Flux-Assisted Preparation of Powdered Bismuth Superconductors
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FLUX-ASSISTED PREPARATION OF POWDERED BISMUTH SUPERCONDUCTORS R. H.Arendt, M. F. Garbauskas and L. L. Schilling General Electric Company, Corporate Research and Development P. 0. Box 8, Schenectady, NY 12301 ABSTRACT Results are presented on efforts to prepare both the 80K, Bi2CaSr 2Cu2Oz (2122), and 110K, (Bi,Pb)2Ca 2Sr2Cu 3Oz (2223), transition temperature bismuth superconducting species in either phase pure or nearly phase pure, powdered form using either equimolar NaCI-KCI or NaBiOz as molten fluxes. Data are presented on the effects of variation of the reaction temperature and duration, solvent content, relative CaO and CuO contents, and of post-synthesis anneals of the product materials. INTRODUCTION The conventional preparation of the bismuth-based oxide superconductors typically involves the solid-state reaction of the cation oxides 1 , or compounds, such as carbonates, which yield the oxides on heating, or of precusor oxide species containing a sub-set of the cations as either compounds or solid-solutions 2 . Common features of these preparations are long times of reaction, the absence of final product phase purity, and aggregation of the product crystals. An alternate approach to the synthesis of complex oxide materials as relatively mono-dispersed powders has been the use of molten salts as a fugitive reaction medium 3 ,4 . The present study evaluated the use of melts of NaBiOz and of the NaCI-KCI eutectic (50:50 molar) as reaction media for the synthesis of both the (2122) and (2223) species. The former solvent was used at low levels and accepted as a second phase impurity since no means could be devised to separate it from the superconductor without simultaneously destroying the desired species. The latter solvent was removed by distilled water-washing the reacted mass, yielding a substantially chloride-free, powdered product. MATERIAL PREPARATION The samples were prepared by reacting powdered Bi20 3 , and PbO (Bi:Pb=1.7:0.3) if present, with separately reacted precusor mixtures of the alkaline earth and copper oxides 2 . Three compositions of the precursor were used: CaSr2Cu205 [1:2:2], Ca 2Sr 2Cu 307 (2:2:3] and Ca2.7s~r2Cu 3 .750 8 .5 [2.75:2:3.75]. The alkali halides were used in their available crystalline forms; the NaBiOz was generated as part of the reaction from anhydrous Na2CO 3 and Bi20 3 . All materials were reagent grade. The reaction mixtures were prepared by dry-milling the powdered materials for 30 minutes with dense zirconia media in a polyethylene jar. The reactions were carried out in shallow, covered Au pans. The 5.0 g samples were plunged into a pre-heated furnace at the reaction temperature, reacted for the desired period, then withdrawn from the furnace. The reaction temperatures and times are contained in the accompanying tables.
Mat. Res. Soc. Symp. Proc. Vol. 169. t1990 Materials Research Society
398
Both the NaBiOz- and chloride-containing samples were ground prior to their evaluation. In addition, a portion of the chloride-based samples were treated with room temperature
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