy phase excellent in Jc characteristics. CONSTITUTION:This method for thermally treating the silver sheath wire of an oxide superconductor consisting mainly of a Bi2Sr2Ca1Cu2Oy phase is characterized by subjecting the wire to a partial melting treatment, quenching the melted wire in a temperature range just above its crystallization temperature, and subsequently gradually cooling the wire. The quenching-stopping temperature is preferably a temperature just above 845 deg.C when the thermal treatment is carried out in the atmosphere, or a temperature just above 850 deg.C when the thermal treatment is performed in oxygen. The gradual cooling is preferably performed at a rate of 10 deg.C/hr at least down to 800 deg.C. Thereby, the Bi (2212) silver sheath wire reduced in impurity phases and having high Jc characteristics can be produced by the simple method by which the combination of the quenching treatment with the gradual cooling treatment is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is mainly composed of 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. The present invention relates to a heat treatment method for producing a Bi-based oxide superconductor wire rod.

[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.

Among oxide superconductors, the Bi-based superconductor is a die-drawing wire in which a calcined raw material powder is packed in a silver tube.
The wire can be made by a so-called powder-in-tube method, which is made into a wire by rolling or the like. Among the Bi-based superconductors, the Bi (2212) wire rod has a highly oriented structure with uniform crystallographic orientation, which is obtained by partially melting it at around 890 ° C and then slowly cooling it, and the decrease in Jc characteristics due to the magnetic field is small. A wire rod is obtained.

For example, Japanese Journal of Applied Ph
As ysics, Vol.29, No.7, L1066-L1068,
Bi ₂ O ₃ , SrCO ₃ , CaCO ₃ , CuO, and Ag ₂ O powders were added to Bi: S.
The mixture was mixed so that the atomic ratio of r: Ca: Cu: Ag was 2: 2: 1: 2: 0.8, and the mixture was calcined at 880 ° C and the obtained powder was filled into a silver pipe, which was drawn and rolled. Then, it is processed into a tape-shaped wire and then heat-treated by partially melting it in a mixed gas of Ar and O ₂ at 880 ° C and gradually cooling it to a maximum temperature of 8.88 × 10 ⁴ A / cm ² (temperature
It is reported that the Jc of 4.2K and magnetic field of 1 Tesla was achieved.

What is common to the heat treatment of Bi (2212) -based 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
See).

[0007]

The problems of the above-mentioned heat treatment methods known so far are as follows. That is, as described above, the Bi (2212) phase is melted at the partial melting temperature, but the other impurity phases such as CaO and Sr-Ca-Cu-O phases are present in the wire as solid particles. Therefore Bi
Above the crystallization temperature of the (2212) phase, a solid-liquid coexisting state occurs,
The impurity phase causes grain growth. In particular, the grain growth rate of the Sr-Ca-Cu-O phase was high, and it grew even at low temperatures below the crystallization temperature.
Sr-Ca-Cu-O grains remain.

As a result, a deviation from a predetermined composition occurs in the wire, and in addition to the Sr-Ca-Cu-O grains, the normal phase B
i ₂ Sr ₂ Cu ₁ O _{y, that} is, the (2201) phase is generated and deteriorates the 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 heat treatment method for producing a silver sheath wire rod mainly composed of Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _y having excellent Jc characteristics.

[0010]

The present invention provides "Bi ₂ Sr ₂ Ca ₁ Cu ₂
In the heat treatment of a silver sheath wire of an oxide superconductor mainly composed of O _y , after the partial melting treatment, the temperature range up to just above the crystallization temperature is rapidly cooled and then gradually cooled. The heat treatment method for conductive wire ”is the main point.

A conventional method may be used as a method of manufacturing the wire before the heat treatment. That is, a raw material powder is mixed in a predetermined ratio and calcined to obtain a powder, which is filled in a silver sheath tube and drawn into a wire by a method such as wire drawing or rolling. It is desirable to add a small amount of silver oxide to the raw material powder.

FIG. 1 shows a heat pattern of the heat treatment method of the present invention.

The rate (R
There are no particular restrictions on s). The Tpm may be in the commonly employed range of 875-895 ° C. The holding time is preferably about 5 to 30 minutes.

