Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-05-03
2001-09-18
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C505S110000, C505S110000, C505S433000, C505S704000, C505S739000
Reexamination Certificate
active
06289576
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for the manufacture of filamentary or wire-like superconductors, particularly for the manufacture of filamentary or wire-like superconductors with a small, circular or flat cross-section.
BACKGROUND OF THE INVENTION
Since the discovery of oxidic high temperature superconduction, materials have been known which have superconducting properties at temperatures up to 125° K. These materials are metallic oxide ceramics produced by sintering processes, i.e. very brittle and difficultly workable materials, which can only be brought into the forms necessary for technical applications with extreme difficulty. For many applications in the fields of electrical engineering and microelectronics, as well as for the manufacture of superconducting solenoid coils as energy stores there is a need for filamentary or strip-like conductors. These conductors are preferably to be manufacturable with diameters down to below 1 mm and with lengths up to several kilometers. Such conductors must be sufficiently flexible that they can be bent without suffering damage about a radius of approximately 20 mm. Particularly in connection with magnet construction, it is also important for the conductors to have a very high current density of above 10,000 A/cm
2
in the magnetic field.
Under Prof. Dr. P. Wachter a dissertation or thesis was produced at ETH Zurich (Diss. ETH No. 10213, Joachim Löhle), describing filamentary conductors for high temperature superconduction. It does not in fact relate to a superconducting material shaped in wire or filament fort, but an enveloped, sintered, oxidic, superconducting ceramic material, particularly YBa
2
Cu
3
O
x
or Bi
2
Sr
2
CaCu
2
O
x
. The filamentary superconductors are manufactured in that by multiple pressing, sintering and recomminution from the corresponding oxides, superconducting ceramic material is filled into a tube or sleeve and then the filled tube or sleeve is drawn in a per se known manner into a wire shape. So that the ceramic material has the desired superconducting properties in the completely drawn wire shape, it must again be sintered in this state (final sintering) under flowing oxygen.
Filamentary conductors with flat cross-sections (strips) are produced by rolling the not yet finally sintered, filamentary conductors and final sintering to the strip shape. The strips have higher current densities than the filamentary conductors with a circular cross-section, which is due to the compaction of the ceramic material and the crystallite orientation during rolling.
For drawing filamentary conductors, use is e.g. made of a metal tube with an external diameter of 10 mm and an internal diameter of 8 mm, into which is filled superconducting ceramic material and which is shaped by multiple drawing to a 0.5 mm external diameter and 0.4 mm internal diameter, a cross-sectional reduction of approximately 8% being obtained during each drawing stage.
According to an analogous method filamentary conductors and strips are produced with more than one superconducting core, in that the metal tube is filled with e.g. nine already drawn filamentary conductors in each case enveloping a core instead of being filled with the ceramic material. By filling a tube with already multicore, filamentary conductors the number of cores can be further increased.
In order that during the necessary final sintering the ceramic material can be brought into contact with the flowing oxygen, the sleeve material must have an adequate oxygen permeability. It is therefore proposed for the production of filamentary conductors to use e.g. silver tubes or palladium-alloyed silver tubes. Apart from an adequate permeability for oxygen, silver also has a sufficiently high melting point to allow a usable sintering temperature and is sufficiently chemically stable not to react during the sintering process with either the oxygen or the ceramic material. During the drawing of the filamentary conductors the silver sleeve is cold-worked and therefore becomes harder. Therefore during the aforementioned, exemplified drawing process between drawing stages, soft annealing must take place one or two times.
Highest demands must be made on the filamentary conductor quality, because defects can greatly influence superconduction. The filamentary conductors and, strips described in the aforementioned dissertation are produced by tiresome manual work, with costs which would be unacceptable for technical or industrial uses. The problems occurring for possible industrial manufacture and in particular with an acceptable drawing speed did not form the task of the aforementioned dissertation and are not referred to in any way therein.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method with which it is possible to manufacture the above-described and similar filamentary superconductors on an industrial scale with a satisfactory precision, drawing speed and yield.
The difficulty encountered in the manufacture of the described, filamentary conductors and strips is due to the fact that silver is a relatively soft and ductile material, which offers a correspondingly low resistance to the drawing process. In addition, only part of the material to be drawn is silver (approximately 50% of the cross-section). The core is of a brittle, not ultrafine homogeneous material, which offers a completely different resistance to the working. As the sleeve material must in particular satisfy the requirements for the final sintering (oxygen permeability and chemical stability), it cannot be replaced in random manner by a material more optimum for the drawing process. Therefore irregularities disturbing or reducing the superconduction in filamentary conductors produced according to the prior art can only be avoided by exercising maximum care, which is unachievable in a technical method with acceptable drawing speeds. An optimization of the mechanical deformation or working by drawing and the material properties for the demanding final sintering cannot be achieved through one and the same material. Thus, the fundamental idea is to use two materials selected in optimum manner for the particular task, which together solve the described problem and enable the goal to be achieved.
In the method according to the invention the aforementioned difficulties are overcome in that the envelope filled with the superconducting material and which is to be worked or shaped is placed in a further envelope, so that there is a combination with an inner and an outer envelope, the inner envelope or sintering sleeve fulfilling in optimum manner the conditions for the final sintering and the outer envelope or drawing sleeve allowing a problem free drawing process, which is rapid compared with what has been hitherto achieved. The outer sleeve or envelope is so designed that during drawing it takes up most of the tensile force, in no way modifies or endangers the inner envelope and following drawing can be removed again from the wire shape produced.
An example of a procedure for the careful removal of the drawing sleeve from the sintering sleeve is an etching process.
Such a drawing sleeve can e.g. be a soft annealed steel tube (St 35), into which is introduced the sintering sleeve filled with the ceramic material in the form of a silver or silver alloy tube. The sleeve combination is subjected to a drawing process on a standard wire drawing machine and for each drawing stage there is once again a cross-sectional decrease of about 8%. Like the silver sintering sleeve)the outer drawing sleeve also becomes hard and brittle due to the cold working during drawing. Therefore the drawing process is interrupted at least once and the steel, now thin-walled drawing sleeve is etched away and the silver sintering sleeve is annealed at approximately 280° C. The sintering sleeve with the superconducting content is then introduced into a new, soft annealed drawing sleeve and then undergoes further drawing stages. The completely drawn product, in which the sintering sleeve enveloping the core has the des
Gramm Rolf A.
Vogt Oscar
Wachter Peter
Arbes Carl J.
Lowe Hauptman & Gilman & Berner LLP
VOCO Draht AG
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