Oxygen transport studies in nanocrystalline ceria films
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Olga Marina Energy Science and Technology Division, Pacific Northwest National Laboratory, Richland, Washington 99352
D.R. Baer Chemical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352
S. Thevuthasan W. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352
P. Nachimuthu Department of Chemistry, University of Nevada, Las Vegas, Nevada 89154; and Lawrence Berkeley National Laboratory, Berkeley, California 94720
D.W. Lindle Department of Chemistry, University of Nevada, Las Vegas, Nevada 89154 (Received 10 September 2004; accepted 14 February 2005)
Oxygen uptake and conductivity were measured by nuclear-reaction analysis and alternating current impedance technique at the intermediate temperature range on sol-gel grown nanocrystalline ceria films with average grain-sizes 7 nm and 38 nm synthesized at 723 and 1173 K, respectively. Higher oxygen uptake and lower ionic conductivity were observed in ceria films with ∼7-nm grain size. High permeation-assisted oxygen diffusion in nanocrystallites combined with oxygen trapping in the disordered region contributed to higher oxygen uptake. However, the lower ionic conductivity in the film resulted from the absence of long-range lattice ordering and inactive grain-boundary/surface oxygen vacancy sites due to oxygenation. The relationship between oxygen uptake and conductivity in ceria is discussed in details by considering grain-size dependent defect density, related surface area, and enhanced oxygen mobility.
I. INTRODUCTION
There is a great deal of interest in the study of oxygen diffusion and transport in fluorite structures in the intermediate temperature (700–1000 K) range,1,2 especially in ceria due to its well-known3 and valuable oxygen storage and conduction properties, which are useful in oxide fuel cells. It is essential to gain fundamental understanding about oxygen diffusion process in pure ceria. Oxygen diffusivity4 in ceria can become even more complex upon grain-size reduction due to enhanced defect density, modified grain boundary contribution, added porosity, and
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0157 J. Mater. Res., Vol. 20, No. 5, May 2005
increase in total grain surface area. Among various synthesis techniques5–11 that produce lower grain sizes, the sol-gel method has an advantage of excellent synthesis control over the average grain size. In ceria-based oxide fuel cell films of average grain size ∼50 nm and below, the sol-gel technique has been proved5 to be more effective than other techniques in creating much stronger bonding between electrode and electrolyte at low temperatures resulting in better electrochemical performance maintaining nanocrystallinity. It is well established12–14 that nuclear-reaction analysis (NRA) is an effective method for determining oxygen uptake and related mobility effects. In this report, we try to interpret these transport properties with the help of
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