Cooling rate after partial melting treatment (Rc in FIG. 1)
Is larger the better. The quenching stop temperature (Tr in FIG. 1), that is, the slow cooling start temperature is preferably changed depending on the atmosphere in which the heat treatment is performed, as described later. Slow cooling rate (Fig. 1
Rj) should be 10 ° C / hr or less. The gradual cooling stop temperature (Ts) shall be 800 ℃ or less. 600 to 750 ℃
In the temperature range of 1, the Bi (2212) phase changes to the Bi (2201) non-superconducting phase and Jc may decrease, so it is preferable to increase the cooling rate.

[0015]

Table 1 shows the partial melting temperature of the silver sheath wire of Bi (2212) and the crystallization temperature of Bi (2212) measured in air and oxygen by DTA (differential thermal analysis).

[0016]

[Table 1]

The difference of 20 to 30 ° C. between the melting temperature and the crystallization temperature of Bi (2212) as shown in Table 1 is considered to be due to the supercooling phenomenon. Therefore, if continuous slow cooling is performed from the partial melting temperature, grain growth of the impurity phase in the solid state, particularly the Sr-Ca-Cu-O phase, actively occurs before the crystallization of Bi (2212) begins. It will be. This is the main cause of the problems of the above-mentioned conventional method.

The method of the present invention is characterized in that during the cooling process from the melting temperature of Bi (2212) to the crystallization temperature, grain growth of the impurity phase does not occur, that is, from the partial melting temperature to immediately above the crystallization temperature. Is to cool rapidly and then slowly. In principle, the quenching rate is preferably as high as possible. As shown in Table 3 described later, the value of Jc is remarkably increased by increasing the cooling rate.

The temperature range for rapid cooling is from the partial melting temperature to Bi
Up to just above the crystallization temperature of (2212). As shown in Table 1, the crystallization temperature changes depending on the heat treatment atmosphere. Since the crystallization temperature is 845 ° C in the atmosphere, it is cooled immediately above that temperature, preferably to 855 ° C which is about 10 ° C higher than this temperature. If it is rapidly cooled to a temperature lower than the crystallization temperature (845 ° C), grain growth of the Bi (2212) phase is not sufficiently performed, the crystal orientation is deteriorated, and Jc is deteriorated.

In an oxygen atmosphere, the crystallization temperature at that time
Immediately above 850 ° C, preferably 10 ° C higher than this, and rapidly cooled to a temperature range of about 860 ° C. Also in this case, rapid cooling to a temperature range lower than 850 ° C. is not preferable for the above reason. In any case, if the quenching temperature range is too high, the impurity phase, especially Sr-
The effect of suppressing the grain growth of the Ca-Cu-O phase becomes small.

Usually, an electric furnace for adjusting the temperature with a program temperature controller is used for such heat treatment, but it is difficult to increase the cooling rate because the heat capacity of the furnace itself is large. In the heat treatment method of the present invention, two-stage cooling is performed in which the material is rapidly cooled to a predetermined temperature and then gradually cooled. A special device is used to obtain a high cooling rate in this rapid cooling step. Is desirable.

FIG. 2 shows an outline of an apparatus suitable for carrying out the heat treatment method of the present invention. The heating device 1 is for heating the first zone for partial melting to the temperature Tpm shown in FIG. The heating device 2 is initially set to the rapid cooling stop temperature Tr shown in FIG. 1 and then gradually cools the sample while lowering the temperature to the slow cooling stop temperature Ts at the slow cooling rate Rj. The sample (wire to be heat treated) 3 is held in the quartz tube 4 in the first zone for a predetermined time to be in a partially molten state,
Then, it is rapidly cooled while moving in the moving zone, and then gradually cooled in the second zone. The cooling rate of the quenching process can be freely adjusted by adjusting the moving speed of the sample in the moving zone. In addition, on the outside of the quartz tube 4 in the movement zone,
If a forced cooling means such as a water cooling pipe is provided, the cooling rate can be adjusted more easily.

