Pulse power technology has important applications in national defense industry. Dielectric capacitor is one of the energy storage components of pulse power devices. Ceramic dielectric capacitors have some advantages like fast charging-discharging speed, good electromechanical performance, anti-aging, and resistance to extreme conditions, which can be used in dielectric energy storage. The existing energy storage properties of BaTiO₃-based dielectric ceramics are generally low at a high electric field, and lead-based ceramic capacitors are increasingly limited because of the presence of lead as a highly toxic element. It is thus necessary for pulse power technology to develop lead-free ceramic capacitors with superior comprehensive energy storage properties. NaNbO₃ (NN) ceramics have a rich structural phase transition and a low theoretical density, which makes their electrical properties highly adjustable and is conducive to the development of lightweight capacitors. Recent work on relaxor ferroelectric, anti-ferroelectric and relaxor antiferroelectric NaNbO₃-based ceramics are performed. Compared with the typical lead-free antiferroelectric AgNbO₃, NN as one of of lead-free antiferroelectric materials has some advantages like low theoretical density (i.e., 4.55 g/cm³), high polarization intensity (i.e., 40 μC/cm²), simple preparation process, and relatively low raw material cost. Developing NN-based relaxor antiferroelectric ceramics with superior comprehensive energy storage properties is still a challenge because of the contradiction between polarization and breakdown strength. Therefore, exploring a relationship between the structure and energy storage properties of NN-based relaxor antiferroelectric ceramics is still a research aspect for NN-based ceramics. In this paper, a strategy of reducing tolerance factor was adopted to stabilize the antiferroelectric phase of NN-based ceramics. 10% (in mole fraction) BiFeO₃(BF) with a low tolerance factor (i.e., 0.954 3) was incorporated into pure NN as a second end member, and the antiferroelectric R phase (Pbnm) of NN ceramics was stabilized at room temperature. The third end member Sr(Ti_0.85Zr_0.15)O₃(STZ) was introduced to further improve the relaxor characteristic of the ceramics, and then the relaxor antiferroelectric phase was constructed in NN-based ceramics. In addition, the phase structure, microstructure, energy storage properties, stability and charging-discharging performance of the ceramics were also analyzed.

The chemical composition of the ceramics was NaNbO₃, 0.9NaNbO₃-0.1BiFeO₃, and (1-x)[0.9 NaNbO₃-0.1BiFeO₃]-xSr(Ti_0.85Zr_0.15)O₃, where x=0.05, 0.10, 0.15. All the ceramics were prepared by a conventional solid phase method. The ceramics were prepared via sintering at 1 230-1 260 ℃ for 2 h. The crystal structure of ceramics was analyzed by a model DX-2700BH X-ray diffractometer (Haoyuan Instrument Co., China) and a model Lab-Ram HR800 spectrometer (HORIBA Co., France) with a laser wavelength of 532 nm. The in-situ temperature test for Raman spectra was performed on a model Linkam THMS600 temperature control platform. The sample used for in-situ electric field Raman spectroscopy test was plated with an electrode by an ion sputtering method. The sputtering current was set to 6 mA and the sputtering time to 2 min to ensure that the sample had both electrical conductivity and the penetration in Raman spectroscopic laser. The natural surface morphology of the samples was determined by a scanning electron microscope, and the grain size of the ceramics was determined using a software named Nano Measurer. After the sample was thinned, the domain structure of the ceramics was characterized by a model JEM-2100F field emission transmission electron microscope. The sample was polished to 0.5 mm thickness, cleaned, and coated with silver electrodes on the upper and lower surfaces of the sample. The dielectric properties of the ceramics were measured at -120 -450 ℃ by a model E4980A precision LCR bridge (Agilent Co., USA). The hysteresis loops of the ceramics with the thickness of 150 μm were tested by a model Precision Premier II ferroelectric tester (Radiant Co., USA).

Based on the XRD patterns, NN, NN-BF and NN-BF-xSTZ ceramics are a pure perovskite structure. NN ceramics show diffraction peaks for phases (110) and (200), manifesting that they are antiferroelectric orthogonal P(Pbcm) phases at room temperature. The orthogonal phase features disappear, and the diffraction peaks of phases (110) and (200) change from split state to single peak with the addition of BF and STZ, indicating that the addition of BF and STZ improves the symmetry of the ceramic and the ceramic transforms into a pseudo-cubic phase. NN ceramics have a dielectric anomaly peak at 400 ℃, which represents a transition from the antiferroelectric P phase to the antiferroelectric R phase. For NN-BF and NN-BF-xSTZ ceramics, the P-R phase transition peak disappears, and a diffuse dielectric anomaly peak appears at -100 ℃. Compared with NN ceramics, the room temperature dielectric constant of NN-BF ceramics increases, which is mainly due to the presence of anti-ferroelectric R phase in the ceramics. The in-situ XRD and Raman measurements show that the symmetry of the local structure of NN-BF-10STZ ceramics has a good stability. After BF and STZ doping, the grain size of the ceramics is reduced and the grain distribution uniformity is improved. The breakdown strength of ceramics can be enhanced due to the effect of decreasing grain size and improving distribution uniformity. STZ doping reduces the size of antiferroelectric domains. They can respond quickly to an external electric field due to the high activity and low energy barrier of nano-domains, thus reducing the remanent polarization, which is conducive to improving the energy storage properties. The transformation from antiferroelectric phase to ferroelectric phase of NN ceramics only occurs in the presence of the electric field. After the introduction of BF, the antiferroelectric R phase of the ceramics is stabilized, and the P-E curve shows a double hysteresis loop. The relaxation characteristics of the ceramics are further enhanced with the introduction of STZ. The P-E loops of NN-BF-10STZ ceramics become more elongated, and the polarization hysteresis is suppressed. The NN-BF-10STZ achieves the optimal energy storage properties (i.e., W_rec of 5.22 J/cm³ and η of 83.92%). Also, NN-BF-10STZ ceramics have a superior stability at 25-120 ℃ and 10-150 Hz.

Pure NN ceramics exhibited a typical square hysteresis loop with low energy storage density and efficiency (i.e., W_rec of 0.14 J/cm³, and η of 6.69%). After the introduction of the second component of BF, the ceramics had a double hysteresis loop like antiferroelectric, and the energy storage characteristics (i.e., W_rec~3.55 J/cm³, η~70.61%) were optimized. Furthermore, the introduction of STZ enhanced the relaxation of the ceramics, and reduced the polarization hysteresis of the hysteresis loop. The energy storage properties were optimized (i.e., W_rec~5.22 J/cm³, η~83.92%). 0.9[0.9NaNbO₃-0.1BiFeO₃]-0.1Sr(Ti_0.85Zr_0.15)O₃ ceramics with the optimal energy storage performance had a stability at different temperatures and frequencies, which could be used as a promising dielectric energy storage material with application prospects.