Two-stage sintering of dense superconducting YBa 2 Cu 4 O 8 ceramics

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Two-stage sintering of dense superconducting YBa2 Cu4 O8 ceramics Motohide Matsuda,a) Yoshihisa Ogawa, Kimihiro Yamashita, and Takao Umegaki Department of Industrial Chemistry, Faculty of Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-03, Japan

Michihiro Miyake Department of Environmental Chemistry and Materials, Faculty of Environmental Science and Technology, Okayama University, 2-1-1 Tsushima-Naka, Okayama 700, Japan (Received 24 June 1996; accepted 9 January 1997)

Dense YBa2 Cu4 O8 (124) ceramics were prepared by the following two-stage sintering: compressed 124 powders were heated to 1203 K for decomposition to YBa2 Cu3 O72x (123) and CuO, and then cooled down to 1073 K and annealed for the re-formation of 124 single phase. The relative densities thus obtained were 93 –95%, as high as those prepared under high oxygen pressure. The densification was considered to take place due to the liquid phase. In the sintered 124, the island-like fine grains of CuO were observed within the 123 matrix. As a result, it was considered that the re-formation of 124 proceeded in each grain. The geometrical phase distribution in the grains is discussed on the basis of superconducting properties from resistivity measurements.

I. INTRODUCTION

From a practical point of view, superconducting 124 has the excellent advantages of high thermal stability, unchangeable oxygen content, and nontoxic components. The 124 ceramics have, however, been weak in densification.1–4 Because low bulk density causes low critical current density, densification is of great importance. Williams et al. have prepared dense 124 ceramics with relative density above 90% which led to high electric conductivity.5 The combination of hot isostatic pressing with the complex sintering process was needed in their preparation. Triscone et al. have obtained the relative density of ca. 94% by sintering under high pressure.6 Both preparations were carried out under high pressure. The present authors have recently shown that the dense 124 ceramics can be prepared even under ordinary pressure.7 The method was so-called twostage sintering, which consists of sintering at 1203 K (first stage) followed by annealing at 1073 K (second stage). The sintering was performed in the instability of 124 according to a previous report.8 X-ray diffraction analyses confirmed the decomposition of 124 of starting phase at the first stage. The decomposition products were 123 and CuO. Although it was considered that the densification was accomplished at the first stage, the driving force for the densification has remained unknown. The 123 and CuO were not confirmed after a)

Author to whom correspondence should be addressed, at the Department of Environmental Chemistry and Materials, Faculty of Environmental Science and Technology, Okayama University, 2-1-1 Tsushima-Naka, Okayama 700, Japan. J. Mater. Res., Vol. 12, No. 5, May 1997

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