Defect chemistry and electrical properties of a Pr-CeO 2 solid solution: From nano- to micro-scale
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Defect chemistry and electrical properties of a Pr-CeO2 solid solution: From nano- to micro-scale S. R. Bishop, J-J. Kim, N. Thompson, and H. L. Tuller Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. ABSTRACT In nano-crystalline ceramics, the grain boundary volume fraction is large relative to that in micro-crystalline materials and can therefore become the dominant factor in determining its electrical, chemical, and mechanical properties. Reduced enthalpies of defect formation for nanocrystalline Pr0.1Ce0.9O2-δ , derived from thermo-gravimetric and impedance spectroscopy measurements, are reported. In addition, observations of cerium carbonate formation on nanoporous materials and implications for thermo-gravimetric analysis are discussed. INTRODUCTION Nanocrystalline cerium oxide ion conductors have been reported to exhibit considerably enhanced electrical conductivities relative to their bulk counterparts [1, 2]. This increased conductivity is attributed to space charge regions adjacent to the grain boundaries exhibiting a strong accumulation of electrons [3]. In addition there is the potential for a decreased reduction enthalpy and hence enhanced defect formation at interfaces relative to the bulk. The system Pr0.1Ce0.9O2-δ, recently characterized in the microcrystalline state by the authors [4, 5], is analyzed in the nanocrystalline state in this work. Pr-cerium oxide (PCO) solid solutions are particularly interesting oxygen ion conductors to study given that both Pr and Ce exhibit valence change, albeit under different conditions of temperature and pO2. This results in PCO transitioning from a mixed ionic electronic conductor (MIEC) at high pO2, to a predominantly ionic conductor and then back into a MIEC with successively reducing pO2 while simultaneously undergoing chemical expansion due to defect formation. This system thus serves as an excellent candidate to study nanocrystalline effects under a variety of defect regimes as well as lattice strain states. EXPERIMENTAL METHODS Details on the preparation and measurement of oxygen stoichiometry (Cahn 2000 microbalance) and electrical conductivity (alpha-A Novocontrol impedance analyzer) in microcrystalline PCO can be found in previous reports [4, 6]. Nanocrystalline powders were prepared using the same Pechini sol-gel method and conditions as used to prepare microcrystalline powders. Current assisted hot pressing (CAHP) at 850 oC for less than one hour was used to densify nanocrystalline samples. The pO2 was controlled using O2/N2 and CO/CO2 mixtures with computer controlled MKS mass flow controllers. Fourier transform infrared (FTIR) spectroscopy in reflectance mode was used to characterize carbonate formation following TGA on a nanoporous sample.
RESULTS AND DISCUSSION The nanopowder, with particle size of approximately 15 nm, observed by transmission electron microscopy, is shown in figure 1. The microstructure for the CAHP densified nanopowder is also shown in the figure. The grains grow
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