Pure NBT exhibits the frequency-dependent Curie temperature, indicating that its dielectric properties are affected by temperature, measurement frequency, and material processing condition. To enhance the dielectric and relaxor properties of NBT, various dopants such as Sr, K, Li and Bi are incorporated into the NBT structure. These modifications significantly alter the dielectric constant and relaxation behavior, demonstrating a dominant influence of dopant on the material properties. Among these, the solid solution of BaTiO₃ (BT) with NBT is widely investigated due to its ability to stabilize the perovskite structure and improve dielectric performance. However, the temperature-dependent stability of dielectric properties remains a critical challenge for high-temperature applications.

In this study, (1-x)(0.75Na_0.5Bi_0.5TiO₃-0.25BaTiO₃) -xBaZrO₃ (NBT-BT-xBZ, x = 0, 0.08, 0.14, and 0.20) ceramics were prepared by a solid-state reaction method. The effect of BaZrO₃ (BZ) addition on the structural, dielectric, and energy storage properties was systematically investigated. In addition, the phase transition and relaxation behaviors were also analyzed based on the modified Curie-Weiss law, Vogel-Fulcher relation, and Lorentz-type empirical law.

The starting materials were powders of high purity Na₂CO₃, Bi₂O₃, TiO₂, BaCO₃, and ZrO₂. The powders were weighed according to a stoichiometric ratio (with 1% excess of Na and Bi) and ground with ethanol in a ball mill at 300 r/min for more than 12 h, and the weight ratio of raw material to ethanol and zirconium balls was 1∶1∶2. The dried material was heat-treated at 850 ℃ for 2 h to promote the formation of NBT-BT-BZ. After further grinding for 12 h, the samples were mixed with a small amount of polyvinyl alcohol (PVA). The samples were sintered in air at 1150 ℃ for 2 h and cooled to room temperature.

The phase composition of the ceramic samples was determined by an model D8 ADVANDCE X-ray diffractometer (D8 ADVANDCEXRD, Bruker AXS Ltd., Germany) with Cu target Kα rays, at X-ray wavelength λ of 1.540 6 Å, 2θ in the range of 10° to 80°, applied voltage of 40 kV, and a current of 500 mA. A silver paste was coated on the two surfaces as electrodes and heat-treated at 700℃ for 10 min. The dielectric properties of the ceramic samples were determined at different frequencies by a model DMS-1000 high-temperature dielectric temperature spectroscope (BALAB Tech. Co., China) with at a ramp rate of 3(°)/min in a temperature range from room temperature to 450 ℃. The overdamped (200 Ω) discharge tests for bulk ceramic samples were performed by a model CFD-005 discharge tester (Gogo (GG) Instruments Technology, China)).

The XRD patterns indicate that all the ceramic samples have a perovskite structure without any detectable secondary phase, proving that zirconium ions can completely enter the lattice and form a solid solution. Based on the locally magnified XRD peaks, the XRD peak shape shifts towards lower angles as a whole as the BZ content increases. This indicates that the overall volume of the crystal cell shows an expansion as the Zr ions replace Ti ions due to different ionic radii of Zr and Ti ions. The SEM images show that the grain size gradually increases with increasing the BZ content. The addition of BZ promotes the grain growth. However, this gradually slows down with the increase of content up to x of 0.20. The limited grain size variation appears in the latter two samples. All the samples show a relatively dense morphology.

The Curie temperature of the NBT-0.25BT ceramic samples is 256 ℃, which is similar to the reported results. The Curie temperature decreases gradually with the increase of BZ additive, and the dielectric temperature spectrum flattens out, indicating that the enhanced structural and temperature stability of the NBT-BT-BZ ceramics. The maximum values of all dielectric constants correspond to temperatures that increase with frequency, indicating a dielectric relaxor behavior. A frequency dispersion is accompanied at near the Curie temperature, which can be ascribed to the thermal evolution of the tetragonal polar nanoregions (PNRs) and the mixing effect of the transition from tripartite to tetragonal PNRs.

The γ values obtained from the experimental data at 100 kHz are 1.79, 1.83, 1.89, and 1.92 for NBT-BT, NBT-BT-0.08BZ, NBT-BT-0.14BZ, and NBT-BT-0.20BZ, respectively. The γ value increases gradually with the addition of the BZ content, showing an enhanced relaxation of the NBT-BT-BZ ceramics. The comparison of discharge current curves and energy density of all the ceramics indicate that the addition of BZ significantly improves the discharge current and energy storage performance. A high discharging energy density (W_d) of 1.6 J·cm^-3 with a fast discharging speed (τ_0.9) of 75 ns is obtained for the ceramic samples with x of 0.14. This can be attributed to an increased relaxation as the BZ content increases.

NBT-BT-BZ ceramics were prepared by a solid-state reaction method. The XRD patterns revealed a phase transition from a tetragonal phase to a pseudocubic phase as the BZ content increased. The dielectric relaxation behavior of the ceramics could be described by three empirical laws (i.e., modified Curie-Weiss law, Vogel-Fulcher relation and Lorentz-type empirical law). The dielectric relaxation followed the modified Curie-Weiss law and the Vogel-Fulcher relationship. The parameters γ and E_a, which were obtained to evaluate the relaxation behavior, increased at a higher BZ content. The Lorentz-type relationship effectively described the temperature dependence of the dielectric constant on both the low- and high-temperature sides within a specific temperature range for all the ceramics.