Superior temperature‐stable dielectrics for MLCCs based on Bi0.5Na0.5TiO3‐NaNbO3 system modified by CaZrO3

An ultra‐wide temperature stable ceramic system based on (1?x) [0.94(0.75Bi_0.5Na_0.5TiO₃?0.25NaNbO₃)?0.06BaTiO₃]?xCaZrO₃ (CZ100x) is developed for capacitor application in this study. All samples exhibit characteristics of pseudocubic structures in XRD patterns. With CaZrO₃ addition, the coupling effect of polar nanoregions (PNRs) is weakening, leading to greatly improved temperature stability of dielectric properties. Among all samples, the most attractive properties are obtained in the composition of CZ10 at <15% variation in dielectric permittivity spanning from ?55°C to 400°C and lower than 0.02 of dielectric loss of between ?60°C and 300°C, accompanied by high DC resistivity (10⁷ Ω m at 300°C, calculated by fitting Jonscher's power law). Furthermore, tentative multilayer ceramic capacitors (MLCCs) composed of CZ10 dielectric and Ag:Pd (70:30) internal electrode layers were fabricated by tape casting and cofiring processes. Temperature‐stable dielectric property in formation of MLCC was successfully realized, with small ΔC/C_25°C (<15%) and loss factor (≤ 0.02) between ?55°C and 340°C. Meanwhile, CZ10‐based MLCC showed temperature‐insensitive energy storage density of 0.31?0.35 J/cm³ and high‐energy efficiency of above 77% at 120 kV/cm in the range of ?55 to 175°C. All of these exhibit wonderful temperature‐stable dielectric properties and indicate the promising future of CZ10 dielectric as high‐temperature ceramic capacitors.     

Large Strain Response and Fatigue‐Resistant Behavior in Ternary Bi0.5Na0.5TiO3–BaTiO3–Bi(Zn0.5Ti0.5)O3 Solid Solutions

A ternary solid solution (1 ? x)(0.88Bi_0.5Na_0.5TiO₃–0.12BaTiO₃)‐xBi(Zn_0.5Ti_0.5)O₃ (BNBZT, BNBZTx) was designed and fabricated using the traditional solid‐state reaction method. The temperature and composition dependence of dielectric, ferroelectric, piezoelectric, and fatigue properties were systematically investigated and a schematic phase diagram was proposed. The substitution with Bi(Zn_0.5Ti_0.5)O₃ was found to shift the phase transition (ferroelectric tetragonal to relaxor pseudocubic phase) to lower temperatures. At a critical composition x of 0.05, large electric‐field‐induced strain response with normalized strain S_max/E_max as high as 526 pm/V was obtained under a moderate field of 4 kV/mm around room temperature. The strain exhibited good temperature stability within the temperature range of 25°C–120°C. In addition, excellent fatigue‐resistant behavior was observed in the proposed BNBZT solid solution after 10⁶ bipolar cycles. These give the BNBZT system great potential as environmental friendly solid‐state actuator.     

Large Electrostrictive Strain in (Bi0.5Na0.5)TiO3–BaTiO3–(Sr0.7Bi0.2)TiO3 Solid Solutions

Relaxor ferroelectrics (0.94 ? x)(Bi_0.5Na_0.5)TiO₃–0.06BaTiO_3?x(Sr_0.7Bi_0.2□_0.1)TiO₃ (BNT–BT–xSBT) (0 ≤ x ≤ 0.5), were prepared by a solid‐state reaction process, and their structures were characterized by the transmission electron microscopy and Raman spectroscopy. The BNT–BT–0.3SBT has a very high electrostrictive strain S = 0.152% with hysteresis‐free behavior, much more than the reported S in other ferroelectrics. S–P² profiles perfectly follow the quadratic relation, which indicates a purely electrostrictive effect with a high electrostrictive coefficient (Q₁₁) of 0.0297 m⁴/C². Even, its Q₁₁ keeps at a high level in the temperature range from ambient temperature to 180°C. The field‐induced large electrostrictive strain of BNT–BT–0.3SBT was attributed to the existence of ferroelectric nanodomains.     

