JPH03201316A - Manufacture of bi oxide superconductor wire rod - Google Patents

Abstract

PURPOSE:To make a critical temperature and a critical current density high and restrain a decrease in Jc in a magnetic field by impregnating a melting material having a specific composition into an elongated fiber convergent material having a specified composition, followed by a thermal treatment at a temperature under a melting point. CONSTITUTION:A melting material having a composition phase (2212 phase) of (Bi, Pb)2Sr2CaCu2Ox in is impregnated into an elongated fiber convergent material having a composition of (Ca, Sr)2CuO3. The resultant impregnated material is cooled down to a temperature under a melting point of the 2212 phase with the application of a temperature gradient (an optimum temperature gradient) is applied along the longitudinal direction thereof. The elongated convergent material in this state is a wire rod where fiber having a composition of (Ca, Sr)2CuO3 is dispersed in the 2212 phase with a crystal surface oriented and solidified in the longitudinal direction. Furthermore, the wire rod is thermally treated at a temperature under the melting point of the 2212 phase. Accordingly, a critical temperature and a critical current density (Jc) become high, and a decrease in Jc in a magnetic field can remarkably be restrained.

Description

Detailed Description of the Invention (Field of Industrial Application) Since the discovery in 1986 that the critical temperature (Tc) of La-5r(Ba)-Cu-0-based oxide exceeds 30K, new oxides have been developed. Superconductors have been discovered one after another, and currently (formerly Pb)-3r-Ca-Cu-0 (Bi) oxides have a critical temperature (Tc) of 100
It has reached the point where it exceeds K, and the liquid nitrogen temperature is 7.
Many institutions are actively conducting research to put this technology to practical use.

The present invention relates to a method for manufacturing a Bi-based oxide superconducting wire,
The superconducting properties of the wire, especially Tc and critical current density (Jc
) aims to effectively improve and improve

(Prior art) The main uses of superconductors are expected to be electromagnets, power transmission lines, etc., and for this purpose it is essential to establish wire technology.

Conventionally, regarding the production of oxide superconducting wires (including tape materials), for example, "Japan Institute of Metals Bulletin, 1987, 26th
1) Powder coating process 2) Doctor blade process 3) Plasma spray process 4) Quenched alloy ribbon oxidation process 5) Solutions of organic acid salts 6) A method in which a suspension of powder or powder is applied onto a tape; 6) A method in which the suspension is mixed with an organic polymer to adjust the viscosity and then spun.

Among these, the highest Jc of Bii oxide superconductors is currently obtained by applying the powder coating method, and depending on the manufacturing conditions, the temperature is 77K and 10% in zero magnetic field.
Cases exceeding 'A/cm2 have also been reported (e.g.
International Superconductivity Symposium (Tsukuba City, 1989) WB
-8, WB-9).

However, this powder coating method still has the following problems.

That is, a Bi-based superconductor has a phase with a composition of (Bi, pb) ZsrZcacuZoX where Tc is about 80 to 90 (hereinafter referred to as 2212 phase), and a phase with a composition of (Bi, Pb)2Sr2Ca2Cu30y where Tc exceeds 100 K. phase (hereinafter referred to as 2223 phase) is the main phase, and among these, it is extremely difficult to extract the latter with a high Tc as a single phase, and in many cases, 7 phases such as 2212 phase and 2223 phase are used. (Ratio r of main beam intensity l of X-ray diffraction =
12223/ (12212+Tzzz*) is 0
< r <60%) or consisted of a 2212-phase monomer.

For this reason, the Bii oxide superconductor has a small temperature margin from the operating temperature of 77K, which poses a practical problem, and even if a single-phase 2223 wire is produced, it is a polycrystalline material with fine grains. Because it is a solid body, it has a property that Jc is extremely reduced by application of an external magnetic field due to weak superconducting coupling between crystal grains caused by a small coherence length.

(Problems to be Solved by the Invention) The present invention overcomes the problems of the conventional technology as described above, and provides a Bi-based material with a Tc exceeding 100 K, a high Jc, and a small drop in Jc in a magnetic field. The purpose of the present invention is to provide an oxide type lightning conductor wire.

(Means for Solving the Problems) The present invention provides a method of adding (Bi, Pb)zsrzcacu to a long bundle of fibers having a composition of (Ca, Sr)2CuO3.
Impregnation with a melt having a composition of zOx, cooling to a temperature below the melting point of the melt with a temperature gradient along its longitudinal direction, and then heat treatment at a temperature below the melting point of the melt. This is a method for manufacturing old-thread superconducting wire.

