High energy storage density realized in Bi0.5Na0.5TiO3-based relaxor ferroelectric ceramics at ultralow sintering temperature

The miniaturization and integration trend of electronic applications requires high energy storage performance, and the development of multilayer ceramic capacitors (MLCC) demands the compatibility between ceramic sintering temperature and co-firing temperature of metal electrodes. Herein, we obtained a high recoverable energy storage density and a low sintering temperature simultaneously in 0.5(Bi_0.5Na_0.5)TiO₃-0.5SrTiO₃-x mol% CuO (0.5BNT-0.5ST-x mol% CuO) via the combination of adding CuO sintering aid and citrate sol-gel synthesis method. The optimum sintering temperature decreases significantly from 1130 °C for x = 0 to 820 °C for x = 2.0. The ceramic of 0.5BNT-0.5ST-1.5 mol% CuO exhibits a large W_rec of 2.20 J/cm³ and η of 72.39% under 230 kV/cm. Furthermore, the same sample also possesses a large C_D of 1740.97 A/cm², an extremely high P_D of 139.28 MW/cm³ and an ultrafast discharge speed of 82 ns. These merits reveal that the ceramic of 0.5BNT-0.5ST-1.5 mol% CuO has great potential in practical MLCC production.     

Preparation of Na0.5Bi0.5TiO3 ceramics by hydrothermal-assisted cold sintering

A recently proposed novel technique, termed “cold sintering process” (CSP), can provide dense ceramic solids at remarkably low temperatures around 180?°C. In a recent work, we successfully obtained dense Na_0.5Bi_0.5TiO₃ ceramics by this method. Bismuth titanate sodium nanoparticles were prepared as the raw material powder by the hydrothermal synthesis route. A hydrothermal precursor solution was used as the transient solvent for cold sintering. Under the combined action of pressure and temperature, the Na_0.5Bi_0.5TiO₃ green body was densified by dissolution-precipitation, and a preliminary densified ceramic sheet was obtained. The amorphous phase in the ceramic sheet was then transformed into a crystalline phase by annealing. Finally, densified Na_0.5Bi_0.5TiO₃ ceramic sheets were obtained, with density of up to 99%, relative permittivity of 681, and dielectric loss of 0.08 at 10?kHz and room temperature. The piezoelectric coefficient d₃₃ of the sample was 52.5?pC/N. The properties of the prepared ceramics were comparable to those of the conventional sintered ceramics.     

Structure and ferroelectric behaviour of Na0.5Bi0.5TiO3-KNbO3 ceramics

Lead-free piezoelectric ceramics, (1?x)Na_0.5Bi_0.5TiO₃-xKNbO₃ (NBT-xKN), with x?=?0.02–0.08 were fabricated by solid-state reaction and sintering. The crystal structures and dielectric properties were measured for different KN contents. All compositions in the unpoled, as-sintered state were found to be single-phase pseudo-cubic. However, typical ferroelectric behaviour, with well-saturated polarisation-electric field hysteresis loops, was observed for certain compositions at high electric field levels. It is shown using high-energy synchrotron X-ray diffraction that the application of the electric field induced an irreversible structural transformation from the nano-polar pseudo-cubic phase to a ferroelectric rhombohedral phase. The changes in lattice elastic strain and crystallographic texture of a poled NBT-0.02KN specimen as a function of the grain orientation, ψ, conform well to those expected for a conventional rhombohedrally distorted perovskite ferroelectric ceramic. The dielectric permittivity-temperature relationships for all compositions exhibit two transition temperatures and a frequency-dependent behaviour that is typical of a relaxor ferroelectric. The transition temperatures and grain size decrease with the increasing KN content.     

BaTiO3 nanowires-induced phase transition and thermally stable strain in (Bi0.5Na0.5)TiO3 piezoelectric ceramics

