Two Different Interactions between Oxygen Vacancies and Dopant Cations for Ionic Conductivity in CaO-Doped CeO2 Electrol
- PDF / 35,529 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 3 Downloads / 142 Views
Two Different Interactions between Oxygen Vacancies and Dopant Cations for Ionic Conductivity in CaO-Doped CeO2 Electrolyte Materials Yuanzhong Zhou*, Xi Chen Department of Physics, Huazhong University of Science and Technology, Wuhan, China *Currently with Fairchild Semiconductor, South Portland, ME ABSTRACT The electrical measurement has demonstrated that conductivity of CaO-doped CeO2 has higher activation energy for low temperature and lower activation energy for high temperature. A model with two different kinds of defect interactions between oxygen vacancy and doped cations has been used to interpret the phenomenon. Diffusion based on hopping of oxygen ions was assumed as the mechanism of electrical conduction. The analysis indicated that at high temperature free oxygen vacancies are dominant and the activation energy is only for oxygen ion hopping. At low temperature, however, oxygen vacancies associated with dopant calcium ions are dominant for high CaO content and the activation energy is the energy for hopping of an oxygen ion plus half of the association energy between one oxygen vacancy and one calcium ion. For low level doping, both free and associated oxygen vacancies are important. INTRODUCTION Ceria-based oxides, doped with ions of alkaline or rare earth elements, have attracted much interest for their high ionic conductivity and low activation energy [1,2]. Numerous experimental works on these materials have shown that maximum ionic conductivity occurs at around dopant level of 10~12mol% for ceria doped with oxides of two-valent metals and below 10mol% for rare earth doped ceria. Two slopes in Arrehenius plots of log(σT) versus reciprocal temperature have been observed in several works on CeO2 doped with calcia and rare-earth oxides [3~5]. Similar results were also obtained in our experimental investigation on CaO-doped ceria, (CeO2)1-x(CaO)x. Several models have been proposed to describe ionic conductivity of CeO2 based oxide materials (e.g. [6,7]). These models might involve multiple nearest neighbor dopant ions or longrange force, which could be too complicated to emphasize dominant defect status under different operation conditions, or did not clearly demonstrate the temperature dependence of the conductivity. In this paper we will demonstrate that a very simple model can be used to interpret the temperature dependence of the ionic conductivity of CaO-doped ceria materials. This model includes two different interactions between oxygen vacancies and dopant cations and could fit measure data well. STRUCTURE AND CONDUCTIVITY Ceria oxide has fluorite-type of structure. In this structure, anions form a simple cubic sublattice and cations form a face-centered cubic sublattice. Each oxygen ion neighbors with four cations and each cation is surrounded by eight oxygen ions. Only half of the body-centered sites in the oxygen sublattice are occupied by cations. This makes the structure be open and be quite favorable for oxygen ions to move in the materials. The diffusivity of oxygen was reported six order
Data Loading...
