High-Resolution Analytical Electron Microscopy Investigation of Metastable Tetragonal Phase Stabilization in Undoped, So
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High-Resolution Analytical Electron Microscopy Investigation of Metastable Tetragonal Phase Stabilization in Undoped, Sol-Gel Derived Zirconia Nanoceramics Vladimir P. Oleshko1, James M. Howe1, Satyajit Shukla2, and Sudipta Seal2 1
Department of Materials Science & Engineering, University of Virginia, Charlottesville, VA 22904 2 University of Central Florida, Advanced Materials Processing and Analysis Center & Mechanical Materials Aerospace Engineering Department, Orlando, Florida 32816 ABSTRACT The mechanisms underlying stabilization of the metastable tetragonal (t)-phase in sol-gel derived, nanocrystalline ZrO2 were studied by high-resolution analytical electron microscopy, utilizing parallel electron-energy loss (PEEL) and energy-dispersive X-ray spectroscopies. The powders were synthesized by hydrolysis of Zr (IV) n-propoxide at ratios of molar concentration of water to Zr n-propoxide, R=5 and 60, respectively, followed by calcination at 400oC. Dense particles of the as-precipitated ZrO2 (R=5) revealed 4-11 nm-sized nanocrystals embedded in the amorphous matrix that may serve as nuclei for the t-phase during calcination. The calcined particles consist of 10-100 nm–sized t-crystals. For as-precipitated ZrO2 (R=60), week aggregates (50-100 nm) of largely amorphous 4-20 nm-sized particles after calcination yield a mixture of t- and monoclinic (m-) nanocrystals. PEELS fingerprints of the band structure with the intensity threshold matching the expected position of a direct bandgap at 4-5 eV allow to differentiate between the amorphous and nanocrystalline ZrO2. Stabilization of t-phase (R=5) with sizes up to 16 times larger than reported earlier is likely due to strain-induced confinement from surrounding growing grains, which suppress the volume expansion associated with the martensitic t-m transformation. For R=60, loose nanoparticle agglomerates cannot suppress the transformation. In this case, the t-phase may be partially stabilized due to a crystal size effect and /or to the presence of m-phase. INTRODUCTION Undoped zirconia (ZrO2) with the metastable tetragonal t-phase is a promising single-phase high-density ceramic for a variety of industrial applications, ranging from corrosion-resistant and thermal-barrier coatings, solid fuel cells, nuclear fuel rods, heterogeneous catalysts to paint additives and gas sensors [1,2]. ZrO2-related research has been recently boosted by a search for new dielectric materials capable of substituting for SiO2 as a gate dielectric in microelectronics [3]. Pure ZrO2 has been also utilized as a dispersed phase in both oxide and non-oxide ceramics to increase fracture toughness, strength, and hardness [4]. At ambient pressure, ZrO2 exhibits three well-defined polymorphs: (1) the monoclinic m-ZrO2 (5.83 g/cm3, space group P21/c), stable from room temperature to 1170oC; (2) the t-ZrO2 (6.10 g/cm3, space group P42/nmc), thermodynamically stable between 1170o and 2370oC; and (3) the cubic c-ZrO2 (6.09 g/cm3, space group Fm 3 m), existing above 2370oC and below the melting point
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