Nanograin Composite Model Studies of Nanocrystalline Gadolinia‐Doped Ceria

Abstract

Nanocrystalline gadolinia‐doped ceria (GDC) specimens with grain sizes ranging from 10 to 100 nm were studied by AC‐impedance spectroscopy over the temperature range of ∼150°–∼300°C, and were analyzed by the nanograin composite model (n‐GCM), which is capable of extracting local properties (grain‐core conductivity, grain‐boundary conductivity, grain‐boundary dielectric constant) and also grain‐boundary width. The grain‐core dielectric constant, a necessary input parameter for the n‐GCM procedure, was measured separately on a microcrystalline GDC specimen sintered from identical powders. In spite of modest increases in grain‐boundary conductivity at the nanoscale, the total conductivity exhibited a monotonic decrease with decreasing grain size. This behavior was attributed to the large increase in the number of grain‐boundary barriers at the nanoscale, which overwhelms the slight increase in grain‐boundary conductivity. An unusual “up‐and‐down” behavior was observed in grain‐boundary conductivity versus grain size, which was accounted for by a similar trend in the preexponential factor versus grain size. Effective grain‐boundary widths, also determined by the n‐GCM, exhibited a similar “up‐and‐down” behavior, which probably reflects the differences in thermal history from specimen‐to‐specimen.