Enhanced energy density in K_0.5Na_0.5NbO₃ doped Bi_1/2Na_1/2TiO₃-6BaTiO₃-based relaxor ferroelectric ceramics by tuning Curie temperature

Abstract BNT-6BT (Bi 0.5 Na 0.5 TiO 3 -6BaTiO 3 )-based ceramics have attracted significant attention for applications in high energy density capacitors. In this investigation, K 0.5 Na 0.5 NbO 3 (KNN) component was added to BNT-6BT ceramics to modify the Curie temperature of (1- x ) (BNT-6BT)- x KNN ceramics and improve their energy density for capacitor applications. Scanning electron microscopy images revealed that the doping of KNN can destroy the long-range ordered structure of the ferroelectric domains in BNT-6BT and form nanopolar regions in the ceramic. The x-ray diffraction and Rietveld refinement results revealed that with an increase in the KNN content of more than 20%, the fraction of the cubic phase in 0.8(BNT-6BT)-0.2KNN increases to more than 80%, suggesting that the ceramic materials transform from a normal ferroelectric to a relaxor ferroelectric. The TEM (transmission electron microscope) analysis further confirmed that cubic and tetragonal phases coexisted. P-E loop measurements demonstrated that both the saturated polarization ( P s ) and remnant polarization ( P r ) values of the (1- x ) (BNT-6BT)- x KNN ceramics decrease with increasing KNN content. This finding suggests that the ceramic material transforms from a normal ferroelectric to a relaxor ferroelectric. According to the P-E loop data, the highest energy density and charge‒discharge efficiency of 0.8(BNT-6BT)-0.2KNN (mole ratio) were 4.3 J cm −3 and 75.4%, respectively, In addition, the energy density and efficiency of the 0.8(BNT-6BT)-0.2KNN ceramic decreased by 16.4% and 7.38%, respectively, when the sample was heated to 125 °C, indicating the temperature stability of the 0.8(BNT-6BT)-0.2KNN ceramic for high energy density capacitor applications. Compared with previously published results, 0.8(BNT-6BT)-0.2KNN ceramic shows superior performance relatively high energy density and high efficiency at relatively low electric fields (<150 kV cm −1 ).