High-temperature dielectric and energy storage properties of Bi0.5Na0.5TiO3-based ceramics modified by Sr0.8Na0.4Nb2O6

Sr_0.8Na_0.4Nb₂O₆ with a tungsten bronze structure is introduced into perovskite-structured 0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ composition (abbreviated as BNT-BT-xSNN, x = 0-0.04). The temperature stability of dielectric properties and energy storage performance is found to be effectively enhanced by Sr_0.8Na_0.4Nb₂O₆ dopant. When x is 0.03, the temperature ranges covering |ε'-ε'_150°C|/ε'_150°C ≤15% and tanδ ≤ 0.02 are 43°C-404°C and 90°C-422°C, respectively. More importantly, ε′ can be retained as high as 3304 at 150°C. Besides, the variances of energy storage density and its efficiency are 6.4% and 5.3%, respectively, in the temperature range from room temperature (RT) to 180°C. Therefore, this work provides a new method of compositional modification in BNT-based materials to improve their temperature stability of dielectric and energy storage properties.     

Nb-doped BaTiO3-(Bi0.5Na0.5)TiO3 ceramics with core-shell structure for high-temperature dielectric applications

Nb-doped 0.90BaTiO₃-0.10(Bi_0.5Na_0.5)TiO₃ temperature-insensitive ceramics with novel core-shell structure were sintered at low temperature by the conventional solid-state reaction method. The beneficial role of Nb in facilitating the formation of core-shell structure because of chemical inhomogeneity is verified, which is responsible for the weak temperature dependence of dielectric properties. Temperature dependence of permittivity measured at different frequencies shows high frequency dispersion at low temperature, while without relaxor characteristic at high temperature. The Vogel–Fulcher model was adopted to study the relaxor behaviour of Nb-doped 0.90BaTiO₃-0.10(Bi_0.5Na_0.5)TiO₃ ceramics at low temperature. The samples with an addition of 1.5?mol% Nb₂O₅ provide a temperature coefficient of capacitance meeting the requirements of the X9R characteristic, and result exhibits an optimum dielectric behaviour of ε_r ～1900, tanδ ～1.8% at room temperature, making the material a promising candidate for high temperature applications.     

Origin of enhanced depolarization temperature in quenched Na0.5Bi0.5TiO3-BaTiO3 ceramics

Na_0.5Bi_0.5TiO₃-BaTiO₃ (NBT-BT)-based lead-free piezoelectric ceramics have been actively studied in recent years as a potential replacement for lead-based materials in ultrasonic applications. However, its relatively low thermal depolarization temperature (T_d) is still an imperative obstacle hindering implementation in practical application. Recently, it was reported that quenching is an effective way to improve T_d of NBT-based ceramics, but the essential mechanism is still unclear. In this study, 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ (NBT-6BT) ceramics were quenched in air and liquid nitrogen, and then annealed in oxygen and nitrogen atmospheres to explore the origin of enhanced depolarization temperature. The results from this study correlate the enhancement of T_d to the residual stress, which induces a stable rhombohedral ferroelectric phase, thereby increasing the thermal depolarization temperature of NBT-6BT. Our results indicate that the residual stress is also an important factor influencing the electrical properties of quenched piezoelectric ceramics, which should be given more attention in future studies.     

Colossal permittivity in BaTiO3-0.5wt%Na0.5Ba0.5TiO3 ceramics with high insulation resistivity induced by reducing atmosphere

The donor-acceptor co-doping grain-fine BaTiO₃-0.5wt%Na_0.5Ba_0.5TiO₃ (BT-0.5wt%NBT) ceramics are obtained by conventional solid-state reaction method and sintered in different atmospheres. The dielectric properties are sensitively influenced by the sintering atmosphere, which the colossal permittivity can be activated and increased by enhanced atmospheric reducibility with the increase of H₂ content. However, the excessive H₂ content can make a significant deterioration in dielectric loss and insulation resistivity. The impedance spectra, XPS and EPR measurements indicate that sintering atmosphere can effectively regulate the concentration and distribution of charge carriers (delocalized electrons, oxygen vacancies and defects), which induce the interfacial polarization and hopping polarization. When sintered in 0.1% H₂/N₂, the sample possesses a relatively good comprehensive performance with colossal permittivity (ε_γ>4×10⁴), ultra-low dielectric loss (tanδ=0.0218) and high insulation resistivity (ρ_v>10¹¹Ω·cm), which related to the moderate values of resistance and conduction activation energy for the grain boundaries.     

