ABSTRACT
Oxide ion conductors find important technical applications in electrochemical devices such as solid-oxide fuel cells (SOFCs), oxygen separation membranes and sensors. Na0.5Bi0.5TiO3 (NBT) is a well-known lead-free piezoelectric material; however, it is often reported to possess high leakage conductivity that is problematic for its piezo- and ferroelectric applications. Here we report this high leakage to be oxide ion conduction due to Bi-deficiency and oxygen vacancies induced during materials processing. Mg-doping on the Ti-site increases the ionic conductivity to ~0.01 S cm(-1) at 600 °C, improves the electrolyte stability in reducing atmospheres and lowers the sintering temperature. This study not only demonstrates how to adjust the nominal NBT composition for dielectric-based applications, but also, more importantly, gives NBT-based materials an unexpected role as a completely new family of oxide ion conductors with potential applications in intermediate-temperature SOFCs and opens up a new direction to design oxide ion conductors in perovskite oxides.
ABSTRACT
Biaxially textured epitaxial thin-film heterostructures of ceria and 8 mol % yttria-stabilized zirconia (8YSZ) were grown using pulsed laser deposition (PLD) with the aim to unravel the effect of the interfacial conductivity on the charge transport properties. Five different samples were fabricated, keeping the total thickness constant (300 nm), but with a different number of heterointerfaces (between 4 and 60). To remove any potential contribution of the deposition substrate to the total conductivity, the heterostructures were grown on (001)-oriented MgO single-crystalline wafers. Layers free of high-angle grain boundaries and with low density of misfit dislocations were obtained, as revealed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) analysis. The crystallographic quality of these samples allowed the investigation of their conduction properties, suppressing any transport effects along grain boundaries and/or interfacial dislocation pathways. Electrochemical impedance spectroscopy (EIS) and secondary ion mass spectroscopy (SIMS) measurements showed that for these samples the interfacial conductivity has a negligible effect on the transport properties.
