Raman spectroscopy of Na0.5Bi0.5TiO3

We have measured the Raman spectra of bismuth sodium titanate in its rhombohedral, tetragonal, and cubic phases, with special attention paid to the phase transitions at 584K and 837K (heating). Both transitions appear to be order-disorder and strongly first order, with large thermal hysteresis. The phonon spectra at temperature slightly below the tetragonal phase are remarkably similar to BaTiO₃ with A₁(To) modes at 130, 269, and 541 cm^-1 (compared with 170, 270, 520 cm^-1 in BaTiO₃) and an E(TO) at 52 cm^-1.     

ZnO‐enhanced electrical properties of Bi0.5Na0.5TiO3‐based incipient ferroelectrics

Bi_0.5Na_0.5TiO₃‐based incipient ferroelectrics with pseudocubic structure generally show weak ferro‐/piezoelectricity but giant field‐induced strains. It is difficult to artificially and smoothly improve the electrical property based on conventional chemical doping or substituting without changing the crystal structure and suppressing the strain. Here, by introducing the semiconductor ZnO into the lead‐free incipient ferroelectric ((Bi_0.5(Na_0.84K_0.16)_0.5)_0.96Sr_0.04)(Ti_0.975Nb_0.025)O₃ (BNT–2.5Nb) to form 0‐3 type composites (BNT–2.5Nb:xZnO), we experimentally illustrate that the resistance and ferro‐/piezoelectric properties can be enhanced significantly with an unchanged crystal structure and only slightly suppressed strains. For example, the remanent polarization and piezoelectric coefficient increase from 4.6 μC/cm² and 8 pC/N for x=0 to 9.0 μC/cm² and 31 pC/N for x=0.3. At the same time, the total strain only decreases from 0.140% for x=0 to 0.108% for x=0.3, whereas the negative strain increases from ?0.003% for x=0 to ?0.010% for x=0.3. And the thermal stability of d₃₃ is enhanced. The corresponding mechanism is attributed to that ZnO can form a local field, preventing the depolarization of field induced macroscopic ferroelectric domains. Our results not only provide a feasible way to tune electrical properties of BNT‐based incipient ferroelectrics, but also may stimulate further work on artificially structured high‐performance ferroelectrics.     

Enhanced energy density and thermal stability in relaxor ferroelectric Bi0.5Na0.5TiO3-Sr0.7Bi0.2TiO3 ceramics

Sr_0.7Bi_0.2TiO₃ (SBT) was introduced into Bi_0.5Na_0.5TiO₃ (BNT) via a standard solid-state route to modulate its relaxation behaviour and energy storage performance. With increasing SBT content, the perovskite structure of BNT transforms from a rhombohedral phase to a weakly polarized pseudo-cubic phase, and the relaxation behaviour is enhanced. In particular, the E_DBS is improved from 120 kV/cm of BNT to 160 kV/cm of 0.6BNT-0.4SBT, which displays a large recoverable energy storage density (W_rec = 2.20 J/cm³), implying a large potential ability of energy storage for the 0.6BNT-0.4SBT ceramic. Moreover, both dielectric properties (28–326 °C) and energy storage properties (20–140 °C) exhibit a good thermal stability for the same 0.6BNT-0.4SBT composition. These characteristics suggest 0.6BNT-0.4SBT ceramic could be a promising candidate to be applied in a pulse power system over a broad temperature range.     

Defect Structure of Doped Lead‐Free 0.9(Bi0.5Na0.5)TiO3−0.1(Bi0.5K0.5)TiO3 Piezoceramics

Cu‐ and V‐doped BNKT10‐based piezoelectric ceramics with up to 0.5 at.% dopant concentration were synthesized and displayed more homogeneous grain growth compared to undoped BNKT10 ceramics. The defect chemistry and defect structure, studied by X‐ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR), indicate a slightly rhombic electronic environment with major unidirectional octahedral distortion of the local environment of Cu. The solubility limit of Cu²⁺ in this material system is lower than 0.05 at.% Cu; above this limit, a Cu segregation at the grain boundaries is prevalent, unlike in PZT and KNN. Here, V was shown to be incorporated into the perovskite lattice and possess oxidation states of +4 and +5, acting both as isovalent and donor dopant, predominantly compensated by A‐site vacancies. A trend toward higher ceramic densities, higher maximum polarization, and higher remanent polarization with increasing Cu concentration was observed. A maximum mechanical coupling factor could be obtained in the case of doping with 0.4 at.% V and 0.1 at.% Cu with a planar coupling of 0.19 and a thickness coupling factor of 0.56.     