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.     

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.     

Effects of K0.5Bi0.5TiO3 addition on dielectric properties of BaTiO3 ceramics

(1?x)BaTiO₃–xK_0.5Bi_0.5TiO₃ (abbreviated as BT–KBT, 0.10≦x≦0.15) dielectric ceramics were prepared by a conventional oxide mixing method. The effects of KBT content on the densification, microstructure and dielectric properties of BT ceramics were investigated. The density characterization results show that the addition of KBT significantly lowered the sintering temperature of BT ceramics to about 1280 °C. The XRD results showed that the phase compositions of all samples were pure tetragonal phases. The dielectric constant and dielectric loss firstly increased and then decreased with the increase of KBT. In addition, dielectric constant and dielectric loss versus frequency were characterized in the frequency range from 100 Hz to 2 MHz. It is found that the dielectric constant and the dielectric loss changed with the increase of KBT contents regularly.     

Reducing dielectric loss in Na0.5Bi0.5TiO3 based high temperature capacitor material

The demand for capacitors exhibiting low sensitivity towards temperature changes and high power peaks has increased significantly. Recently, Na_0.5Bi_0.5TiO₃ (NBT) based ceramics became excellent candidates for such extreme temperature capacitors. The dielectric loss of these materials is, however, difficult to control because of the complex defect chemistry of NBT based ceramics. Therefore, it is the limiting factor for high temperature applications. In this work, we present a strategy to increase the upper temperature limit for low dielectric loss. The addition of BiAlO₃ to Na_0.5Bi_0.5TiO₃-BaTiO₃-CaZrO₃ reduces the loss and sensitivity towards Bi evaporation during synthesis. For unmodified samples, the relative permittivity (ε_r = 581, at 1 kHz) varies less than 15 %, while the dielectric loss stays below 0.02 between -68 and 368 °C. With the addition of BiAlO₃, the temperature range of low loss extends from -68 to 391 °C at even higher permittivity (ε_r = 628, at 1 kHz).     

Temperature-stable dielectric ceramics based on Na0.5Bi0.5TiO3

Multiple ion substitutions to Na_0.5Bi_0.5TiO₃ give rise to favourable dielectric properties over the technologically important temperature range ?55?°C to 300?°C. A relative permittivity, ε_r,?=?1300?±?15% was recorded, with low loss tangent, tanδ?≤?0.025, for temperatures from 310?°C to 0?°C, tanδ increasing to 0.05 at ?55?°C (1?kHz) in the targeted solid solution (1–x)[0.85Na_0.5Bi_0.5TiO₃–0.15Ba_0.8Ca_0.2Ti_1-yZr_yO₃]–xNaNbO₃: x?=?0.3, y?=?0.2. The ε_r-T plots for NaNbO₃ contents x?<?0.2 exhibited a frequency-dependent inflection below the temperature of a broad dielectric peak. Higher levels of niobate substitution resulted in a single peak with frequency dispersion, typical of a normal relaxor ferroelectric. Experimental trends in properties suggest that the dielectric inflection is the true relaxor dielectric peak and appears as an inflection due to overlap with an independent broad dielectric peak. Process-related cation and oxygen vacancies and their possible contributions to dielectric properties are discussed.     

Composition dependence of dielectric properties,elastic modulus,and electroactivity in (carbon black‐BaTiO3)/silicone rubber nanocomposites

The dielectric properties, elastic modulus, and electromechanical responses of dielectric elastomers (DEs) consisting of silicone rubber and carbon black (CB) incorporated with BaTiO₃ (BT) were studied. When compared with single filler/rubber composites, the resulting three‐component nanocomposites yielded very abnormal phenomena. They might be attributed to the interactions between the two kinds of fillers. The increase in concentration of CB (BT) would play a destructive role to the network structure formed by BT (CB) particles. The maximum electromechanical strain of the nanocomposites achieved at mass fraction m_CB = 0.03 and m_BT = 0.06. The resultant electromechanical strain would be attributed to the large dielectric permittivity in the three‐component nanocomposites, in which the BT particles themselves have a high dielectric permittivity and the electrical networks of CB particles have a contribution on the increase in dielectric permittivity of the three‐component nanocomposites. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Low hysteresis and temperature stable electrostrain in 0.97(0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-0.03AgNbO3/xZnO composite ceramics

