|About this Abstract
||2010 Electronic Materials Conference
||TMS 2010 Electronic Materials Conference
||Q6, Surface-Interface Conductivity in Thin Film Gd-doped CeO2
||Matthew Swanson, Lakshmi Krishna, Natee Tangtrakarn, Madhana Sunder, P.D. Moran
|On-Site Speaker (Planned)
The ionic conductivity of a Gadolinium doped Ceria (GDC) interface/surface at temperatures between 300-700<SUP>o</SUP>C is of interest due to the application of the material as an electrolyte for low temperature Solid Oxide Fuel Cells (SOFCs). It has been asserted in the literature that the exceptionally high ionic conductivity observed in thin GDC films is due to the substrate/film interface or surface acting as a high conductivity path for ions [1, 2]. Those studies were not performed on single crystal GDC and, therefore, did not completely isolate the ionic conduction along the film surface and substrate/film interface. A study to extract the surface and interface contributions to the total ionic conductivity of GDC films has not been reported. This work addresses the need for such an analysis.
The approach taken in this work has been to fabricate a series of GDC single-crystal films of varying thicknesses on Al<SUB>2</SUB>O<SUB>3</SUB> substrates by RF magnetron sputtering and then to measure their conductivities over the 300-700<SUP>o</SUP>C temperature range. These data are analyzed to extract the differences in carrier concentration and activation energy of the single-crystal films as a function of film thickness. An analysis is performed to extract separate activation energies and specific conductivities for the bulk and interface/surface components of the total conductivity.
In contrast to the assertions in the literature concerning ultrathin GDC films it is found that as the film thickness is decreased, so that the interface and surface conduction mechanism is more predominate, the total ionic conductivity decreases rather than increases. This data is analyzed in the framework of changes in the temperature-dependent carrier concentration and mobility of the surface and interface regions. Analysis of these data suggests that the temperature-dependent, oxygen non-stoichiometry can significantly affect the ionic conductivity of the GDC surface at temperatures lower than around 600<SUP>o</SUP>C. The GDC surface becomes disproportionally resistive at lower temperatures as vacancies are replaced with oxygen atoms, effectively reducing the carrier concentration. This reduction in the surface carrier concentration leads to a drop in total film conductivity by a factor of three from the 500 to 100 nm film at 300<SUP>o</SUP>C.