Environment-friendly lead-free piezoceramics with high strain response and extremely excellent stability in a wide operating temperature range are critically important in practical actuator applications. Here, we develop a new strategy to tune the electrostrictive strain behavior in Bi_0.5Na_0.5TiO₃ (BNT)-based ceramics via using high aspect ratio BaTiO₃ nanowires (BT NWs) as a modifier. The addition of BT NWs generates a crossover from a typical ferroelectric (BT conventional spherical particles) to a complete ergodic relaxor (ER) phase at ambient temperature, accompanied by a large electrostrictive strain of ∼0.17% with d₃₃*(S_max/E_max) = 284 pm/V. Such a high electrostrictive strain is extremely thermally stable with only <7% fluctuation from 27 °C to 120 °C. In addition, the BT NWs-modified ceramics also exhibit acceptable fatigue endurance (<30% up to 10⁵ cycles) and frequency dependence (<20% at 10Hz–100Hz). These achieved exceptional performances can be ascribed to the BT NWs-driven complete ER phase at room temperature. The findings of this study can inspire enhanced interest in nanowires as a viable modifier to BNT-based materials due to promising potential for practical actuator applications in a wide temperature range.     

Structural evolution and electrical properties of Na0.5Bi0.5TiO3-CoFe2O4 ceramics with embedded structures

Lead-free 0.9Na_0.5Bi_0.5TiO₃ − 0.1CoFe₂O₄ (0.9NBT-0.1CFO) composite ceramics with novel embedded microstructures were synthesized by an in-situ sol-gel method. The structural evolution, ferroelectricities and dielectricities were studied. The embedded structures were formed around 1000 °C − 1100 °C with contributions of the strong grain boundary mobility and small size of CFO grains, and then were destroyed at 1150 °C. The 0.9NBT-0.1CFO ceramic sintered at 1100 °C showed the optimal ferroelectric hysteresis loop with a remanent polarization of ~ 52 μC/cm². Anomaly peaks in the temperature dependent permittivity curves were observed in all ceramics with embedded structures. These Debye-like type peaks generated by the interface effects of NBT and CFO appeared around ~620 °C, and could reach relatively large values of ~15000 at 100 kHz. The change rate of permittivity between zero magnetic field and 500 Oe reached 2.4%, indicating a noteworthy magnetoelectric effect.     

Influence of secondary phases on ferroelectric properties of Bi0.5Na0.5TiO3 ceramics

The effects of secondary phases on ferroelectric properties of Bi_0.5Na_0.5TiO₃ (BNT) have been studied. Ceramic powders were prepared by solid state reaction employing different sintering temperatures and characterized by X-ray diffraction (XRD), Scanning Electron Microscopy and impedance spectroscopy. The perovskite structure was detected by XRD; together with small peaks corresponding to a secondary phase assigned to the Na₂Ti₆O₁₃-based phase in calcined powders. In addition, morphology and the content of the secondary phase were modified by the sintering temperatures, affecting the ferroelectric properties, and ac and dc conductivities. We believe that our results can benefit not only the understanding of BNT ceramics, but also expand the range of applications.     

Two-step sintering of 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 lead-free piezoelectric material

Two-step sintering (TSS) as an efficient sintering method for obtaining dense microstructure while preventing excess grain growth was used for sintering 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ composition which is located near the morphotropic phase boundary of this binary system. In order to compare the obtained microstructure and piezoelectric properties, conventional single step sintering (SSS) was also examined. Microstructure evolution during sintering at different temperatures was investigated to find the optimum sintering temperature. Ferroelectric hysteresis loop as well as unipolar strain behavior of optimally sintered ceramics was studied. According to density measurement and microstructure studies of the prepared ceramics, TSS resulted in finer and more dense and uniform microstructure compared to SSS method. As a result remnant polarization of TSSed ceramic was increased by 35% and its coercive field was decreased by 16%. The inverse piezoelectric coefficient of the SSSed and TSSed was obtained 220 and 300 p.m./V, respectively. These values are high enough for practical applications such as actuators. The obtained results clearly showed that TSS is capable of sintering 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ at temperatures lower than which is required for SSS method. Therefore the composition stoichiometry is maintained after sintering and denser microstructure without abnormal grain growth is obtained which is responsible for improved electrical properties of the piezoceramics.     

Analysis of high temperature reduction process of Na0.5Bi0.5TiO3-based ceramics

Lead-free metamaterials with enormous effective apparent piezoelectric response has been fabricated by applying an asymmetric chemical reduction to Na_0.5Bi_0.5TiO₃ (NBT)-based ceramics. To achieve high performance, optimization of the reduction conditions is required. In this study, we analyzed the effect of reduction temperature and time on the reduction thickness of NBT-based ceramics. We found that the reduction reaction between NBT-based ceramics and graphite is an interface reaction rate-controlled process. The reduction thickness has a linear relationship with the reaction time at a fixed reduction temperature. The lower activation energy of NBT-based ceramics than that of lead-based materials indicates the lead-free ceramics are easier to be reduced. The effect of the reduction on the flexoelectric-like response was further explored, and the maximum response (?>1 mC/m) was measured in the ceramics having a reduction-thickness-to-total-thickness ratio of around 0.28. This study provides a guideline to optimize the fabrication conditions of the NBT-based metamaterials.     

