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

Thermally stable, stoichiometric, cubic yttria-stabilized zirconia (YSZ) thin-film electrolytes have been synthesized by reactive pulsed dc magnetron sputtering from a Zr–Y $(80/20at.%)$ 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 $∼20nm$ 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 $∼6nm$ 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.