Electrical Conductivity in Praseodymium-Cerium Oxide
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EE10.8.1
Electrical Conductivity in Praseodymium-Cerium Oxide Todd S. Stefanik and Harry L. Tuller Crystal Physics and Electroceramics Laboratory Department of Materials Science and Engineering, Massachusetts Institute of Technology Cambridge, MA 02139, USA ABSTRACT The electrical conductivity of PrxCe1-xO2-δ (PCO) for 0 ≤ x ≤ 0.20 was examined over a wide range of temperatures and oxygen partial pressures. A defect model based on multiple Pr valence states was found to be qualitatively consistent with the observed data. A unique pO2-dependent ionic conductivity is observed at high pO2 values in compositions containing low levels of Pr (0 ≤ x ≤ 0.01). In compositions containing higher amounts of Pr (0.05 ≤ x ≤ 0.20), formation of a Pr induced impurity band results in a significant electronic conductivity at high pO2 values. INTRODUCTION Doped cerium oxide has received widespread attention owing to its high ionic conductivity at reduced temperatures and the drive to reduce solid oxide fuel cell operating temperatures below the useful limit of yttria stabilized zirconia.[1] Ceria-based electrolyte materials are typically doped with fixed valence acceptor ions such as Gd, Sm, and Y. Charge compensation is achieved through the creation of oxygen vacancies, both enhancing ionic conductivity and suppressing electron concentration. However, if a multivalent ion such as Pr is added to the ceria lattice, both electronic and ionic conductivity can be enhanced, yielding a mixed electronic-ionic conductor. Such materials are of interest in applications such as solid oxide fuel cell electrodes[2] and oxygen separation membranes.[3] The relative ease with which Pr reduces also allows for large oxygen nonstoichiometries in the PCO system.[4] This quality makes the material attractive as the sensing element in resonance-based mass sensitive sensors[5] and sorption compressors.[6] Preliminary studies in our labs have also shown PCO solid solutions to be interesting surface-effect sensor materials, potentially exhibiting enhanced selectivity over conventional sensing materials. Despite the interesting properties exhibited by PCO, relatively few studies of defects and transport in this system exist. The electrical conductivity of PCO was studied as a function of temperature in air,[2,7,8] but these studies provide little insight into the defect structure of the system. In a preliminary report, PCO with 0 ≤ x ≤ 0.05 was examined as a function of temperature and pO2 and a defect model was suggested.[5] In this study, measurements were extended to include a wider range of solid solutions and conditions of temperature and pO2.
EE10.8.2
EXPERIMENTAL DETAILS Powders of PCO ranging from 0 to 20% cation substitution of Ce by Pr were prepared via coprecipitation of nitrate salt solutions into oxalic acid. The resulting oxalate powders were washed, dried, and calcined to 700°C for 1 hour. The resulting powders were single phase and nanocrystalline as determined by X-ray peak broadening (crystallite size ~20nm). These powders were isostatica
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