Enhanced ferroelectric properties and thermal stability of Mn‐doped 0.96(Bi0.5 Na0.5)TiO3‐0.04BiAlO3 ceramics

(Bi_0.5Na_0.5)TiO₃–BiAlO₃ lead‐free materials exhibit excellent ferroelectric properties, but its depolarization temperature is relatively low which is the major obstacle limiting the material's practical application. In this study, the effects of Manganese (Mn) modification on the microstructure, ferroelectric properties and depolarization behavior of 0.96(Bi_0.5Na_0.5)(Ti_1?xMn_x)O₃–0.04BiAlO₃ ceramics were investigated. It was found that the average grain size was enlarged and ferroelectric properties were enhanced with small Mn addition, meanwhile the tangent loss decreased. The remnant polarization (P_r) of the samples reached an optimal value (~41 μC/cm²) as Mn content increased up to 0.7 mol%, whereas further addition resulted in the decrease in P_r. Moreover, appropriate Mn addition (x=0.7%) can improve the depolarization temperature from 140°C to 161°C determined from thermally stimulated depolarization currents measurement.     

Thermally‐induced phase transformations in Na0.5Bi0.5TiO3–KNbO3 ceramics

The structures and functional properties of Na_0.5Bi_0.5TiO₃–xKNbO₃ (NBT‐xKN) solid solutions, with x in the range from 0.01 to 0.09, were investigated using a combination of high‐resolution synchrotron X‐ray powder diffraction (SXPD) and ferroelectric property measurements. For low KN contents, an irreversible transformation from cubic to rhombohedral phases was observed after the application of a high electric field, indicating that the polar nanoregions (PNRs) in the unpoled state can be transformed into metastable long‐range ordered ferroelectric domains in the poled state. In contrast, the near‐cubic phase of the unpoled ceramics was found to be remarkably stable and was retained on cooling to a temperature of ?175°C. Upon heating, the field‐induced metastable ferroelectric rhombohedral phase transformed back to the nanopolar cubic state at the structural transformation temperature, T_ST, which was determined as approximately 225°C and 125°C for KN contents of 3% and 5% respectively. For the field‐induced rhombohedral phase in the poled specimens, the pseudo‐cubic lattice parameter, a_p, exhibited an anomalous reduction while the inter‐axial angle increased towards a value of 90° on heating, resulting in an overall increase in volume. The observed structural changes were correlated with the results of temperature‐dependent dielectric, ferroelectric and depolarization measurements, enabling the construction of a phase diagram to define the stable regions of the different ferroelectric phases as a function of composition and temperature.     

Enhanced Piezoelectric and Dielectric Responses in 92.5%(Bi0.5Na0.5) TiO3 ‐7.5%BaTiO3 Ceramics

Structure and piezoelectric coefficient (d₃₃) of lead‐free 7.5% mole BaTiO₃‐doped (Bi_0.5Na_0.5) TiO₃ (BNT‐7.5%BT) polycrystalline piezoceramics have been characterized systematically as a function of poling electric (E) field. Dielectric permittivity and loss were also measured as functions of frequency and temperature. The piezoelectric coefficient d₃₃ after poling at E = 35 kV/cm can reach d₃₃~186 pC/N, which is the highest value reported among (1?x) BNT–xBT compositions. A prior poling E field can reduce rhomobherdal lattice distortion, and enhance tetragonal phase and polarization ordering, that contribute significantly to the rapid raise of d₃₃ and lower depolarizing temperature (T_d). The reduced dielectric permittivity for the poled sample is attributed to ordered state and the pinning of field‐induced nanodomain walls by the presence of oxygen vacancies.     

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.     

Zr‐Substituted (Na0.5Bi0.5)1‐xBax TiO3‐Based Solid Solutions

This study investigates the phase formation of Zr‐substituted [(Na_0.5Bi_0.5)_0.80Ba_0.20](Ti_1?yZr_y)O₃ system during its solid‐state synthesis. The synthesis was found to be a multistep process accompanied by the formation of a number of intermediate phases which depend on the solid solution's composition and sintering temperatures. Single‐phase solid solutions were obtained when the sintering temperature was increased to 1000°C–1100°C. Increase in content of substituting Zr‐ions tends to linearly increase in the size of the crystal cell leading to reduction in the tolerance factor which results in the increase in stability of the antiferroelectric phase relative to the ferroelectric phase.     

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.     

