Phase diagram of Cu 2 O-CuO-Y 2 O 3 system in air

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I.

INTRODUCTION

HIGH-Tc superconducting material, YBa2CU306+x, which was discovered by Wu e t al.,tl] can usually be prepared by solid-state sintefing or by directional solidification. In order to obtain specimens with good quality by these preparation techniques, the phase diagram is, of course, indispensable. Until now, several experimental works have been performed to clarify the phase diagram of the Y203-BaO-CuO ternary system and related binary diagrams, t2-71 However, these diagrams seem to be quite insufficient for practical use. The establishment of more correct diagrams is urgently requested. For a basic understanding, it is necessary to first make clear the terminal binary diagrams. In our previous paper, the phase diagram of the BaO-CuO binary systemfTl was reported precisely. Here, the phase diagram of the Cu20-CuOY203 ternary system in air is reported. In the Cu20-CuO-Y203 system, the compound of Y2Cu205 was reported to exist in air by Arjomand and Machin. tsl The crystal structure of Y2Cu205 is orthorhombic with the space group of Pna2~ (33) and the lattice constant of a = 1.0779, b = 0.3496, and c = 1.2456 nm. Arjomand and Machint8j also prepared the compound YCuO 3 at 1073 K under 400 atm oxygen pressure, which has a hexagonal symmetry with a = 0.531 and c = 1.322 nm, respectively. With decreasing the oxygen partial pressure, another compound, YCuO2, was found to exist by Ishiguro e t al. t9] They prepared this compound by heating the Y2Cu205 at 1373 K in a stream of N2 and air under 0.01 oxygen partial pressure. The CuO phase is sensitive to the oxygen partial pressure and changes easily to Cu20, depending on the temperature and oxygen partial pressure. The phase diagram of Cu20CuO has been reported by Vogel and Pocher l~~ and reproduced by Gadalla e t al. t~ll as a function of oxygen partial pressure. After the discovery of the high-To superconducting material, Roth e t al. t21 reported on the preliminary diagram of the Y203-CUOx binary system. The melting point of the Y2Cu205 in air was examined by quenching the portion of the compound held in a small open Pt tube from a vertical quench furnace. Their phase diagram is, however, not correct in part, as made clear in the present paper.

WEI ZHANG, Graduate Student, and KOZO OSAMURA, Professor, are with the Department of Metallurgy, Kyoto University, Sakyo-ku, Kyoto 606, Japan. Manuscript submitted August 8, 1989. METALLURGICALTRANSACTIONSA

II.

EXPERIMENTAL PROCEDURE

Y203 and CuO powders with purities above 99.9 pct were used in this work. An appropriate amount of Y203 and CuO powders was mixed by an agate mortar for 4 hours and calcined at 1173 K for 48 hours to get an equilibrium state. Sintered specimens were cooled to room temperature by air cooling. The existing phases were identified by the powder X-ray diffraction with a Cu-K~ characteristic X-ray and electron probe microanalyzer (EPMA). Microstructures were observed by scanning electron microscope (SEM) and optical microscope. Thermal analysis experiment was performed using the thermogr

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