Electricity: conductors and insulators – Conduits – cables or conductors – Superconductors
Reexamination Certificate
1999-03-29
2002-09-10
Paladini, Albert W. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Superconductors
C174S015500, C029S599000
Reexamination Certificate
active
06448501
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to superconductors, and more particularly to an armored spring-core superconducting cable and method of construction.
BACKGROUND OF THE INVENTION
Superconducting materials have the unique material property of having zero electrical resistance. In other words, superconducting materials can conduct electricity with no loss of energy. Superconducting materials exhibit this unique material property only when cooled below their respective critical temperature. Commercial applications of superconducting materials are generally limited to high temperature superconducting materials, which have a higher critical temperature than low temperature superconducting materials. High temperature superconducting materials, such as Bi-2212 and Bi-2223, are generally perovskites, a crystalline ceramic in which a combination of metal atoms are arranged in a crystal lattice containing planes of oxygen atoms. Each metal atom is chemically bonded to at least one oxygen atom, so that the overall material is an oxide ceramic. The superconducting material Bi-2212 has the chemical composition Bi
2+x
(Sr,Ca)
3
Cu
2
O
8+d
, and the superconducting material Bi-2223 has the chemical composition Bi
2+x
(Sr,Ca)
4
Cu
3
O
10+d
. The slight shifts x, d from integer composition refer to the empirical fact that the best properties are obtained when there is a slight excess of the indicated atom in the crystal.
In high-temperature superconductors, superconducting current flows in the planes of oxygen atoms within each crystal grain of the superconducting perovskite crystal. This leads to the necessity to achieve compaction of the grains within the conductor and alignment of the grains so that current can be transferred from grain to grain along the conductor.
The desired perovskite phase in high temperature superconductive materials, such as Bi-2212 and Bi-2223, is very difficult to form. If any of the fabrication conditions, such as the relative proportions of the several metal atoms, i.e., stoichiometry, the temperature cycle, and the oxygen content, vary from optimum conditions, other non-superconducting phases will be formed and the material will not be superconducting.
In many commercial applications, the superconducting material is packed into sheathes, i.e., tubes, of silver metal and the sheathes are formed into strands, either in the form of flat ribbons or round wires. The silver sheath, or silver matrix, contains the superconducting material while permitting oxygen to diffuse readily into and out of the silver sheath during the high-temperature heat treat when the reactions take place to form the superconducting phase. The best properties have been attained in superconducting strands that each contain many superconducting filaments of the superconducting material. A typical superconducting strand is formed by a multi-step procedure as follows.
Finely ground oxides or nitrates of the constituent metal atoms are mixed in the desired stoichiometry. When the oxides are used, they are mixed as finely ground solid powders. When the nitrates are used, each nitrate is dissolved to saturation in water, the saturated solutions are titrated in the desired ratio into a mixture solution, the solution mixture is evaporated or freeze dried or spray-evaporated to yield a nitrate powder mixture, and finally the nitrate powder mixture is baked at high temperature, typically at 600° C. for 12 hours, to evolve the nitrate and leave a uniformly dispersed oxide mixture. The oxide mixture is then sintered by baking at a high temperature, typically ~850° C., at which the oxide mixture reacts to form the desired perovskite phase.
The sintered material, consisting largely of the desired perovskite phases, is ground to a fine powder and packed into a silver sheath. A number of packed silver sheathes are combined and then cold worked, by drawing or roll pressing, to form a metallurgically bonded strand. The strand is further drawn or rolled to reduce its cross-sectional area to the desired size. In the case of Bi-2223, a further heat treat is performed, typically ~830° C. for more than 24 hours, to promote the growth of long filamentary crystals of the perovskite phase within each filament of the strand. The strand is then cold worked again to reduce porosity and increase the alignment of the crystal grains within each the superconducting filament. If the strand is to be formed into a superconducting coil, such as used electrical or electromagnetic devices, it may be wound into a superconducting coil in this state of processing, with electrical insulation provided to insulate adjacent turns within the coil.
The strand or coil is then subjected to a final precision heat treatment cycle in a strictly controlled atmosphere. In the case of Bi-2212, this final heat treat is again at ~830° C. in an atmosphere containing 20% oxygen for 6 days, in which the metal oxides diffuse and react to form large aligned grains of perovskite phase within each filament of superconducting material. The control of temperature and oxygen within the strand or coil during this final heat treatment is critical to the performance of the superconducting strand. In the case of Bi-2223, this final heat treat is at lower temperature, typically ~400° C., and has the purpose of relieving stress within the superconducting crystals and the silver sheath.
Superconducting strands may be used in any suitable electrical or electromagnetic device, such as electric motors, generators, energy storage devices, transformers, magnetic bearings, high strength magnets used in magnetic resonance imaging, and the like. Superconducting coils generally comprise one or more superconducting strands that are wound around a core. Electricity flowing through the superconducting strands produce a magnetic field within the core. The strength of the magnetic field can be increased by increasing the number of times the superconducting strand is wrapped around the core and by increasing the current flowing through the superconducting strand. As will be discussed in greater detail below, the magnetic field produces a physical load on each individual superconducting wire, which is generally referred to by those skilled in the art as Lorentz stress.
Lorentz stresses produce an operational, or mechanical, load that acts to push the individual windings of superconducting strands away from the core. Lorentz stresses are produced by the magnetic field acting on the superconducting materials. The maximum Lorentz stress within a coil increases by the square of magnetic field strength produced by that coil. The operational load is transferred outward from each winding to each outwardly successive winding of superconducting strands. The magnetic field strength of the superconducting coil is generally limited by the operational loads that the outermost superconducting strands can support. Accordingly, the number of layers of superconducting strands that can be wound in a coil of superconducting strands is limited by the operational load that can be supported by a superconducting strand before its current capacity is degraded, which in turn limits the strength of the magnetic field that can be achieved in a conventional superconducting coil.
A technical disadvantage of conventional superconducting strands is that the silver sheathes that contain the superconducting material are soft and have low tensile strength. The operational loading from Lorentz stress must be supported by the silver sheathes. For this reason the silver is sometimes hardened to increase the strength of the strand. The hardening is typically achieved either by alloying with other metals with the silver, or by dispersing an insoluble material, typically an oxide, such as Al
2
O
3
, in the silver. The alloy or dispersion hardening may substantially increase the cost of the silver, and accordingly increase the cost of the superconducting strand. Even hardened silver has only moderate tensile strength compared to structural metals like stainless st
McIntyre Peter M.
Soika Rainer H.
Cook Stacy S.
Galasso Raymond M.
Paladini Albert W.
Simon Galasso & Frantz, PLC
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