Enhanced pyroelectric properties in (Bi0.5Na0.5)TiO3–BiAlO3–NaNbO3 ternary system lead‐free ceramics

High pyroelectric performance and good thermal stability of pyroelectric materials are desirable for the application of infrared thermal detectors. In this work, enhanced pyroelectric properties were achieved in a new ternary (1?x)(0.98(Bi_0.5Na_0.5)(Ti_0.995Mn_0.005)O₃–0.02BiAlO₃)–xNaNbO₃ (BNT–BA–xNN) lead‐free ceramics. The effect of NN addition on the microstructure, phase transition, ferroelectric, and pyroelectric properties of BNT–BA–xNN ceramics were investigated. It was found that the average grain size decreased as x increased to 0.03, whereas increased with further NN addition. The pyroelectric coefficient p at room temperature (RT) was significantly increased from 3.87 × 10^?8Ccm^?2K^?1 at x = 0 to 8.45 × 10^?8Ccm^?2K^?1 at x = 0.03. The figures of merit (FOMs), F_i, F_v and F_d, were also enhanced with addition of NN. Because of high p (7.48 × 10^?8Ccm^?2K^?1) as well as relatively low dielectric permittivity (~370) and low dielectric loss (~0.011), the optimal FOMs at RT were obtained at x = 0.02 with F_i = 2.66 × 10^?10 m/V, F_v = 8.07 × 10^?2 m²/C, and F_d = 4.22 × 10^?5 Pa^?1/2, which are superior to other reported lead‐free ceramics. Furthermore, the compositions with x ≤ 0.03 exhibited excellent temperature stability in a wide temperature range from 20 to 80°C because of high depolarization temperature (≥110°C). Those results unveil the potential of BNT–BA–xNN ceramics for infrared detector applications.     

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.     

Low‐Temperature Sintering of Bi0.5(Na,K)0.5TiO3 for Multilayer Ceramic Actuators

We investigated the influence of CuO amount (0.5–3.0 mol%), sintering temperature (900°C–1000°C), and sintering time (2–6 h) on the low‐temperature sintering behavior of CuO‐added Bi_0.5(Na_0.78K_0.22)_0.5TiO₃ (BNKT22) ceramics. Normalized strain (S_max/E_max), piezoelectric coefficient (d₃₃), and remanent polarization (P_r) of 1.0 mol% CuO‐added BNKT22 ceramics sintered at 950°C for 4 h was 280 pm/V, 180 pC/N, and 28 μC/cm², respectively. These values are similar to those of pure BNKT22 ceramics sintered at 1150°C. In addition, we investigated the performance of multilayer ceramic actuators made from CuO‐added BNKT22 in acoustic sound speaker devices. A prototype sound speaker device showed similar output sound pressure levels as a Pb(Zr,Ti)O₃‐based device in the frequency range 0.66–20 kHz. This result highlights the feasibility of using low‐cost multilayer ceramic devices made of lead‐free BNKT‐based piezoelectric materials in sound speaker devices.     

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.     

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.     

Ferroelectric‐quasiferroelectric‐ergodic relaxor transition and multifunctional electrical properties in Bi0.5Na0.5TiO3‐based ceramics

A‐site substituted 0.88(Bi_0.5Na_0.5)_1?x(Li_0.5Nd_0.5)_xTiO₃–0.12BaTiO₃ (BNTLNx–BT12) ceramics were synthesized using a conventional solid‐state reaction route. The structural transformation and miscellaneous electrical properties were systematically investigated. The A‐site modification induced two sequence transitions from ferroelectric tetragonal (T) to quasi‐ferroelectric pseudocubic (PC) phase, followed closely by the second transition from non‐ergodic to ergodic relaxor (NR‐ER), and finally to dynamic polar nanoregions (PNRs). The significant enhancement in piezoelectric activity, strain response, broad plateau‐like maximum dielectric permittivity over a large temperature range and energy‐storage level at different compositions may be attributed to the compositionally‐induced T‐PC to NR‐ER transition and the alignment of dynamically‐fluctuating PNRs, respectively. The evolution of multifunctional electrical properties, associated with the variations in structure/microstructure, might provide a new insight to investigate the underlying mechanism of structure‐electrical properties relationship in ferroelectric solid solutions.     

Structural Modulation of Na0.5Bi0.5TiO3 in Hydrothermal Synthesis

In this study, the high‐quality cubic‐shaped single‐crystalline Na_0.5Bi_0.5TiO₃ (NBT) has been successfully synthesized by a hydrothermal method, in which their grain size can be modulated effectively by both mineralizer concentration and reactant concentration. Our result shows that the formation of fine cubic‐shaped NBT can be promoted by increasing the mineralizer concentration, while be inhibited by increasing the reactant concentration as a whole. However, further higher mineralizer concentration or lower reactant concentration may lead to a smaller grain size in NBT. From our results, it is suggested that, to synthesize a big single crystalline of NBT, a suitable mineralizer and reactant concentration should be carefully considered. Interestingly, it is also found that the one‐dimensional nanostructure may be more inclined to form Na–Ti–O (NT) compounds rather than NBT phase. And the growth mechanism and a possible growth model have been presented. Our result presents a simple and green technical preparation route for the low‐cost and high‐quality NBT single crystalline with self‐designed shape and size.     

