Growth kinetics of high T c phase in Bi-Sr-Ca-Cu-O thick films
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Thick films of B i - S r - C a - C u - 0 on (100) MgO substrates have been prepared by the screen printing technique with the starting composition of 1112. The annealing process involves a two-step heat treatment. The first-step temperature is chosen between 874 °C and 898 °C for 45 min followed by slow cooling (2 °C/min) to 864 °C. In the second step, the films are kept at 864 °C for a time varying from 0 to 104 h. The Tc (R = 0) and intensity of the characteristic peaks of the high Tc and the low Tc phases in the x-ray diffraction pattern are found to be dependent on the first-step temperature and the duration of the second step. The process of crystallization and the growth of plate-like grains as a function of annealing time at the second step are studied using Scanning Electron Microscopy (SEM). Kinetics of the growth of the high Tc phase has been explained in terms of a reaction mechanism involving the first and second annealing steps. I. INTRODUCTION The screen-printing technique is one of the widely used techniques to fabricate thick films for microelectronic applications. Recently the screen-printing technique has been used successfully to prepare Y-Ba-Cu-0 (YBCO)1-2 and Bi-Sr-Ca-Cu-0 (BSCCO)3"5 superconducting films. BSCCO superconductors have several advantages over YBCO, such as compositional stability and inertness to moisture. BSCCO is known to exist in three phases with nominal compositions 2201, 2212, and 2223 having Tc of 20, 85, and 110 K, respectively.6"9 Since the discovery of the BSCCO system by Maeda et al.,w efforts have been made to obtain a single high Tc 2223 phase in bulk as well as in thin/thick films. It is relatively easier to prepare single 2212 phase material in bulk or film form as compared to single high Tc 2223 phase material. Several starting compositions have been used in an attempt to prepare a single phase 2223 BSCCO superconductor, but in general these lead to multiphase material. The growth of the high Tc 2223 phase in the BSCCO system has been found to be very susceptible to the heattreatment conditions and environment. Sintering under low oxygen pressure is conducive to the growth of the high Tc phase.11 Annealing of the BSCCO material near its melting temperature and heating it for a longer duration12 is necessary for growing a substantial amount of the high Tc phase. It has also been shown that addition of extra Ca and Cu to the ideal composition of 2223 enhances the growth of the high Tc phase.13 Takano et al.u have observed that Pb substitution for Bi stabilizes and promotes the growth of the high Tc phase. Pb addition is found to lower the melting temperature and favors the diffusion of constituents for the growth of the high Tc phase. Moreover, the addition of Ag in B i ( P b ) - S r - C a - C u - 0 material alters15-16 the J. Mater. Res., Vol. 7, No. 8, Aug 1992 http://journals.cambridge.org
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optimum oxidation and annealing conditions, promotes the formation of the high Tc phase, reduces the porosity of the material, and increases the critic
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