Abstract:
At temperatures located in a cryogenic temperature range, nickel steel is used for components, which can be subjected to high levels of mechanical stress and which have ferromagnetic properties at these temperatures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and hereby claims priority to German Application No. 10050371.3 filed on Oct. 11, 2000, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a device preferably operating in the temperature range of liquid air. The material, which inevitably has to be able to withstand mechanical load and/or temperature changes and embrittlement, of the component of the device which can be subjected to high mechanical loads must also have required ferromagnetic properties and/or a high magnetic conductivity for the intended magnetic flux guidance and/or flux amplification. 
     It is known from, e.g. Reed, Materials at Low Temperatures, pages 388/389, to use nickel steel with a nickel content of between 3.5 and 9% for components which can be subjected to mechanical loads for cryogenic applications, i.e. for the temperature range &lt;173 K. Examples of such components include tanks which are required for storing and transporting liquefied gases, such as for example liquid air. Nickel steel with a nickel content of 12% has also already been developed for temperatures below 77 K and especially for use in connection with liquid helium at 4 K. It is essential for these materials that the transition temperature from ductile to brittle be reduced to below the intended application temperature. The abovementioned reference by Reed also refers to additives which may be present in the nickel steel. 
     However, for the use of a material which corresponds to the invention, it is not only its temperature-dependent mechanical property which is of crucial importance, but also it is necessary for the material which is mechanically suitable for the temperature range mentioned also to have a ferromagnetic property. 
     The latter property is crucial, for example, when the material is to be used for magnetic flux guidance and/or flux amplification in machines and machine parts which include superconducting elements. Examples are contactless bearings which also include superconducting structural elements which hold a rotating machine shaft in a floating position without contact in a bearing shell by magnetic repulsion forces. In this context, reference is made, for example, to German Utility Model U 9403202.5 “Magnetische Lagerungseinrichtung mit Hoch-T c -Supraleitermaterial” and to U.S. Pat. No. 5,777,420 “Superconducting Synchronous Motor Construction”. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to use a material which, at the low operating temperatures required, has sufficient strength, even in the long term, for the expected mechanical loads, e.g. does not become brittle at the low temperatures, and moreover has ferromagnetic properties for the purpose of magnetic flux guidance and/or amplification, for a component of the device which is of relevance in this context. 
     It was determined as a result of tests being carried out that a nickel steel X8Ni9, DIN 1.5662, US standard ASTM A353, A553 has ferromagnetic properties for the abovementioned purposes even at temperatures below 77 K. This is a temperature range which is preferably of relevance to the invention. Even at these low temperatures, this material has so little tendency to become brittle that it does not suffer any ductile fracture even under mechanical load. Nickel steel containing from 3.5 to 8% of nickel also has a sufficiently high ferromagnetic property for the above-mentioned applications to enable it to be used for elements for magnetic flow guidance and/or amplification. The same is also true of nickel steel containing 12 to 13% of nickel. 
     Not only does the mechanical strength of the material with ferromagnetic properties required in accordance with the object play a role in, for example, rotating machine parts, on account of the centrifugal forces which occur, but also reliably mechanical stable material is advantageous or even imperative for the encapsulation and holding of magnet with high-temperature superconductor coils. This is the case, for example, if the holder also acts as a flux-guiding means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 shows a cross section through a rotor of an electric motor with superconducting field excitation in the rotor and a rotor core as component made from material which is used in accordance with the invention. 
     FIG. 2 shows a high magnetic field device with superconducting magnet field coil, as is known in principle, for example, from DE-C 19813211. 
     FIG. 3 shows a magnetic bearing of a rotor shaft using high-temperature superconductor material, as is known in principle from DE-C 4436831. 
     FIG. 4 shows a superconducting magnet coil as Is known from WO 96/08830. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     In FIG. 1,  1  denotes, as the device of the invention, the rotor, which in FIG. 1 is illustrated in cross section perpendicular to the axis, of an electric motor with superconducting excitation winding. The superconducting coils, which are arranged or designed in stepped form, are shown in one half of the illustration and denoted by  2 . They include, in a known way, strip conductor windings made from a high-temperature superconducting material used for this purpose. Materials of this type are well known. A current which is excited in a coil  2  of this type generates a magnetic field with a magnetic field H or magnetic flux density B which in the inner cross-sectional region of the coil is directed axially with respect to the coil. This rotor also includes, in a manner which is known for rotors for electric motors, a core  3  which guides the magnetic flux, as a component which is advantageously made from ferromagnetic material.  4  denotes magnetic flux guidance of this type in this core  3 . With the structure of the in this case 4-pole rotor which is illustrated, there are then two north poles N and two south poles S as poles of the core  3  or rotor  1  of the motor, as is known. According to the invention, this core  3  preferably consists of nickel steel, with a nickel content of 9 to 13%, or is DIN standard steel X8Ni9 DIN 1.5662. 
     FIG. 2 shows a magnet  11  with superconducting coils  12  which is known in terms of its design principles. These coils are located in a cryostat housing  13  with a correspondingly thermally insulated holder. Two steel rings as a component of the invention are denoted by  14  and  15  and in this case are used for magnetic flux guidance and shaping. These components are exposed to considerable magnetically induced mechanical forces. Therefore, these rings are made from nickel steel. This flux-guidance measure makes it possible to improve the homogeneity of the magnetic field between the poles  16  of the magnet  11 . 
     FIG. 3 shows a magnetic floating bearing, which is known from the abovementioned patent, of a rotor shaft  31 .  34  denotes annular permanent magnets which are in this case arranged on the shaft  31  and are axially polarized as shown. These permanent magnets  34  are oriented in such a way with respect to one another that the axial magnetic field of two adjacent magnets  34  are opposite one another in these magnets and are then together oriented radially outside the magnets. 
     For radial guidance of the magnetic flux  134  which originates from the permanent magnets, the rings made from ferromagnetic material, which are denoted by  33  and are illustrated in section, are provided on the shaft  31 . 
     A layer or sleeve of superconducting material is denoted by  35 . It is positioned on or at the inner side of the sleeve-shaped outer bearing shell. This superconducting material is cooled to the low temperature required for superconduction by a coolant, e.g. liquid nitrogen. This coolant flows inside the coolant passages which are denoted by  132  and are in this case provided in the stationary outer bearing shell  36 . 
     The electrical shielding currents which occur in the superconducting material when the bearing shaft with the permanent magnets on it rotates effect the magnetomotive forces required for the floating bearing of the shaft. 
     The gap between the rotating parts and the stationary parts of an arrangement of this type is known to be made as small as possible. However, this has the effect that the permanent magnets and the rings  33  are also cooled to low temperatures. For this reason, it is provided that the rings  33 , as components which are subject to mechanical loads as a result of the centrifugal force which occurs, preferably consist of ferromagnetic nickel steel. 
     As illustrated, the magnetic field coil  40  shown in FIG. 4 includes superconducting coils  42  positioned on top of one another in the axial direction. At the ends of the coils there are rings  43 , the shape and dimensions of which are matched to the coils and which preferably consist of ferromagnetic nickel steel used in accordance with the invention, as the component. The magnetic flux which is known to emanate from this coil produces optimized flux guidance. 
     The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.