Ultra‐Wide Temperature Stable Dielectrics Based on Bi0.5Na0.5TiO3–NaNbO3 System

The microstructure, phase structure, ferroelectric, and dielectric properties of (1?x)Bi_0.5Na_0.5TiO₃‐xNaNbO₃ [(1?x)BNT‐xNN] ceramics conventionally sintered in the temperature range of 1080°C–1120°C were investigated as a candidate for capacitor dielectrics with wide temperature stability. Perovskite phase with no secondary impurity was observed by XRD measurement. With increasing NN content, (1?x)BNT‐xNN was found to gradually transform from ferroelectric (x = 0–0.05) to relaxor (x = 0.10–0.20) and then to paraelectric state (x = 0.25–0.35) at room temperature, indicated by P–I–E loops analysis, associated with greatly enhanced dielectric temperature stability. For the samples with x = 0.25–0.35, the temperature coefficient of capacitance (TCC) was found <11% in an ultra‐wide temperature range of ?60°C–400°C with moderate dielectric constant and low dielectric loss, promising for temperature stable capacitor applications.     

Microstructure and Electrical Properties of Nonstoichiometric 0.94(Na0.5Bi0.5+x)TiO3–0.06BaTiO3 Lead‐Free Ceramics

0.94(Na_0.5Bi_0.5+x)TiO₃–0.06BaTiO₃ (x = ?0.04, 0, 0.02; named NB_0.46T‐6BT, NB_0.50T‐6BT, NB_0.52T‐6BT, respectively) lead‐free piezoelectric ceramics were prepared via the solid‐state reaction method. Effects of Bi³⁺ nonstoichiometry on microstructure, dielectric, ferroelectric, and piezoelectric properties were studied. All ceramics show typical X‐ray diffraction peaks of ABO₃ perovskite structure. The lattice parameters increase with the increase in the Bi³⁺ content. The electron probe microanalysis demonstrates that the excess Bi₂O₃ in the starting composition can compensate the Bi₂O₃ loss induced during sample processing. The size and shape of grains are closely related to the Bi³⁺ content. For the unpoled NB_0.50T‐6BT and NB_0.52T‐6BT, there are two dielectric anomalies in the dielectric constant–temperature curves. The unpoled NB_0.46T‐6BT shows one dielectric anomaly accompanied by high dielectric constant and dielectric loss at low frequencies. After poling, a new dielectric anomaly appears around depolarization temperature (T_d) for all ceramics and the T_d values increase with the Bi³⁺ amount decreasing from excess to deficiency. The diffuse phase transition character was studied via the Curie–Weiss law and modified Curie–Weiss law. The activation energy values obtained via the impedance analysis are 0.69, 1.05, and 1.16 eV for NB_0.46T‐6BT, NB_0.50T‐6BT and NB_0.52T‐6BT, respectively, implying the change in oxygen vacancy concentration in the ceramics. The piezoelectric constant, polarization, and coercive field of the ceramics change with the variation in the Bi³⁺ content. The Rayleigh analysis suggests that the change in electrical properties of the ceramics with the variation in the Bi³⁺ amount is related to the effect of oxygen vacancies.     

Composition‐ and Temperature‐Dependent Large Strain in (1−x)(0.8Bi0.5Na0.5TiO3–0.2Bi0.5K0.5TiO3)– xNaNbO3 Ceramics

Ternary solid solutions of (1 ? x)(0.8Bi_0.5Na_0.5TiO₃–0.2Bi_0.5K_0.5TiO₃)– xNaNbO₃ (BNKT–xNN) lead‐free piezoceramics were fabricated using a conventional solid‐state reaction method. Pure BNKT composition exhibited an electric‐field‐induced irreversible structural transition from pseudocubic to ferroelectric rhombohedral phase at room temperature. Accompanied with the ferroelectric‐to‐relaxor temperature T_F‐R shifted down below room temperature as the substitution of NN, a compositionally induced nonergodic‐to‐ergodic relaxor transition was presented, which featured the pinched‐shape polarization and sprout‐shape strain hysteresis loops. A strain value of ~0.445% (under a driving field of 55 kV/cm) with large normalized strain of ~810 pm/V was obtained for the composition of BNKT–0.04NN, and the large strain was attributed to the reversible electric‐field‐induced transition between ergodic relaxor and ferroelectric phase.     