[0023]

Example: Bi ₂ O ₃ , SrCO ₃ , CaCO ₃ , Cu as raw material powder
Powders of O ₂ and Ag ₂ O were used, and these were Bi: Sr: Ca: Cu: Ag.
Were weighed so that the atomization ratio was 2: 2: 1: 2: 0.8, and mixed in a mortar. The reason for adding Ag ₂ O is to improve the orientation of Bi (2212) and the Jc characteristics.

The above-mentioned mixed powder is calcined in the air at 800 ° C. for 12 hours and then pulverized and mixed.
It was calcined for 12 hours to obtain a raw material powder for wire rod. This raw material powder is 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 the wire is 3 mm wide and 0.1 mm thick by die drawing and roll rolling.
Processed into a tape-shaped wire.

The above tape-shaped wire was heat-treated in the atmosphere and oxygen in the heat pattern shown in FIG. The conditions are as follows.

Partial melting temperature (Tpm): 885 ° C (holding for 10 minutes) Rapid cooling rate (Rc): 240 ° C / hr Rapid cooling stop temperature (Tr): As shown in Table 2, Slow cooling rate (Rj): 5 ° C / hr Slow cooling stop temperature (Ts): 800 ℃ After slow cooling was stopped, the furnace was cooled to room temperature.

Table 2 shows the quenching stop temperature (Tr) in air and oxygen atmosphere, and Jc (4.2K, 1Te) of the obtained wire.
(in sla) and the area ratio (%) of Sr-Ca-Cu-O are shown.
The area ratio is the area ratio of the entire Sr-Ca-Cu-O grains in the total cross-sectional area of the wire.

No. A1 and S1 in Table 2 are 885 ° C without quenching.
This is based on the conventional heat treatment method of gradually cooling from the partial melting temperature to 800 ° C.

In the samples (No. A2 to A4, S2 to S4) rapidly cooled to a temperature in the range of 875 ° C. to 855 ° C. in air and oxygen, the Sr-Ca-Cu-O area ratio was decreased and Jc was improved. ing.

However, the samples (No. A5 to A7, S5 to S7) rapidly cooled to below the crystallization temperature of Bi (2212) both in air and in oxygen.
Shows that the orientation is poor and Jc is drastically reduced.

Sr-Ca-Cu-O changes into a Bi (2212) phase by a solid phase diffusion reaction during crystallization and gradual cooling to 800 ° C., but Comparative Examples Nos. A5 to A7 and S5 to In S7, this reaction becomes difficult, and conversely, the area ratio of Sr-Ca-Cu-O is increasing.

In Table 3, the quenching stop temperature (Tr) is kept constant at 865 ° C. and the quenching rate (Rc) when heat-treated in oxygen and Jc
And the Sr-Ca-Cu-O area ratio are shown. As Rc increases, Sr-Ca-Cu-O decreases and Jc also improves. sample
From No. C7 to C9, the equipment shown in Fig. 2 was used to increase the quenching rate. Larger Jc is obtained in these examples.

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

EFFECTS OF THE INVENTION According to the heat treatment method of the present invention, Bi (2212), which has a small amount of impurity phases and high Jc characteristics, is a simple method of cooling from a partially molten state by a combination of rapid cooling and slow cooling. It is possible to manufacture a silver sheath wire rod.

[Brief description of drawings]

FIG. 1 is a diagram showing a heat pattern of a heat treatment method of the present invention.

FIG. 2 is a diagram showing an outline of an apparatus suitable for carrying out the heat treatment method of the present invention.

[Explanation of symbols]

1: Heating device for partial melting zone, 2: Heating device for slow cooling zone, 3: Sample 4: Quartz tube

Claims (2)

[Claims] 1. In the heat treatment of a silver sheath wire of an oxide superconductor mainly composed of Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _y , the temperature range up to just above the crystallization temperature is rapidly cooled after the partial melting treatment. A method for heat treating an oxide superconducting wire, characterized by gradually cooling thereafter. 2. The quenching stop temperature is directly above 845 ° C. when the heat treatment is performed in the atmosphere, and immediately above 850 ° C. when the heat treatment is performed in oxygen,
The heat treatment method according to claim 1, wherein the gradual cooling is performed to at least 800 ° C at a cooling rate of 10 ° C / hr or less.