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J. Phys. Chem. C 2009, 113, 14658–14662
High-Pressure Response of Zirconia Nanoparticles with an Alumina Shell F. X. Zhang, M. Lang, and R. C. Ewing* Department of Geological Sciences, UniVersity of Michigan, Ann Arbor, Michigan 48109
J. Lian Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180
Z. W. Wang Cornell High Energy Synchrotron Source, Cornell UniVersity, Ithaca, New York 14853 ReceiVed: May 4, 2009; ReVised Manuscript ReceiVed: June 22, 2009
Nanosized (12-17 nm) zirconia particles with a thin alumina shell (2640 K). Both the tetragonal and cubic structures of ZrO2 are related to the fluorite structure in which Zr is in 8-fold coordination. The high-temperature phases can be stabilized by doping with other oxides,3 such as MgO, CaO, or Y2O3. Recently, the high-pressure behavior of ZrO2 has been extensively studied and several new phases have been discovered.16-28 The orthorhombic phase at high pressure and high temperature is quenchable and has a very large bulk modulus (444 GPa),18 suggesting that it may be a super hard material. In addition, due to the structural similarity, the high-pressure behavior of zirconia can provide insights into pressure-induced phase transitions in SiO2, which is a very important Earth material.16 However, there is still a controversy concerning the sequence of phase formation for pure ZrO2 at high pressures and temperatures. At room temperature, at least three high-pressure phases have been reported.16-18,22 The monoclinic ZrO2 transformed to an orthorhombic phase (orth-I) at pressures below 5 GPa, and another orthorhombic phase (orth-II) was found at higher pressures. The orth-II phase appears to be isostructural with cotunnite (PbCl2),10,16,18 and the Zr4+ has an increased coordination number (>8). Both the monoclinic and orth-I phases of ZrO2 have a distorted fluorite structure, and the phase transition between them is displacive. The phase transition from orth-I to orth-II is a nucleation and growth process, and increased temperature can * To whom correspondence should be addressed. E-mail: rodewing@ umich.edu.
greatly enhance the kinetics of the phase transition.17,22,29 The orth-II phase was found to transform to a new tetragonal structure at pressures greater than 35 GPa.30 The structure and electronic properties of various polymorphs of zirconia have also been widely studied by quantum mechanical calculations.9,20,27,28 However, some experimental results have indicated that more phases may occur for ZrO2 under high-pressure and hightemperature conditions or during the quenching process.17 Particle size, another important parameter, can change the stability of phases under ambient conditions or high pressures.31-33 Zirconia has been found to have a high-temperature tetragonal structure when the grain size is less than several tens of nanometers.2 The pressure-induced phase transition in the nanosized ZrO2 was found to be different from samples of larger grain size.19,26 The difference in the phase stability and phase transition behaviors at high pressure between micro- and nanosized materials is due to the competition between surface energy and bulk energy. Composite nanoparticles with “shells” of a different material have attracted more and more attention in recent years because of their wide applications in the fields of catalytic and biomedical engineering.34,35 From a structural point of view, the strong shell can protect the metastable phase in the core and make the phase behavior of the core material different from both bulk and nanoparticles. In this study, the structural behavior of nanosized ZrO2 particles with a thin Al2O3 shell (
