Ionic conductivity and thermal stability of magnetron-sputtered nanocrystalline yttria-stabilized zirconia

Abstract

Thermally stable, stoichiometric, cubic yttria-stabilized zirconia (YSZ) thin-film electrolytes have been synthesized by reactive pulsed dc magnetron sputtering from a Zr–Y (80/20 at. %) alloy target. Films deposited at floating potential had a ⟨111⟩ texture. Single-line profile analysis of the 111 x-ray diffraction peak yielded a grain size of ∼20 nm and a microstrain of ∼2% regardless of deposition temperature. Films deposited at 400 °C and selected bias voltages in the range from −70 to −200 V showed a reduced grain size for higher bias voltages, yielding a grain size of ∼6 nm and a microstrain of ∼2.5% at bias voltages of −175 and −200 V with additional incorporation of argon. The films were thermally stable; very limited grain coarsening was observed up to an annealing temperature of 800 °C. Temperature-dependent impedance spectroscopy analysis of the YSZ films with Ag electrodes showed that the in-plane ionic conductivity was within one order of magnitude higher in films deposited with substrate bias corresponding to a decrease in grain size compared to films deposited at floating potential. This suggests that there is a significant contribution to the ionic conductivity from grain boundaries. The activation energy for oxygen ion migration was determined to be between 1.14 and 1.30 eV.