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Publications (3)4.86 Total impact

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    ABSTRACT: To investigate the fundamental aspects of vacancy ordering in oxygen-transporting ceramic membranes, we have performed atomic resolution analysis of individual domains in brownmillerite-type SrCoO3−δ. Electron energy loss spectroscopy indicates that the Co valence state in adjacent planes can be 2+ and 4+. This charge localization is accompanied by oxygen deficiency and the formation of ordered octahedral and tetrahedral coordinated Co sites. At microdomain boundaries, Z-contrast images reveal a structural relaxation of the octahedral site with the reduction of the Co valence state from 4+ to 3+ and the incorporation of extra oxygen vacancies.
    Journal of the American Ceramic Society 12/2004; 85(4):969 - 976. · 2.43 Impact Factor
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    ABSTRACT: The atomic-scale structure, composition, and chemistry of grain boundaries in two fluorite-structured ceramic materials were characterized by a combination of Z-contrast imaging and electron energy-loss spectroscopy (EELS). In the case of a symmetric 24° [001] tilt bicrystal of yttria-stabilized-zirconia (YSZ), a shift in the zirconium M-edge onset and a change in the yttrium and zirconium M-edge ratios at the boundary indicate an increase in the number of electrons in the boundary plane. A detailed study of the structure and composition indicates that this is caused by an increase in the number of oxygen vacancies in the grain boundary core that is partially compensated by yttrium segregation. Studies of grain boundaries in an industrial Gd-doped ceria ceramic reveals similar changes in vacancy/dopant profiles indicating that these effects may be generic to grain boundaries in fluorite-structured materials.
    Journal of the American Ceramic Society 08/2002; 85(9):2359 - 2363. · 2.43 Impact Factor
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    ABSTRACT: The ordering of vacancies is critical to the performance of oxide ceramics as electrodes in oxygen transporting membranes and in solid oxide fuel cells. Here, we show how a combination of atomic-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) can be employed to quantify vacancy ordering in these materials. In particular, the direct nature of the Z-contrast imaging technique reveals ordering of vacancies in the form of a periodic intensity modulation. These intensity modulations can be used to position the electron probe at defined locations for the acquisition of EELS (which has the same atomic resolution as the image). As we know the location from which the spectrum was acquired, we can perform detailed analysis of the spectral fine structure to quantify the number of vacancies. Multiple scattering analysis performed with self-consistent potentials has, for example, allowed the differences in occupancy/atomic arrangements between hexagonal, perovskite and brownmillerite structures to be correlated with processing conditions. An additional benefit of the self-consistent potentials used in this analysis, is that the relationship between the oxygen vacancies and the valence state of the 3d transition metals (measured from the L-edge) can also be addressed.