Oxygen Diffusivity and Defect Transport in Pure and Yb Doped Nano-crystalline Ceria
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Oxygen Diffusivity and Defect Transport in Pure and Yb Doped Nano-crystalline Ceria Laxmikant Saraf, V. Shutthanandan, S. Thevuthasan, C. M. Wang, K.T. Koch, J. A. Andreasen, O. Marina and Y. Zhang Pacific Northwest National Laboratory, Richland, WA 99352
ABSTRACT Oxygen (18O) diffusivity in sol-gel synthesized nano-crystalline ceria films of average grain size of 3 nm and 7 nm, annealed at 300 0C and 450 0C for one hour respectively is examined by nuclear reaction analysis (NRA). Diffusivity and electrical transport properties measured by a. c. impedance spectroscopy were compared with microcrystalline ceria film of average grain size 38 nm annealed at 900 0C for one hour. Effect of enhanced oxygen diffusion along with reduced ionic transport in nano-crystalline ceria and reduced oxygen diffusion along with enhanced ionic transport in microcrystalline ceria are correlated to long range ordering, grain boundary scattering and defect density. Enhancement in the conductivity with reduction in activation energy from 1 eV to 0.5 eV in the case 4 atom% ytterbium (Yb) doped ceria compared to pure ceria is a result of increased oxygen vacancies taking part in the defect transport.
INTRODUCTION A unique ability of ceria to gain or lose oxygen ion has valued importance in the area of solid oxide fuel cells [1] and catalysts [2]. In solid oxide fuel cell, high efficiency intermediate temperature energy conversion from chemical to electric state reducing the environmental pollutants makes pure and doped ceria an attractive material for electrolytes. Ceria is known as pure ionic conductor [3]. Grain size reduction in ceria introduces defects and results in CeO2-x enhancing its electronic conductivity [4]. Thus, overall conductivity in pure ceria can be dramatically improved in the nano-crystalline form with an introduction of electronic conductivity. Such mixed (electronic/ionic) conductors are extremely useful at anode site in the development of solid oxide fuel cells [5]. Ceria, in the bulk form is also known for its high oxygen diffusivity of 10-5 cm2 s-1 at 970 0C for oxygen vacancies [6]. This underscores the importance of studying modified diffusivity mechanism in nano-crystalline ceria due to introduction of defects. It is valuable to gain fundamental understanding of modifications in the diffusivity mechanism due to presence of large number of interfaces and paths of disordered regions available in the nano-crystalline (ceria) form. Such a high oxygen diffusivity in ceria can be extremely useful in fabricating oxygen sensors [7]. Establishing a correlation between oxygen diffusivity and ionic transport in nano-crystalline ceria would also greatly strengthen fundamental understanding of mixed conductors. Oxygen storage and related vacancy/interstitial migration in doped ceria also had been center of focus in the recent years [8]. Upon size reduction, apart from doping concentration, the nature and impact of ionic conduction in doped ceria relies upon grain dynamics. Ionic radius of ytterbium (Yb) is close to th
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