Electricity: conductors and insulators – Conduits – cables or conductors – Superconductors
Reexamination Certificate
2001-09-18
2003-05-20
Paladini, Albert W. (Department: 2125)
Electricity: conductors and insulators
Conduits, cables or conductors
Superconductors
C505S230000, C505S231000, C505S704000
Reexamination Certificate
active
06566609
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxide superconducting wire and a method of preparing the same, and a cable conductor which is formed by assembling such oxide superconducting wires, and more particularly, it relates to an oxide superconducting wire which can carry a heavy current in ac application and a method of preparing the same, and a cable conductor which is formed by assembling such oxide superconducting wires.
2. Description of the Background Art
The principal feature of an oxide superconductor resides in that the same is in a superconducting state also at a temperature exceeding the liquid nitrogen temperature. Therefore, a wire consisting of such an oxide superconductor is expected for application to a superconducting device, as a material which can be used under cooling with liquid nitrogen.
The inventors have developed a tape-shaped Bi-based Ag-coated multifilamentary wire, which is prepared from filaments of an oxide superconductor with a stabilizer of silver. A Bi-based Ag-coated wire can be prepared by charging a metal pipe with raw material powder serving as a precursor for a Bi oxide superconductor, wire-drawing the pipe and thereafter repeating rolling and a heat treatment a plurality of times.
On the other hand, a multifilamentary wire can be prepared by charging metal pipes with raw material powder, wire-drawing the same, engaging a plurality of such wires in a metal pipe for forming a multi-filamentary substance, further wire-drawing the same and thereafter repeating rolling and a heat treatment a plurality of times.
Among such preparation steps, the rolling step is effective for improving the orientation of crystal grains in the Bi superconductor having a plate-type crystal structure, strengthening bonding between the crystal grains and improving the density of the filaments, and regarded as being indispensable for attaining a high critical current density in preparation of a Bi-based Ag-coated wire.
Further, the aspect ratio of a section of the wire is increased by this rolling, whereby the aspect ratio of a section of each filament is also increased. This is advantageous for growth of the plate-type crystals, and a high critical current density is consequently attained.
On the other hand, the heat treatment step for the purpose of sintering is also indispensable for forming the superconductor, attaining crystal growth and strengthening bonding between the crystal grains, since the oxide superconductor is ceramics.
The Bi-based Ag-coated wire which is prepared in the aforementioned manner is excellent in bending property and capable of preparing a long wire having a critical current density exceeding 10
4
A/cm
2
, and hence the same is expected for application to a superconducting cable or magnet.
In ac application of such an oxide superconducting wire, however, ac loss resulting from a fluctuating magnetic field in driving comes into question. In a cable conductor which is formed by assembling superconducting wires, on the other hand, there arises a new problem to be solved such as a drift phenomenon resulting from ununiformity between impedances of the wires, which cannot be caused in dc application. Due to a drift caused in such a manner, further, loss upon formation of the conductor is disadvantageously increased beyond the sum of ac loss values of strands.
As to such problems caused in ac application, various countermeasures have generally been studied in relation to metal superconducting wires, for example. In more concrete terms, countermeasures of arranging high resistance barrier layers around or between filaments, preparing an extra-fine multifilamentary wire from superconducting filaments, increasing the specific resistance of a matrix and the like are studied in order to reduce ac loss. In order to suppress a current drift by uniformalizing the impedances of the filaments or wires in a conductor for an ac magnet, on the other hand, countermeasures of twisting the filaments or wires, dislocating the wires or filaments and the like are studied.
In order to attain a heavy current, further, a countermeasure of further twisting primary stranded wires each prepared by twisting superconducting strands to attain a flat-molded multinary structure or the like is studied.
While a countermeasure of further twisting primarily stranded wires to attain a multinary structure or the like must be taken also in employment of the aforementioned Bi-based Ag-coated wire for ac application similarly to the metal superconducting wire, however, it is impossible to implement the aforementioned multinary structure through an oxide superconducting wire by a method which is absolutely identical to that for the metal superconducting wire. This is because a Bi-based Ag-coated multifilamentary wire indispensably requires rolling and sintering processes as described above, while no such rolling and sintering steps are required for preparing a metal superconducting wire.
Namely, it is difficult to twist wires of a Bi oxide superconductor after sintering, since the Bi oxide superconductor is ceramics which is weak against bending distortion. Even if such wires can be twisted, a high critical current density cannot be attained. Further, it is difficult to twist wires in which aspect ratios of sections are increased by rolling. Even if such wires can be twisted, a number of clearances are defined in the stranded wire as compared with that prepared by twisting round wires, and a high critical current density cannot be attained.
SUMMARY OF THE INVENTION
In order to solve the aforementioned problems, an object of the present invention is to provide an oxide superconducting wire which maintains a high critical current density and has a small current drift with small ac loss when the same carries an alternating current and a method of preparing the same, and a cable conductor which is formed by assembling such oxide superconducting wires.
According to an aspect of the present invention, an oxide superconducting wire is provided. This oxide superconducting wire is a flat-molded stranded wire which is formed by twisting a plurality of metal-coated strands consisting of an oxide superconductor, and is characterized in that the flat-molded stranded wire has a rectangular sectional shape, and a section of each strand forming the flat-molded stranded wire has an aspect ratio of at least 2.
Throughout the specification, the term “aspect ratio” indicates the ratio of the thickness to the width in a cross section of the oxide superconducting wire.
Superconducting filaments provided in the strands can be brought into flat shapes having a high aspect ratio by setting the strands at an aspect ratio of at least 2. Consequently, a superconducting wire having a high critical current density can be obtained. In particular, the aspect ratio of the superconducting filaments is preferably around 10. The section of each strand preferably has an aspect ratio of not more than 20. It is difficult to increase the aspect ratio of the strands beyond 20 in case of twisting and molding the same.
According to the present invention, the strands are completely dislocated due to the twisting, whereby the impedances of the strands forming the stranded wire can be equalized to each other.
According to the present invention, further, the stranded wire has a rectangular sectional shape. Thus, the wire can be densely wound to be advantageously compacted when the same is applied to a coil or a cable.
Preferably, the metal coatings of the strands consist of silver or a silver alloy, and coating layers consisting of a material having higher resistance than silver are provided on the outer peripheries of the metal coatings.
Due to the presence of such coating layers, the strands can be prevented from bonding in the stranded wire, so that ac loss is effectively reduced.
The material having higher resistance than silver is prepared from a high resistance metal material or an inorganic insulating material, for example.
When no such coating layers consisting of a materia
Fujikami Jun
Ohmatsu Kazuya
Saga Nobuhiro
Sato Ken'ichi
Paladini Albert W.
Pennie & Edmonds LLP
Sumitomo Electric Industries Ltd.
LandOfFree
Oxide superconducting wire does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Oxide superconducting wire, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Oxide superconducting wire will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3043646