Diffusion calculations for the 80-K-to-110-K Bi(Pb)SrCaCuO superconducting phase transformation
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ARTICLES Diffusion calculations for the 80-K-to-110-K Bi(Pb)SrCaCuO superconducting phase transformation Wen Zhu,a) Chu Kun Kuo, and Patrick S. Nicholson Ceramic Engineering Research Group, Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7 (Received 8 September 1998; accepted 30 August 1999)
A diffusion model is proposed to fit the measured chemical-transformation rates of the Bi(Pb)CaCuSrO 80-K phase to 110-K phase. Diffusion coefficients and activation energies in PO2 ⳱ 0.08 and 0.21 atm are reported. The low diffusion rates and high activation energies suggest cation diffusion controls the transformation.
I. INTRODUCTION
Significant effort has been made to synthesize the single-phase (Bi,Pb)2Sr2Ca2Cu3Ox (110 K or 2223) superconductor because of its high critical temperature (110 K) and promising applications. The phase formation is sluggish, and single- or nearly single-phase usually involves 50–100 h, sometimes even several hundred hours.1–3 Phase formation has been explained by diffusion,4–8 nucleation-growth,9,10 chemical reaction,11–15 and low-level mobile liquid droplet16 mechanisms. A systematic investigation of Bi1.84Pb0.34Sr1.91Ca2.00Cu3.04Ox nominal composition revealed the 110 K phase formed preferably in oxygen partial pressure and temperature ranges, 0.08 艋 PO2 艋 0.21 atm and 820 艋 T 艋 860 °C, via chemical reaction between the 80 K phase [Bi 2 Sr 2 Ca 1 Cu 2 O 8 (2212)] and Ca–Cu-containing phases.17 The presence of small amounts of liquid catalyzed this reaction, but increasing liquid phase at high temperatures promoted decomposition of the 110 K product. Though liquid phase was observed in grain boundaries, no “mobile” droplets were detected by SEM. Previous studies suggest that diffusion is the ratecontrolling step in the 80-K-to-110-K phase transformation. Diffusion of various species has been studied in superconductors systems. Oxygen ion diffusion in YBa 2 Cu 3 O x (YBCO) 18–20 and BiSrCaCuO (BSCCO) 21–25 superconductors has been meaured by O18 tracer, in situ resistivity, thermogravimetry, and oxygen ion conductivity methods. The O18 ion diffusivity in 80 K polycrystal was determined to be 10−12–10−7 cm2 s−1 at 400–800 °C. Cation diffusion in YBCO26,27 and BSCCO28,29 has been investigated via tracer method. Their diffusivities are several orders of a)
Present address: IREQ-Hydro-Que´bec, 1800 boul. Lionel Boulet, Varennes, Que´bec, J3X-1S1, Canada. J. Mater. Res., Vol. 14, No. 11, Nov 1999
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magnitude smaller than oxygen ion diffusivities in the corresponding phases. Table I lists the diffusivities for several cations in the YBCO and BSCCO systems at 850 °C. The reported Sr diffusivities differ by 2 orders of magnitude. Chen28 considered this difference was due to the sample preparation method used in Ref. 29. The diffusion of cations in the single crystals of YBCO and BSCCO is anisotropic; i.e., the diffusivities in the c-axis direction
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