Proton Conduction on YSZ Electrolyte Thin Films Prepared by RF Magnetron Sputtering

Abstract

Yttria stabilized zirconia (YSZ) is known as solid electrolyte materials with oxide ion conduction and is used for sensor and solid oxide fuel cells (SOFCs). Recently, Wagner et al. has reported proton conductivity in YSZ single crystals at high temperature. Furthermore, Scherrer et al. has also reported proton conduction in YSZ thin films prepared by various technique. However, the existence of proton conduction of YSZ thin film at intermediate temperature (IMT) region has not been clarified thus far. In this study, we present the structural and electrical properties at IMT region of YSZ thin films prepared by RF magnetron sputtering. YSZ thin films were deposited on Al2O3 substrates at various temperatures by RF magnetron sputtering using ceramics target. The 8 mol % Y2O3-doped ZrO2 target was prepared by solid phase reaction. The flow rate of Ar gas and the deposition pressure were set at 0.67 sccm, 8.0 × 10-3 Torr. The structural property of the YSZ thin films were characterized as a function of film thickness by powder X-ray diffraction (XRD) analysis. The conductivity of the deposited thin film was measured by AC impedance method at the temperature region of 200-500 ℃. The measurement frequency region was from 0.1 Hz to 32 MHz. In order to probe conducting carriers, the conductivity was measured by changing oxygen gas partial pressure (P O2). The electronic structure was characterized by photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS). The lattice constant of the (111)-oriented thin film increases with increasing film thickness due to relaxation of lattice distortion. The 80 nm thick film exhibits the oxide ion conduction with the activation energy of ~ 1.13 eV. Although the conductivity of 160 nm thick film dose not depend on atmosphere during measurement, the 80 nm and 120 nm thick films have higher conductivity in atmosphere than that in dry atmosphere. The activation energy of 80 nm thick film is ~ 0.76 eV and of 120 nm thick film is 0.82 eV. The electrical conductivity depends on film thickness. These results indicate that the thin film has proton conduction at the surface in the temperature region.