Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-03-26
2003-01-28
Barrera, Ramon M. (Department: 2832)
Metal working
Method of mechanical manufacture
Electrical device making
C335S216000, C174S125100, C505S884000, C505S887000, C505S924000
Reexamination Certificate
active
06510604
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates generally to superconducting cables experiencing reduced strain, and more specifically to such superconducting cables that are composed of A-15 materials.
2. Discussion of the Related Art
Superconductors are phases that exhibit extremely low (practically zero) electrical resistance below their critical temperature and critical magnetic field. Superconducting cables have been used in a variety of applications, predominantly in superconducting electromagnetic magnets in which a superconductor is wound into a coil. Superconducting magnets have been used in applications including, for example, devices used for nuclear magnetic resonance (NMR) spectroscopy, magnetic resonance imaging (MRI), superconducting magnetic energy storage (SMES) and magnetic mine sweeping, as disclosed in, for example,
Superconducting Magnets,
M. N. Wilson, Oxford University Press, New York, N.Y. (1983) (hereinafter “Wilson”) and
Case Studies in Superconducting Magnets,
Y. Iwasa, Plenum Press, New York, N.Y. (1994) (hereinafter “Iwasa”).
To wind a coil, of course, the material defining the coil must be bent. The smaller the coil, the more the material defining the coil must be bent. Since superconducting magnets in many cases are made of a relatively small coil, the superconducting material defining the coil must be bent significantly. Even in the case of relatively large coils, bending superconductors to make coils according to prior art methods can be problematic due to the relatively large cross-sectional size of the superconductors typically used in these applications. One reason that superconducting magnets might desirably be small is in NMR applications where the intensity of the magnetic field is critical. Stored energy of the magnet system and its overall cost scale directly with the size of the bore of the superconducting magnet where intense magnetic fields are produced. In general, for two superconducting magnet systems designed and fabricated to produce a given magnetic field strength, the system with a smaller superconducting magnet bore will be less costly to fabricate and operate. In order to wind a superconductor around a magnet bore, the superconductor must be bent significantly. The lower limit on the radius of curvature to which a superconductor, such as in a superconducting cable, can be wound within a magnet system, such as an NMR, MRI, or other practical magnet systems, is usually determined by the irreversible strain (defined below) of the superconductor.
Superconducting wires are typically comprised of a plurality of superconducting filaments disposed within a matrix that is typically formed of an electrically conducting material, such as metals and metal alloys. Typically, superconducting cables are formed of a plurality of intertwined wires including superconducting wires. Superconducting cables are often used for large current applications and may include additional wires that are not superconducting in order to provide physical support to the cable and/or to act as a current stabilizing medium should superconductivity of any of the superconducting wires be interrupted.
When a superconductor or superconducting cable is bent, strain is induced on the superconducting filaments. Since many superconductors are brittle, bending them can cause them to break. That is, in winding superconducting coils, if the strain surpasses the irreversible strain of the material from which the superconducting filaments of the cable are formed, the potential magnetic field of the system can not be achieved. Hence, for a given superconductor or superconducting cable, there is a lower limit on the radius of curvature to which the superconductor or superconducting cable can be wound within the magnet system, dependant on the irreversible strain of the superconducting filaments within the superconductor or superconducting cable.
Known superconductors must be cooled to be made superconducting and must be kept cool to remain superconducting, for example, in a bath of liquid helium. The intensity of the magnetic field produced by a superconducting magnetic generally scales with the number of turns of the superconductor or superconducting cable present. Generally, a superconductor, such as superconducting wire or cable, is wound around a support structure or coil form a number of times in order to produce a desired magnetic field. In order to eliminate undesirable electrical current flow between the windings and/or turns, superconductors are advantageously electrically insulated. In conventional systems involving brittle superconductors, electrical insulation of the superconductors, such as superconducting wires or cables, is typically performed after the superconductor is wound around the support or coil form. This method is limited in its efficiency because it cannot always optimize the ratio of conductor to non-conductor present in the windings.
A15 superconductors are known intermetallic compounds (defined below) that have relatively high critical temperatures and critical magnetic fields compared to other conventional superconducting alloys, so it is desirable to employ A15 superconductors in many magnet systems, particularly such systems that are designed for use with magnetic fields of above 10 Tesla, typically from about ten to about 24 Tesla. While A15 superconductors can provide certain superior performance characteristics, these are inherently brittle and have relatively low irreversible strains. Therefore, monolithic A15 superconductors (those which comprise a continuous medium or whose members are bonded together) typically cannot be wound to a small enough radius of curvature to be useful for winding into coils in fabricating many magnet systems. In an attempt to overcome this problem, a “wind-then-react” (or “wind-and-react”) method has been commonly used to incorporate A15 superconductors into magnet systems. As described in Wilson and Iwasa, the “wind-then-react” method involves winding unreacted cables around a support or coil form and subsequently heating the entire magnet system to cause a reaction within the unreacted cables to form superconducting filaments (filaments comprising a superconducting phase) within the cables. However, this approach has several disadvantages in many cases. For example, since heating occurs after the cable is wound within the magnet system, the various components of the magnet system should be compatible with the temperatures used during the formation of the superconducting phase (e.g., about 925 K for Nb
3
Sn). This can severely limit the choice of materials from which various components of the magnet system can be formed. For example, the magnet system often cannot include aluminum or its alloys since these melt at the temperatures used during formation of the superconducting filaments. Another important disadvantage is the difficulty and expense of applying insulation to a magnet winding to effectively coat the individual conducting wires or cables to prevent electrical current flow between the windings/turns. Hence, the “wind-then-react” method can result in higher cost and complexity in preparing the magnet system, while resulting in a system that may offer inferior performance.
An alternative to the “wind-then-react” method is the “react-then-wind” technique. As discussed in Wilson and Iwasa, the “react-then-wind” method involves forming the superconducting filaments within superconducting cables by heat reacting and subsequently winding the cables into the magnet system. Since heating of the cables occurs prior to their incorporation into the magnet system, the “react-then-wind” method allows for a broader range of materials from which the components of the magnet system can be formed. However, the low irreversible strain of many superconductors has precluded the broad use of the “react-then-wind” method or systems with small bores with these superconductors. Instead, the “react-then-wind” method has typically been confined to systems such as superconductors having tape-like cross-sec
Massachusetts Institute of Technology
Wolf Greenfield & Sacks P.C.
LandOfFree
Superconducting cables experiencing reduced strain due to... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Superconducting cables experiencing reduced strain due to..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Superconducting cables experiencing reduced strain due to... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3028159