Photoluminescence and Temperature Dependent Electrical Properties of Er‐Doped 0.94Bi0.5Na0.5TiO3‐0.06BaTiO3 Ceramics

Er‐doped 0.94Bi_0.5Na_0.5TiO₃‐0.06BaTiO₃ (BNT‐6BT: xEr, x is the molar ratio of Er³⁺ doping) lead‐free piezoceramics with x = 0–0.02 were prepared and their multifunctional properties have been comprehensively investigated. Our results show that Er‐doping has significant effects on morphology of grain, photoluminescence, dielectric, and ferroelectric properties of the ceramics. At room temperature, the green (550 nm) and red (670 nm) emissions are enhanced by Er‐doping, reaching the strongest emission intensity when x = 0.0075. The complex and composition‐dependent effects of electric poling on photoluminescence also have been measured. As for electrical properties, on the one hand, Er‐doping tends to flatten the dielectric constant‐temperature (ε_r‐T) curves, leading to temperature‐insensitive dielectric constant in a wide temperature range (50°C–300°C). On the other hand, Er‐doping significantly decreases the ferroelectric‐relaxor transition temperature (T_F–R) and depolarization temperature (T_d), with the T_F–R decreasing from 76°C to 42°C for x = 0–0.02. As a result, significant composition‐dependent electrical features were found in ferroelectric and piezoelectric properties at room temperature. In general, piezoelectric and ferroelectric properties tend to become weaker, as confirmed by the composition‐dependent piezoelectric coefficient (d₃₃), planar coupling factor (k_p), and the shape of polarization‐electric field (P–E), current‐electric field (J–E), bipolar/unipolar strain‐electric field (S–E) curves. Furthermore, to understand the relationship between the T_F–R/T_d and the electrical properties, the composition of x = 0.0075 has been intensively studied. Our results indicate that the BNT‐6BT: xEr with appropriate Er‐doping may be a promising multifunctional material with integrated photoluminescence and electrical properties for practical applications.     

Temperature‐stable dielectric and energy storage properties of La(Ti0.5Mg0.5)O3‐doped (Bi0.5Na0.5)TiO3‐(Sr0.7Bi0.2)TiO3 lead‐free ceramics

A new type of (0.7?x)Bi_0.5Na_0.5TiO₃‐0.3Sr_0.7Bi_0.2TiO₃‐xLaTi_0.5Mg_0.5O₃ (LTM1000x, x = 0.0, 0.005, 0.01, 0.03, 0.05 wt%) lead‐free energy storage ceramic material was prepared by a combining ternary perovskite compounds, and the phase transition, dielectric, and energy storage characteristics were analyzed. It was found that the ceramic materials can achieve a stable dielectric property with a large dielectric constant in a wide temperature range with proper doping. The dielectric constant was stable at 2170 ± 15% in the temperature range of 35‐363°C at LTM05. In addition, the storage energy density was greatly improved to 1.32 J/cm³ with a high‐energy storage efficiency of 75% at the composition. More importantly, the energy storage density exhibited good temperature stability in the measurement range, which was maintained within 5% in the temperature range of 30‐110°C. Particularly, LTM05 show excellent fatigue resistance within 10⁶ fatigue cycles. The results show that the ceramic material is a promising material for temperature‐stable energy storage.     

Effect of closed pores on dielectric properties of 0.8Na0.5Bi0.5TiO3-0.2K0.5Bi0.5TiO3 porous ceramics

Porous 0.8Na_0.5Bi_0.5TiO₃-0.2K_0.5Bi_0.5TiO₃ ceramics are fabricated via the pore-forming agent method with polymethyl methacrylate (PMMA) and stearic acid (SA) as pore forming agents, and microstructure observations demonstrate that the porosity, pore shape, and pore sizes can be controlled by the synthesis technology. The dielectric properties of porous ceramics are found not only correlated to the pore-matrix composite model, but also have a significant grain-size effect. Based on the Zener Theory, pining forces exerted by pores on the grain boundary are calculated, to explain the shape effect of pores on grain boundary migration. A phase-field simulation is carried out to investigate pore shape effect on the grain size regulation in porous polycrystalline, and simulation results are in good agreements with experiential results as well as theoretical calculations. Thus, a modified equation is proposed to predict the effective permittivity of the porous piezoelectric ceramics by considering effects of porosity, pore shape and grain size.