Microstructure, nanochemistry and transport properties of Y-doped zirconia and Gd-doped ceria
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EE4.4.1
Microstructure, nanochemistry and transport properties of Y-doped zirconia and Gd-doped ceria G.Petot-Ervas, C.Petot , J.M.Raulot, J.Kusinski*, I.Sproule**, M.Graham** CNRS-UMR 8580, SPMS, Ecole Centrale Paris, Grande voie des Vignes, 92295 Châtenay-Malabry, Cedex (France), *Academy of Mining and Metallurgy, Krakow, Poland **CNRC, Ottawa, Canada Abstract - In this work we have shown the influence of the microstructure and nanochemistry on the transport properties of Y2O3-(9mol%) stabilized zirconia and Gd2O3 (10 mol%)-doped ceria. Zirconia (YSZ) samples show transport properties (DO, sgb) which increase with the grain size, while they decrease in ceria (GSC). This difference was attributed to the presence of glassy grain boundary precipitates in YSZ. On the other hand, it was shown that kinetic demixing processes during cooling, at the end of sintering, play an important role on the grain boundary properties of these oxides. I-Introduction The recent works of Badwall [1] and Aoki et al.[2] have related the transport properties of YSZ to the sample microstructure. Badwall commensurated his result to relocation of glassy precipitates to triple points and concludes that the anionic conduction is restricted to grain boundary regions not wetted by amorphous phases. Aoki et al. explain their results by segregation effects occurring during cooling, at the end of sintering. According to the available results, further progress to improve the electrolyte performance will depend upon a better understanding of the relationships between the microstructure and the associated properties. In the present work, experiments have been performed with samples prepared from different starting powders sintered under different conditions. 2. Experimental The Y-doped zirconia samples have been prepared from powders isostatically pressed at 2000 or 4000 bar [3,4]. The ZC samples were sintered from commercial submicronic Y2O3 (9.9 mol%) doped-zirconia powder, whose main impurity is SiO2 (~0.42 wt%). The ZF samples were sintered from nanometric Y2O3 (9.0 mol%) doped-zirconia powder prepared by the freeze-drying method [3,4]. Their primary impurity was SiO2 (~1.0 wt% in samples A, ~1.6 wt% in samples B). The Gd-doped ceria samples have been prepared from two batches of commercial powders (L and H) doped with 10 mole % Gd203. The electrical conductivity was obtained by complex impedance spectroscopy [3], in the -2 6 frequency range 10 - 20x10 Hz, with and without a bias voltage. The oxygen diffusion coefficient was determined by the electrochemical method [5]. The principle of the method is to place the opposite sides I and II of the sample, coated with the same electrode material, between reversible electrodes subjected to different PO2 ( POI and POII ). 2
2
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In open circuit conditions, the chemical potential of the electrons is the same at the cell terminals (me) I = (me ) II 2 and the cell generates an open circuit voltage, given by : 2
II
PO2 RT I log E open =(he) II - (he )2I = F II = t F ion 2 2 2 I 4F
(1) PO2 In shor
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