Dielectric Relaxor Evolution and Frequency‐Insensitive Giant Strains in (Bi0.5Na0.5)TiO3‐Modified Bi(Mg0.5Ti0.5)O3–PbTiO3 Ferroelectric Ceramics

The 0.45Bi(Mg_0.5Ti_0.5)O₃–(0.55 ? x)PbTiO₃–x(Bi_0.5Na_0.5)TiO₃ (BMT–PT–xBNT) ternary solid solution ceramics were prepared via a conventional solid‐state reaction method; the evolution of dielectric relaxor behavior and the electrostrain features were investigated. The XRD and dielectric measurements showed that all studied compositions own a single pseudocubic perovskite structure and undergo a diffuse‐to‐relaxor phase transition owing to the evolution of the domain from a frozen state to a dynamic state. The formation of the above dielectric relaxor behavior was further confirmed by a couple of measurements such as polarization loops, polarization current density curves, as well as bipolar strain loops. A large strain value of ~0.41% at a driving field of 7 kV/mm (normalized strain d₃₃* of ~590 pm/V) was obtained at room temperature for the composition with x = 0.32, which is located near the boundary between ergodic and nonergodic relaxor. Moreover, this electric field‐induced large strain was found to own a frequency‐insensitive characteristic.     

Large Electromechanical Strain in Lead‐Free Binary System K0.5Bi0.5TiO3‐Bi(Mg0.5Ti0.5)O3

Solid solution formation in the lead‐free binary system (1?x)K_0.5Bi_0.5TiO₃?xBi(Mg_0.5Ti_0.5)O₃ has been studied for compositions x ≤ 0.12. X‐ray powder diffraction shows single‐phase perovskite for x < 0.1, and a mixed phase region between tetragonal and pseudocubic phases for compositions 0.04 ≤ x ≤ 0.06. Large electromechanical strains of ~0.3% at fields of 50 kV/cm are recorded in the mixed phase region, with d₃₃* (S_max/E_max) values of ~600 pm/V. The materials sustain polarization at low electric fields with remnant polarization ~18 μC/cm² and coercive field ~20 kV/cm for x = 0.06. Relative permittivity‐temperature plots display relaxor characteristics, with peak temperature ~340°C.     

Dielectric and Raman Spectroscopic Studies of Na0.5Bi0.5TiO3–BaSnO3 Ferroelectric System

A series of lead‐free perovskite solid solutions of (1 ? x) Na_0.5Bi_0.5TiO₃(NBT)—x BaSnO₃(BSN), for 0.0 ≤ x ≤ 0.15 have been synthesized using a high‐temperature solid‐state reaction route. The phase transition behaviors are studied using dielectric and Raman spectroscopic techniques. The ferroelectric to relaxor phase transition temperature (T_FR) and the temperature corresponding to maximum dielectric permittivity (T_m) are estimated from the temperature‐dependent dielectric data. Dielectric studies show diffuse phase transition around ~335°C in pure NBT and this transition temperature decreases with increase in x. The disappearance of x‐dependence of A₁ mode frequency at ~134 cm^?1 for x ≥ 0.1 is consistent with rhombohedral‐orthorhombic transition. In situ temperature dependence Raman spectroscopic studies show disappearance and discontinuous changes in the phonon mode frequencies across rhombohedral (x < 0.1)/orthorhombic (x ≥ 0.1) to tetragonal transition.     

Preparation and Electric Properties of Bi0.5Na0.5TiO3–Bi(Al0.5Ga0.5)O3 Lead‐Free Piezoceramics

A new lead‐free BNT‐based piezoelectric ceramics of (1 ? x)Bi_0.5Na_0.5TiO₃–xBi(Al_0.5Ga_0.5)O₃ (x = 0, 0.02, 0.03, 0.04, and 0.05) were synthesized using a conventional ceramic fabrication method. Their structures and electrical properties were investigated. All the samples show a typical ferroelectric P(E) loops and S(E) curves at room temperature. The optimal properties are obtained at the composition of the x = 0.03. The substitution of Bi(Al_0.5Ga_0.5)O₃ enhances piezoelectric constant and increases Curie temperature from 58 pC/N and 310°C of pure BNT to 93 pC/N and 325°C of the x = 0.03. The temperature‐dependent P(E) loops and S(E) curves of 0.97BNT–0.03BAG indicate that phase transition from ferroelectric to antiferroelectric takes place over a very wide temperature region from 80°C to 180°C. The results show that the introduction of BAG improves the electrical properties of BNT.     

