Process for producing a multifilamentary superconducting tape

Details Process for producing a multifilamentary superconducting tape Process for producing a multifilamentary superconducting tape

1996-05-24

2001-08-14

Arbes, Carl J. (Department: 3729)

Metal working

Method of mechanical manufacture

Electrical device making

C174S125100, C505S110000, C505S430000, C505S431000

Reexamination Certificate

active

06272730

ABSTRACT:

In a general aspect, this invention relates to a process for producing a multifilamentary conductive tape of the type comprising a plurality of layers of a high-temperature superconducting material incorporated in mutually spaced relationship within a metal matrix.
More particularly, the invention relates to a process wherein a plurality of metal tubes, including a precursor of a superconducting material, are subjected to at least one plastic deformation step and at least one sintering step.
In the following description and in the subsequent claims, the term: high-temperature superconducting material, is used to indicate any material, such as for instance ceramics based on mixed oxides of copper, barium, yttrium or bismuth, lead, strontium, calcium, copper, thallium and mercury, comprising a superconducting phase having a substantially null resistivity at a temperature, defined as critical temperature or T
c
, higher than 20° K (about −253° C.).
The term: precursor, is used to indicate in the following either any compound or mixture of compounds capable to form—subsequent to one or more heat treatments—a superconducting material as defined hereinabove, or a compound or mixture of compounds wherein the superconducting phase has already been formed.
Lastly, the term: electrically conductive element, is used to indicate in the following any element capable of carrying electric current, such as for instance cables or windings for magnet coils or for electric equipment in general.
As is known, in the field of electric power transmission, one of the more difficult problems to solve is that of making use of the so-called superconducting materials increasingly advantageously, either from a technological or from an economic point of view.
Although these materials are well known, in fact, their diffusion has been limited so far to some specific practical applications, such as for instance the manufacture of magnets for NMR apparatuses, for which cost is not a discriminating factor.
Actually, the cost savings due to the smaller power dissipated by the superconductor is so far more than counter-balanced by the costs of the refrigeration necessary to keep the superconductor below its critical temperature.
In order to solve the aforementioned problem, while part of the research has been oriented towards the experimentation for new superconducting materials, another part has attempted to improve either the characteristics of the existing materials, or the performances of the conductors incorporating the materials presently available.
As to the geometric characteristics of the conductors, it has been observed, for instance, that the optimal geometry is that of the thin tape having a thickness of from 0.05 to 1 mm.
In this case, in fact, the conductor including the ceramic superconductor—which is known to be very fragile—achieves on the one hand an improved resistance against the various mechanical bending stresses to which it undergoes during all the operations connected to the manufacture, handling and installation of the cable containing the same, and, on the other hand, achieves better performances as to its critical current density, because of the more favourable orientation and compactness degree of the superconducting material.
For various reasons and in particular to achieve a still greater mechanical resistance, the above conductors are generally constituted by a plurality of tapes, each formed by a core of superconducting material enclosed in a metal casing, generally silver or silver alloys, coupled to one another to produce a multifilamentary composite structure.
According to the prevailing methodology, known in the art under the term: “powder-in-tube”, this multifilamentary composite structure is obtained starting from a plurality of metal tubes filled with a suitable powder precursor, which are enclosed in their turn in another external metal tube or a billet, so as to obtain a compact bundle of tubes which undergo several subsequent plastic deformation treatments, of extrusion and/or drawing first and then of rolling and/or pressing, until the desired tape-shaped structure is obtained.
Between a rolling treatment and the following one, the tape being processed is subjected to one or more heat treatments, in order to promote the formation of the superconducting ceramic material starting from its precursor and, above all, to promote its sintering, i.e. the mutual “welding” of the grains of the superconductor powder.
The need of incorporating the superconducting material into a plurality of metal tubes, and, especially, the need of enclosing the latter in an external casing in order to carry out the above-identified plastic deformation and sintering treatments on the multifilamentary composite structure, however, limit to a large extent the overall quantity of superconducting material that can be incorporated in a tape, with a corresponding limitation of the maximum current density achievable in conditions of superconductivity or technical critical current density “J
e
”, defined as the ratio between the critical current and the total cross section of the multifilamentary composite structure.
The technical problem underlying the present invention is therefore to provide a multifilamentary conductive tape, useable, for instance, as a superconducting element within a cable, which tape shows an improved technical critical current density J
e
using the same quantity of superconducting material.
According to a first aspect thereof, the present invention therefore relates to a process for producing a superconducting multifilamentary tape of the aforementioned type, which is characterized in that said plastic deformation step is carried out on each individual tube of said plurality of tubes, so as to obtain a corresponding plurality of elementary tapes, structurally independent from one another, and in that it comprises at least a coupling step of said elementary tapes, including a heat treatment at a temperature and for a time sufficient to cause a stable mutual connection between said tapes.
According to the invention, in fact, it has surprisingly been observed that a tape having the required multifilamentary structure may be obtained—without using any external containing casing—by submitting each tube, filled with superconducting material or a precursor thereof, to a first plastic deformation treatment, so as to obtain a plurality of structurally independent elementary tapes which are then irreversibly coupled and welded to one another by means of a heat treatment at a temperature and for a time sufficient to cause a stable mutual connection thereof.
In contrast to the constant teaching of the prior art, in fact, it has been observed that, even without the use of any external containing casing, such heat treatment allows to weld the tapes to one another so effectively that all the following plastic deformation and/or sintering treatments, as well as all the subsequent operations carried out on the multifilamentary tape may be performed without any problem of separation or mutual sliding of the individual starting tapes.
According to the invention, it has been noticed that—when the metal matrix is constituted by silver or silver alloys—optimum adhesion characteristics between the tapes incorporating the superconducting material or a precursor thereof may be achieved when the tapes—stacked, arranged side by side or anyhow in contact with one another—are subjected to a heat treatment at a temperature of not less than 600° C. for a time of not less than 50 hours.
According to a preferred embodiment, the tapes incorporate a precursor of the chosen superconducting material, and the step of coupling the tapes is advantageously performed simultaneously with one of the sintering steps necessary to promote the growth of the oriented crystalline structure of the final superconducting material.
It has been observed, in fact, that the temperature and the time necessary to carry out sintering of the precursor may ensure those minimum values necessary to obtain an adequate adhesion between th

Affiliated with

Also associated with

Rating

Process for producing a multifilamentary superconducting tape does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Process for producing a multifilamentary superconducting tape, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for producing a multifilamentary superconducting tape will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2505074