Na_0.5Bi_0.5TiO₃-based solid solutions are one of the most promising lead-free piezoelectric candidates since they can be easily tailored to exhibit large electrostrain. However, the large hysteresis and temperature-dependence of the electrostrain response are longstanding obstacles for their practical applications. In the present study, 0–3 type composites were developed with 0.97(0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃)-0.03AgNbO₃ (NBT-6BT-3AN) matrix phase and ZnO inclusions. The optimum addition of 5 wt.% ZnO in the composites leads to reduction in hysteresis of electrostrain by 35% in comparison with pure NBT-6BT-3AN at room temperature. Meanwhile, electrostrain of the composites maintains superior temperature stability, with a variance of less than ±10% in the temperature range between 25 and 125 °C. The reason for the improvement of electrostrain is proposed to be attributed to the ergodic/nonergodic mixture phases induced by residual stress between the inclusion and matrix, as well as the phase evolution caused by the incorporation of Zn²⁺ into the matrix. Therefore, this work provides a new strategy to improve the electromechanical properties of Na_0.5Bi_0.5TiO₃-based ceramics.     

Enhanced piezoresponse and electric field induced relaxor-ferroelectric phase transition in NBT-0.06BT ceramic prepared from hydrothermally synthesized nanoparticles

0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ (NBT-0.06BT) nanoparticles were synthesized by hydrothermal method and subsequently used to prepare NBT-0.06BT ceramics. After poling at the electric field of 3.5 kV/mm, the piezoelectric coefficient (d₃₃) and electromechanical coupling factor (κ_p) reached 171 pC/N and 0.31, respectively. The NBT-0.06BT ceramics also exhibited a large remanent polarization of 46.10 μC/cm² and electric field induced strain of 0.243% at 8 kV/mm corresponding to normalized strain d₃₃^*=303 pm/V (S_max/E_max). The effects of polarization on crystalline phase, microstructures, dielectric properties and domain structures were investigated to reveal the origin of enhanced piezoresponse and electric field induced strain in as-prepared NBT-0.06BT ceramics. It can be observed that the tetragonal phase in NBT-0.06BT ceramics was enhanced and polar nanoregions were transformed into tweed-like structures and some lamellar domains after E-field poling. The dielectric response of NBT-0.06BT ceramics also exhibited an electric-field-induced relaxor-to-ferroelectric phase transition.     

Structure,Infrared Reflectivity and Microwave Dielectric Properties of (Na0.5La0.5)MoO4–(Na0.5Bi0.5)MoO4 Ceramics

A series of temperature‐stable microwave dielectric ceramics, (1?x)(Na_0.5La_0.5)MoO₄–x(Na_0.5Bi_0.5)MoO₄ (0.0 ≤ x ≤ 1.0) were prepared by using solid‐state reaction. All specimens can be well sintered at temperature of 580°C–680°C. Sintering behavior, phase composition, microstructures, and microwave dielectric properties of the ceramics were investigated. X‐ray diffraction results indicated that tetragonal scheelite solid solution was formed. Microwave dielectric properties showed that permittivity (ε_r) and temperature coefficient of resonant frequency (τ_f) were increased gradually, while quality factor (Q × f) values were decreased, at the x value was increased. The 0.45(Na_0.5La_0.5)MoO₄–0.55(Na_0.5Bi_0.5)MoO₄ ceramic sintered at 640°C with a relative permittivity of 23.1, a Q × f values of 17 500 GHz (at 9 GHz) and a near zero τ_f value of 0.28 ppm/°C. Far‐infrared spectra (50–1000 cm^?1) study showed that complex dielectric spectra were in good agreement with the measured microwave permittivity and dielectric losses.     

High permittivity and excellent high-temperature energy storage properties of X9R BaTiO3–(Bi0.5Na0.5)TiO3 ceramics

