The Influence of Morphological Aspects in the Reaction Pathway and Superconductive Properties of Bi2223-Ca 2 CuO 3 Ceram
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Moliner 50, 46100-Burjassot (Valencia), Spain. ABSTRACT A variation of the solid-state matrix technique has been developed as a procedure for the preparation of 2223 bismuth superconducting phases [(BiPb) 2 Sr 2Ca 1 -nCunO 4 +2n, n=3]. The adequate processing of samples of nominal compositions 2223+x 0021, obtained both by this procedure and by the polymer matrix method, has allowed the modification of the microstructure of the calcium copper excess, with no noticeable change in that of the 2223 grains. The study of the phase evolution in the processing of the samples has shown that the formation rate of the 2223
phase is faster when the particle size of the calcium cuprate is smaller. A dependence of the intergranular superconducting properties on the secondary phase is observed, appearing as a maximum in Jc0]4cer at x=0.2 and x=0.5 for samples prepared by solid state matrix and polymer matrix methods, respectively. INTRODUCTION We have recently described two new routes allowing the fast obtention of the n=3 phase, namely freeze-dryingI and polymer-matrix processing. 2 In both cases, the presence of an excess of calcium and copper was found not only to play a remarkable kinetic role in the 2223 formation, but also has a clear influence in the intergranular superconducting properties. We report here preliminary results of a systematic study on the dependence of the reaction pathway and intergranular superconducting properties with respect to the secondary phase excess 3 and microstructure. In order to reach this objective, a variation of the solid state matrix technique 4 has been developed as alternative preparative procedure. The use of this method, comparatively
with the polymer matrix technique, has allowed the modification of the microstructure of the calcium-copper excess without a noticiable difference in the microstructure of the 2223 grains. By processing samples obtained by both methods, using the same experimental conditions, results can be compared to furnish the required information. EXPERIMENTAL
All the samples studied in this work have been prepared by the polymer matrix technique (method I) and by the solid state matrix procedure (method II). Details of the preparation by the method I have been described elsewhere. 2 The nominal composition of the samples is Bi:Pb:Sr:Ca:Cu=l.84:0.34:2:2+2x:3+x (x=0.0, 0.1, 0.15, 0.2, 0.5, 1.0). The starting reagents used for the method II are the 2201 phase (Bi:Pb:Sr:Cu= 1.84:0.34:2:1), obtained by the polymer matrix technique, and calcium cuprates, obtained by thermal decomposition of the bimetallic formate precursors CaCu(HCOO) 4 and Ca2 Cu(HCOO) 6 .5 The molar relation among the reagents is 2201 +2CaCuO 2+xCa 2CuO 3 , to give samples of the same nominal composition as those prepared by the polymer method. The samples were then pressed at 200 MPa to obtain pellets of 12.0 mm of diameter, and sintered at 850'C in air for 40 hours, with two intermediate grindings and pelletizations (with the same conditions). For susceptibility measurements, the samples were cut
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