Grain Size and Chemical Composition Effects on the Grain Boundary Resistance of Ceria
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Grain Size and Chemical Composition Effects on the Grain Boundary Resistance of Ceria Xiao-Dong Zhou, Harlan U. Anderson, and Wayne Huebner Electronic Materials Applied Research Center, Department of Ceramic Engineering University of Missouri-Rolla, Rolla, MO 65401, U. S. A. ABSTRACT Studies related to the effects of grain size (30nm – 5.0µm) on the electrical conductivity of undoped CeO2 and Ce0.90Gd0.10O1.95 were performed. A series of impedance spectra as a function of temperature and grain size were analyzed. It was found that the ratio of the grain boundary resistance to the total resistance became lower with decreasing grain size, increasing temperature or increasing Gd content. For the case of Gd doped CeO2, the source of the grain boundary resistance may be due to the trapping of oxygen ions in the grain boundary area.
INTRODUCTION Solid oxide fuel cells (SOFC) that can be operated over the intermediate temperature (IT) regime (500ºC ~ 700ºC) are of great importance for the commercialization of these the cells. Compared to the conventional high temperature cell operation (>800ºC), the intermediate temperature operation requires an extremely strict material selection. A low resistance of the electrolyte is a key component in IT-SOFC in order to achieve a useable current density. Due to the higher oxygen ion conductivity of Ce0.90Gd0.10O1.95 (0.025 Ω-1·cm-1 at 600°C) compared to zirconia-based materials (50% of the theoretical value (7.1g/cm3 for CeO2). Various sintering temperatures were used with a heating rate of 3°C per min and a holding time of four hours at the maximum temperature. Scanning electron microscopy (SEM) was used to investigate the raw materials and sintered ceramics. Four-point dc and two-point ac impedance spectroscopy (IS) measurements were performed using a Solartron 1260 frequency response analyzer with a 1296 interface, with an applied voltage of 0.01V over a frequency range of 1Hz to 1MHz. Thick film Ag or Pt (T > 900ºC) electrodes (Electro-Science Lab, 9912-G) were used for all measurements. Data were collected for temperatures ranging between 300 and 1000°C. All electrical measurements were performed only after a stable value of the dc conductivity was achieved. Z-plot software (Scribner Associates Inc.) was used to develop appropriate equivalent circuits for modeling the impedance data.
RESULTS AND DISCUSSION The influence of temperature and impurity Figure 1 shows a plot of the Rgb/Rt ratio vs. temperature along with a plot of ln(σT) vs. 10000/T analyzed from the 2-probe ac impedance spectra on the CeO2 with an average grain size ~5µm. By assuming small polaron conduction, the plot of ln(σT) vs. 1/T will give the activation energy (Ea). The calculated activation energy and the enthalpy of oxygen vacancy formation are listed in Table I. Lower activation energy of the grain (Ea, g ~ 1.4eV) was observed in the measuring temperature regime, compared to the total activation energy (Ea) and the grain boundary activation energy, (Ea, gb) (~ 2.1eV). The difference between Ea, g a
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