Method of preparing a multifilamentary bismuth oxide superconducting wire

Abstract: Powder of not more than 1 µm in mean particle diameter is prepared to contain a mixture of superconducting phases mainly composed of 2212 phases of Bi-Sr-Ca-Cu or (Bi, Pb)-Sr-Ca-Cu and non-superconducting phases which is obtained by calcining and pulverizing raw material powder at least once, this powder is heat treated at a high temperature and thereafter coated with a metal to prepare a round wire by deformation processing, thereafter a tape type or flat type wire is prepared by deformation processing, then the wire is heat treated under conditions for allowing phase transformation of the 2212 phases of main superconducting phases to 2223 phases with facilitation of grain growth, thereafter the as-formed 2223 phases are highly densified by deformation processing or pressurization, and the wire is again heat treated so that the 2223 phases are strongly bonded with each other and the non-superconducting phases are finely dispersed. Thus, a bismuth oxide superconducting wire having a high critical current density is obtained.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of preparing a multifilamentary bismuth oxide superconducting wire, and more particularly, it relates to an improvement for increasing the critical current density of such a bismuth oxide superconducting wire. Description of the Background Art In recent years, superconductive materials of ceramics, i.e., oxide superconductive materials are watched with interest as those exhibiting higher critical temperatures. In particular, a bismuth oxide superconductive material is expected for practical use due to a high critical temperature of about 110 K. It is known that a bismuth oxide superconductor has phases having critical temperatures of 110 K, 80 K and 10K. It is also known that non-superconducting phases partially appear when a 110 K-phase superconductor is prepared in particular. In relation to such a bismuth oxide superconductor, it is further known that a 110 K phase has a 2223 composition of Bi-Sr-Ca-Cu or (Bi, Pb)-Sr-Ca-Cu and a 80 K phase has a 2212 composition of the same components. On the other hand, an oxide superconducting wire is generally prepared by filling a metal pipe with oxide raw material powder, working the same into a wire through deformation processing such as extrusion, wire drawing or rolling, and thereafter heat treating the wire. In order to work an oxide superconductor into an oxide superconducting wire and apply the same to a superconducting magnet, a wire for a device or a power cable, for example, it is important to attain a high critical current density, in addition to a high critical temperature. A current density exceeding at least 1000 A/cm 2 must be attained in order to put an oxide superconducting wire into practice. When a superconductor is worked into a long oxide superconducting wire, further, such a current density must be substantially uniformly attained over the longitudinal direction of the superconducting wire. While it is effective to employ a bismuth superconductor, particularly that containing Bi partially replaced by Pb, in order to increase the critical temperature, the current density of such a bismuth superconductor generally remains at about 100 to 200 A/cm 2 at the most. In practice, however, it is necessary to attain a current density of at least 10 times thereof, and such a high current density must be substantially uniformly attained over the longitudinal direction of the superconducting wire. EP-A-0467 238 which is not pre-published prior art according to Art. 54 (3) EPC is concerning a monofilamentary bismuth oxide superconducting wire. EP-A-0 379 960 discloses a method of producing a monofilamentary bismuth oxide superconducting wire. This method comprises a step of covering raw material powder having a 2223 phase with a metal sheath. Summary of the Invention Accordingly, an object of the present invention is to provide a method of preparing a multifilamentary bismuth oxide superconducting wire, which exhibits a higher critical current density. The above problem is solved by the method of claim 1. In the step (2) which is carried out in advance of the step (3), the powder is preferably heat treated in the atmosphere, under decompression or in an inert gas, for the purpose of degassing. In the step (6), the round wires are preferably worked into tape type or flat type wires whose thickness is not more than 20 % of the diameter of the round wires by single deformation processing. According to the present invention, it is possible to finely disperse non-superconducting phases in parts of the 2223 phase in a superconductor portion of the as-obtained bismuth oxide superconducting wire. Thus, current paths are not hindered by the non-superconducting phases but connected with each other, whereby a bismuth oxide superconducting wire exhibiting a high critical current density is obtained. Since superconducting phases are formed with no hindrance by the non-superconducting phases, it is possible to obtain a multifilamentary bismuth oxide superconducting wire which has a substantially uniform critical current density over the longitudinal direction of the wire. According to the present invention, therefore, obtained is a multifilamentary bismuth oxide superconducting wire having a high critical current density substantially uniformly over the longitudinal direction, whereby such a superconducting wire can be applied to a cable or a magnet with no problem. According to the present invention, the aforementioned high critical current density is attained particularly since the powder is prepared to be not more than 1 µm in mean particle diameter in the step (1), the composite is subjected to deformation processing (or pressurization) a plurality of times through the steps (4), (6) and (8) after the step (3), a plurality of said round wires is coated with a metal in steps (5), and the same is heat treated a plurality of times through the steps (7) and (9). When the powder is heat treated at a high temperature in the step (2) for degassing, which is carried out in advance of the step (3), bonding properties at the grain boundaries of the 2223 phases are improved and the superconducting wire is prevented from swelling, whereby the superconductor portion contained in the superconducting wire is prevented from cracking etc. Thus, it is possible to remove a factor for reducing the critical current density in the superconducting wire. When the plurality of round wires is strongly worked into a tape type or flat type wire whose thickness is not more than 20 % of the diameter of the round wires in the step (6) through single deformation processing, a cross-directional flow of the metal-coated powder is further prompted. Since such a cross-directional flow is caused with further constraint by upper and lower rolls or the like as compared with a longitudinal flow, the powder is further densified to provide a higher critical current density. By step (5) of coating a plurality of the said round wires with a metal for obtaining a multifilamentary wire between the steps (4) and (6), the superconductor is sectionally distributed in a plurality of portions in the as-obtained bismuth oxide superconducting wire. Thus, a prescribed critical current density can be provided by a plurality of superconductor portions, and the thickness of each superconductor portion can be reduced. Therefore, it is possible to improve distortion resistance of the critical current density of the bismuth oxide superconducting wire. DESCRIPTION OF THE PREFERRED EMBODIMENT Example Bi 2 O 3 , PbO, SrCO 3 , CaCO 3 and CuO were so blended that Bi, Pb, Sr, Ca and Cu were in composition ratios of 1.83:0.42:1.99:2.02:3.03, and this mixture was repeatedly subjected to heat treatments at 700°C for 8 hours and 800°C for 10 hours and a further heat treatment in a decompressed atmosphere of 1064 Pa (8 Torr) at 760° C for 8 hours, and pulverized after the respective heat treatments. This powder was further heat treated at 860°C for 5 hours. The as-obtained powder contained about 20 % of the 2223 phase in the 2212 phase and non-superconducting phases. This powder was further pulverized in a wet ball mill, to obtain powder of 0.7 µm in mean particle diameter. The as-formed powder was degassed in a decompressed atmosphere of 1064 Pa (8 Torr) at 700°C for 40 minutes. Then the powder was filled up in a silver pipe of 12 mm in outer diameter and 10 mm in inner diameter, and drawn into 1.8 mm in outer diameter. 36 such wires were filled up in a silver pipe of 16.5 mm in outer diameter and 13.5 mm in inner diameter, which in turn was drawn into 1.8 mm in outer diameter and again drawn into 1.4 mm in outer diameter. This wire was rolled into 0.24 mm in thickness through a single process, and heat treated in the atmosphere at 845°C for 50 hours. The as-formed tape type wire was further rolled into 0.2 mm in thickness, and heat treated at 840°C for 50 hours. The as-obtained wire exhibited a critical current density of 24000 A/cm 2 at the liquid nitrogen temperature, to prove that a high critical current density can be attained according to the inventive method. A superconductor portion of this wire was examined to ascertain that the 2212 phase were phase-transformed by the first heat treatment to the 2223 phase, which were grain-grown into 0.1 to 0.3 µm in thickness and several ten um in width and length, and that these were highly densified by the rolling from 0.24 mm to 0.2 mm in thickness and strongly grain-bonded by the second heat treatment.

