Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-12-12
2004-12-14
Dunn, Tom (Department: 1725)
Stock material or miscellaneous articles
Composite
Of inorganic material
C505S126000
Reexamination Certificate
active
06830839
ABSTRACT:
The invention relates to a method for the heat treatment of a superconductor material, the heat treated superconductor material based on (Y/Rare Earth) BaCuO having good material properties and especially a high remanent induction and which contains just one grain or just a few grains or one or more magnetic domains, and the use of the heat treated material.
When developing superconducting materials, the aim is to produce superconducting and magnetic properties which are as high as possible. There are particular advantages to be gained if texturised polycrystalline or superconducting materials comprising just one or just a few magnetic domains are employed. The magnetic properties thereof are best, when the shaped body only comprises one magnetic domain.
One particular field of employment for so-called massive shaped bodies made of high temperature superconductors is that of non-contact making, self-stabilising magnetic bearings. Such bearings contain arrangements of permanent magnets and high temperature superconducting shaped bodies:
If a high temperature superconducting shaped body is located in the field of a permanent magnet at a temperature above its transition temperature T
c
, then a magnetic flux will pass therethrough. If, in this case, the superconductor is cooled to temperatures below the transition temperature then a portion of the magnetic flux will remain frozen into the superconducting material. In this state, it is only possible to displace the high temperature superconducting shaped body by the application of force. The stability of such a bearing is so much the greater, the larger the amount of magnetic flux that can be frozen into the superconducting material, i.e. the higher the maximum value of the remanent induction.
Improved magnetic properties such as e.g. higher values of the remanent induction and of the levitation force enable the construction of e.g. magnetic bearings having larger gaps between the superconducting components and the permanent magnets or the electromagnets. In the case of motors incorporating superconducting components for example, greater unbalances or deviations from the ideal run of the rotors can thereby be permitted. In the case of the bearing arrangement for the inertial mass in flywheel storage devices based on such bearings, the safety reserves can be increased, and simpler bearing geometries, which are more economical and technically less complicated to construct, can be accepted due to such an enlargement of the gap.
There is a need for shaped bodies having good superconducting and magnetic properties as well as adequate mechanical stability but which, in addition, should possibly not have too great a volume of the shaped body for a given value of the remanent induction and thus exhibit the highest possible value of magnetising force, whereby they can be more easily fitted in the corresponding apparatus. Above all, there is a need to produce strong magnets for the temperature range extending up to 77 K, the temperature of fluid nitrogen, which, on the one hand, use shaped bodies having surface areas of just a few square centimetres, but which, on the other hand, use considerably larger, possibly segmented, shaped bodies such as e.g. rings.
One way of increasing the remanent induction of such a shaped body could lie in the enlargement of the magnetic domains by processing larger, single domain shaped bodies. However, due to the very low growth rates in the phase (Y/Rare Earth)
1
Ba
2
Cu
3
O
7-x
(=phase 123), this is associated with very long processing times. In addition, the growth front in shaped bodies based upon (Y/Rare Earth) BaCuO having an extensively homogenous starting composition becomes unstable with grains of increasing size. Morita et al., Mater. Sci. Eng. B53, 1998, 159-163 counteracts this by using shaped bodies having deliberately arranged non-homogenous starting compositions in which an intense chemical gradient is produced in the shaped body. Shaped YBaCuO bodies having a large surface area, Dy-containing shaped body placed thereon were produced. In turn, a crystal nucleus containing Sm was seated on the Dy-rich shaped body. Hereby, maximum values of the remanent induction of 1080 mT at 77 K and 0 T were achieved. In this process, the Dy-rich shaped body is initially texturised. Thereafter, this shaped body serves as a large surface area crystal nucleus for texturising the Y-rich shaped body. This means that two complete texturising processes, together with all of the necessary process steps associated therewith, must be carried out here, this thereby resulting in a considerable increase in the processing time.
Moreover, it is known from the production of shaped bodies having a Y basis that a complete intergrowth of the central grain can only rarely be achieved. In general, this central grain is surrounded at the sides and below by a non-texturised layer of approximately 1 to 2 mm thickness in which the particles that have been displaced forwardly by the growth front solidify in the residual smelt. It is assumed that this was also the case when growing Dy-rich shaped bodies by the methods described by Morita et al. In such cases, it is not possible to effect the texture transfer process from the Dy-rich shaped bodies to the Y-rich bodies. The Y-rich shaped body would then grow polycrystalline and exhibit correspondingly poor superconducting properties. In the case of this process, there was then a very high rejection quota.
The production of suitable preliminary materials for the process and also the processing of the combined shaped bodies are very complicated procedures and introduce an additional lengthening of the processing times. Moreover, for some applications, it is necessary to keep the size of the superconducting shaped bodies as small as possible, especially if they are to be fitted into existing constructions of apparatus.
An enlargement of the magnetic domains without increasing the size of the shaped body can also be achieved in the case of shaped bodies incorporating cracks or/and other point defects by healing such point defects using the method described in the German patent application 198 41 925.2. By virtue of the reference thereto, this patent application is considered to be included in full in the present application.
Furthermore, shaped bodies based upon SmBaCuO are known from Ikuta et al., Supercond. Sci. Techn. 11, 1998, 1345-1347, these bodies containing a high proportion of Ag
2
O and having a remanent induction of up to 1700 mT. However, such an Sm-rich superconducting material can only be produced with great difficulty and in the absence of air since the superconducting phase Sm-123 is not stable under such conditions. The production of the shaped body must therefore be undertaken in a protective gas atmosphere having a very low partial pressure of oxygen. Furthermore, a comparison with YBaCuO type shaped bodies was drawn in
FIG. 2
of this publication, the remanent induction thereof being not even half as large as that of shaped bodies based upon SmBaCuO.
Consequently, the object of the invention is to propose a method by means of which such superconducting materials having a high remanent induction, a high levitation force or/and a high critical transport current density can be produced. Furthermore, it is advantageous if these shaped bodies can be produced in as simple and reliable a manner as possible.
This object is achieved by a method for the heat treatment of shaped bodies made of a superconducting material based on (Y/Rare Earth)BaCuO, which is characterised in that a coating consisting of a coating material is applied to at least one part of the surface of the shaped body, whereby the coating material melts at least partially at a lower temperature than the material of the shaped body or/and is flowable at a lower temperature than that material and, possibly hereby, flows out over the surface of the shaped body, whereby the shaped body together with the applied coating material is heated to a temperature at which the material of the shaped body does not yet
Bäcker Michael
Bock Joachim
Freyhardt Herbert
Leenders Andreas
Ullrich Martin
Cooke Colleen P.
Dunn Tom
Nexans Superconductors GmbH
Sughrue & Mion, PLLC
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