Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – Superconductor having metal connect – pad – connect structure,...
Reexamination Certificate
2001-05-09
2003-06-24
Dunn, Tom (Department: 1725)
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
Superconductor having metal connect, pad, connect structure,...
C505S236000, C505S703000, C174S125100
Reexamination Certificate
active
06584333
ABSTRACT:
DESCRIPTION
The invention relates to a high-temperature superconductor component with a particular cross-sectional area, which has a current-carrying section, the current-carrying section being in contact, in a safety region, with a safety conductor in such a way that in the safety region the current flowing on transition of the superconductor to normal conduction can be taken up without damage by the safety conductor in at least 1 second and rerouted, as well as a process for its production.
With the discovery of a ceramic material which exhibits the phenomenon of superconductivity at a comparatively high temperature, Bednorz und Muller (Z. Phys. B. 64, 189 (1986)) achieved the first significant improvement in the superconductivity transition temperature in the 1980s. The material used by Bednorz und Muller had a nominal composition La
2−x
M
x
CuO
y
, in which M stood for calcium, barium or strontium, x typically varied around values between 0 and 0.3 and y was dependent on the production conditions. The highest superconductivity transition temperature was measured with the materials in which M stood for strontium and x stood for approximately 0.15 to 0.20. These materials had a transition temperature in the range of between approximately 40 and approximately 50 K (Cava et al., Phys. Rev. Letters, 58, 408 (1987)). In March 1987, Chu et al., Phys. Rev. Letters, 58, 405 (1987) reported that a material with the composition Y
1.2
Ba
0.8
CuO
y
exhibited a superconductivity transition temperature which lay between approximately 90 and 100 K.
Since these discoveries, another series of materials have been found which exhibit superconductivity at temperatures lying above the boiling point of liquid nitrogen. With these discoveries, superconductor technology therefore made a leap forward to wider applications, since current conduction without resistance no longer depended exclusively on extremely expensive and sensitive cooling of the superconductor material using liquid helium (boiling point: 4 K), but could be achieved even with substantially cheaper liquid nitrogen at a correspondingly high temperature.
However, a disadvantage with the newly discovered materials was that they were ceramic materials with which conventional processing methods for “conductors” could not be used. The ceramic superconductors have no degree of flexibility or ductility, were brittle and were not shapable. Machining of these materials therefore generally needed to be carried out with expensive diamond cutting tools at high cost.
This changed at least partially with the introduction of the so-called fusion casting process. This process, which is described in EP-B 0 462 409, opens up the possibility of producing, for example, cylindrical hollow moldings with different dimensions, for example thick-walled tubes with comparatively small diameter and fairly large length, as could be used in a power engineering application.
With growing understanding of the phenomena taking place in super-conductivity, as well as further development of the technology for processing ceramic superconductors, methods were developed making it possible to form the ceramic high-temperature superconductor material into flexible wires or strips in order to open up a range of application for high-temperature superconductor materials at least almost equivalent to normal conductors. For example,
IEEE Transactions on Magnetics, Vol.
25
, No.
2, March 1989 describes that strips of superconductor material can be obtained in the form of silver-clad wires. To that end, the corresponding materials giving rise to superconductors are for example mixed, calcined and sintered and then put into a silver tube. Through a pulling process, the diameter of the silver tubes is reduced in steps until a wire or a thin strip which can be processed with appropriate flexibility is created. The wires and strips which can be obtained in this way are generally also subjected to further treatment in order to produce or to optimize the required superconductor properties. When appropriate, with particular types of high-temperature temperature superconductors, it is also possible to put powder that is already superconducting into the corresponding silver tubes and then process them to form strips or wires. This method is generally known as the power in tube method (PIT). In such high-temperature superconductors in wire or strip form, the superconductor material is in the form of superconducting “grains”. Although the current can flow very readily within the grains, current flow from grain to grain is, however, possible only along the grain boundaries which have inferior superconductor properties to the grains themselves. The grain boundaries are therefore generally referred to as weak links. Another disadvantage with such wire or strip systems is that the continuous cross-sectional areas of the high-temperature superconductor material are very small. Customary wires or strips have, for example, a cross-sectional area of about 0.4 mm
2
, of which generally only about 30% corresponds to high-temperature superconductor material. It is therefore obvious that the critical current that can be achieved with such systems is very limited and is insufficient for many applications.
The use of silver in connection with high-temperature superconductors is described, for example, by M. Itoh, H. Ishigaki, T. Ohyama, T. Minemoto, H. Noijiri and M. Motokawa in
J. Mater. Res., Vol.
6, 11, November 1991. They have shown that containing silver powder improves the electrical properties of a high-temperature superconductor material, in the present case based on Y, Ba, Cu and O. The authors describe high-temperature superconductors containing up to 28% by weight in powder form.
One already well-established and intensively investigated possible use for tubular components made of high-temperature superconductor material is represented by inductive current limiting. For example, U.S. Pat. No. 5,140,290 describes a device for inductive current limiting of an alternating current, in which the current to be limited flows through an induction coil. A hollow cylinder or a high-temperature superconductor is arranged in the interior of this coil, and a soft magnetic material with high permeability is arranged concentrically inside. In normal operation, or with rated current, the superconductivity of the hollow cylinder shields its interior, so that the impedance of the induction coil is very low. The current then flows circularly in the hollow cylinder, a flow of current does not take place along the longitudinal axis of the hollow cylinder.
With overcurrent, for example due to a mains short circuit, the superconductivity disappears and the impedance of the induction coil reaches its maximum current-limiting value. Such a voltage and current application when there are temporary overcurrents above the critical current and with electric voltages of a few mV/cm to V/cm lead to so-called “hot” points. Owing to minor nonuniformities in the material of the high-temperature superconductor, local peaks in the electric voltage occur. These lead to enhanced energy dissipation and therefore to heating at this point. The result is an increasing local peak in the resistance and therefore generation of a voltage drop. With prolonged application, such an effect leads to destruction of areas of the high-temperature superconductor.
Such “hot” points occur routinely in high-temperature superconductor materials since, during the production of such a material, variations in quality along the high-temperature superconductor, which have lower current-carrying capacity, may occur. “Hot” points generally have a small extent of about 0.01 to 0.5 mm. The overcurrents which occur are extra-ordinarily brief and range from periods of about 10 to 100 ms.
In order to stabilize such a component, DE-A 44 18 050 proposes to apply an about 1 to 10 &mgr;m thick silver layer to the surface of the superconductor. In order to mechanically stabilize the superconductor, an elastic steel wire winding is proposed which is fixed us
Bock Joachim
Brommer Guenter
Gauss Stephan
Grom Markus
Holzem Johannes
Cooke Colleen P.
Dunn Tom
Nexans Superconductors GmbH
Sughrue & Mion, PLLC
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