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of Me3+ (Me = Y, Gd, Nd, La) doped perovskites (BaCeO3,
BaZrO3, SrZrO3) exhibit high proton conductivities
after exposure to water vapor, and thus have potential as electrolytes
in mid-temperature fuel cells. My early work in this system focused
on the relationship between microstructure, defect chemistry and proton
transport. My group has shown, for example, that the conductivity
of Gd-doped BaCeO3 is highly sensitive to slight changes
in stoichiometry, which may occur as a result of extreme processing
conditions. However, glassy grain boundary phases, which were
predicted to result from either excess barium content or dopant incorporation
onto the Ba rather than Ce site, are not relevant to the transport mechanism.
Our transmission electron microscopy studies revealed the grain boundaries
in polycrystalline BaCe0.9Gd0.1O3 and
BaCe0.85Gd0.15O3 to be free of any
secondary phases that could influence proton transport. In a comprehensive
study of microstructural influences on proton transport, we measured
the conductivities of samples with grain sizes ranging from 1 to 11
mm, and established that the neither the bulk nor the specific grain
boundary conductivity depend on grain size . Moreover, we demonstrated
quite clearly that the grain boundary impedance is in fact higher
than it is the bulk. Thus, for fuel cell and other applications,
materials with large grain sizes are preferable in order to minimize
the total grain boundary volume and maximize overall conductivity.
As a part of this study, we also developed a complete methodology for
determining the specific grain boundary conductivity from impedance
measurements, without the need for direct microstructural investigations.
Normally, when one performs an impedance measurement, one obtains a
value for the effective grain boundary resistance, but is
unable to convert this to a specific grain boundary resistivity because
the grain size and thus the number of grain boundaries that contribute
to this effective resistance is not known. If one makes the assumption
that the bulk and grain boundary dielectric constants are equal (and
they certainly do not differ by more than an order of magnitude), we
have demonstrated that excellent estimates of the specific grain boundary
resistivity can be obtained directly from the impedance data, in the
absence of any microstructural knowledge. This has significant
impact for enabling the grain boundary properties of different samples
prepared by different routes to be directly compared.
efforts aim on improving the chemical stability of barium cerate in
CO2 containing atmospheres, as well as expanding our investigations
to related chemical systems, with a particular emphasis on developing
mixed electron and proton conductors.
Department of Energy; General Motors; EPRI; HRL (Hughes Research Labs);
Honeywell-Allied Signal; Defense Advanced Research Projects Agency (DARPA)
Y. Jones, J. Wu, L.P. Li and S. M. Haile, "Hydrogen Content in
Doped and Undoped BaPrO3 and BaCeO3
by Cold Neutron Prompt-Gamma Activation Analysis," J. Appl.
Phy. 97, 114908-1-4. (2005).
Wu, R. A. Davies, M. S. Islam, and S. M. Haile, "Atomistic
Study of Doped BaCeO3: Dopant Site-Selectivity
and Cation Non-stoichiometry," Chem. Mat. 17,
A. Mary, C. Y. Jones, G. J. Snyder and S. M. Haile, "Nonstoichiometry,
Structure and Electrical Properties of 'SrPrO3',"
Chem. Mat. 17, 5146-5154 (2005).
Babilo and S. M. Haile, "Enhanced
Sintering of Yttrium doped Barium Zirconate by Addition of ZnO,"
J. Amer. Cer. Soc. 88, 2362-2368 (2005).
Wu, S. M. Webb, S. Brennan and S. M. Haile, "Dopant
Site Selectivity in BaCe0.85M0.15O3-d
by Extended X-ray Absorption Fine Structure," J. Appl. Phys.
97, 054101 (2005).
Wu, L. P. Li, W. T. P. Espinosa, and S. M. Haile, "Defect Chemistry
and Transport Properties of BaxCe0.85M0.15O3-d,"
J. Mat. Res. 19, 2366-2376 (2004).
Haile, G. Staneff and K.H. Ryu "Non-Stoichiometry, Grain Boundary
Transport and Chemical Stability of Proton Conducting Perovskites,"
J. Mat. Sci. 36 (2001) 1149-1160.
Ryu and S.M. Haile "Chemical Stability and Proton Conductivity
of Doped Perovskite Oxides in the BaCeO3-BaZrO3
System," Solid State Ionics 125 (1999) 355-367.
M. Haile, D. L. West and J. Campbell "The Role of Microstructure
and Processing on the Proton Conducting Properties of Gadolinium-Doped
Barium Cerate," J. Mat. Res. 13 (1998) 1576-1595.
Shima and S. M. Haile "The Influence of Cation Non-Stoichiometry
on the Properties of Undoped and Gadolinia-Doped Barium Cerate,"
Solid State Ionics 97 (1997) 443-447.