Superconductor technology: apparatus – material – process – High temperature – per se – Having tc greater than or equal to 150 k
Patent
1989-05-17
1991-12-24
Roy, Upendra
Superconductor technology: apparatus, material, process
High temperature , per se
Having tc greater than or equal to 150 k
29599, 420503, 428930, 505704, 505740, 505781, H01B 3914, H01B 1200
Patent
active
050752855
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a sheathing material for superconducting wires according to the preamble of claim 1.
Technology Review
In technologically usable superconducting wires, the superconducting oxide must be surrounded by a metal sheath. This metallic sheathing material performs several important tasks.
One function of the sheathing material is to minimize heating as the result of local losses of the superconducting property of the oxide and thus contributes to the retention of the superconductive property of the entire unit when the transition temperature is exceeded. Another function of the sheathing is to support the brittle superconducting oxide against the usually unavoidable mechanical stresses which occur during its manufacture and also as a result of the Lorentz force during operation of a superconductor.
Moreover, the sheathing material must be able to air-tightly seal the superconducting core against the atmosphere at temperatures below 300.degree. K. so that its oxygen content remains unchanged during storage as well as during operation.
All prior art high field superconductors at B.sub.C2 >15 Tesla are distinguished by extraordinary brittleness. This property quite significantly influences the manufacturing process for wires. Wires containing high field superconductors such as YBa.sub.2 Cu.sub.3 O.sub.7 or oxides derived from it (substitution of Y by rare earths) can at present be manufactured only by powder technology methods.
In this case, the already formed (pre-reacted) superconducting phase is filled into a metal tube, is compressed and shaped by deformation processes such as hammering, rolling or drawing into a thin wire having a diameter of about 1 mm.
It is not yet known how the deformation process takes place in detail within the superconducting phase, but it has been possible to detect the results of the deformation by measuring the transition temperature T.sub.c and the critical current density J.sub.c.
Although for YBa.sub.2 Cu.sub.3 O.sub.7 and oxides derived therefrom T.sub.c remains constant, the percentage of the superconducting phase and the critical current density J.sub.c decrease with increasing degree of deformation. The superconducting properties disappear completely if the deformation is extensive. To recover the original superconducting properties at a later time, recovery heat treatments are necessary which are customarily performed at temperatures above 800.degree. C., frequently at about 900.degree. C. to a maximum of 940.degree. C., which is followed by an oxidation heat treatment.
In the past, the sheathing material for the production of high field superconducting wires has almost exclusively been pure silver (Proc. 1. European Workshop on High T.sub.c Superconductors and Potential Applications, P. Dubots et al, page 133; G. Barani et al, page 137; R. Flukiger et al, page 131; and S. Jin et al, Appl. Phys. Lett. 51 (1987), page 203).
Pure silver is well suited for this purpose for several reasons. It is ductile, its melting point in air lies at 960.8.degree. C. and in a pure oxygen atmosphere at 939.degree. C.; it does not undergo a reaction with the oxidic superconductor if it is heat treated and finally it is permeable to oxygen at temperatures above 400.degree. C. From an economic point of view, the price of silver is considered to be still acceptable for the intended purpose.
Although Jin et al propose a sheathing material of copper including a diffusion barrier of nickel/gold, copper is less suitable due to its irreversible formation of copper oxide.
For a superconductor of the YBa.sub.2 Cu.sub.3 O.sub.7 type, the above mentioned recovery heat treatment is usually performed at temperatures barely below the melting point of silver in an oxygen atmosphere. This recovery heat treatment is then followed by an oxidation heat treatment at about 400.degree. C. to 700.degree. C. at which the oxygen loss in the oxidic superconducting material YBa.sub.2 Cu.sub.3 O.sub.7 --which is about 2% at 900.degree. C.--is replaced again.
SUMMARY O
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Jin et al., Appl. Phys. Letts. 51 (Jul. 20, 1987) 203-4.
Dubots et al. in Europ. Workshop . . . High Tc Superconductors . . . , Jul. 1-3, 1987, pp. 133-135.
Tachikawa et al, Proc. IEEE, 77 (Aug. 1989) 1124.
Kernforschungszentrum Karlsruhe GmbH
Roy Upendra
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