Insight into insulation degradation mechanism of Al2O3 involved with positive and negative defects

Abstract

Al₂O₃ with the desired electrical insulating properties and thermal shock resistivity has been extensively applied in the field of solid oxide fuel cells and oxygen sensors. However, degradation of the insulating Al₂O₃ layer is an intractable issue in practical applications. In this study, different point defect structures of Al₂O₃ were realized with the substitutional doping effect of ZrO₂ and MgO. The MgO dopant provides positively charged oxygen vacancies, whereas the ZrO₂ dopant tends to trigger negatively charged vacancy formation at Al³⁺ sites. The oxygen vacancy concentration of Al₂O₃ exhibits the following trend: MgO-doped Al₂O₃ > Al₂O₃ > ZrO₂-doped Al₂O₃. Furthermore, the densification morphology, insulating properties, and oxygen vacancy migration of Al₂O₃ have been confirmed to be largely affected by the extrinsic factors. This study indicates that oxygen vacancy migration depends on the applied electric field at high temperatures. As the voltage and temperature increase, oxygen vacancy migration shows obvious electric-field-dependent characteristics, and its aggregation macroscopically shows hole defects. The defect position of Al₂O₃ is nonstoichiometric Al₂O_3-x with poor crystallinity. Therefore, it is believed that the oxygen vacancy migration triggered by the second phase directly determines the insulation performance and causes the degradation of Al₂O₃ materials.