Dielectric capacitors are indispensable for pulsed power systems due to their high power density, ultrafast charge/discharge capability (>µs), and excellent fatigue resistance. However, their widespread adoption is fundamentally limited by poor energy storage performance, characterized by inadequate recoverable energy density (W_rec) and efficiency (η). To meet the demands of high-performance applications, it is imperative to simultaneously enhance W_rec, η, and breakdown strength (E_b). Among environmentally friendly candidates, BiFeO₃-BaTiO₃ (BF-BT) relaxor ferroelectric ceramics stand out due to their high Curie temperature (T_C ≈ 830 ℃) and large maximum polarization (P_max). However, the intrinsic low E_b of BF-BT ceramics caused from the volatilization of Bi³⁺ and the change in the valence state of Fe³⁺ hinder their energy storage potential. To address these challenges, this work was to use a dual-strategy approach with compositional tailoring and two-step sintering for the preparation of (0.7-x)BF-0.3BT-xSAN relaxor ferroelectric ceramics. The introduction of Sr(Nb_0.5Al_0.5)O₃ into the 0.7BF-0.3BT system was to enhance relaxor behavior and refine the grain size, synergistically boosting both E_b and polarization response. Furthermore, the effect of SAN on the microstructure, dielectric, and energy storage properties was systematically investigated. This could provide research ideas and theoretical basis for improving the energy storage performance of BF-BT ceramics based relaxor ferroelectric ceramics.

The raw materials powders of Bi₂O₃, Fe₂O₃, BaCO₃, TiO₂, SrCO₃, MnO₂, Nb₂O₅ and Al₂O₃ were analytical purity. These powders were precisely weighed according to a stoichiometric ratio, then ground with ethanol in a ball mill at 400 r/min for 12 h prior to drying. The mixed powder with a small amount of sintering aids (MnO₂) was subsequently calcined at 850 ℃. The mixture was further ground for 12 h and dried. The dried powder was compacted by cold isostatic pressing (200 MPa, 2 min) into circular discs (Ф10 mm×1 mm). These discs were placed in covered alumina crucibles to minimize Bi volatilization and thermally treated at 1050-1110 ℃ for 2 min, and then sintered at 1020-1050 ℃. The ceramic sheet was polished for microstructure characterization. After polishing for microstructural analysis, silver electrodes were screen-printed on the both surfaces of the 0.7 mm thick sintered ceramics and fired at 700 ℃ for 20 min for electrical characterization.

The crystal structure of the ceramics was analyzed by X-ray diffraction (XRD). The surface morphology and grain distribution were examined by field emission scanning electron microscopy (FE-SEM) and performed by a software named Nano Measure. The microscopic domain structure of the ceramics was characterized by piezoelectric force microscopy (PFM). The Raman spectra in the range of 90-900 cm^-1 were determined by Raman spectroscopy. The polarization-electric field hysteresis loops (P-E) were measured by a ferroelectric tester on disk-shaped samples (with the sizes of Ф1.5 mm × 0.08 mm).

The XRD patterns reveal that all the (0.7-x)BF-0.3BT-xSAN ceramics exhibit an asymmetric singlet pattern, indicating that the crystal structure is a pseudo-cubic structure. The content of the second phase gradually increases with the increase of x due to the solid solubility limitation of SAN in BF-BT ceramics. The addition of SAN causes the XRD diffraction peaks shifting to higher angles, indicating a significant contraction of the lattice volume. The Raman peaks of ceramics do not show a significant shift in the Raman full spectrum, but the peak half width gradually increases, which is attributed to the increase in structural disorder. The Raman fitting results further indicate that increasing SAN weakens both A—O and B—O bonds, thereby reducing unit cell polarity and increasing local disorder in the BO₆ octahedra. This enhanced disorder facilitates the establishment of local random electric fields, which in turn promotes the formation of polar nanoregions. The SEM images manifest that the SAN results in a significant grain size reduction, especially as the x exceeds 0.14. The PFM results demonstrate that ceramics of matrix and 0.54BF-.3BT-0.16SAN exhibit both strong and weak piezoelectric response nanodomains, and the nanodomains in 0.54BF-0.3BT-0.16SAN exhibit a weaker piezoelectric response. All the ceramics exhibit significant frequency dispersion and diffusion phase transition in their dielectric results, which imply a strong relaxation behavior. Polar, which can be attributed to a synergistic effect from the fine grain size, moderates a dielectric constant, and the presence of highly active nanodomains. Moreover, the measurements of P-E loops under varying freqization-electric field (P-E) hysteresis loops show a significant decrease in the polarization strength of the ceramic with the increase of SAN, which is attributed to the increase of relaxor characteristics. The Weibull distribution fitting results reveal that the breakdown strength (E_b) initially increases and then decreases with increasing x. Specifically, the 0.54BF-0.3BT-0.16SAN ceramic achieves the maximum E_b of 370 kV/cm. The results of cycling tests reveal that the ceramic exhibits robust frequency, thermal, and fatigue stability.

The (0.7-x)BF-0.3BT-xSAN relaxor ferroelectric ceramics with a pseudo-cubic structure were prepared by a two-step sintering method. The addition of SAN led to a contraction of unit cell and a concurrent weakening of both A—O and B—O bonds, consequently reducing unit cell polarity and enhancing local disorder within the BO₆ octahedra. The results of dielectric test indicated that (0.7-x)BF-0.3BT-xSAN ceramics had strong relaxation properties. The 0.54BF-0.3BT-0.16SAN ceramic simultaneously achieved the superior energy storage performance (i.e., W_rec of 3.90 J/cm³ and η of 85.5%) at E_b of 370 kV/cm. Furthermore, the 0.54BF-0.3BT-0.16SAN ceramic exhibited the excellent frequency (i.e., 1-1000 Hz), temperature (i.e., -20 ℃ to100 ℃), and cycling stability (i.e., 10⁵ cycles).