Phase Equilibria in the System Tl-Ca-Ba-Cu-O. I. Stability of the 2122 Phase Under Conditions of Oxygen Annealing
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PHASE EQUILIBRIA IN THE SYSTEM TI-Ca-Ba-Cu-O. I. STABILITY OF THE 2122 PHASE UNDER CONDITIONS OF OXYGEN ANNEALING. L. P. Cook, C. K. Chiang, W. Wong-Ng, L. J. Swartzendruber, and L. H. Bennett, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899.
ABSTRACT Results are presented for 850'C oxygen annealing experiments (up to 62 hr) on the 2122 (TI:Ca:Ba:Cu) oxide phase, with and without intermediate grinding. Samples were characterized by powder X-ray diffraction and a.c. magnetic susceptibility. The 2122 phase appears to be thermodynamically stable in oxygen under conditions of both extensive grinding, and, annealing of long duration. However, onset T.'s vary from 71 K to 109 K for apparently single-phase 2122 samples with nearly identical X-ray powder patterns, depending upon the heat treatment.
INTRODUCTION Early experiments in our laboratory indicated the sensitivity of the high T. thallium-containing phases, both in terms of structural stability and superconducting properties, to conditions of synthesis. These phases do not appear to form readily if the thallium is added by vapor transport; rather, presence of solid or liquid thallium oxide may be required. Under mildly reducing conditions, the 2122 and 2223 phases form rapidly from mixtures of T1 2 0 3 , CaO and barium cuprate, but disappear upon further treatment, to yield a complex mixture of (Ba,TI,Ca) cuprates, which do-not have the desired superconducting properties. Initially it was thought that the 2122 and 2223 phases might be metastable under all conditions relative to formation of alternate, kinetically hindered, layered structures with lower Gibbs energies of formation. However we have, since then, demonstrated the long-term stability of the 2122 phase in air at 850°C [1]. Yet our measured Tc values for the air-annealed 2122 phase are significantly lower (95-101 K) than the in generally accepted value of 110 K, spite of excellent agreement with the Rot._ cove published X-ray pattern [2]. We observed 0, a relation
between
the amount of grinding
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and reheating and the sharpness of the Tc transition, as measured via a.c. magnetic susceptibility.
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With this as backgound, the present study was undertaken to a) confirm the phase stability of the 2122 oxide phase under conditions of oxygen annealing, and b) determine how the Tc values for the 2122 composition could be increased to above those of the air-annealed specimens.
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Fig.l. Experimental setup for oxygen annealing of the 2122 phase.
Mat. Res. Soc. Symp. Proc. Vol. 169. ,1990 Materials Research Society
138
EXPERIMENTAL METHODS A diagram of the experimental setup used in oxygen annealing experiments on the 2122 phase is shown in Fig. 1. The oxygen was passed through a moisture and CO 2 absorbant prior to introduction into the equilibration chamber. To prepare the 2122 compound, use was made of the fact that its composition plots on a line between BaCuO2 and CaO. Hence the starting mate
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