Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – Superconducting wire – tape – cable – or fiber – per se
Reexamination Certificate
2003-07-11
2004-11-16
Bos, Steven (Department: 1754)
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
Superconducting wire, tape, cable, or fiber, per se
C428S114000, C428S930000, C505S234000
Reexamination Certificate
active
06819948
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a superconducting tape and in particular to a composite superconducting tape.
The invention has been developed primarily for use with high load current carrying cables and will be described hereinafter with reference to that application. It will be appreciated, however, that the invention is not limited to this particular field of use.
BACKGROUND OF THE INVENTION
It is known to use superconducting tapes to make coils, magnets, transformers, motors and generators as well as current carrying cables. Superconductors are materials which exhibit, at sufficiently low temperatures, substantially zero electrical resistance for direct current. The temperature at which a substance becomes superconducting is called the transition or critical temperature. Early superconductors had critical temperatures that necessitated refrigeration with liquid helium. However, due to low thermodynamic efficiency, this is an expensive an generally uneconomic arrangement. More recently it has been known to utilise “ceramic” superconductors with critical temperatures up to around 135K.
Superconductors with critical temperatures over 77K are of particular interest as they can be operated with liquid nitrogen refrigeration at relatively high thermodynamic efficiency and relatively low cost.
Superconductors can only carry so much current before the voltage drop per unit of length begins to increase in an exponential form. The so called “critical” current, I
c
, is defined for precision as the maximum current that can be carried before the voltage drop increases above 1 microvolt per centimeter.
Tapes comprising superconducting material, and referred to as superconducting tapes, are already known, and comprise one or many superconducting filaments in a medium of silver or silver alloy. The main class of superconducting tape are known as powder-in-tube or PiT tape. These are made by drawing or otherwise reducing a tube of silver or silver alloy filled with a powder form of the superconducting oxide or its precursor. The tubes are also rolled or otherwise formed into a thin tape.
Multifilamentary tapes are mostly made by grouping filled tubes in a common silver or silver alloy sheath at art intermediate stage of reduction.
One important superconducting oxide is known as Bi-2223, and is a compound oxide of bismuth, strontium, calcium and copper. Certain limited substitutions can be made, as required. Bi-2223 can also be considered a cuprate salt.
Known tapes usually have a thickness of between around 0.2 mm and 0.3 mm, and a width of between 2 mm and 5 mm. The superconducting filaments must be thin to obtain an adequate critical current and are typically around 10 to 40 microns in thickness. Moreover, the filaments typically have an aspect ratio of at least 1:10.
The filaments comprise many plate-like grains and, for good performance, the grains should be aligned as much as possible in the same crystallographic orientation. The relative orientation is often referred to as the grain alignment or “texture”. Well textured filaments allow a high critical current, and give overall flexibility to the whole tape. If the filaments are too thick, then they will degrade at relatively large bending radii of the tape.
A superconducting tape's flexibility can be measured in terms of the reversible bending radius achievable when such bending is carried out at room temperature. The reversible bending radius may be defined as the largest bending radius which causes more than 5% degradation in the critical current, Ic, as measured on the unstrained tape. Other criteria which are used to define the quality of the tape and the amount of current that a superconducting tape can carry is the critical current density, J
c
, and the engineering current density, J
c
. The critical current density, J
c
, is defined by J
c
=I
c
/A
sc
−
where A
sc
is the total superconductor area in the cross-section of the tape, and the engineering current density J
c
is defined by J
c
=I
c
/A
tape
, where A
tape
is the total cross-sectional area of the tape. The higher J
c
and J
c
the better. The ratio of the cross-sectional area of the superconductor to the cross-sectional area of the whole tape is called the Fill Factor, FF, and, therefore, we can deduce that FF=J
c
/J
c
.
When tapes are used to carry alternating current, rather than direct current, the superconducting tapes do not exhibit zero power loss, though losses are small compared to those exhibited by normal metallic conductors. This power loss resulting from the carrying alternating current rather than direct current is called “AC loss”.
For an individual tape, the AC loss can be of the order of 100 microwatts per meter per Amp of critical current squared. To achieve the maximum performance from these tapes the AC losses should be minimised. This is particularly true when they are used in superconducting cables where the current loads are high.
Composite tapes are sometimes made by forming a stack of individual tapes and wrapping the stack with one or more flexible wrapping tapes to keep it together. These wrapping tapes are generally a metal and more often silver.
Inevitably there are some gaps and/or overlapping between the turns of the wrapping tape. If such a wrapped stack were to be rolled then the gaps or overlapping would create kinks in the filaments, which destroys local grain alignment leading to degradation of the overall critical current density J
c
.
It is an object of the present invention, at least in the preferred embodiment, to overcome or substantially ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative.
DISCLOSURE OF THE INVENTION
According to a first aspect of the invention there is provided a composite superconducting tape comprising a multiplicity of stacked and diffusion bonded superconducting tapes in which all elongate components extend longitudinally and a compatible metal tape is bonded to at least one exposed major surface.
The preferred embodiments of the invention making use of the combination described above to achieve a tape of high superconductor cross-sectional area and A thin and wide filament structure. This is highly desirable for a high critical current density.
Preferably, the filament thickness is as thin as is required to provide a desired critical current density, wherein the fill factor of the tape is about 40%.
Preferably also, the composite tape consists solely of the stacked and bonded constituent tapes. The composite tape has exposed major surfaces which are defined by those surfaces of the constituent tapes which are not abutted against an adjacent surface. More preferably, the exposed major surfaces of the composite tape covered with a metal. More preferably, the tape is of silver or silver alloy. In other embodiments, metal tapes other than those of silver are used.
In a preferred form, the superconductor filaments are at least 10 &mgr;m away from the exposed major surfaces. More preferably, use is made of the thin metal tape to allow this to be achieved without incorporating unnecessary amounts of silver elsewhere in the composite tape.
Preferably also, the metal tape is used to control mechanical stresses. More particularly, some composite tapes are intended, in use, to be curved always in one direction such that one of the major exposed surfaces will always be convex and the opposed major exposed surface will always be concave. In such circumstances a metal tape is applied to the major surface that will be convex. More preferably, the major side that will be concave either does not include a metal tape or includes a second metal tape that is weaker than the first metal tape. In some embodiments the stronger tape is a silver alloy tape and the weaker tape a pure silver tape. In other embodiments the tapes differ in thickness.
Preferably, the metal tape is flat has a width not substantially greater than that of the superconducting tapes. In some embodiments the metal tape has a width of less than
Bos Steven
Cooke Colleen P.
Fay Kaplun & Marcin LLP
Metal Manufacturers Limited
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