Composition design of BNBT-ST relaxor ferroelectric ceramics in superparaelectric state with ultrahigh energy density

BNT-6BT-based lead-free ceramics have gained significant attention due to their relatively large piezoelectric coefficients and tunable morphotropic phase boundary conditions. In this investigation, SrTiO3 (ST) was added to BNT-6BT ceramics to fabricate the BNT-6BT-xST (BNBT-xST) composition. The impact of ST content on the phase structure and dielectric permittivity of the BNBT-xST ceramics was studied. The XRD refinement data show that with increasing ST content, the crystal structure of BNBT-xST ceramics was transformed from the morphotropic phase structure (coexisting tetragonal (P4/mmm) and pseudo-cubic (Pmm) phase) to the pseudo-cubic phase. The dielectric permittivity data of the BNBT-80ST showed that the Tm (the maximum permittivity temperature) moved to −61.9 °C, which caused the room temperature to fall within the range of Tm < T < TB (the Burns temperature). This makes the BNBT-80ST a superparaelectric state in relaxor ferroelectrics (RFE). Furthermore, the maximum energy density and the charge-discharge efficiency of the BNBT-80ST achieved 5.48 J/cm3 and 87% at 495 kV/cm, respectively. The energy density of the BNBT-80ST was improved by 2.97-fold and 49% compared with that of the BNBT-30ST and BNBT-70ST. This is mostly attributed to weakened ferroelectricity and improved paraelectricity in the BNBT-80ST. This investigation demonstrates that BNBT-xST ceramics can be adjusted to a superparaelectric state with an ST content of 80%. This makes the material to possess both a large polarization and a high breakdown field. This material holds great promise for high energy density capacitor applications.