Microstructural Evolution in a CeO2-Gd2O3 System
Abstract Microstructural evolution in a CeO2-Gd2O3 system at atomic and nanoscale levels with increasing Gd concentration has been comprehensively investigated by transmission electron microscopy. When the Gd concentration was increased from 10 to 80 at.%, the phase transformation from ceria with fluorite structure to solid solution with C-type structure was not a sudden change but an evolution in the sequence of clusters, domains, and precipitates with C-type structure in the fluorite-structured matrix. Moreover, the ordering of aggregated Gd cations and oxygen vacancies in these microstructural inhomogeneities developed continuously with increasing Gd concentration. This microstructural evolution can be further described based on the development of defect clusters containing Gd cations and oxygen vacancies.
Ionic conductivities and high resolution microscopic evaluation of grain and grain boundaries of cerium-based codoped solid electrolytes
Abstract Doped CeGdO and codoped CeGdOSmO compositions were synthesized, giving rise to nanoparticulate powders. Ionic conductivities at bulk and grain boundaries of the sintered samples were determined, exhibiting increased conductivity in the samaria-codoped samples. Scanning electron microscopy (SEM) showed a significant reduction in the grain size of samaria-codoped electrolytes. This reduced grain size of the codoped samples caused a reduction in Schottky barrier height, increasing oxygen vacancy concentration in the space-charge layer of the grain boundary and culminating in greater ionic conductivity in the boundary region. For the gadolinium doped samples, high resolution transmission electron microscopy images at grains showed the presence of large cluster of defects (nanodomains), hindering the movement of charge carriers and reducing ionic conductivity. However, the samaria-codoped system displayed better homogeneity at atomic level, resulting in reduced oxygen vacancy ordering and, consequently, smaller nanodomains and higher bulk (grain) conductivity. The reduced grain sizes and smaller nanodomains caused by codoping favor the ionic conductivity of ceria-based ceramics, doped with gadolinia and codoped with samaria.
Ionic conductivities, sintering temperatures and microstructures of bulk ceramic CeO2 doped with Y2O3
Abstract Ionic conductivities of CeO 2 :Y 2 O 3 bulk ceramic were investigated with different sintering temperatures and correlated with the resulted microstructure. Lower sintering temperatures ( T ≤1400°C) were found to give much higher overall DC conductivities (e.g., σ DC ∼7×10 −3 S/cm at 700°C for a 4% Y 2 O 3 -doped sample sintered at 1400°C). The samples sintered at lower temperatures showed higher grain boundary conductivities than those sintered at the traditional sintering temperature, 1500°C. The model involving non-resistive grain boundaries can be employed to explain the lower grain boundary resistivities in our samples of low sintering temperatures. These samples were examined by scanning transmission electron microscopy (STEM) with energy dispersive X-ray (EDX) and electron energy loss spectroscopy (EELS). Most of the boundaries (>90%) were found precipitate-free in the small grain samples, and a higher Y/O ratio was observed at all these boundaries examined. The lower sintering temperatures suppress grain growth giving rise to small grain size (below 1 μm). The finer grain size provides large grain boundary areas for impurities to precipitate and solutes to segregate. Under such condition, there are insufficient impurities to form continuous precipitate layers at all boundaries, such as ion transport blocking layers at boundaries are not fully formed. At the same time, there are insufficient Y Ce ′ ions for all the boundaries in the fine grain samples while the mobility of the Y Ce ′ ions is low at low sintering temperatures to form well developed space charged regions at these boundaries to abate transboundary ionic transport. These three combined effects have abated some of most resistive mechanisms for ionic transport across boundaries.