Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-05-03
2003-03-11
Dunn, Tom (Department: 1725)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S107000, C505S220000, C505S231000
Reexamination Certificate
active
06531233
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the use of superconducting wires and more particularly relates to methods of joining such superconducting wires.
2. Description of the Prior Art
Superconductivity is the phenomenon where, when a particular material, such as a wire, is subjected to successively colder temperatures, it undergoes a state transition where all electrical resistance disappears, i.e. the material can conduct electricity without generating any heat, i.e., loss of energy. At this stage the material is said to have become a superconductor. This transition only occurs in specific metals/alloys and compounds. The state of superconductivity for a given superconductor is a function of its temperature, background magnetic field, and transport electric current. A superconductor carries current without resistance when it is below its critical temperature (Tc) and is in an environment, often referred to as background, where the magnetic field is less than its critical magnetic field (Bc). The limit of superconducting transport current in a superconductor depends on how much the operating temperature (To) and background magnet field (Bo) are below its Tc and Bc. The lower the To and Bo, the higher the limit in transport current.
There are currently two known types of superconductors: Type I and Type II. Compared to the Type I, the Type II superconductors carry more current at higher temperatures and exhibit an “upper critical magnetic field” (Bc2) which depending on the material can be as high as a few tens of tesla (T). [see for example, Stability of Superconductor, by Lawrence Dresner, Plenum Press, New York, N.Y., 1995, Chapter 1]. Practically all superconductors in commercial use are Type II.
A useful class of product made of superconductors is superconducting wires. Superconducting wires, and cables made from them, are widely used in fabrication of electromagnets, hereafter referred to as magnets, that generate magnetic fields higher than 1 tesla.
Magnets are produced by winding wires in various coil geometries. When electric current passes through the winding, a magnetic field is produced. When the wire is a superconductor, no electric power is lost in the magnet and it is called a superconducting magnet.
A current market for superconducting magnets is in devices used for Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR) spectroscopy. These devices require constant and stable magnetic fields.
A constant and stable magnetic field can be produced by flow of current through a superconducting magnet in which the ends of the superconducting wires that make up the magnet are joined together by a superconducting joint. In an ideal superconducting joint, the transport current from one wire must enter into another without electrical resistance. In such a case, current circulates in the windings of the superconducting magnet without appreciable loss of energy in the magnet, or the joint, and therefore the magnetic field that the magnet produces remains constant and the superconducting magnet is said to be in a “persistent mode” providing the desired constant and stable magnetic field. Thus, superconducting joints are vital components of superconducting magnets that are used in MRI and NMR devices.
For reasons that relate mainly to stability of superconductors, superconducting wires used in most magnet application are multifilamentary (MF) composites [see for example Stability of Superconductor, by Lawrence Dresner, Plenum Press, New York, N.Y., 1995]. In a multifilament superconductive wire the superconducting current is carried by superconducting filaments.
FIG. 1
illustrates a cross-section of superconducting filaments
24
spaced apart from one another within matrix
28
forming multifilamentary wire
20
such as used in most superconducting magnets in use today which filaments can be made from Niobium-Titanium (Nb—Ti) alloy. The Tc and Bc
2
for Nb—Ti are about 10 Kelvin (K) and about 10 T, respectively. Nb—Ti alloy is ductile and basically insensitive to strain and its use in fabricating MF wires, and subsequently in a magnet, is straightforward and comparatively less expensive than other materials.
Superconducting magnets for operation at fields higher than about 10 T rely principally on the use of type A15 superconductors. Among the A15 superconductors the Nb
3
Sn based wires are most practical for large scale production. This is basically due to the fact that fabrication of Nb
3
Sn conductors is less complicated, and more economical than others. Almost all operating A15 magnets to date have used Nb
3
Sn conductors. The Tc and Bc
2
for Nb
3
Sn are about 18 K and about 23 T, respectively. With proper alloying of the Nb
3
Sn phase, for example by Ta, its critical properties can be improved. Other A15 superconductors such as Nb
3
Al that have better superconducting properties than Nb
3
Sn are under development.
Nb
3
Sn, like other A15 type superconductors, is an intermetallic compound and is inherently brittle. Therefore Nb
3
Sn does not lend itself to normal conductor fabrication methods where a given material undergoes significant plastic deformation. For most applications in magnet technology, Nb
3
Sn superconductors are produced by a two-step process in which a multifilamentary composite wire that contains Nb and Sn in separate regions is formed into wire and then, during a subsequent reaction heat treatment at, for example, 650C-750C, the Nb
3
Sn is formed by solid state reaction. Fabrication of Nb
3
Sn MF wires and their use in magnets is relatively more difficult and expensive than MF Nb—Ti wires.
A high field superconducting magnet for “high end” NMR spectrometer, for example 14T for a 600 MHz device, is typically comprised of a number of nested coils (solenoids). Please refer, for example, to J. E. C. Williams, S. Pourrahimi, Y. Iwasa, and L. J. Neuringer,“A 600 MHz Spectrometer Magnet,” IEEE Trans. Vol. Mag-25, pp. 1767-1770, 1989. Currently, for both economy and practicality, a high field superconducting NMR magnet uses a few coils that use Nb—Ti MF wires and a few coils that use Nb
3
Sn MF wires. The Nb—Ti coils combine to produce a field of up to about 10T and the Nb
3
Sn coils add the remaining increments of the magnetic field.
FIG. 2
shows a simplified drawing of the nested coils of Nb—Ti
12
and Nb
3
Sn
14
that make up such a high field NMR magnet. A typical high field NMR magnet requires a number of superconducting joints
16
that connect through superconducting connectors
18
the nested coils to form a single unit magnet system. Magnets used in MRI devices also require superconducting joints that connect the multiple superconducting magnet modules of an overall superconducting magnet systems. Superconducting joints are also key components of “persistent switches” that are used in both NMR and MRI devices. A description of a persistent switch is given for example in Superconducting Magnets, M. N. Wilson, Oxford University Press, New York, N.Y. (1983), Chapter 11.
With improvements in economy of fabrication of A15 superconductors and the advantage that they can operate at relatively higher temperatures, next generation high field NMR magnets, or MRI magnets, may use A15 superconductors and coils exclusively.
In most conventional MF superconducting wires
20
, as for example in
FIG. 1
, the superconducting filaments
24
are disconnected from one another for the most part. Therefore, to fabricate a superconducting joint between two MF wires often superconducting filaments
24
of the wires
20
are accessed at the wire ends by separating filaments
24
from their matrix
28
. This is often done by dissolving matrix
28
. In a conventional superconducting joint between two MF superconducting wires the current transfers from the filaments of one wire to the filaments of the other wire, at wire ends, through a superconducting medium. Such connection can be achieved, for example, by dissolving the metallic matrix at the wire ends and encapsulating the fila
Pourrahimi Nadder
Pourrahimi Shahin
Cooke Coleen P.
Dunn Tom
Nitkin William
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