Large strain and low hysteresis in (0.64-x) Bi0.5Na0.5TiO3- 0.36Sr0.7Bi0.2TiO3- x(K0.5La0.5)(Ti0.9Zr0.1)O3 piezoceramics

In this work, (0.64-x)Bi_0.5Na_0.5TiO₃- 0.36Sr_0.7Bi_0.2TiO₃- x(K_0.5La_0.5)(Ti_0.9Zr_0.1)O₃ lead-free piezoceramics were designed and fabricated by a conventional solid-phase sintering process. It is found that large strains (0.33 %), low hysteresis coefficients (32 %), and large dynamic d₃₃* (367 p.m./V) were obtained at x = 0.01. The large strain originates from the reversible transition of the relaxor to the long-range ferroelectric order in the electric field. When the ferroelectric and relaxor phases coexist in a proper ratio, they can provide a favorable condition for the easier movement of the domains and improve the strain properties. In addition, after 10⁵ cycles, the bipolar strain loop of x = 0.01 content changed slightly, demonstrating excellent fatigue resistance. This work provides a new way to design piezoelectric ceramics with large strain and low hysteresis.     

Improved electric-field-induced strain and strong photoluminescence properties in novel Er3+-modified 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 multifunctional ceramics

Recently, the progress of electronic devices toward miniaturization has strongly promoted development of multifunctional materials possessing multiple desirable properties. In this study, we develop and fabricate 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃-xEr multifunctional ceramics which show simultaneously considerable electric-field-induced strain and bright green light emission properties. With the introduction of Er³⁺, the ceramics gradually transform from non-ergodic relaxor phase to ergodic relaxor phase which could reversibly transform to ferroelectric phase under the electric field. As a result, with improving Er³⁺ content, the shape of the polarization-electric field loops of the ceramics become pinched, and it is obvious that the negative strain disappears while the positive strain gradually increases and reaches a maximum value 0.46% at x = 1.2 mol%. Besides, After the ceramics are poled, the light emission peak are greatly enhanced attributed to the decreased crystal symmetry and increased domain size, and is the strongest at x = 1.2 mol%. These results indicate that 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃-xEr ceramics are good candidates for developing multifunctional optoelectronic devices.     

Electric-field-temperature phase diagram and electromechanical properties in lead-free (Na0.5Bi0.5)TiO3-based incipient piezoelectric ceramics

In this work, the (1-x)(0.8Na_0.5Bi_0.5TiO₃-0.2K_0.5Bi_0.5TiO₃)-xSrTiO₃ (NKBT-xST) incipient piezoelectric ceramics with x = 0–0.07 (0ST-7ST) were prepared by the solid-state reaction method and their structural transformation and electromechanical properties were investigated as a function of ST content. As the ST content increases, the long-range ferroelectric order is disrupted, and the ferroelectric-relaxor phase transition temperature (T_FR) shifts to around room temperature for NKBT-5ST ceramics, accompanied by a relatively high electrostrain of 0.3% at 6 kV/mm. The large strain response associated with the vanished ferroelectric properties around T_FR can be attributed to the reversible relaxor-ferroelectric phase transition. The electric-field-temperature (E-T) phase diagrams were established, and the transition between the two field-induced long-range ferroelectric states were found to take place via a two-step switching process through an intermediate relaxor state. The threshold electric field to trigger the conversion between ferroelectric state and relaxor state depends strongly on the dynamics of polarization relaxation, which is influenced by temperature and composition.     

