Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2000-04-13
2003-11-11
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C505S110000, C029S599000
Reexamination Certificate
active
06646528
ABSTRACT:
The invention relates to a process for the production of a coil made of a high-temperature superconductor material. Superconducting coils are used for the assembly of transformers for heavy currents with a strength of usually much more than 50 A, of magnets in particular for research purposes, in high-energy physics, in ore extractors, in the fabrication of semiconductor materials and for medical purposes such as e.g. magnetic resonance imaging, and for resistive current limiters.
Coils made of a high-temperature superconductor material, e.g. based on bismuth-(lead)-strontium-calcium-copper oxide (═BSCCO and PbBSCCO, respectively) or rare-earth element(s)-alkaline earth element(s)-copper oxide (═YBCO), are already known. Since, in the latter class of material, yttrium is usually, and also in the scope of the present application, counted among the rare-earth elements, since yttrium is normally regarded as the most important or only rare-earth element for this class of material, and since Ba is the most important and often only alkaline earth element (B for barium), the term “YBCO” will be used below for this class of material.
Coils which are made of wound superconducting wire now usually have a coil length of from 50 mm to 110 mm and a superconducting wire length of from 40 mm to 80 m, for example an external coil diameter of 49 mm and for example an internal coil diameter of 13 mm. As high-temperature superconductors, they are now normally prepared from a BSCCO material containing large proportions of the phases BSCCO 2212 or BSCCO 2223 with encapsulation in a silver alloy. Low-temperature superconducting coils normally contain niobium-titanium, niobium-tin or niobium-aluminum. Such coils are now normally used at the temperature of liquid helium, 4.2 K, or liquid nitrogen, 77 K, as magnets.
They can be used as high-temperature superconducting working coils in superconductor magnets together with low-temperature superconductor coils in DC operation. These magnet systems are preferably used for creating very uniform magnetic fields and are employed, in particular, in magnetic resonance imaging MRI. They are also necessary for creating strong deflecting magnetic fields in particle accelerators.
They can also be employed as AC coils in transformers, in order to be used as a secondary or primary coil, in transformers of the core or shell type, for AC voltage conversion.
Superconducting coils can also be used as resistive current limiters, in particular for AC, in order to avoid the creation of high short-circuit currents, especially in power stations, and to prevent destruction of plant components such as generators and transformers. In this case, the extraordinarily short response times are in particular advantageous.
Very few superconducting coils are now used in practice. They are wound from a high-temperature superconducting wire that has been prepared using the oxide powder in tube method (OPIT). The metal cladding usually consists of an alloy with an electrically conductive noble metal whose effect, during operation, is that some of the current carried leads to the formation of shielding currents and hence to additional electrical losses, the AC losses.
AC power loss is converted into heat, and must then be removed by the cooling system. In the superconductor material, the magnetic self-fields are also constantly changed along with the polarity reversal of the alternating current; the energy then dissipated—known as hysteresis losses—contributes substantially to the AC losses. Thin wire filaments lead in this regard to lower AC losses than large thicknesses. The AC losses are therefore substantially dependent on frequency, and on the thickness or diameter of the superconducting article or filament.
The alternating magnetic fields associated with the alternating current induce eddy currents in a conventional electrical conductor such as metallic conductors, and hence for example in silver alloys. Because of the normal-conducting properties of the metallic material, this causes resistive losses according to Ohm's law. However, the AC losses increase as the resistance of the normal conductor decreases. The AC losses in silver alloys at 20 K are therefore actually significantly higher than 77 K. Lastly, AC coupling losses can also occur in the case of closely adjoining articles, such as e.g. in a filament bundle. All three loss mechanisms increase exponentially with n=3, and therefore drastically with the current and linearly with the frequency. The values of the AC current loss are also dependent on the specimen geometry and conductor arrangement, and can therefore be compared only under standardized measurement conditions.
Attempts have been made to reduce these current losses by reducing the proportion of metal used, and optionally also fitting insulating interlayers or selecting less electrically conductive alloys. Nevertheless, the level of shielding currents is still high.
With OPIT wire, coils are usually made which, because of the wire dimensions, can only carry relatively small currents, of the order of up to about 20 A, so that very many windings are normally needed. They can be produced e.g. with high-temperature superconducting wires that have been made using the OPIT method. With the OPIT method, a tube containing predominantly silver is filled with especially fine-grained powder having the chemical composition of a superconductor which is then, e.g. by rolling, reduced in cross-section, compacted, textured, annealed and converted into the desired superconductor material, or further crystallized. These wires often have a diameter of from 0.1 to 0.3 mm including their metal cladding. They are almost always clad by a metal tube containing silver. The method is comparatively expensive and takes a very long time in all; the pure process time is normally now longer than 1 month. The coils made therefrom have the disadvantage that they are very expensive to produce and—owing to the superconductor powder quality used and the subsequent mechanical and heat treatment stages—very great performance differences occur, possibly to the extent of losing the superconducting properties at 77 K.
Because of the now often still too low current-carrying capacity and excessive AC losses of many superconductor components, their use is limited. Further development of such components is needed so that even higher currents can flow through these components superconductively and with low loss, or without loss.
When the critical current density Jc is exceeded, the superconductivity collapses and the superconductor becomes a normal conductor. This is associated with stronger heating of the conductor and possibly melting of the superconducting material.
In order to produce high-temperature superconductors with lower AC losses, or high critical current densities, it is necessary to optimize the superconducting material in terms of purity, phase purity, phase composition, degree of crystallization and orientation.
Particularly large cross sections or large widths, that is to say large thicknesses, would be advantageous because of the consequently much higher critical current density and current-carrying capacity. During production, non-superconducting foreign articles and gas inclusions in the cross section are to be avoided, since they impair the electrical properties.
High-temperature superconductor materials based on YBCO would be particularly advantageous for use in coils because of their particularly favorable values of critical current density and current-carrying capacity; but they cannot yet be drawn suitably to form wires.
U.S. Pat. No. 4,970,483 describes a YBCO coil that, inter alia, was produced by isostatic compression and sintering of a tube section and subsequent sawing, no stabilization having been used during the processing. Such coils are therefore to be handled and processed with the utmost care, with a high risk of causing irreparable damage being run.
The object was therefore to propose a process for the production of superconducting coils, with which it
Bock Joachim
Brommer Guenter
Ehrenberg Juergen
Aventis Research & Technologies
Donovan Lincoln
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