Nanocrystalline yttria powders

A. Suresh, M.J. Mayo, and W.D. Porter, Thermodynamics of the tetragonal-to-monoclinic phase transformation in fine and nanocrystalline yttria-stabilized zirconia powders, J. Mater. Res. 18(12), 2912-2921 (2003). 

Nanocrystalline yttria (Y2O3) powders with most suitable characteristics for the fabrication of yttria crucibles were synthesized by the sol-gel method [85]. The combustion synthesis method was used starting with high-purity (99.9%) yttrium nitrate hexahydrate and citric acid. Different fuel to oxidant (citric acid/nitrate) ratios (0.25, 0.5, 0.75, 1.0, and 1.1) were tested. These mixtures were heated at 373 K until a viscous gel was formed, and its decomposition into powder formation was realized by thermal treatment at 473 K for 3 h. In all mixtures, yttria powders were obtained by thermal treatment at 1073 K. Scanning electron microscopy (SEM) showed that these powders were porous, whereas high-resolution transmission electron microscopy (HRTEM) revealed that they consist of randomly oriented cuboidal nanocrystallites with an average crystallite size of about 17-30 nm. In order to obtain dense ceramics, the powders were compacted as pellets at 120 MPa and were sintered at 1673 K. Pellets with a sintered density as high as 98-99% of the theoretical density could be obtained from powders prepared with fuel to oxidant ratios (/ ) of 0.75 and 1.0. 

The present work was focused on the synthesis of nanocrystalline components for electrochemical cells via the cellulose-precursor technique. This method was used to prepare nanostructured intermediate-temperature (IT) SOFC anodes made of a series of cermets comprising gadolinia-doped ceria (CGO), yttria-stabilized zirconia (YSZ), Gdi.86Cao.i4Ti207.5 (GCTO) pyrochlore, metallic nickel and copper. Perovskite-type SrFcojAlo.sOs.s (SFA) powder, also obtained via the cellulose precursor, was applied onto membranes of the same composition to enhance specific surface area and electrocatalytic activity in the reactors for methane conversion [3]. 

A citrate gel combustion method has been reported to synthesize nanosized ytterbium doped yttria [290]. Nanocrystalline powders single-phase... 

In the sol-gel method, the nature of the complexant (fuel) is also important The sintered density and the activation energies of compacts obtained from microwave-assisted gel-combined combustion process for a series of fuels (citric acid, urea, and glycine) showed different results. This was well evidenced by determining the crystal structure, particle size, specific surface area, and morphology of nanocrystalline 8mol% yttria-stabilized zirconia (YSZ) powders [23]. 

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