Influence of sintering conditions on piezoelectric properties of KNbO3 ceramics

A homogeneous KNbO₃ (KN) phase was formed in specimens that were sintered at 1020 °C and 1040 °C, without formation of the K₂O-deficient secondary phase, indicating that the amount of evaporation of K₂O during sintering was very small. However, the KN liquid phase was formed during sintering and assisted the densification of the KN ceramics. A dense microstructure was developed in the specimen sintered at 1020 °C for 6 h and abnormal grain growth occurred in this specimen. A similar microstructure was observed in the specimens sintered at 1040 °C for 1.0 h. The dielectric and piezoelectric properties of the KN ceramics were considerably influenced by the relative density. The KN ceramics sintered at 1020 °C for 6 h, which showed a large relative density that was 95% of the theoretical density, exhibited promising electrical properties: ɛ^T₃₃/ɛ_o of 540, d₃₃ of 109 pC/N, k_p of 0.29, and Q_m of 197.     

Nanoscale origins of small hysteresis and remnant strain in Bi0.5Na0.5TiO3-based lead-free ceramics

BiAlO₃-doped Bi_0.5Na_0.5TiO₃-Bi_0.5K_0.5TiO₃ (BA-doped BNT-BKT) ceramics are greatly concerned due to their sufficient electric-field-induced strain with small hysteresis and remnant strain for high precision positioning devices and other actuators. In this paper, the structural analysis especially the high-resolution transmission electron microscope (HRTEM) is used to reveal the origin of excellent properties obtained in 0.96(0.75BNT-0.25BKT)-0.04BA, which exhibits a large strain of 0.21% at ∼70 kV/cm, a small strain hysteresis of only 24% and a near-zero remnant strain. Using HRTEM, the antiferroelectric nano-domains composited by three variants of in-phase a⁰a⁰c⁺ octahedral tilting coexisted with the remnant ferroelectric nano-domains of anti-phase a⁻a⁻a⁻ octahedral tilting are directly identified. Then a continuous tilting model is proposed to interpret the gradually transitional tilting involving nano-domains leading to the small hysteresis and near-zero remnant strain. The findings may pave a way for further optimizing the properties through creating stable antiferroelectric nano-domains in BNT-based ceramics and the analogues.     

Nanoscale origins of small hysteresis and remnant strain in Bi0.5Na0.5TiO3-based lead-free ceramics

BiAlO₃-doped Bi_0.5Na_0.5TiO₃-Bi_0.5K_0.5TiO₃ (BA-doped BNT-BKT) ceramics are greatly concerned due to their sufficient electric-field-induced strain with small hysteresis and remnant strain for high precision positioning devices and other actuators. In this paper, the structural analysis especially the high-resolution transmission electron microscope (HRTEM) is used to reveal the origin of excellent properties obtained in 0.96(0.75BNT-0.25BKT)-0.04BA, which exhibits a large strain of 0.21% at ～70 kV/cm, a small strain hysteresis of only 24% and a near-zero remnant strain. Using HRTEM, the antiferroelectric nano-domains composited by three variants of in-phase a⁰a⁰c⁺ octahedral tilting coexisted with the remnant ferroelectric nano-domains of anti-phase a^?a^?a^? octahedral tilting are directly identified. Then a continuous tilting model is proposed to interpret the gradually transitional tilting involving nano-domains leading to the small hysteresis and near-zero remnant strain. The findings may pave a way for further optimizing the properties through creating stable antiferroelectric nano-domains in BNT-based ceramics and the analogues.     

Densification behaviors of Al2O3 ceramics during flash sintering

The densification behaviors of MgO-doped-Al₂O₃ ceramics in the flashing stage and the steady stage were investigated using the classic kinetic model. The results show that the most densification of MgO-doped Al₂O₃ was completed during the flashing stage. The densification mechanism transferred from particle rearrangement resulted from Columbic force among particles under the effect of electrical field in the flashing stage to the lattice diffusion in the steady stage. Therefore, the densification rate in the steady stage dramatically decreased. Additionally, the estimated densification activation energy in the steady stage of flash sintering is 396 kJ/mol, much lower than the activation densification of lattice diffusion measured from conventional sintering, likely due to the effect of electric field/current-induced point defects on the diffusion.     