Improved dielectric energy storage performance of Na0.5Bi0.5TiO3–Sr0.7Nd0.2TiO3 lead-free ceramics by adding an appropriate amount of AgNbO3

Based on the significant advantages of dielectric ceramics in high power energy storage, (1-x) (0.55Bi_0.5Na_0.5TiO₃-0.45Sr_0.7Nd_0.2TiO₃)-xAgNbO₃ (NBSNT-xAN) ceramics were prepared by traditional solid phase method. The introduction of AN in NBSNT ceramics not only increased the degree of relaxation, but also refined the grain size, enhanced the BDS, and finally improved the energy storage performance. It is found that the NBSNT-0.5AN ceramics obtained an effective energy storage density as high as 3.08 J/cm³ and an efficiency of 79.94%. In addition, good thermal stability and temperature stability were exhibited in the range of 30–120 °C and 10–350 Hz, and at the same time, it performed very well in the pulsed test at room temperature and variable temperatures. This provides a design idea for the miniaturization and integration of energy storage ceramic materials.     

Structure,Ferroelectric, Piezoelectric,and Ferromagnetic Properties of BiFeO3‐BaTiO3‐Bi0.5Na0.5TiO3 Lead‐Free Multiferroic Ceramics

Lead‐free multiferroic ceramics of BiFeO₃‐BaTiO₃‐Bi_0.5Na_0.5TiO₃ have been prepared by a conventional ceramic technique. The microstructure, multiferroic, and piezoelectric properties of the ceramics have been studied. The ceramics sintered at 1000°C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and tetragonal phases is formed at x = 0.02. After the addition of Bi_0.5Na_0.5TiO₃, two dielectric anomalies are observed at high temperatures (T_m ~ 510°C–570°C and T₂ ~ 720°C). The phase transition around T_m becomes wider gradually with increasing x. The ferroelectricity, piezoelectricity, and ferromagnetism of the ceramics are significantly improved after the addition of Bi_0.5Na_0.5TiO₃. High resistivity (~1.3 × 10⁹ Ω·cm), strong ferroelectricity (P_r = 27.4 μC/cm²), good piezoelectricity (d₃₃ =140 pC/N, k_p = 31.4%), and weak magnetic properties (M_r =0.19 emu/g) are observed.     

Enhanced optical transmittance and energy-storage performance in NaNbO3-modified Bi0.5Na0.5TiO3 ceramics

Dielectric ceramics with both excellent energy storage and optical transmittance have attracted much attention in recent years. However, the transparent Pb-free energy-storage ceramics were rare reported. In this work, we prepared transparent relaxor ferroelectric ceramics (1 − x)Bi_0.5Na_0.5TiO₃–xNaNbO₃ (BNT–xNN) by conventional solid-state reaction method. We find the NN-doping can enhance the polarization and breakdown strength of BNT by suppressing the grain growth and restrained the reduction of Ti⁴⁺ to Ti³⁺. As a result, a high recoverable energy-storage density of 5.14 J/cm³ and its energy efficiency of 79.65% are achieved in BNT–0.5NN ceramic at 286 kV/cm. Furthermore, NN-doping can promote the densification to improve the optical transmittance of BNT, rising from ∼26% (x = 0.2) to ∼32% (x = 0.5) in the visible light region. These characteristics demonstrate the potential application of BNT–xNN as transparent energy-storage dielectric ceramics.     

Dielectric Properties of Ba0.8Ca0.2TiO3–Bi(Mg0.5,Ti0.5)O3–NaNbO3 Ceramics

Ceramics in the system 0.45Ba_0.8Ca_0.2TiO₃–(0.55?x)Bi(Mg_0.5Ti_0.5)O₃–xNaNbO₃, x = 0–0.02 were fabricated by a conventional solid‐state reaction route. X‐ray powder diffraction indicated cubic or pseudocubic symmetry for all samples. The parent 0.45Ba_0.8Ca_0.2TiO₃–0.55Bi(Mg_0.5Ti_0.5)O₃ composition is a relaxor dielectric with a near‐stable temperature coefficient of relative permittivity, ε_r = 950 ± 10% across the temperature range 80°C–600°C. Incorporation of NaNbO₃ at x = 0.2 extends the lower working temperature to ≤25°C, with ε_r = 575% ± 15% from temperatures ≤25°C to >400°C, and tan δ < 0.025 from 25°C to 400°C. Values of dc resistivity ranged from ~10⁹ Ω·m at 250°C to ~10⁶ Ω·m at 500°C. The properties suggest that this material may be of interest for high‐temperature capacitor applications.     

