Electrochemical removal of segregated silicon dioxide impurities from yttria stabilized zirconia surfaces at elevated temperatures

Here we report on the electrochemical removal of segregated silicon dioxide impurities from Yttria Stabilized Zirconia (YSZ) surfaces at elevated temperatures studied under Ultra High Vacuum (UHV) conditions. YSZ single crystals were heated in vacuum by an applied 18 kHz a.c. voltage using the ionic conductivity of YSZ. The crystals were annealed in vacuum and atmospheres of water or oxygen from 10− 5 mbar to 100 mbar in the temperature range of 1100 °C to 1275 °C. The surface was after annealing analyzed by X-ray Photoelectron Spectroscopy (XPS) without exposing the crystal to atmosphere between annealing and XPS analysis. Silicon enrichment of the surface was only observed at oxygen and water vapor partial pressures above 25 mbar and 10 mbar, respectively. No silicon was observed on crystals annealed in vacuum and at oxygen and water vapor partial pressures below 10 mbar. The YSZ seems to get partially electrochemically reduced by the a.c. voltage when no oxidation substances are present. The absence of silicon on the surfaces annealed in vacuum or at low oxygen or water vapor partial pressures was attributed to electrochemical reduction of silicon dioxide to volatile silicon monoxide on the YSZ surface. This was demonstrated by silicon enrichment of a gold foil placed behind the YSZ crystal surface while annealed. The results suggest a fast way to clean YSZ for trace silicon dioxide impurities found in the bulk of the cleanest crystals commercially available.

Research Highlights
► Single crystal yttria stabilized zirconia is reduced by a.c. voltage in UHV.
► Silicon dioxide enrichment on the YSZ surface is temperature, atmosphere and pressure dependent.
► Segregated silicon dioxide impurities can be removed electrochemically from the surface of the YSZ crystal 1.