Superconductor technology: apparatus – material – process – High temperature – per se – Having tc greater than or equal to 150 k
Patent
1989-11-27
1993-08-17
Bell, Mark L.
Superconductor technology: apparatus, material, process
High temperature , per se
Having tc greater than or equal to 150 k
505782, 505783, 505784, 501123, 501126, 264 65, H01B 1206, H01L 3912, C01G 300, C04B 4189
Patent
active
052368890
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a process of producing (Bi or Tl)--Ca--(Sr or Ba)--Cu--O based superconducting ceramics, which causes little variation in the composition upon heat treatment such as calcination so that excellent superconducting ceramics materials can be prepared.
BACKGROUND ART
Bi--Ca--Sr--Cu--O based, Tl--Ca--Sr--Cu--O based, Tl--Ca--Ba--Cu--O based and Tl--Ca--Ba--Cu--O based superconducting ceramic materials (hereinafter, abbreviated as (Bi,Tl)--Ca--(Sr,Ba)--Cu--O based superconducting ceramic materials) have recently been proposed. It is well known that these ceramic materials are prepared by a process including the steps of providing starting powders, i.e., powders of Bi oxide (Bi.sub.2 O.sub.3), Tl oxide (Tl.sub.2 O.sub.3), Ca carbonate (CaCO.sub.3), Sr carbonate (SrCO.sub.3) or Ba carbonate (BaCO.sub.3), and Cu oxide (CuO), compounding and mixing them in a predetermined compounding ratio to obtain a mixed powder, subjecting the mixed powder repeatedly two or three times to calcining at a temperature of from 700.degree. to 800.degree. C. for a certain retention time and grinding to form a (Bi or Tl)--Ca--(Sr or Ba)--Cu--O based superconducting oxide powder, press-molding the superconducting oxide powder under ordinary conditions, and then sintering the powder.
However, the compositions of (Bi or Tl)--Ca--(Sr or Ba)--Cu--O based superconducting oxide powders prepared by the above--described conventional process tends to vary because volatile components, such as Bi and Tl, evaporate during calcination. This results in inadequate superconducting characteristics.
Also, when a carbonate powder, such as Ca carbonate, Sr carbonate, or Ba carbonate is used as starting powders, carbon remains even after repeated calcination of the powders. It is difficult to reduce the carbon content to 0.5% or less by weight by, for example, 5 repetitions of calcination and grinding. When such high-carbon-content (Bi or Tl)--Ca--(Sr or Ba)--Cu--O based superconducting oxide powder is used as a starting material for preparing superconducting ceramic materials, the carbon residue deposits at grain boundaries during calcination, giving inadequate superconducting characteristics.
DISCLOSURE OF INVENTION
Accordingly, the present inventors have intensively investigated in order to produce (Bi or Tl)--Ca--(Sr or Ba)--Cu--O based superconducting ceramics characterized by little variation of the composition from the foregoing viewpoint, and as a result they have found that when a mixing powder obtained by compounding and mixing low vapor pressure (less-volatile) components of the starting powders, that is, powders of Ca, Sr or) Ba, and Cu compounds in a predetermined compounding ratio is subjected to primary calcination at a high temperature of from 850.degree. to 1050.degree. C. to form complex oxides of Ca, Sr or Ba, and Cu (hereinafter, the oxide being referred to as a Ca--(Sr or Ba)--Cu--O based oxide), the Ca--(Sr or Ba)--Cu--O based oxide does not vary in their compositions upon high temperature calcination because it has no volatile component. Further, it has been found that when a volatile component, e.g., Bi.sub.2 O.sub.3 powder or Tl.sub.2 O.sub.3, is compounded and mixed with the Ca--(Sr or Ba)--Cu--O based oxide in a predetermined compounding ratio, and subjected to secondary calcining at a temperature of from 500.degree. to 820.degree. C. a (Bi or Tl)--Ca--(Sr or Ba)--Cu--O based superconducting oxide powder is obtained which has little variation in composition. Furthermore, it has been found that superconducting ceramics materials prepared using the above-obtained superconducting oxide powders as starting material have high superconductivity characteristics.
Also, it has been found that when a carbonate type of compound is used as the low vapor pressure component, Ca--(Sr or Ba)--Cu--O based oxides produced by this method contain little or no carbon because the carbonate compound is completely decomposed in a high temperature calcination treatment, that a (Bi or Tl)--Ca--(Sr or Ba)--Cu--
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Ouchi Yukihiro
Sakurai Takeshi
Sugihara Tadashi
Takeshita Takuo
Bell Mark L.
Bonnar C. M.
Mitsubishi Materials Corporation
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