Large Piezoresponse and Ferroelectric Properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–Bi(Mg0.5Ti0.5)O3 Thin Films Prepared by Chemical Solution Deposition

Bulk ceramic 72.5 mol%(Bi_0.5Na_0.5)TiO₃–22.5 mol%(Bi_0.5K_0.5)TiO₃–5 mol%Bi(Mg_0.5Ti_0.5)O₃ (BNT–BKT–BMgT) has previously been reported to show a large high‐field piezoelectric coefficient (d₃₃* = 570 pm/V). In this work, the same composition was synthesized in thin film embodiments on platinized silicon substrates via chemical solution deposition. Overdoping of volatile cations in the precursor solutions was necessary to achieve phase‐pure perovskite. An annealing temperature of 700°C resulted in good ferroelectric properties (P_max = 52 μC/cm² and P_r = 12 μC/cm²). Quantitative compositional analysis of films annealed at 650°C and 700°C indicated that near ideal atomic ratios were achieved. Compositional fluctuations observed through the film thickness were in good agreement with the existence of voids formed between successive spin‐cast layers, as observed with electron microscopy. Bipolar and unipolar strain measurements were performed via double laser beam interferometry and a high effective piezoelectric coefficient (d_33,f) of approximately 75 pm/V was obtained.     

Morphotropic Phase Boundary in Solution‐Derived (Bi0.5Na0.5)1−xBaxTiO3 Thin Films: Part II Functional Properties and Phase Stability

The analysis of the functional properties (ferroelectric, dielectric, and piezoelectric) of chemical solution deposited thin films of the lead‐free (Bi_0.5Na_0.5)_1?xBa_xTiO₃ (BNBT) solid solution prepared from solution precursors with and without Na⁺ and Bi³⁺ excesses has been performed in this work. At room temperature a nonergodic relaxor ferroelectric state has been found. The switched polarization of the films is not stable at room temperature, poor remnant polarization, associated with an enhancement of the induced domains randomization produced by the films constraints. The depolarization temperature for the switched polarization allowed us to build up a tentative phase diagram for these BNBT films. Both the better functional properties and the agreement of the depolarization temperature with the freezing temperature of the relaxor Volger–Fulcher behavior permit to locate the center of the morphotropic phase boundary region close to x = 0.055 in the stoichiometric films and x = 0.10 for the films with Na⁺ and Bi³⁺ excesses. Based on these results, the possible applications of these films are discussed.     

Preparation and Electric Properties of Bi0.5Na0.5TiO3–Bi(Mg0.5Ti0.5)O3 Lead‐Free Piezoceramics

Lead‐free BNT‐based piezoceramics, (1?x)Bi_0.5Na_0.5TiO₃–xBi(Mg_0.5Ti_0.5)O₃ [(1?x)BNT–xBMT] (0.00 ≤ x ≤ 0.06) binary system, were synthesized using a conventional ceramic fabrication method. Effect of Bi(Mg_0.5Ti_0.5)O₃ (BMT) substitution on room temperature (RT) crystal structure, and temperature dependence of electric properties were investigated. The XRD indicates that a pure perovskite phase is formed. The introduction of BMT decreases E_C of BNT from 7.3 to 4.0 kV/mm, and increases d₃₃ from 58 pC/N to 110 pC/N for the x = 0.05. The system shows a typical ferroelectric (FE) polarization loop P(E) and butterfly bipolar strain‐electric S(E) curve at RT. For the composition of 0.95BNT–0.05BMT antiferroelectric (AFE) phase appears near 80°C, characterized by a constricted P(E) loop and altered bipolar S(E) butterfly, and gradually prevails with increasing temperature. Temperature dependence of dielectric constant shows that T_C increases from 310°C for pure BNT to 352°C for the x = 0.05. The results indicate that the piezoelectric properties of BNT have been improved by means of Bi(Mg_0.5Ti_0.5)O₃ substitution.     

