Dielectric ceramic capacitors are essential components in next-generation advanced pulse power systems due to their ultra-fast charging/discharging rates and remarkable power density. As a typical lead-free perovskite material, Sr_0.7Bi_0.2TiO₃ (SBT) has a larger permittivity rather than SrTiO₃ because Bi modification favors producing a high polarization due to the intense hybridization between the 6s² orbitals of Bi³⁺ and O 2p orbitals. Moreover, it also has a superior ferroelectric relaxor behavior with a diffused maximum dielectric constant in a wide temperature range, benefiting from Bi³⁺ off centering and Sr site vacancies. However, SBT ceramic still has a critical challenge for improving dielectric breakdown strength (E_b). Some previous studies indicate that adding some oxides can effectively inhibit grain growth and increase E_b. In this paper, the SBT ceramics were synthesized by a conventional solid-state sintering method with ZrO₂ as an additive. A part of Zr⁴⁺ was deposited on the grain boundary in the form of oxides to inhibit grain growth. The breakdown strength and the energy storage density of the ceramics with different ZrO₂ contents were analyzed.

Bi₂O₃(99.5%, in mass. The same below), SrCO₃(99.5%), TiO₂(99.8%), and ZrO₂(99.9%) were used as raw materials. The raw materials were weighed according to the stoichiometric ratio. The materials were mixed with anhydrous ethanol and ground in a ball mill with zirconium balls for 12 h. The slurry was dried in an oven. After pre-firing at 850 ℃ for 5 h, the powder was further ground for 24 h according to the corresponding mass ratio Sr_0.7Bi_0.2TiO₃+xZrO₂ (SBT-xZr, x= 0, 0.5%, 1.0%, 2.0%, 5.0%, in mass fraction). The dried ground powder was pressed into sheets with a thickness of 0.8 mm and a diameter of 12 mm, and then sintered at a rate of 4 ℃/min at 1 200-1 250 ℃ for 3 h, and then cooled down naturally.

Based on the XRD patterns and mapping energy spectra, element Zr partially dissolves in the SBT lattice in the form of Zr⁴⁺, and the rest exists in the form of ZrO₂ on the grain boundary and surface, showing the formation of a 0-3 type composite structure. The dielectric constant of SBT-2%Zr ceramic consistently exhibits a high value, while the low dielectric loss mitigates excess waste heat, which is beneficial to the energy storage performance. The heightened energy storage density primarily stems from the increased E_b value. With the augmentation of ZrO₂ content, the E_b value of the SBT-2%Zr increases, reaching a peak of 580 kV/cm, which is 27.6% higher than that of pure SBT ceramic. Consequently, the recoverable energy storage density (W_rec) experiences an elevation from 4.07 J/cm³ for the ceramic without x to 5.57 J/cm³ for the ceramic with x of 2%, while energy efficiency (η) maintains 88.97%. However, for the ceramic with x of 5%, despite achieving a high E_b value, the introduction of excessive ZrO₂ leads to a notable reduction in maximum polarization (P_max), preventing the attainment of the maximum W_rec value. The average grain size decreases from 2.30 μm to 1.31 μm, facilitating the attainment of more highly insulated grain boundaries. Also, the band gap energy (E_g) increases from 2.94 eV for the SBT ceramic to 3.02 eV for the SBT-5%Zr ceramic. This signifies that electrons require a larger applied electric field to achieve the valence band and conduction band transition. The expansion of the impedance circle indicates a higher resistivity of the material, and the elevated activation energy (E_a) effectively restrains the transmission of charged carriers. These interrelated mechanisms are crucial factors contributing to the substantial enhancement of E_b. In addition, SBT-2%Zr ceramic also shows an excellent fatigue stability (i.e, 5×10⁴ cycles, W_rec change <1%) and a good charge-discharge performance (i.e., discharge energy density W_d=2.15 J/cm³, current density C_D=1 060.51 A/cm² and power density P_D=169.68 MW/cm³).

A lead-free 0-3 type relaxation ferroelectric ceramic based on SBT was prepared by a stable solid-state reaction method. The ceramics doped with different concentrations of ZrO₂ exhibited an original crystal structure with some Zr⁴⁺ in the lattice. The remaining Zr⁴⁺ existed in the form of a second phase, predominantly appearing at the boundaries and surfaces of grains. The increase of energy storage density mainly depended on the increase of E_b, and the possible physical mechanisms (i.e., grain size, band gap, impedance and activation energy) cooperated to promote the increase of E_b. SBT-2%Zr ceramic had the optimum energy storage performance, i.e., recoverable energy storage density (W_rec=5.57 J/cm³) and energy efficiency (η=88.97%). After 5×10⁴ cycles, SBT-2%Zr ceramic with a good performance stability (i.e., W_rec change amplitude <1%) could be used as a promising candidate material for pulse power energy storage capacitors.