Process for prorducing thallium-containing high-T.sub.c supercon

Details Process for prorducing thallium-containing high-T.sub.c supercon Process for prorducing thallium-containing high-T.sub.c supercon

1996-09-17

1998-09-01

Kopec, Mark

Superconductor technology: apparatus, material, process

Processes of producing or treating high temperature...

Heating, annealing, or sintering

505482, 505492, 505220, C04B 3564, C04B 3550

Patent

active

058011269

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a process for producing thallium-containing oxide-ceramic superconductors, by means of which massive ceramic components and also precursor materials for the "oxide powder in tube" method (OPIT) can be obtained.
It is already known that a plurality of high-T.sub.c super-conducting oxide compounds are formed in the pseudoquaternary system Tl.sub.2 O.sub.3 /CaO/BaO/CuO. This is primarily dependent on the ratio of the metal oxides used in each case, on the temperatures used and on the oxygen partial pressure during firing. Unlike the case of the structurally related Bi compounds, in the case of thallium-containing superconductors there are two classes of compounds having the general, nominal compositions: compounds Tl.sub.0.1 Pb.sub.0.5 Ca.sub.n-1 Sr.sub.2 Cu.sub.n O.sub.z.
In the case a) the compounds having n=2 and n=3 and in the case b) those having n=1, 2 and 3 form superconductors having critical transition temperatures (T.sub.c) above the boiling point of liquid nitrogen (T=77K). The discovery and characterization of these compounds is described, for example, by Z. Z. Sheng et al. in Nature, vol. 332, p. 55, (1988). The highest transition temperature T.sub.c known hitherto is 125K for the thallium-rich, lead-free compound of the type Tl-2223 from the above-mentioned series b). The transition temperatures for Tl-1223 from the series a) are in the range 118K<T.sub.c <122K. According to the studies of D. H. KIM et al., published in Physica C 177, p. 431, (1991), there is a clear relationship between the critical current density and the spacing of the so called CuO planes in the crystal structure, as a function of the details of the production method.
This is related to the melting behavior of the magnetic flux lattice, particularly in the superconductors built up of layers. In the case of the thallium-containing superconductors, the best magnetic coupling between the layers is observed in the case of the compounds of the abovementioned 183, p. 67, (1991)!. At a temperature of 77K and a magnetic field of 1 T, intragranular, critical currents of the order of J.sub.c =10.sup.5 Lett. 60, p. 1019, (1992)!. The critical transport current densities are at present, owing to the grain boundary effects (weak links), still far below this value, namely in the region of a few 100 A/cm.sup.2. For the above reasons, the Tl superconductors of the series a) (Tl-1223 and Tl-1122) are of greater practical importance for use in power applications in the temperature range above 77K. They are even superior to the bismuth-containing superconductor ceramics in this temperature range, because there only well characterized compounds analogous to series b), containing Bi in place of Tl and Sr in place of Ba, are known. These are compounds of the nominal composition Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.Y (n=2, T.sub.c =92K) and Bi.sub.2 Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.x (n=3, T.sub.c =110K) in which the spacing between the CuO planes is in principle greater than in the comparable thallium compounds of the series a).
A further, industrially significant compound is YBa.sub.2 Cu.sub.3 O.sub.7 (T.sub.c =92K). Although the spacing between the CuO planes is there even less than in the case of the thallium-containing superconductors, the grain boundary effects are significantly more pronounced for a comparable texture, which is apparent in a stronger magnetic field dependence of J.sub.c.
The actual preparation of oxide ceramic superconductors is carried out by means of the solid state reaction at high temperatures. In the case of the thallium-containing superconductor ceramics, the properties achieved at low temperatures depend essentially on the ratio of the metal atoms weighed out in the starting mixture, the selection of the starting components themselves, the oxygen partial pressure of the surrounding gas atmosphere and finally on the temperatures used and the Tl.sub.2 O vapor pressure during firing. The preparation of the TlCa.sub.2 Ba.sub.2 Cu.sub.3 O.sub.8.5+x super-conducting ceramic is described by K. C

REFERENCES:
Goretta et al "Processing TlBa.sub.2 Ca.sub.3 Cu.sub.2 O.sub.y Powders" Supercon, Science & Tech., vol. 5, No. 9, Aug. 1992 pp. 534-537.
Hervieu et al "A New Member of the Thillium Superconductive Series", The 1212 Oxide . . . Journal of Solid state Chem., 75, pp. 212-215, 1988 (No Month).

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