High-T superconductor and process for preparing it

Superconductor technology: apparatus – material – process – High temperature – per se – Bismuth containing

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505725, 505775, 505780, 505785, 505125, 505500, 505501, 252518, H01B 1200, H01L 3912

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054139805

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BRIEF SUMMARY
The present invention relates to a high-T.sub.c superconductor, in particular a ceramic superconductor of a type which contains at least one trivalent element, at least one divalent element, at least one further element, in particular a transition metal element such as copper or niobium, and oxygen. Typical representatives of such ceramic high-T.sub.c superconductors are represented, for example, by the formulae ME.sub.2-y R.sub.2 O.sub.x, ME.sub.2-y R.sub.3 O.sub.x and M.sub.2 E.sub.3-y O.sub.x, where or yttrium,
The transition element component R is preferably composed entirely or at least partly of copper. In the compound named as second above, it is preferable that 6,2<x<7.2.
The invention further relates to a process for preparing ceramic high-T.sub.c superconducting materials. It is particularly suitable for preparing high-T.sub.c superconductors of the above type but can be applied quite generally, that is to say, for example, also to the La-Sr-Nb-O system.
The term "high-T.sub.c superconductor" should in this case be understood to mean superconducting materials whose critical temperature T.sub.c is above 30K.
It is known that the ceramic high-T.sub.c superconductors of the abovementioned types have a strongly anisotropic crystal structure and their superconducting properties, such as the critical current density and the critical field strength, are strongly directionally dependent with reference to the crystal structure. Efforts are therefore made to prepare single crystals which are as large as possible from the materials mentioned. However, according to the present prior art, fairly large volumes of these ceramic superconducting materials can economically be prepared only in polycrystalline form and the individual crystallites or grains of the polycrystalline material have then to be crystallographically similarly aligned if the optimum superconducting properties are to be exploited.
A publication by Wu and Ruckenstein in MATERIALS LETTERS, Volume 5, No. 11.12, October 1987, pages 432-435 discloses that the critical current density and the critical field strength of YBa.sub.2 Cu.sub.3 O.sub.7 is particularly high in the direction of the .vertline.001.vertline. planes, i.e. of the Cu-O planes and that these planes can be aligned in polycrystalline material by pressing and subsequent sintering perpendicularly to the pressing direction.
A publication by Omori et al., JAPANESE JOURNAL OF APPLIED PHYSICS, Volume 26, No. 8, August 1987, pages L1421-L1422-L1423 discloses the preparation of oriented orthorhombic YBa.sub.2 Cu.sub.3 O.sub.7-x polycrystals from tetragonal platelet-type crystals by grinding, pressing and sintering in an oxygen atmosphere.
A publication in Science-Vol. 238 (1987), pages 1655-1656 discloses that oriented grains of an yttrium-barium-copper oxide superconductor can be prepared by fusion at 1,300.degree. C., controlled cooling and subsequent heat treatment for the purpose of oxidizing. This made it possible to increase the critical current density by several powers of ten.
Accounts of Chemical Research, 21, No. 1, pages 1 to 7 discloses the doping of the Ba sites of a YBCO superconductor with alkali-metal ions (K, Rb, Cs), with YBa.sub.2 Cu.sub.3 O.sub.7-y being obtained. Replacing the Cu by Ni or Co reduced the critical temperature T.sub.c considerably.
High-T.sub.c superconductors having the formula LaBa.sub.2 Cu.sub.3 O.sub.7-x and RBa.sub.2 Cu.sub.3 O.sub.7-x (orthorhombic; R=Y, Sin, Eu, Gd, Dy, Ho, Tm, Yb, Lu; O<x<0.2) are disclosed in Nature, Vol. 329, 17.9. 1987, pages 227-229.
The known processes for preparing structured or oriented polycrystalline ceramic high-T.sub.c superconducting materials are complicated, time-consuming and not always readily reproducible. It has hitherto only been possible to prepare single crystals made of high-T.sub.c superconductors with relatively small dimensions. Accordingly, the object of the present invention is to provide a polycrystalline high-T.sub.c superconducting material containing grains which are crystallographically al

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Ishizawa et al "Superconducting Properties of Highly Oxidized Superconductor . . . " Jap. Journ. of App Physics, vol. 26, No. 5, pp. L676-677 May 1987.
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