Space Charge Layers in Polycrystalline Cerium Oxide
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Space Charge Layers in Polycrystalline Cerium Oxide
Andreas Tschöpe Universität des Saarlandes, Technische Physik, Gebäude 43B, 66041 Saarbrücken, Germany
ABSTRACT
The effect of space charge layers in polycrystalline cerium oxide was analyzed by comparing experimental results of grain size-dependent electrical conductivity with theoretical models. Modeling included the calculation of space charge segregation of acceptor ions and of the effective electrical conductivity of polycrystalline cerium oxide in both the macroscopic and mesoscopic range of grain sizes. It is shown that an L-3 power law for the electronic conductivity in the nm-regime is characteristic for the equilibrium space charge model and different from the scaling behavior of alternative models. The origin of space charge potential was investigated by numerical calculation of the electrical potential in a two-phase model. It was found, that a positive excess charge at grain boundaries of cerium oxide is caused by an enhanced oxygen deficiency at the grain boundary core. The influence of acceptor ion doping in the dilute limit and of non-equilibrium distribution of acceptor ions on electrical conductivity was also studied.
INTRODUCTION
Cerium oxide is a mixed ionic/electronic conductor (MIEC). Both, electronic and ionic charge carriers are generated in thermodynamic equilibrium during partial reduction of ceria to nonstoichiometric CeO2-x. The ratio of electronic to ionic conductivity can be manipulated by the addition of lower-valent cations. These acceptor ions participate in the charge balance with the result, that electronic conductivity decreases and ionic conductivity increases when the concentration of acceptor ions is raised. Defect chemistry of the bulk phase, which combines the law of mass action for defect equilibria with the requirement of local charge neutrality allows to calculate the concentration of charge carriers and conductivity for a given temperature, oxygen partial pressure and acceptor concentration [1-3]. In the recent past, the effect of grain size on ionic and electronic conductivity in cerium oxide has been investigated, as it may provide additional options for tailoring materials properties. The major differences found in nanocrystalline cerium oxide as compared to microcrystalline counterparts were (i) a predominantly electronic rather than ionic conductivity, (ii) an enhanced absolute value of electronic conductivity, and (iii) a reduced effective activation energy for electronic conductivity [3-9]. The conclusion, that electronic conductivity was dominating in the investigated nanocrystalline samples, was first based on the observation of a distinct oxygen partial pressure dependence, and was later confirmed by measurements of thermopower [5,10] and measurements of conductivity using ion-blocking electrodes [9]. An enhanced oxygen deficiency at grain boundaries has been suggested as origin of the large electronic conductivity
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and the reduced activation energy for σel in nanocrystalline ceria. In th
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