Diffraction studies of cubic phase stability in undoped zirconia thin films
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Diffraction studies of cubic phase stability in undoped zirconia thin films S.C. Moulzolf and R.J. Lad Laboratory for Surface Science and Technology, Department of Physics and Astronomy, University of Maine, Orono, Maine 04469-5764 (Received 6 August 1999; accepted 18 November 1999)
Pure stoichiometric ZrO2 films were deposited on amorphous silica substrates by electron beam evaporation of Zr in the presence of an electron cyclotron resonance oxygen plasma. Grain size, strain, and texture were analyzed by x-ray diffraction and reflection high-energy electron diffraction. Films grown at room temperature are polycrystalline and exist in the cubic phase. Growth at elevated temperatures produces coexisting cubic and monoclinic phases and shows a maximum critical grain size of ∼10 nm for stabilization of the cubic phase. Pole figure analysis indicates a preferred cubic [200] fiber axis for room-temperature growth and dual monoclinic {111} and {111} in-plane textures for films grown at 400 °C. Postdeposition annealing experiments confirm the existence of a critical grain size and suggest mechanisms for grain growth.
I. INTRODUCTION
Bulk zirconia exhibits three equilibrium solid phases: monoclinic to ∼1100 °C, tetragonal to 2370 °C and cubic up to the melting point of 2680 °C.1 Through appropriate processing and cation doping, metastable crystallites of the high-temperature phases can exist at room temperature in bulk solids.2 Using surface free-energy arguments, Garvie has suggested that a grain size effect can explain the existence of tetragonal phase zirconia well below the bulk transformation temperature without added dopants.3 Furthermore, Garvie’s calculations show that the critical grain size at a given temperature is increased by the presence of intrinsic stresses. In this context, critical grain size is defined as the maximum crystallite size above which transformation from a metastable to the bulk equilibrium phase spontaneously occurs. Zirconia is typically stabilized in the cubic or tetragonal phases by addition of a dopant such as yttria, calcia, or magnesia.4 However, the mechanisms responsible for tetragonal and cubic phase stability are not well understood and can depend critically on amount and type of dopant.5 Thin films present an opportunity for stabilization of the high-temperature phases of zirconia at room temperature without the aid of dopants. For example, Aita et al. have produced polycrystalline tetragonal6 and cubic7 zirconia by use of a nanolaminate architecture with alumina and titania interruption layers, respectively. In previous work, we have shown that films consisting of epitaxially oriented cubic zirconia grains can be synthesized at low coverages on (0112) and (0001) sapphire by substrate J. Mater. Res., Vol. 15, No. 2, Feb 2000
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lattice matching.8,9 Films were deposited with epitaxial orientations of (001) c-ZrO2 㛳 (0112) sapphire with [100] c-ZrO2 㛳 [1210] sapphire and (111) c-ZrO2
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