Impedance Spectroscopy of (Bi1/2Na1/2)TiO3–BaTiO3 Ceramics Modified with (K0.5Na0.5)NbO3

The electrical and dielectric properties of (1 ? x)(0.94Bi_1/2Na_1/2TiO₃–0.06BaTiO₃)–x(K_0.5Na_0.5NbO₃) with x = 0, 0.03, 0.09, 0.18 have been investigated by impedance spectroscopy over a wide temperature range. The dc conductivity of the ceramics follows the Arrhenius law with an activation energy ranging from ~1.20 to 1.50 eV. Measurements under different atmospheres show the materials exhibit n‐type semiconducting behavior at elevated temperatures. The presence of a highly polarizable phase for all compositions is revealed by electric modulus (M″) spectra. The Burns temperature decreases with increasing KNN content. The change in temperature‐dependent permittivity with composition is explained by the difference in thermal evolution of polar nanoregions induced by the addition of KNN.     

Na0.5Bi0.5TiO3-based ferroelectric relaxor with high thermal stability and anti-fatigue for charge-discharge properties

Apart from discharge energy density (W_r) and discharging time (t_0.9), thermal stability and anti-fatigue for charge-discharge performance are also the important performance indexes for dielectric pulsed capacitor. Na_0.5Bi_0.5TiO₃ based ceramics are usually accompanied by huge electric field-induced strain when appling electric field, resulting in the fatigue phenomenon and thermal accumulation effect in the cycling process. In this work, Na_0.5Bi_0.5TiO₃-xNaNbO₃ (NBT-xNN) ferroelectric relaxor ceramic has been prepared by the solid state reaction process. The effect of NaNbO₃ content on microstructures, impedance spectroscopy, electric-field-induced strain and charge-discharge performance of NBT-xNN ceramics have been investigated systematically. Results indicate the proper percent of NaNbO₃ could favor the formation of polar nanoregions (PNRs), which leads to the diffusion of phase transition and the diminution of electromechanical strain. Therefore, the high thermal stability and anti-fatigue for charge-discharge property has been achieved in NBT-xNN ceramics. An enhanced discharging energy density of 2.44 J cm^?3 along with discharge time of 0.31 μs could be obtained in the NBT-xNN with x = 0.3, and a very stable discharge energy density of 2.06 J cm^?3 concomitantly with discharge time less than 0.37 μs could be gained in a wide temperature range of 20–150 °C with a fluctuation of ±4% after 10⁴ charge/discharge cycles. This work would contribute to the development of charge-discharge system, especially dielectric capacitor, for green pulsed power devices.     

Enhanced dielectric energy storage performance of A-site Ca2+-doped Na0.5Bi0.5TiO3–BaTiO3–BiFeO3 Pb-free ceramics

(1-x) (0.98Na_0.5Bi_0.5TiO₃–0.01BaTiO₃–0.01BiFeO₃)–xCaTiO₃ (NBB-xCT) ceramics were produced using traditional solid-state synthesis methods. The surface morphology, domain structure, and electrical properties of the ceramic samples were systematically studied. In addition, the temperature and frequency stabilities of the NBB-15CT sample at 200 kV/cm were tested. Generally, NBB-xCT ceramics exhibit a typical single perovskite phase structure. The results indicate that the NBB-15CT ceramics showed a high energy density of 3.14 J/cm³ at 250 kV/cm. The piezoresponse force microscopy (PFM) results showed that the addition of CT broke the macrodomains of the 0.98Na_0.5Bi_0.5TiO₃-0.01BaTiO₃-0.01BiFeO₃ ceramic and helped to form nanodomains, leading to an improved energy storage performance. The above performance indicates that the specimens possess very good temperature-and frequency-dependent energy storage performances at 30–150 °C and 1–100 Hz. Moreover, the electric energy storage and release in the NBB-15CT ceramic indicated that the power density could reach 55.30 MV/cm³ at 180 kV/cm. Therefore, the NBB-15CT ceramic is a promising material for electrical capacitors.