Enthalpy of formation of yttria-doped ceria
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Solid solutions (1 − x)CeO2 − xYO1.5 (0 艋 x 艋 0.36) were prepared by coprecipitation and sol-gel methods. Their enthalpy of formation relative to the end-members, fluorite-type cubic CeO2 and C-type YO1.5 was determined by oxide melt solution calorimetry. The enthalpy of drop solution shows a roughly linear trend with composition. Extrapolation to x ⳱ 1 gives the transition enthalpy of C-type to cubic fluorite YO1.5 as 22.2 ± 6.7 kJ/mol. This linear behavior is in contrast to the strong curvature seen in the ZrO2 − YO1.5 and HfO2 − YO1.5 systems. The slightly positive enthalpy of formation of CeO2 − YO1.5 is strikingly different from the strongly negative enthalpies of formation of ZrO2 − YO1.5 and HfO2 − YO1.5. The thermodynamics of CeO2 − YO1.5 is analyzed in terms of defect association and oxygen vacancy distribution. Specifically, the association of oxygen vacancies with the tetravalent cations in the zirconia and hafnia systems, in contrast to the preference of vacancies for nearest neighbor yttrium sites in the ceria systems, may explain the different energetics.
I. INTRODUCTION
Ceria and ceria-based materials have been investigated intensively due to their electrical and chemical applications, such as electrodes and electrolytes in solid oxide fuel cells (SOFCs), catalysts for gas phase oxidation and reduction reactions, and oxygen buffers in automotive three-way catalysts.1–3 Pure CeO2 has a fluorite structure, an open structure allowing a high ionic mobility, up to the melting point, 2477 °C.4 When a tetravalent oxide, such as c-ZrO2, c-HfO2, ThO2, and CeO2, is doped with a trivalent or divalent oxide to form a cubic solid solution with fluorite structure, oxygen vacancies form to compensate the lower valence of the dopant. Therefore, fluorite oxides doped with trivalent or divalent oxides often show good ionic conduction, which makes them competitive candidates for the electrolytes in SOFCs.5 Although yttria-stabilized zirconia (YSZ) is now the most commonly used electrolyte, it requires a high operating temperature over 1000 °C. Because the ionic conductivity of gadolinia-doped ceria (GDC) was found to be about one order of magnitude higher than that of YSZ at 800 °C, it may be possible to lower the operating temperature of SOFCs to 600–800 °C by making electrolytes with some ceria-based materials.6 The major drawback a)
Present address: Nuclear Materials and Technology Division, NMT-16, M5 G72, Los Alamos National Laboratory, Los Alamos, New Mexico 875451 b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0017 144
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 1, Jan 2005 Downloaded: 13 Mar 2015
of ceria-based electrolytes is that Ce4+ can be reduced to Ce3+ under reducing conditions at high temperatures. The resulting electronic conductivity is detrimental to the function of the electrolyte in SOFCs.4 The conductivity of doped-ceria is a function of dopant type, dopant concentration, temperature, and oxygen partial pressure. As the dopan
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