J. Am. Cem. zyx Soc., zyx 71 [61177l-73 (W90) Rapid Formation of the 110 K Phase in Bi-Pb-Sr-Ca-Cu-0 through Freeze-Drying Powder Processing z Kang-HoSong, Hua-kun Liu, Shi-xue Dou,* and Charles C. Sorrell* School of Materials Science and Engineering, University of New South Wales, Kensington, New South Wales zy 2033, Australia Three zyxwvutsrq techniques for processing Bi-Pb-Sr-Ca-Cu-0 (BPSCCO) powders were investigated: dry-mixing, sol-gel formation, and freeze-drying. It was found that sintering for 120 h at 850°C is required to form nearly single-phase (Bi, Pb)2SrzCazCu30,,-, by dry-mixing, whereas sintering for 30 h at 840°C was sufficient to form the 110 K (2223) phase when freeze-drying was used. The sol-gel route was found to be intermediate in efficiency between these two techniques. Freeze-drying provided highly reactive, inti- mately mixed, and carbon-free precursors. The presence of carbonates in the uncalcined powders was the major cause of phase segregation and sluggishness of the 110 K phase formation. [Key words: superconductors, freeze-drying, mixing, sol-gel, processing.] I. Introduction T IS zyxwvutsrqponmlk well-known that it is difficult to produce single-phase IBi zSr2Ca2Cu3010-y (2223), despite the use of variable start- ing compositions and heat-treatment conditions.' Partial Pb substitution for Bi promotes the formation of and stabilizes the 110 K phase,' but long sintering times and close tempera- ture control are required to assure major 110 K phase devel- opment when conventional ceramic-processing techniques, such as solid-state reaction, are used. In particular, when CaC03 and SrC03 are used as starting materials, large segre- gated particles of Sr-Ca-Cu-0, Ca-Cu-0, and Cu-0 phases are commonly found in the sintered materials. In coprecipitation and codecomposition techniques, the use of carbonates can be avoided, but it is difficult to control the stoichiometry with the former. It is difficult to achieve a uniform composition in the product with the latter. Further- more, oxalic acid used in the coprecipitation process forms carbonates, such as SrC03 and CaC03, directly as the first stage of decomposition. The sol-gel procedure has been used to produce ultrafine powders with uniform particle-size distribution. However, the dissociation of citric acid used in this process may also yield carbonates. Freeze-drying has been used to produce high-quality pow- ders of YBa2C~307-r,3.4 but there do not appear to be any re- ports discussing the freeze-drying of BPSCCO. Freeze-drying is an advantageous technique in that a solution is flash frozen to prevent segregation, thus giving mixing at nearly the atomic level without recourse to the use of precipitating agents or the risk of carbonate formation. P. K. Gallagher-contributing editor Manuscript No. 198003. Received November 3, 1989; approved January Supported by Metal Manufactures, Ltd. (S.X.D.), and the Common- "Member, American Ceramic Society. 26, 1990. wealth Department of Industry, Technology, and Commerce (H.K.L.). 11. Experimental Procedure Samples of BPSCCO were prepared through three different routes: dry-mixing, sol-gel formation, and freeze-drying. The precise stoichiometries attained were not compared through postmixing assays. In the dry-mixing procedure, BPSCCO was prepared by hand-mixing, calcining, and sintering Bi203, PbO, SrC03, CaC03, and CuO in the cation ratio Bi/Pb/Sr/Ca/Cu = 1.6/0.4/1.6/2/3. The details of this procedure have been de- scribed previously.' In the sol-gel method, the appropriate amount of citric acid, depending on the viscosity, was added to a solution of Bi, Sr, Ca, Cu, and Pb nitrates. The viscous solution was evaporated under vacuum at 50" to 60°C for 24 h, dried in an oven at 110°C for 12 h, and calcined at 650°C for 10 h. The fine powder was pressed into pellets and sintered in air at 840°C for 10 to 120 h. To avoid using and forming carbonates, precursors were prepared through a freeze-drying route. Metal nitrate solutions were made by weighing and dissolving Biz03 in nitric acid; Pb(NO3)2, Sr(NO&, Ca(N03)2. H20, and Cu(N03)~. 3H20 in distilled water, separately; and mixing the five solutions in the cation ratio Bi/pb/Sr/Ca/Cu = 1.6/0.4/1.6/2/3. The nitrate solution was flash frozen by at- omization into liquid nitrogen. The frozen solution was placed in a freeze drierZ with a programmable controller and dried under vacuum (=0.13 Pa torr)). The dried pow- der was transferred to an alumina crucible, placed in a muffle furnace preheated to 200"C, and calcined in air at 830°C for 10 h. The calcined powder had an average particle size of 5 to 10 pm. The powder was then hand ground using a porcelain mortar and pestle, pressed into pellets, and sintered in air at 840°C for 10 to 50 h. The electrical resistivity was measured on bar-shaped speci- mens by the standard four-probe direct-current technique using a current of 10 mA. X-ray diffraction (XRD) patterns were obtained with a powder diffractometer' using CuKa ra- diation. Microstructural and compositional analyses were performed by scanning electron microscopy (SEM)* with en- ergy dispersive spectrometry (EDS).# 111. Results and Discussion Previous work' has shown that full development of the Pb- free 110 K phase 2223 may require sintering times of up to 10 d, although partial Pb substitution for Bi can accelerate this time. Figure 1 shows the temperature dependence of the normalized resistivity for Pb-doped BPSCCO samples in which the powders were prepared by dry-mixing. T, measure- ments with EDS analyses show that sintering for 120 h at 840°C is sufficient to form the 110 K phase in Pb-doped *Model F.D 400, Dynavac Engineering Pty., Ltd., Wantirna South, Victo- ria. Australia. . .-, - _-_ - - 'Model PW 1140/00, Phili s Electronics Instruments, Inc., Mahwah, NJ. 'Model JXA-840, JEOL, ftd., Tokyo, Japan. 'Model AN10000, Link Systems, Ltd., High Wycombe, Buckinghamshire, England, U.K. 1771