(Function) In the Bi-based superconductor, m of (Ca, Sr)2CuO,
Impurities that become a predominant substance are easily formed, and the 2223 phase grows by diffusion of Sr, Ca, and Cu from the interface between the impurity and the 2212 phase (D. Shi et al.,
Appl, Phys, Lett, 55 (7L (1989
) 699, written by Watada et al., 2nd International Superconductivity Symposium (1988, Tsukuba City, PPC-28)), we have arrived at this invention.

Here, the composition of (Ca, Sr)2CuO:+ is 22
Since it has a higher melting point than the 2212 phase, in this invention, first, a fibrous long condensate having the composition (Ca, Sr)zcu03 is impregnated with a molten material having the 2212 phase Mi precipitate. Then, this impregnated material is cooled to a temperature below the melting point (approximately 900°C) of the 2212 phase while applying a temperature gradient (preferably temperature gradient = 10 to 500°C/cm) along its longitudinal direction (preferably Cooling rate: o, i ~500° (:
/min).

The reason why the preferred temperature gradient is 10 to 500°C/cm is that directional solidification of the liquid phase does not occur when the temperature is less than 10°C/cm, but in order to create a temperature gradient that exceeds <500°C/cm. This is because there are difficulties in manufacturing the device. The reason why it is preferable to set the cooling rate to about 0.1 to 500°C/min when cooling the impregnated material to a temperature below the melting point of the 2212 phase is because directional solidification occurs at a cooling rate higher than this range. This is because obtaining a cooling rate smaller than this range involves difficulties in manufacturing the device and lowers productivity.

In this state, the long convergent substance has (CaSr) in the solidified 2212 phase with the ab plane of the crystal oriented in the longitudinal direction.
It is a wire rod in which fibers having a composition of 2CuO3 are dispersed.

By further heat-treating the above wire at a temperature below the melting point of the 2212 phase, Ca, Sr, and Cu diffuse from the fibrous m-oxide into the 2212 phase, resulting in the excitation of the 2223 group strength with Tc exceeding 100 K. A conductive wire material can be obtained.

Suitable conditions for heat treatment include: The higher the temperature, the faster the ion diffusion rate < 2223 phase is likely to be formed, but if it exceeds the melting point of the impregnated material, the oriented and solidified structure will be disturbed, which is undesirable, so the temperature should not exceed the melting point of the impregnated material. 800~870°
It is preferable to keep the temperature in the temperature range of C for a long time of 24 hours or more.

Since this wire is based on a structure obtained by unidirectionally solidifying a molten material, it has superior crystal orientation compared to conventional powder coating processing methods. In addition, since the crystal grain size is greatly developed, the volume ratio of grain boundaries is smaller than that of wire rods manufactured according to conventional methods, and the influence of weak superconducting bonds between crystal grains is small. Therefore, the value of Jc is improved and the drop in the magnetic field is also very small.

The method for producing a fibrous bundle having the composition of (Ca, Sr)zcuoz is as follows: 1) Melt the solid phase composition having the above composition and blow it away with compressed air or centrifugal force to a diameter of 5 to 20 μm. The fibers are then knitted into long bundles.

2) A solid phase composition having the above composition is poured into a pot made of a high melting point metal such as W or MO or a special refractory and has a large number of nozzles at the bottom and melted, and the molten material flowing out from the nozzles is rapidly cooled. Wind it up while converging it.

3) A solid phase composition having the above composition is melted, a single crystal fiber is produced by pulling it up through a thin nozzle, and the fiber is bundled.

4) Prepare a solid phase composition powder having the above composition with a particle size of 0.5 μm or less in advance, disperse it in a dimethyl sulfoxide solution of polyvinyl alcohol with a high degree of polymerization, and then disperse it in linear form in methanol. It is extruded as a product, wound up, continuously spun, heat-treated, and then bundled.

etc. are preferable. Although the diameter of the bundle does not affect the superconducting properties, it is set to about 0.5 to 2 mm for convenience in later use as a wire.

In addition, the temperature at which the above-mentioned bundle is impregnated with a molten material having a composition of 2212 is theoretically the highest at the melting point of the bundle and the lowest at the freezing point of the 2212 phase, but in reality it depends on the viscosity of the liquid, the liquid phase and the solid phase. The optimum temperature is determined by the wettability of the fibers and the geometrical arrangement of the fibers within the bundle.

The present invention will be explained in more detail with reference to Examples below, but the present invention is not limited to the specific conditions in the Examples below.

