An investigation of conductivity, microstructure and stability of HfO2–ZrO2–Y2O3–Al2O3 electrolyte compositions for high-temperature oxygen measurement

In search of better ionically conducting ceramics for high temperature oxygen fuel cells and sensors, the conductivity and microstructure of the HfO2–ZrO2–Y2O3 system with 15 mol% of Y2O3 and the HfO2–ZrO2–Y2O3–Al2O3 system with 50 mol% of Al2O3 have been investigated with X-ray diffractometry (XRD), scanning electron microscopy (SEM) and conductivity measurements as a function of temperature. The stability of electrolyte compositions was studied by continuously monitoring conductivity as a function of time at 1000°C. A majority of the investigated samples exhibited linear Arrhenius plots of the lattice conductivity as a function of temperature. In the HfO2–ZrO2–Y2O3–Al2O3 electrolyte systems the parameter pe′ was measured at a temperature range of 1000–1400°C. The HfO2–ZrO2–Y2O3–Al2O3 electrolyte systems have also showed better thermal shock resistance than the ZrO2–Y2O3 systems. A comparison between the ageing of ZrO2- and HfO2-based electrolyte systems, as a result of long time annealing at a temperature of 1000°C, indicated that the degradation of the HfO2-based system at a temperature of 1000°C and above is 1.5 times lower than the degradation of the ZrO2-based systems.