In order to clarify the optimal sintering conditions, the effects of processing parameter on phase evolution, physical, microstructure, dielectric, ferroelectric, and energy storage density properties of bismuth sodium titanatestrontium bismuth titanate ceramics (or BNT-SBT) were investigated. The studied ceramics were fabricated via a conventional mixed oxide method and sintered at temperatures ranging from 1100-1175°C under normal atmosphere for 3 h dwell time with a heating/cooling rate of 5°C/min. The XRD data revealed that the coexisting rhombohedral and tetragonal phases were observed in all of the ceramics. With increasing sintering temperature, the cubic-rich phase was dominated; and the average grain size tended to increase. For the ceramics sintered at 1150°C, the good density (5.74 g/cm 3 ), dielectric ( max = 3510, tan δ = 0.0501, T F-R = 73.80°C, T m = 273°C), and ferroelectric (P r = 3.05 µC/cm 2 , E c = 7.69 kV/cm) were obtained. In addition, the obtimum sintering temperature of 1150°C was also found to improve the energy storage density properties (W = 0.94 J/cm 3 , η = 89.93% at 125°C, and E = E max ).

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Ferroelectric energy storage materials play essential roles in pulsed power systems and in renewable energy-storage devices. [1][2][3] Lead-containing ferroelectric ceramics, especially (Pb, La)(Zr, Ti)O 3 (PLZT) have been used as a necessary material for energy storage systems due to their excellent energy storage properties. [3][4][5][6] However, as a toxic metal, lead causes harm to the natural environment and human health. Considering these negative effects, leadfree ferroelectric materials, with excellent energy storage properties need to be developed to replace the lead-based materials for energy storage applications. [3,7,8] After receiving intense attention, the density of the stored energy is still unsatisfactory and the efficiency and the thermal stability also need to be improved, which has been a longstanding challenge for developing leadfree ferroelectric materials.Currently, many lead-free ferroelectric materials, for example, SrTiO 3 (ST)-based ceramics, [3] BaTiO 3 (BT)-based ceramics, [9][10][11] BiFeO 3 (BF)-based ceramics, [12,13] AgNbO 3 (AN)-based ceramics, [14] and (Bi 0.5 Na 0.5 )TiO 3 (BNT)-based ceramics [15][16][17][18][19] have been studied and have seen an improvement in their energy storage properties. Bismuth sodium titanate (Bi 0.5 Na 0.5 TiO 3 or BNT) ferroelectric ceramics have aroused extensive research as a potential energy storage material, owing to its large spontaneous polarization (P S ) of over 40 μC cm À2 , which derives from the hybridization of Bi 6p and O 2p orbitals. [20,21] However, BNT ceramics have low energy storage efficiency due to their large remnant polarization (P r ) at room temperature, which severely limits the energy storage performance. [19][20][21][22] Previous researchers have doped chemical compounds into BNT-based ceramics to form a relaxor ferroelectrics state with breaking ferroelectric long-range order, which resulted in reduced P r and subsequently led to increased energy storage efficiency. [17,18,23]

In this research, temperature dependence on the phase evolution, physical, microstructure, mechanical, dielectric, and electric field-induced strain responses of the Bi0.41Na0.35Sr0.21TiO3 or BNST ceramics were investigated. The BNST ceramic was prepared by a conventional mixed oxide method and sintered at various temperatures from 1100 °C to 1175 °C in order to clarify the optimal sintering temperature for all propereties. X-ray diffraction results showed that all ceramics exhibited a single perovskite without any secondary phases. A mixed between tetragonal and rhombohedral phases were identified for all ceramics. Grain size tended to increase with increasing the sintering temperature. The mechanical improvement was related with the change in densification. The optimum relative density (95.83 %), mechanical (HV = 6.43 GPa, HK = 5.12 GPa, E = 107.57 GPa, and KIC = 2.42 MPa.m1/2), and dielectric properties (ε′ = 1752, tan δ = 0.0514, and δA = 100 K), were obtained for the ceramic sintered at 1125 - 1150 °C. In addition, the obtimum sintering temperature of 1125 - 1150 °C were also found to improve the electric field-induced strain response (Smax = 0.14 % and d*33 = 285 pm/V), and the electrostrictive coefficient (Q33 = 0.0199 m4/C2).

Homogeneous 0.722(Bi0.5Na0.5TiO3)-0.228(SrTiO3)-0.05(AgNbO3) (BNT-ST-AN) ceramics with various amounts of potassium fluoride (KF) added were prepared by the solid-state combustion technique. The ceramics presented a single perovskite phase with coexisting rhombohedral (R), cubic (C) and orthorhombic (O) phases. The amount of the R phase decreased while the percentage of the C+O phase increased when KF addition increased from 0.0 to 3.0 mol%. The smallest grain size, the highest density and maximum dielectric constant (em) were achieved with a KF addition of 1.5 mol%. Following this design composition of the ceramics, the highest recoverable energy-storage density (Wrec ~1.60 J/cm3) and η above 85.8% at a low electric field (100 kV/cm) were obtained from BNT-ST-AN with KF addition at 1.5 mol% because this composition contained a morphotropic phase boundary (MPB) region and had the smallest grain size, which gave the lowest remnant polarization (Pr) and a large maximum polarization (Pm). Additionally, BNT-ST-AN with KF addition at 0.15 mol% exhibits stability over a wide range of temperatures (25–150°C) at a low electric field (100 kV/cm), which shows great potential in pulse-power system applications.