Highly-reliable dielectric capacitors with excellent comprehensive energy-storage properties using Bi0.5Na0.5TiO3-based relaxor ferroelectric ceramics

The development of capacitors with high reliability and good comprehensive performances is of great significance for practical applications. In this work, lead-free relaxor ferroelectric (FE) ceramics of (1-x)(0.5(Bi_0.5Na_0.5)TiO₃-0.5SrTiO₃)-xBi(Mg_2/3Nb_1/3)O₃ ((1-x)(BNT-ST)-xBMN) were prepared by a conventional solid-state reaction method. The introduction of BMN was found to enhance local structure disorder, leading to the significantly reduced size of FE nanodomains, which is responsible for the slim polarization-electric field hysteresis loops. A giant energy-storage density of 6.62 J/cm³ and a high efficiency of 82 % can be achieved simultaneously under a moderate electric field of 34 kV/mm at x = 0.08. It also exhibits high discharge density ～ 2.74 J/cm³, large power density ～ 248 MW/cm³ and ultrafast discharge rate ～ 28 ns at 20 kV/mm in addition to excellent temperature (10–130 °C) and frequency (1–100 Hz) stabilities. These results demonstrate that the (1-x)(BNT-ST)-xBMN ceramic system is a promising lead-free candidate for advanced pulsed power capacitor applications.     

Large electric field-induced strain in ternary Bi0.5Na0.5TiO3-BaTiO3-Sr2MnSbO6 lead-free ceramics

The large electric-field-induced strain of Bi_0.5Na_0.5TiO₃-BaTiO₃ based ceramic make it a potential replacement for lead-based ferroelectrics in actuator applications. Herein, a ternary system (1-x)(Bi_0.5Na_0.5)_0.935Ba_0.065TiO₃-xSr₂MnSbO₆ (BNBT6.5-xSMS) ceramic was fabricated using conventional solid-state reaction. It was found that the ferroelectric to relaxor phase transition temperature T_F-R gradually shifted to lower temperature by increasing SMS contents. The ferroelectricity and piezoelectricity of BNBT6.5 were highly affected by trace amount of SMS doping. For composition BNBT6.5-0.003SMS, where T_F-R was near room temperature, a large electric-field-induced unipolar strain of ~0.4% with high normalized strain (S_max/E_max) of 728 pm/V, which is comparable to lead-based ferroelectric/antiferroelectric ceramics, was achieved owing to the reversible electric-field-induced phase transition between a non-polar relaxor phase to a polar phase with long-range ferroelectric order.     

Thermally-stable large strain in Bi(Mn0.5Ti0.5)O3 modified 0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3 ceramics

(1-x)[0.8Bi_0.5Na_0.5TiO₃-0.2Bi_0.5K_0.5TiO₃]-xBi(Mn_0.5Ti_0.5)O₃ (x = 0–0.06, BNKMT100x) lead-free ferroelectric ceramics were prepared via solid state reaction method. Bi(Mn_0.5Ti_0.5)O₃ induces a structure transition from rhombohedral-tetragonal morphotropic phases to pseudo-cubic phase. Moreover, the wide range of compositions within x = 0.03–0.055 exhibit large strain of 0.31%–0.41% and electrostrictive coefficient of 0.027–0.041 m⁴/C². Especially, at x = 0.04, the large strain and electrostrictive coefficient are nearly temperature-independent in the range of 25–100 °C. The impedance analysis shows the large strain and electrostrictive coefficient originate from polar nanoregions response due to the addition of Bi(Mn_0.5Ti_0.5)O₃.     

Improved dielectric energy storage performance of Na0.5Bi0.5TiO3-based lead-free relaxation ferroelectric ceramics achieved by domain structural regulation and enhanced densification

There is still a problem of low energy storage density in dielectric capacitors which is a core component of power systems. For the improvement of the energy storage density, the linear dielectric material CaTiO₃ (CT) was introduced in Na_0.5Bi_0.5TiO₃ (NBT) ceramics in this paper. By modifying the A site, a new relaxor ferroelectric ceramic was successfully synthesized and attained a recoverable density (W_rec) of 2.34 J/cm³ at x = 0.18. Moreover, the preparation process was optimized in this paper. Through the viscous polymer process (VPP) route, the energy density (W_A) of 82NBT-18CT_VPP ceramic further reaches 6.45 J/cm³ at 340 kV/cm, with efficiency (η) up to 75% and a W_rec of 4.82 J/cm³. At the same time, the change of W_rec is small at temperature (30–150 °C) and frequency (1 Hz–300 Hz), which demonstrates its excellent stability. The discharge power density reaches about 180 MW/cm³ and the discharge time is 0.117 μs, which indicates its excellent pulse discharge performance. The results show that 82NBT-18CT lead-free relaxation ferroelectric material is expected to become ideal for high-energy storage applications.