The role of microstructure and processing on the proton conducting properties of gadolinium-doped barium cerate
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The role of microstructure and processing on the proton conducting properties of gadolinium-doped barium cerate Sossina M. Hailea) Department of Material Science, California Institute of Technology, Pasadena, California 91125
David L. Westb) and John Campbell Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195-2120 (Received 19 August 1997; accepted 12 January 1998)
The influence of grain boundary conductivity and microstructure on the electrical properties of BaCe0.85 Gd0.15 O32d have been examined. Grain sizes were varied by sintering at various temperatures. Impedance data were analyzed using the brick layer model, and some new consequences of this model are presented. The specific grain boundary conductivity exhibits an activation energy of ,0.7 eV, and for similar processing routes, is independent of grain size. An isotope effect was observed, indicating that protons (or deuterons) are the mobile species. TEM investigations showed the intergranular regions to be free of any glassy phase that could account for the differences in bulk and grain boundary properties. Single-crystal fibers, grown by a modified float zone process, were notably barium deficient, and exhibited a low conductivity, comparable to that of polycrystalline Ba0.96 Ce0.85 Gd0.15 O32d .
I. INTRODUCTION
Proton conduction in doped perovskite oxides has been the subject of extensive investigation ever since Iwahara and co-workers demonstrated the utility of such materials in hydrogen sensors and other solid state ionic devices.1 Proton incorporation into such materials has been generally recognized to occur by a two-step process.1–3 The first, the introduction of trivalent ions on the normally quatravalent ion site (e.g., replacement of Ce41 in BaCeO3 with Gd31 ), creates vacant oxygen sites, or, in oxidizing atmospheres, creates electronic holes. The second, the exposure of the material to H2 O or H2 containing atmospheres, leads to the occupation of previously vacant oxygen sites by hydroxyl groups and the uptake of the remaining protons at other oxygen sites, or (again, for highly oxidized atmospheres) to the exchange of electronic holes with protons. In contrast to the general consensus on the mechanism of bulk proton incorporation, there are unresolved questions with respect to the chemical and electrical properties of the grain boundary in these materials. Indeed, it has been proposed that the grain boundaries in barium cerate, in particular, are responsible for the overall high conductivity of this material. Such a conclusion would naturally drive the development of nanocrystalline materials. In the present work we demon-
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Author to whom correspondence should be addressed. Present address: Department of Materials Science and Engineering, University of Illinois, Urbana-Champagne, Illinois 61801.
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http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 6, Jun 1998
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