Chemical Preparation of Superconducting Oxides Using Conventional Chelating Agents
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CHEMICAL PREPARATION OF SUPERCONDUCTING OXIDES USING CONVENTIONAL CHELATING AGENTS Cyril Chiang and C. Y. Shei Materials Research Laboratories, Hsinchu, 31015, Taiwan, ROC.
Industrial Technology Research Institute,
ABSTRACT Samples of Bij. 4Pb 0. 6Sr2 Ca2Cu3O, were prepared by sol-gel technique. The formation of polymeric chelate was made possible by oxalic acid in which metal cations are homogeneously distributed. The functional groups in the chelating agent determine the cation absorption rate that pH value is no more critical. XRD and SQUID showed that the compound has a nearly pure 2223 phase and Tr was measured at 110 K. The synthesis is reproducible and the process is amenable for scaling up.
1. INTRODUCTION It is known that there are two Bi-Sr-Ca-Cu-O superconducting phases with critical temperature of 110 K (2223 phase) and 80 K (2212 phase).[l] Conventional solid state reaction of oxides with nominal composition of Bi 2 Sr2 Ca2 Cu3Oy does not guarantee the formation of a single high-T, phase. Takano et al. discovered that the partial substitution of Pb for Bi may stabilize the 2223 phase.[2] It was also revealed that additional Ca and Cu could accelerate the 2223 phase formation.[3] Solid state reaction has slow kinetics because of the diffusion length. In order to prepare mainly high-T. phase, the use of chemical precursors is essential in order to reduce the path between particles. The sol-gel technique generates ultrafine precursor powders permitting to lower the processing temperatures. Based on the formation of citrate complexes, the sol-gel process has been reported to prepare ceramic superconductors in the Y-Ba-Cu-O system. [4] Sol-gel was not considered practical for the Bi-Sr-Ca-Cu-O system because of the low solubility of bismuth salt. Scientists reported that EDTA (ethylene-diamine-tetra-acetic acid) is capable of retaining bismuth cations in aqueous solution but the acidity has to be carefully normalized.[5] This paper describes a chelate of mixed cations, regardless of the bismuth, with a hydroxycarboxylic acid. The synthesis allows us to prepare pure 110 K phase without being required to monitor the pH in a small range. In our study, sol is suppressed since bismuth cations are not completely complexed and the process is more likely a gelation step. Oxalic acid (HOOC-COOH) was used to chelate metal cations from corresponding metal nitrates. The exclusion of bismuth from the complexed precursor reduces the bismuth-loss during the sintering step, which consequently helps to form 110 K phase more accurately. The amount of the chelating agent has an impact on the organic decomposition, which determines a "clean" reaction. The molar ratio of oxalic acid to total nitrate anions from the metal salts was fixed at one half, in contrast to the claimed amount of two.[6] The following sections explain the advantages of the synthesis.
2. EXPERIMENTAL The gel preparation was carried out with Bi(N0 3 ) 3 "5HzO, Pb(N0 3 )2 , Sr(NOs) 2 , Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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