Claims: A method of preparing a multifilamentary bismuth oxide superconducting wire, comprising the steps of: 1) preparing a powder of not more than 1 µm in mean particle diameter containing a mixture of superconducting phases mainly composed of the 2212 phase of Bi-Sr-Ca-Cu or (Bi, Pb)-Sr-Ca-Cu and non-superconducting phases obtained by calcining and pulverizing raw material powder at least once; and heat treating the powder to contain above 20% of the 2223 phase besides the 2212 phase and non-superconducting phases, 2) first heat treating said powder; 3) coating said powder with a metal; 4) preparing a round wire by deformation-processing the as-formed composite of said powder and said metal coat; 5) coating a plurality of said round wires with a metal; 6) deformation-processing said plurality of said round wires for preparing a multifilamentary tape type or flat type wire; 7) further heat treating said wire in this state under conditions for allowing phase transformation of said 2212 phase contained in said superconducting phases to the 2223 phase and facilitating grain growth; 8) densifying said 2223 phase by deformation processing or pressurization; and 9) bonding the grains of said 2223 phase with each other and finely dispersing said non-superconducting phases by a third heat treatment. A method of preparing a bismuth oxide superconducting wire in accordance with claim 1, wherein said powder is heat treated in the atmosphere, under decompression or in an inert gas for degassing in said step of first heat treating said powder at a high temperature in advance of said step of coating said powder with a metal. A method of preparing a bismuth oxide superconducting wire in accordance with claim 1, wherein said plurality of round wires is worked into a tape type or flat type wire whose thickness is not more than 20% of the diameter of said plurality of round wires by single deformation processing in said step of preparing a multifilamentary tape type or flat type wire.

Inventors: HIKATA TAKESHI, KATO TAKESHI, MUKAI HIDEHITO, SATO KENICHI, SHIBUTA NOBUHIRO, UEYAMA MUNETSUGU, HIKATA, TAKESHI, KATO, TAKESHI, MUKAI, HIDEHITO, SATO, KENICHI, SHIBUTA, NOBUHIRO, UEYAMA, MUNETSUGU, HIKATA, TAKESHI, C/O OSAKA WORKS, KATO, TAKESHI, C/O OSAKA WORKS, MUKAI, HIDEHITO, C/O OSAKA WORKS, SATO, KENICHI, C/O OSAKA WORKS, SHIBUTA, NOBUHIRO, C/O OSAKA WORKS, UEYAMA, MUNETSUGU, C/O OSAKA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1992
Main classification: C04B35
IPC codes: C04B35, H01B13, C01G29, C01G1, H01L39, B21F19, H01B12