(Example) Example 1 CaCO3+ 5rCOt, CuO powder as Ca:
After weighing and mixing to give a molar ratio of Sr:Cu-1,9:0.1:l,
It was calcined at °C for 12 hours. The obtained calcined product was heated to 2800°C by high-frequency heating using tungsten as a susceptor (heating medium), melted, and blown out from a nozzle to produce glass fibers with an average diameter of 0.1 mm. The fibers were bundled into a long bundle with a diameter of 1 mm.

An apparatus as shown in FIG. 1 was used to impregnate this long bundle with a melt having a composition of 2212. In addition, number 1 in the same figure is a crucible with a wire drawing opening 1b at the bottom 1a,
2 is the liquid phase of 2212M1 precept, and 3 is a pulley around which the long convergence material p is wound. The crucible 1 is made of an Au-Pd alloy that has excellent corrosion resistance against Bi-based superconductors.
First, in this crucible 1, B12 (1++PbO, SrCO
3, CaC0:+, CuO powder Bi: Pb:
Sr: Ca: Cu=1.9: 0.1 +
The mixture was weighed and kneaded so as to have a molar ratio of 2:1:2 and calcined in the air at 800°C for 12 hours, and heated to approximately 950°C using a Kanthal heater. Liquid phase 2 was prepared. The prefabricated fiber bundle p passes through the liquid phase 2 from the pulley 3 and has a diameter of 5 mn+.
It is pulled out at a speed of 1 cm/min in the direction of the arrow through the pull-out port 1a of about 100 mm. The temperature T2 of the drawing port 1b is maintained near the melting point of the liquid phase 2 (approximately 900'C), but since the viscosity of the liquid phase is high, the liquid phase 2 in the crucible 1 flows out from the drawing port 1b. The liquid phase 2 impregnated into the bundle P solidifies under a temperature gradient of about 100' (:/cm) between the bottom 1a of the crucible 1 and the withdrawal port lb.

Next, the drawn wire was heat treated at 840°C and 4811r in an air atmosphere, and the properties of the obtained Bi-based superconducting wire were investigated. The results are shown in Table 1.

Comparative example BIKO: II pbo, 5rCO, , CaC0,
, , CuO powder as Bi:Pb:Sr:Ca
: Cu = 1.9 : 0.1 : 2 : 2
: After weighing and mixing so that the molar ratio was 3, the mixture was calcined at 800°C and 1211r in the air atmosphere and pulverized. This calcined powder was packed into a silver pipe with an internal diameter of 8 mm and a wall thickness of 1 mm.
Repeated stretching and rolling to create a width of 3mm and a thickness of 0.1mm.
After forming it into a tape shape, it was heat-treated in an air atmosphere at 840°C for 48 hours to obtain a Bi-based superconducting wire. The properties of this wire are also shown in Table 1.

As is clear from Table 1, it was confirmed that the Bi-based superconducting wire according to the present invention has higher Tc and Jc than those made by the conventional method, and the decrease in Jc in a magnetic field has been significantly improved. According to SEM (scanning electron microscopy) observation, the Bi-based superconducting wire according to the present invention exhibited remarkable crystal growth in the longitudinal direction compared to those made using the conventional method, and exhibited a dense microstructure with almost no visible bores. This large grain growth makes the volume fraction of the grain boundary portion relatively small, which reduces the influence of weak superconducting bonds between grains, and appears to improve the reduction in Jc in the magnetic field.

(Effects of the Invention) Thus, according to the present invention, it has become possible to manufacture a Bi-based superconducting wire having a high ratio of 2223 phase with Tc > 100, which was difficult in LJ according to the prior art. Furthermore, since the volume fraction of grain boundaries became smaller, the decrease in Jc in a magnetic field was significantly improved. Therefore, this invention makes a significant contribution to the field of application of superconducting materials such as electromagnets and power transmission lines.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an example of a method for impregnating a long bundled material with a molten material of 2212 sets. 1... Au-Pd crucible 1a... Crucible bottom 1b... Wire drawing port 2... Liquid phase with 2212 composition 3... Pulley P... Long bundled object

Claims (1)

[Claims] 1. (Bi, Pb)_2Sr_2CaCu_2
It is characterized by impregnating a melt with a composition containing O_x, cooling it to a temperature below the melting point of the melt with a temperature gradient in its longitudinal direction, and then heat-treating it at a temperature below the melting point of the melt. A method for producing a Bi-based oxide superconducting wire.