Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1996-09-25
2002-06-04
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C174S125100, C505S230000, C505S231000
Reexamination Certificate
active
06397454
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to oxide superconductor assemblies in which oxide superconducting elements are electrically decoupled from one another. The invention further relates to oxide superconductor conductors having high transverse matrix resistivity and methods of their making.
BACKGROUND OF THE INVENTION
Many applications of high temperature oxide superconductors involve conductor performance in time-varying magnetic fields with very stringent AC loss requirements. Mitigation of AC losses in superconducting oxides involves control over filament dimension, conductor matrix dimensions, matrix resistivities perpendicular to the magnetic field (transverse resistivity) and the critical current.
AC losses may be attributed to three different phenomena: hysteresis loss, eddy current loss, and coupling loss. Hysteresis loss describes the effect of a magnetic field on a superconducting filament due to hysteresis magnetization. Eddy current losses represent current loops in the matrix that create a magnetic field which opposes a change in the applied field. Coupling losses are similar to eddy current losses where a significant portion of the current loop may be loss-less inside two or more superconducting filaments in a multifilament composite.
Hysteresis loss is generated only in the superconductor filament and is predominantly proportional to filament dimension. Therefore, to mitigate hysteresis loss, it is desirable to reduce the filament diameter. Filament diameters on the order of 140 &mgr;m or less are considered minimally effective in the mitigation of hysteresis loss in oxide superconducting composites.
Eddy currents are generated only in the matrix and coupling currents involve current loops between two or more filaments connected through the matrix in a multifilamentary composite. Both eddy current losses and coupling losses are inversely proportional to the matrix resistivity. For coupling currents, the relevant resistivity is transverse to the filament axis. In addition, coupling losses depend upon the superconductor filament critical current density and the twist pitch of the filaments in the multifilamentary composite. Thus, AC losses due to coupling and eddy currents may be mitigated by decreasing the twist pitch of the superconducting filaments and increasing the resistivity (particularly the transverse resistivity) of the matrix.
Losses may be effectively mitigated only for twist pitches that are short relative to the diameter of the conductor composite. The twist pitch is defined as the longitudinal distance over which a filament traverses in a complete revolution around the conductor back to its angular starting point. As the twist pitch approaches the wire diameter, the angle of the filaments increases rapidly, as does the torsional strain, and the dependency of loss on twist pitch weakens.
The effective transverse resistivity is complicated by the unusual oxide superconductor grain morphology. Typically, both the overall composite and the superconducting filaments within the composite are aspected, with a cross sectional width, w, to thickness, d, ratio (w/d) on the order of 10 or more. Power losses are inversely proportional to the filament thickness (which scales as 1d) or width depending upon field orientation. To achieve the same loss level in a composite with an aspect ratio of 10, the matrix resistivity must be at least 100 times greater than in a non-aspected conductor.
Thus it is desirable to use fine dimension, twisted filaments in oxide superconducting composites having a high matrix resistivity. Because twisting becomes ineffective for twist pitches that approach the conductor's cross sectional dimensions and introduce filament strain, the most effective means of mitigating AC losses is to increase transverse resistivity of the matrix.
Attempts to prepare oxide superconducting composites with high matrix resistivities have been reported in the prior art. Many of the reported composites use a matrix alloyed with an element selected to reduce the overall conductivity of the matrix, e.g., Ag—X, where X is Au, Al, etc. Shiga et al. in U.S. Pat. No. 5,296,456 describe an oxide superconducting composite in which a ceramic superconductor is sheathed in a noble metal. The noble metal is alloyed with metals such as Zn, In, Cd, Cu, Mg, Be, Ni, Fe, Co, Cr, Ti, Mn, Zr, Al, Ga and rare earth elements to form a low conductivity layer. However, such alloys are not effective in reducing the conductivity of the matrix to levels considered effective for the mitigation of power losses. Further, both the longitudinal and transverse resistivity of the matrix is reduced. This is disadvantageous because it inhibits a high conductivity electrical shunt or current bypass should the superconducting pathway fail.
Sumitomo Electric Co. in EP 638,942 describe a twisted, multifilamentary oxide superconducting composite, in which each individual filament strand is surrounded by a 10 wt % Au/Ag alloy layer having a higher resistivity than the silver matrix. Such a composite suffers from several disadvantages. First, the Au—Ag alloy is of insufficient resistivity to mitigate AC losses in magnetic fields of 0.1-0.2 T, which is of interest in many applications. Secondly, the high resistance layer is directly surrounding the superconducting filament, precluding a more conductive silver matrix to act as a longitudinal electrical shunt in the event that a filament loses superconductivity. In addition, the resultant cable is very expensive to make.
Wagner et al. in U.S. Pat. No. 4,990,491 discloses a multifilamentary low temperature superconducting (LTS) wire (Nb
3
Sn) with an outer NiO coating. Copper or bronze clad filaments are plated with a metallic nickel layer, which is then converted to NiO in the heat treatment used to form the Nb
3
Sn. The architecture of the cable permits insulation of one multifilamentary strand from another, but does not decouple each superconductor filament as is required to mitigate coupling losses. Further, the AC losses in oxide superconductor composites are quite different than in low temperature superconductor (LTS) composites. The oxide superconductor filaments are usually larger and the matrix resistivity is smaller than in LTS composites. Weak links and the anisotropy of the high T
c
superconductor grains produce a critical current density (J
c
) that varies with the local filament chemistry and grain orientation. Thus, composite geometries useful in the LTS field are not readily applicable to the oxide superconductor composites.
Other references report the use of insulating layers or sheets in the construction of a multifilamentary oxide superconductor composite. EP 503,525 describes a multifilamentary composite in which an intermediate layer made up of a high resistance metal, such as CuNi, is placed between multifilamentary tapes making up the composite. Only low level loss reduction is achieved. While this reduced coupling between multifilamentary tapes, it does not satisfactorily reduce losses within each tape.
In a similar fashion, EP 650,205 describes multifilamentary oxide superconductor composite tapes prepared from multiple tape layers spirally wound on a cable form. In order to reduce AC losses due to coupling between multifilamentary tapes, an intermediate insulating layer is wound between individual tape windings. As in EP 503,525, this architecture may reduce coupling between multifilamentary tapes, but it does not reduce losses within each tape.
Thus, the prior art attempting to increase the resistivity between individual superconducting filaments for the mitigation of AC losses has not been satisfactory. There remains a need to provide an oxide superconductor composite which possesses sufficient transverse matrix resistivity to reduce AC power losses, but which retains sufficiently low longitudinal resistivity in contact with the superconducting filament to serve as a conductive shunt. Furthermore, such a composite should be prepared under a cost-effective, manufacturing condition.
SUMMARY OF THE INVENTION
It
Barnes William L.
Christopherson Craig J.
DeMoranville Kenneth
Seuntjens Jeffrey M.
Snitchler Gregory L.
American Superconductor Corp.
Arbes Carl J.
Choate Hall & Stewart
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