Dielectric relaxation and high recoverable energy density in (1-x)(0.3BiFeO₃-0.7SrTiO₃)-xK_0.5Na_0.5NbO₃ ceramics

(1-x)(0.3BiFeO₃-0.7SrTiO₃)-xK_0.5Na_0.5NbO₃ (BFO-STO-xKNN, x=0–0.05) ceramics were synthesized by solid state sintering for energy storage application. The grain size of BFO-STO-xKNN increased with KNN content until x=0.01 and then decreased as x increased further. In contrast, the lattice parameter of BFO-STO-xKNN decreased initially with KNN addition and then increased as x increased further. The initial decrease in lattice parameter was explained by the reduced chemical expansion associated with oxygen vacancies, while the latter increase was due to larger ionic sizes of the dopants. The dielectric response in samples without KNN addition resulted from the Maxwell-Wagner effect with the occurrence of a low frequency (500 Hz) peak at room temperature, while the dielectric relaxation in KNN added samples appeared to have the Debye type with the relaxation peak occurring at a high frequency beyond 10⁶ Hz. The BFO-STO samples free of KNN showed a finite DC conductivity due to electronic hopping between Fe²⁺/Fe³⁺, which was absent in KNN added samples, and hence they showed extremely low DC conductivity. KNN-added BFO-STO exhibited slim hysteresis loops ideal for energy storage application. A high recoverable energy density (W_rec) of 3.20 J/cm³ with 88.0% efficiency (η) was achieved with the x=0.01 samples at the applied field of 273 kV/cm. The x=0.01 samples also showed good thermal stability of W_rec and η, which varied only 4.0% and 0.8%, respectively, over the temperature range between 25 and 100 °C.