Phase transformation studies on YSZ doped with alumina. Part 2: Yttria segregation

Yttria and alumina segregations in annealed alumina-doped YSZ at 800 °C and 1000 °C for different annealing times were discussed. Segregation of yttria into the grain boundaries of YSZ was detected which leads to its deficiency in the YSZ grains. Low dopant concentration in YSZ grains may result in cubic and/or tetragonal to monoclinic phase transformation. Hence, variation of the mechanical and electrical properties of YSZ as a function of annealing temperature was studied. Finally, the material was studied as an oxygen sensor implemented in a real engine where it exposed to the exhaust gas for 50,000 h. Microstructural investigations show that these working conditions lead to the formation of monoclinic phase; cubic YSZ is fully removed whereas tetragonal phase is partially disappeared which confirms the proposed effect of yttria segregation upon the sensor lifetime. This study reveals the importance of considering the degradation of electrolyte material in the high temperature applications such as gas sensing.