Characterization of Un-stabilized Orthorhombic Zirconia Synthesized at Ambient Temperature and Pressure
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Characterization of Un-stabilized Orthorhombic Zirconia Synthesized at Ambient Temperature and Pressure Miriam P. Trubelja1, Donald Potter2, Claudia Rawn3, Karren More3, Joseph J. Helble4 1
Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3136
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Department of Materials Science and Engineering, University of Connecticut, Storrs, CT 062693136 3 High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6062 4 Thayer School of Engineering at Dartmouth, 8000 Cummings Hall, Hanover, NH 03755-8000 ABSTRACT Bulk structures of un-stabilized ZrO2-x with x in the 0 < x < 0.44 range under ambient pressure exist in three different structures (monoclinic, tetragonal and cubic). At ambient temperature and elevated pressures above 3.5 GPa, zirconia, at these compositions, a fourth phase is found, the orthorhombic structure. A dilute sol-gel method was used to produce nanoscale zirconia particles containing the unstabilized orthorhombic cotunnite structure for use in this project. Extensive characterization of this material indicates that the critical factor in determining the synthesized structures appears to be the number and placement of oxygen vacancies. These results also indicate that surface energy alone is not the controlling factor in determining the crystal structure synthesized. INTRODUCTION While zirconia exists as the stable monoclinic phase at ambient pressure and temperature, metastable forms of the material also exist under these conditions when the crystallites are in the nanoscale range. These metastable particles are generally tetragonal or cubic phases [1-7]. The first method ever developed for synthesizing the unstabilized orthorhombic cotunnite (Ortho II) structure [8] at ambient temperature and pressure was devised by Trubelja et al. [9, 10]. They carried out an exploration of a broad range of conditions which permitted the synthesis of particles in the orthorhombic, tetragonal and monoclinic phases. As synthesis conditions were carefully characterized by Trubelja et al [9], this work focused on structural characteristics which might account for the formation of the orthorhombic cotunnite and brookite structures. Two possible theories were considered, these being the surface energy, put forth by Garvie [3, 4] and a second the placement and size of the oxygen vacancy population within the structure, proposed by Hoskins and Martin [11-13]. Work by Bokhimi et al. [14] and others [9, 15, and 16] demonstrated that both unstabilized metastabled tetragonal and monoclinic zirconia can coexist at the same average primary particle sizes, d < 30 nm. Using X-ray diffraction (XRD) and Rietveld refinement to analyze the zirconia
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structures, Bokhimi et al. [14] determined that the hydroxyl population is effectively equal to the number of oxygen vacancies in the tetragonal structure. This made it possible for them to use Fourier Transform Infra-Red spectroscopy to correlate the hydroxyl populations (i.e. oxygen vacancies) with the amount of the tetragonal
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