Electricity: conductors and insulators – Conduits – cables or conductors – Superconductors
Reexamination Certificate
1997-05-05
2001-09-04
Paladini, Albert W. (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Superconductors
C174S12900B, C505S231000, C505S431000
Reexamination Certificate
active
06284979
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to cabled conductors comprising superconducting ceramics and to a method for using them. Superconductors have gained increasing attention for having the potential to improve the efficiency of electric power and magnetics applications. When held below the critical temperature, current and magnetic field which defines its superconducting state, every superconductor is able to carry DC currents with very little energy loss. Equally important for most conductor applications is the ability of superconductors to carry very high currents, with current densities of thousands of times that of conventional copper conductors. Certain of the ceramic superconductors have the potential to maintain these properties at cryogenic temperatures in the general range of the boiling point of nitrogen. However, several different types of resistive losses can occur in ceramic superconductors. Unlike low transition temperature superconductors, which transition abruptly between the high performance superconducting and low performance normal, i.e., resistive, states if one of their critical values is exceeded, the ceramic superconductors do not. Consequently, for ceramic superconductors, a measurement procedure, known as the offset criterion or the electric field (E) criterion, described, for example, in
High T
c
superconductors and critical current measurement,
Cryogenics, Vol 30, pp 667-677 (August 1990), and
Offset criterion for determining super
-
conductor critical current,
Appl. Phys. Lett. 55(9) (28 August 1989), both of which are herein incorporated in their entirety by reference, is often used to establish the the critical current value, i.e. the current value at which the transition between superconducting and non-superconducting states is considered to occur in ceramic superconductors. This procedure defines the critical current density (J
c
) as the current density (J) where the tangent to the electric field (E) vs J curve (for a specified temperature and magnetic field level) at a given electric field level, such as 1 &mgr;V/cm, extrapolates to zero electric field. For brevity, the procedure is typically referenced by the value of its electric field criterion. In the transition regime, resistive losses gradually increase to their non-superconducting values. Above the critical current value, the transition regime is known as the flux flow state.
In time varying magnetic fields or currents, all conductors, including ceramic superconductors in their superconducting and flux flow states, have losses which may include hysteresis and various types of coupling losses. These vary with frequency, AC and DC current amplitude and conductor geometry. Periodic multifilamentary, multistranded structures with short repeat lengths have been shown to minimize AC losses for low transition temperature superconductors, as described, for example, in “
Superconducting Magnets
” by Martin Wilson (1983,1990), pp 197, 307-309, which is herein incorporated by reference, and in conventional copper conductors. Both low transition temperature supercondutors and conventional copper conductors are commonly fabricated into well-known cabled forms, such as Litz cables, Rutherford cables (a type of Litz cable), Roebel cables, or braids for use in time-varying magnetic fields or currents. U.S. Pat. No. 3,764,725 issue Oct. 9, 1973 to Kafka, U.S. Pat. No. 4,857,675 issued Aug. 15, 1989 to Marancik et al, U.S. Pat. No. 1,144,252 issued Jun. 22, 1915, all of which are herein incorporated in their entirety by reference, teach the use of braided Litz, Rutherford, and Roebel cabled forms, respectively. Typically, for low transition temperature superconductors, techniques such as twisting and bending are used to transpose both the filaments in a conductor strand (to minimize filament coupling losses) and the strands in a multistrand conductor (to minimize strand coupling losses) about their central longitudinal axes.
It has been proposed that similar periodic geometries would minimize certain types of AC losses in ceramic superconductors so they would be desirable for any electrical or magnetic application involving time-varying currents or magnetic fields. For example, U.S. Pat. No. 5,038,127 issued Aug. 6, 1991 to Dersch, describes two periodic arrangements of coated conductors intended to reduce eddy current losses.
However, ceramic superconductors have physical limitations, namely anisotropy and low critical strain values, which typically create very high resistive losses in long lengths of high winding density, tightly cabled conductor. Critical strain is rarely an issue for conventional metal cables. Anisotropy is not a design constraint for cables made of either conventional metal or low transition temperature superconductors.
The superconducting ceramics which have shown greatest promise for electrical and magnetic applications at relatively high temperatures are anisotropic superconducting compounds which require texturing in order to optimize their current-carrying capacity. The current-carrying capacity of any composite containing one of these materials depends significantly on the degree of crystallographic alignment, or “texturing”, and intergrain bonding of the superconductor grains induced during the composite manufacturing operation. Suitable texturing methods, all of which are well known in the art, include, for example, various heat treatments to obtain reaction-induced texturing, various deformations to obtain deformation-induced texturing, growth on a textured substrate material, and magnetic alignment. For example, known techniques for texturing the two-layer and three-layer phases of the bismuth-strontium-calcium-copper-oxide family of superconductors are described in Tenbrink, Wilhelm, Heine and Krauth,
Development of Technical High
-
Tc Superconductor Wires and Tapes, Paper MF
-1,
Applied Superconductivity Conference, Chicago
Aug. 23-28,1992), and Motowidlo, Galinski, Hoehn, Jr. and Haldar,
Mechanical and Electrical Properties of BSCCO Multifilament Tape Conductors, paper presented at Materials research Society Meeting,
Apr. 12-15, 1993, Kase et al,
IEEE Trans. Mag.
27(2), 1254(1991), and U.S. Ser. No. 08/041,822 filed Sep. 8, 1994, entitled “Torsional Texturing of Oxide Superconducting Articles”, all of which are herein incorporated in their entirety by reference. Some techniques for forming and texturing the yttrium family of oxide superconductors are described, for example, in L. J. Masur et al,
Physica C
230 (1994) 274-282, M. Fukutomi et al,
Physica C
219 (1998) 333-339, and V. Chakrapani, D. Balkin, P. McGinn,
Applied Superconductivity, Vol.
1,
No.
1/2,(1993), pp. 71-80, all of which are herein incorporated in their entirety by reference. Suitable final heat treatment processes for BSCCO 2223, which are believed to contribute to intergrain bonding via partial melting and crack healing are described, for example, in copending applications U.S. Ser. No. 08/041,822 filed Apr. 1, 1993 and entitled “Improved Processing for Oxide Superconductors”, U.S. Ser. No. 08/198,912, filed Feb. 17, 1994 and also entitled “Improved Processing for Oxide Superconductors”, and in U.S. Ser. No. 08/553,184, filed Nov. 7, 1995 and entitled “Processing of Oxide Superconducting Cables”, all of which are herein incorporated in their entirety by reference.
The desirable crystallographic structure of well-textured ceramic superconductors causes them to have extremely anisotropic current carrying capability, with the highest current flowing in the directions lying in the crystallographic plane containing the a and b direction vectors of the aligned grains, or in other words, orthogonal to the c direction of each grain. Critical current and critical magnetic field may be as much as an order of magnitude lower in a “bad” direction of a well-aligned oxide superconductor than in a “good” direction lying in the crystallographic plane containing the a and b direction vectors of the grains. It is conventional to refer to the set of directions in a crystallographic pla
Barnes William L.
Malozemoff Alexis P.
Otto Alexander
Riley, Jr. Gilbert N.
Seuntjens Jeffrey M.
American Superconductor Corporation
Fish & Richardson P.C.
Paladini Albert W.
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