Enhanced energy density in K0.5Na0.5NbO3 doped Bi1/2Na1/2TiO3-6BaTiO3-based relaxor ferroelectric ceramics by tuning Curie temperature

Abstract BNT-6BT (Bi0.5Na0.5TiO3-6BaTiO3)-based ceramics have attracted significant attention for applications in high energy density capacitors. In this investigation, a K0.5Na0.5NbO3 (KNN) component was added to BNT-6BT ceramics to modify the Curie temperature of BNT-6BT-xKNN ceramics and improve their energy density for capacitor applications. Scanning electron microscopy (SEM) 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 (XRD) and Rietveld refinement results revealed that with an increase in the KNN content of more than 20%, the fraction of the cubic phase in BNT-6BT-20KNN 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 (Ps) and remnant polarization (Pr) values of the BNT-6BT-xKNN 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 BNT-6BT-20KNN were 4.3 J/cm3 and 75.4%, respectively. In addition, the energy density and efficiency of the BNT-6BT-20KNN ceramic decreased by 16.4% and 7.38%, respectively, when the sample was heated to 125 °C, indicating the temperature stability of the BNT-6BT-20KNN ceramic for high energy density capacitor applications. Compared with previously published results, the BNT-6BT-20KNN ceramic is superior in terms of relatively high energy density and high efficiency.