Metal-Organic Decomposition and Microstructure Development in Ba 2 ycu 3 o 7-X Films from Metal Trifluoroacetate Precurs
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METAL-ORGANIC DECOMPOSITION AND MICROSTRUCTURE DEVELOPMENT IN Ba2 YCu3O,, FILMS FROM METAL TRIFLUOROACETATE PRECURSORS PAUL C. MCINTYRE, RAYMOND C. CHIU, MICHAEL J. CIMA, AND WENDELL E. RHINE Ceramics Processing Research Laboratory, Massachusetts Institute of Technology, Cambridge, MA. ABSTRACT Decomposition of metal-organic precursors to Ba 2YCu3O,.. films is difficult because of the high reaction temperature required to decompose the BaCO3 intermediate. The recently proposed use of metal trifluoroacetate (TFA) solutions offers an alternative path to barium-containing superconducting films. The TFA salts decompose to the metal fluorides forming BaF2, eliminating BaCO3 from the system. Ultimate conversion to BYC, however, is shown not only to depend on hydrolysis of the BaF2 at high temperatures, but also hydrolysis of copper trifluoracetate at low temperatures to prevent the volatilization of Cu(TFA)2 . These processes result in unique microstructural behavior which can be characterized by electon microscopy and Auger spectroscopy. Effects due to substrate interactions have been eliminated by use of a chemically inert substrate material, BaZrO 3. INTRODUCTION A disadvantage associated with preparing Ba2YCu3O,., from metal-organic precursors is the stability of BaCO3, which forms as an intermediate compound during decomposition. Inability to decompose BaCO, is the probable cause of the carbon contamination that is frequently detected in these films [1]. The metal trifluoroacetate (TFA) system recently used by Gupta et al. [2] appears to avoid the formation of BaCO3 during decomposition of the precursor. The TFA salts decompose to barium, yttrium, and copper fluorides below 400C, and these are subsequently converted to the oxides at higher temperatures by reaction in a moist furnace atmosphere. Initial investigations indicated that films prepared from this precursor had relatively good electrical properties (T,(O)>90 K; J,>1000 A/cm2 at 77K) and were highly textured with the c-axis of the defect-perovskite unit cell normal to the film plane. The mechanism by which this c-axis orientation develops was not understood, but was linked to the presence of BaF 2 in the film at the start of the high temperature calcination. The strong c-axis texture and improved transport properties observed in BYC films prepared by laser ablation and electron beam evaporation with BaF, as the barium source [3, 4] tend to support this hypothesis. The texture observed in TFA precursor-derived films is not related to epitaxy, as it occurs when films are deposited on oriented single crystals or on polycrystalline substrates [2]. In this work, the chemical and microstructural changes that occur during heat treatment of the TFA-derived films were investigated. Issues addressed included the stability of BaF, during the high temperature calcination and the effect of furnace atmosphere on film microstructure and transport properties. Polycrystalline BaZrO3, a dielectric that has been shown to be chemically inert with respect to BYC over the tempera
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