Compositions: ceramic – Ceramic compositions
Patent
1991-08-19
1993-05-04
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
505782, 505785, 501126, 252518, H01L 3924, C04B 3500
Patent
active
052082148
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a multiphase ceramic superconductor which contains the nonsuperconducting phase strontium indate and at least one superconducting phase composed of the elements bismuth, strontium, calcium, copper and oxygen, and also a process for its production.
The compound strontium indate, SrIn.sub.2 O.sub.4, has been known for a fairly long time (Z. Naturforschung 19b (1967) 955).
It is known that ceramic superconductors which are based on the elements bismuth, strontium, calcium, copper and oxygen generally contain only small proportions of a phase which is superconducting at 110K.
The object was therefore to find a process for increasing the proportion of the phase which is superconducting at 110K in the superconductor. The present invention solves this problem.
The invention is based on the discovery that the proportion of the phase which is superconducting at 110K and which is composed of the elements bismuth, strontium, calcium, copper and oxygen is increased if the phase formation takes place in the presence of strontium indate.
A process has now been found for producing a superconductor which contains at least one superconducting phase based on the elements bismuth, strontium, calcium, copper and oxygen, in which process a mixture of the oxides of bismuth, strontium, calcium and copper or corresponding oxide precursors is mixed and, at the same time, an atomic ratio of the elements is maintained which corresponds to an overall composition Bi.sub.4 (Sr,Ca).sub.k Cu.sub.k-2 O.sub.x, where k is a number from 4 to 8 and the Sr/Ca atomic ratio is 1:9 to 9:1, and the mixture is heated for a prolonged period in the presence of oxygen. The process is characterized in that generally 0.428 to 9 times the amount of strontium indate powder is added to the mixture before heating. Preferably, three times the amount of indate is mixed in.
Instead of strontium indate, the mixed-crystal compound strontium/calcium indate having all the Sr/Ca atomic ratios, for example even pure calcium indate, CaIn.sub.2 O.sub.4, may be used. The indates mentioned are inert under the conditions of synthesis of the superconductor. The proportion of indate added is therefore found again in the ceramic superconductor obtained.
The (Ca,Sr)In.sub.2 O.sub.4 may also be prepared in situ from the alkaline earth metal oxides and indium oxides or indium carbonate, for example in accordance with the equation O.sub.4 +CO.sub.2. an excess of alkaline earth metal oxide which is equimolar with the amount of indium oxide. It is important that indate added is in finely powdered form and is thoroughly mixed with the indium-free mixture.
By the specified process it is possible to prepare superconductors which contain, in addition to strontium indate, at least one of the superconducting phases Bi.sub.4 (Sr,Ca).sub.4 Cu.sub.2 O.sub.x or Bi.sub.4 (Ca,Sr).sub.6 Cu.sub.4 O.sub.x or Bi.sub.4 (Ca,Sr).sub.8 Cu.sub.6 O.sub.x, the Ca/Sr atomic ratio being 1:9 to 9:1 in each case.
The novel superconductors can be produced by thoroughly mixing oxides or oxide precursors of the elements Bi, Sr, Ca and Cu together with SrIn.sub.2 O.sub.4 and heating the mixture to temperatures of at least 700.degree. C.
During the reaction, the atomic ratio of the metals used does not change to a first approximation. The atomic ratio used therefore corresponds to the desired oxide composition.
As oxide precursors, it is generally possible to use compounds which react to form the corresponding oxides at the reaction temperature, in particular the hydroxides and nitrates. The acetates, formates, oxalates and also carbonates of the metals mentioned may furthermore be used. For example, calcium oxide, strontium carbonate, bismuthic acid, bismuth(III) oxide, bismuth(V) oxide Cu.sub.2 O and CuO may be used.
The reaction mixture does not need to melt or needs to melt only partially. In this case, however, it is necessary to maintain the reaction temperature for prolonged period.
In the latter case, the synthesis temperature is preferably between 700.degree. and 900.de
REFERENCES:
Japanese Journal of Applied Physics, Maeda et al., pp. L209-L210. Feb. 1988.
Nicolas et al, "Effect of Indium Subtitution in the 80K and 110K Superconductor of the Bi--Si--Ca--Cu--O System", Supercond. Sci. Technology 2(6) pp. 304-307, Dec. 1989.
Bell Mark L.
Bonner C. M.
Hoechst Aktiengesellschaft
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