Flash Sintering Behavior and Microstructural Evolution of High Entropy Me_xRE_1-xO_2-δ Ceramics

High-entropy fluorite ceramics (HEOs) of the Me_xRE_1-xO_2-δ type are attracting increasing attention in high-temperature structural materials due to their excellent high-temperature stability, low thermal conductivity and unique oxygen ion conductivity. Conventional sintering (CS) of these materials typically requires temperatures exceeding 1600 ℃ and prolonged holding times, leading to high energy consumption and abnormal grain growth. Flash sintering (FS) uses electric field and Joule heating to reduce sintering temperature (<1000 ℃) and time to minutes, lowering energy, suppressing grain coarsening, and enhancing functionality. However, the comprehensive effects of process parameters on the microstructural evolution and defect dynamics of HEOs remain insufficiently understood. This study investigates the sintering behavior and microstructural evolution of Me_xRE_1−xO_2−δ ceramics under an applied electric field, revealing the densification mechanisms driven by multi-physical field coupling. The results demonstrate that under an electric field of 700 V/cm, the furnace temperature for the FS method is reduced by 561 ℃ compared to the CS method, yielding an average grain size between 1.5 and 3.0 μm. With increasing electric field strength, both the onset temperature and incubation time for FS method exhibit an overall decreasing trend. Simultaneously, electron paramagnetic resonance (EPR) spectroscopy reveals a substantial increase in oxygen vacancy concentration within the ceramic matrix following the application of the electric field. Driven by the strong electric field, intense electrochemical reduction disrupts the phase stability of the HEO matrix, leading to the preferential segregation of the LaYbO₃ phase. As the electric field strength further increases, the remaining rare-earth (RE) elements undergo deep solid solution into this segregated phase to form a LaREO_3−δ solid solution. This evolution ultimately triggers lattice expansion and abnormal grain growth within the HEO matrix.