Defect chemistry and proton conductivity in Ba-based perovskites

by Wu, Jian

Abstract (Summary)
The site incorporation mechanism of M3+ dopants into A2+B4+O3 perovskites controls the overall defect chemistry and thus their transport properties. For charge balance reasons, incorporation onto the A2+ site would require the creation of negatively charged point defects, such as cation vacancies, whereas incorporation onto the B4+ site is accompanied by the generation of positively charged defects, typically oxygen vacancies. Oxygen vacancy content, in turn, is relevant to proton conducting oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant oxygen sites. This work proposes that, on the basis of X-ray powder diffraction studies, electron microscopy, chemical analysis, thermal gravimetric analysis, AC impedance spectroscopy, extended X-ray fine structure (EXAFS) and atomistic simulation, that nominally B-site doped barium cerate can exhibit dopant partitioning partially as a consequence of barium evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively impact proton conductivity. As a consequence of the greater ability of larger cations to exist on the Ba site, the H2O adsorption and proton conductivities of large-cation doped barium cerates are lower than those of small-cation doped analogs. A series of dopants, La, Nd, Sm, Gd and Yb are adopted in doped BaCeO3 with the composition BaCe0.85M0.15O3-[delta]. Yb doped BaCeO3 yields the highest proton conductivity among all the doped samples. Compositional non-stoichiometry, which is closely tied to sample processing, is studied in a BaxCe0.85M0.15O3[plus or minus delta] series. It is indicated that low temperature synthesis is beneficial to reduce barium evaporation at elevated temperatures and in turn increase the proton conductivity. The chemical stability of BaCeO3 is investigated and Zr is used to stabilize BaCeO3 in CO2-rich atmosphere effectively. This result helps to commercialize doped BaCeO3 as the electrolyte material for SOFCs.
Bibliographical Information:

Advisor:William Johnson; Sossina Haile; George Gavalas; George Rossman; William Goddard

School:California Institute of Technology

School Location:USA - California

Source Type:Master's Thesis

Keywords:materials science


Date of Publication:10/27/2004

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