Dielectric capacitors are used in pulse power systems due to their advantages of high energy storage efficiency, long storage lifetime, and fast charge/discharge speed. Among the major dielectric materials, ceramic-based dielectrics have a superior temperature stability and a longer lifetime rather than polymer-based dielectrics. Dielectric energy storage ceramics have attracted recent attention. Among various dielectric energy storage ceramics, SrTiO₃ ceramic with a linear dielectric behavior at room temperature is a promising dielectric energy storage material with a high breakdown strength and a low dielectric loss. However, SrTiO₃ ceramic has a relatively low dielectric constant and lacks a spontaneous polarization, resulting in a limited energy storage density. Therefore, SrTiO₃ ceramic cannot meet requirements for specific energy storage application. Improving the polarization strength while maintaining the high breakdown strength through effective structural adjustment can be an effective method to enhance its energy storage performance. In this paper, we fabricated high-performance (Sr_0.42Na_0.15Bi_0.29Ca_0.07□_0.07)TiO₃ (SNBCT) energy storage ceramics with a small amount of ZnO as doping species. In addition, the effect of ZnO doping on the structure and energy storage properties of the ceramics were also investigated.

SNBCT-x%ZnO energy storage ceramics were prepared by a conventional ceramic preparation process. The raw materials were weighed and then mixed in alcohol by grinding in a ball mill for 24 h. The slurry was dried and calcined at 950 ℃ for 4 h to obtain the powders. After further grinding for 24 h, the powders were mixed with polyvinyl alcohol (PVA) binder solution and then pressed into pellets with 10 mm in diameter and 0.5 mm in thickness. After burning off the PVA, the disk samples were sintered at 1 140 ℃ for 2 h. The samples used for dielectric performance tests were polished and coated with silver electrodes, and the samples used for energy storage and charge/discharge performance testing were thinned to a thickness of 0.1 mm, and gold electrodes with a diameter of 2 mm were sputtered on their surfaces.

The structure of the ceramic was analyzed by a model D8 advance X-ray diffractometer (Bruker Co., Germany). The microstructure of the ceramics was determined by a model Merlin compact field emission scanning electron microscope (Carl Zeiss Co., Germany). The ferroelectric properties of the ceramics were analyzed by a model TF Analyzer 3000 ferroelectric analyzer (aixACCT Co., Germany). The temperature-dependent dielectric properties were characterized by a model Wayne Kerr 6500B impedance analyzer (WayneKerr Co., UK). The charge-discharge characteristics were analyzed by a model CFD-003 plus pulsed charge-discharge system (Tongguo Technology Co., China).

ZnO-modified SNBCT samples show a perovskite structure with a cubic phase. A trace of impurity peak appears in the XRD pattern as ZnO doping amount increases to 1.0%, corresponding to a small amount of the secondary Bi₂Ti₂O₇ phase (JCPDS 032-0118). All the ceramics show a uniform and dense microstructure.

The ZnO doping has little influence on the microstructure of the ceramics as the average grain size in these compositions changes slightly.

For the electrical properties, the doping of ZnO weakens the ferroelectric properties of SNBCT ceramics, which is manifested by a decreased maximum polarization strength P_max. In addition, the doping of ZnO improves the breakdown strength, thus enhancing the energy storage performance of SNBCT ceramics. For the sample doped with 0.5% ZnO, a high recoverable energy-storage density (W_rec) of 5.16 J/cm³ and a high energy-storage efficiency (η) of 88.2% in a large breakdown electric field of 468 kV/cm are obtained. Compared with the SrTiO₃-based energy storage ceramics, a great energy storage performance of the studied sample is obtained. A simulation model containing grains and grain boundaries is proposed to explain the enhanced breakdown strength, and the results show that fine grains with many grain boundaries hinder the propagation of breakdown cracks. For the charge-discharge performance, the sample doped with 0.5% ZnO shows a discharge energy density (W_d) of 2.61 J/cm³ and a discharge rate (t_0.9) of 227 ns in an electric field of 280 kV/cm.

All the samples exhibited a perovskite structure dominated by a cubic phase, and provided a slim P-E loops. ZnO doping effectively improved the breakdown strength of the SNBCT matrix, thus enhancing the energy storage performance. The effective energy storage density (W_rec) of the sample reached 5.16 J/cm³ and the energy storage efficiency (η) was 88.2% in the electric field of 468 kV/cm for the sample doped with 0.5% ZnO. A simulation model containing grains and grain boundaries was established and the extension of breakdown cracks in an external electric field was simulated, which explained the enhanced breakdown strength. In addition, the sample doped with 0.5% ZnO had a good charge-discharge performance, showing a discharge energy density (W_d) of 2.61 J/cm³ and a discharge rate (t_0.9) of 227 ns in an electric field of 280 kV/cm. Meanwhile, the sample exhibited a good temperature stability. These results indicated that the SNBCT-x%ZnO ceramics could be promising materials for pulse power capacitor applications.