Charge transfer at oxygen/zirconia interface at elevated temperatures - Part 9: Room temperature

Abstract

The present paper reports the reactivity between the surface of zirconia and oxygen at room temperature. The experimental studies, performed with polycrystalline yttria-stabilised zirconia (YSZ), include in situ monitoring of charge transfer at the oxygen/zirconia interface using work function measurements and determination of mass transport at the oxygen/zirconia interface using the incorporation of oxygen isotope O-18 and its analysis using secondary ion mass spectrometry. Oxidation of YSZ at room temperature is found to result in oxygen chemisorption, leading to the formation of singly ionised molecular oxygen species, and subsequent oxygen incorporation into the lattice. Oxygen incorporation takes place by lattice diffusion from the entire surface exposed to the gas phase, resulting in oxygen penetration of depths of the order of 2 nm over 60 days, and diffusion of oxygen species along the surface from the adsorption sites towards grain boundaries with subsequent rapid diffusion of oxygen into the bulk along grain boundaries. These processes take place in the absence of an electronic conductor. The present findings contradict the widely held perceptions that YSZ at room temperature is quenched and not reactive, and that oxygen incorporation into YSZ requires the presence of a triphase boundary (TPB) formed by the gas phase (oxygen), electronic conductor and oxygen ionic conductor (zirconia).