We fabricated x(Bi_0.5Na_0.5)TiO₃–(1−x)[BaTiO₃–(Bi_0.5Na_0.5)TiO₃–Nb] (BNT-doped BTBNT-Nb) dielectric materials with high permittivity and excellent high-temperature energy storage properties. The initial powder of Nb-modified BTBNT was first calcined and then modified with different stoichiometric ratios of (Bi_0.5Na_0.5)TiO₃ (BNT). Variable-temperature X-ray diffraction (XRD) results showed that the ceramics with a small amount of BNT doping consisted of coexisting tetragonal and pseudocubic phases, which transformed into the pseudocubic phase as the test temperature increased. The results of transmission electron microscopy (TEM) showed that the ceramic grain was the core-shell structure. The permittivity of the 5 mol% BNT-doped BTBNT-Nb ceramic reached up to 2343, meeting the X9R specification. The discharge energy densities of all samples were 1.70-1.91 J/cm³ at room temperature. The discharge energy densities of all samples fluctuated by only ±5% over the wide temperature range from 25°C to 175°C and ±8% from 25°C to 200°C. The discharge energy density of the 50 mol% BNT-doped BTBNT-Nb ceramic was 2.01 J/cm³ at 210 kV/cm and 175°C. The maximum energy efficiencies of all ceramics were up to ~91% at high temperatures and were much better than those at room temperature. The stable dielectric properties within a wide temperature window and excellent high-temperature energy storage properties of this BNT-doped BTBNT-Nb system make it promising to provide candidate materials for multilayer ceramic capacitor applications.     

Impedance spectroscopy of Na0.5Bi4.50+xTi4Oy (x = -0.02, 0, 0.02) ceramics with excellent dielectric properties

The electric and dielectric properties of Na_0.5Bi_4.50+xTi₄O_y (x = −0.02, 0, 0.02) prepared by conventional mixed oxide route have been investigated by impedance spectroscopy (IS) over a wide temperature range. Single-phase bismuth layer-structured perovskite patterns were observed through X-ray diffraction of the three samples Na_0.5Bi_4.5Ti₄O₁₅, Na_0.5Bi_4.48Ti₄O_y, and Na_0.5Bi_4.52Ti₄O_y. The results show that the relative permittivity (εr) increases with the increase in temperature and reaches its maximum at about 675℃. With the continuous increase in temperature, the permittivity decreases gradually. Both relative permittivity and dielectric loss show great stability at the low-temperature zone. The ceramic of x = 0.02 with Ea of 1.09 eV has the maximum oxygen ionic transport number between 600 and 800℃ for all samples. And at this time, it has the maximum electrical conductivity. All the results indicated that Na_0.5Bi_4.50+xTi₄O_y (x = −0.02, 0, 0.02) ceramics were promising base materials for high-temperature capacitor because of their high dielectric properties.     

Enhancing the dielectric and energy storage properties of lead-free Na0.5Bi0.5TiO3–BaTiO3 ceramics by adding K0.5Na0.5NbO3 ferroelectric

A novel strategy of enhancing the dielectric and energy storage properties of Na_0.5Bi_0.5TiO₃–BaTiO₃ (NBT–BT) ceramics by introducing a K_0.5Na_0.5NbO₃ (KNN) ferroelectric phase is proposed herein, and its underlying mechanism is elucidated. The lead-free KNN ceramic decreases the residual polarisation and increases the electric breakdown strength of the NBT–BT matrix through the simultaneous modification of its A-sites and B-sites. The obtained NBT?BT?x?KNN ceramics have a perovskite structure with unifying grains. A bulk 0.9NBT–BT–0.1KNN ceramic sample with a thickness of 0.2 mm possesses a high energy storage density of 2.81 J/cm³ at an applied electric field of 180 kV/cm. Moreover, it exhibits good insulation properties and undergoes rapid charge and discharge processes. Therefore, the obtained 0.9NBT–BT–0.1KNN ceramic can be potentially used in high-power applications because of its high energy density, good insulation properties, and large discharge rate.     

In situ hot-stage TEM of the phase and domain evolution in quenched Na1/2Bi1/2TiO3–BaTiO3

Quenching relaxor ferroelectric 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ (NBT-6BT) enhances the depolarization temperature (T_d), linked to the stabilization of ferroelectric order. The thermal evolution of the domain structure and phase assemblage provides insights about the onset of ferroelectric order in quenched materials. Unpoled furnace cooled and quenched NBT-6BT ceramics were studied using in situ temperature-dependent transmission electron microscopy. The rhombohedral to tetragonal structural transition in furnace cooled and quenched samples occurs in a comparable temperature range of 120°C–220°C. While the tetragonal phase is characterized by polar nanoregions (PNRs) and no domain contrast in the furnace cooled state, the quenched composition exhibits an increased fraction of lamellar domains, which are partially stable up to 300°C, thus benefiting the delayed depolarization. This is further corroborated by the dielectric data indicating earlier freezing of PNR dynamics in the quenched state. The reversibility of the phase transition is demonstrated by successive cooling, where quenched NBT-6BT features an increased grainy PNR contrast after the experiment, followed by a kinetically delayed coalescence of PNRs back into lamellar domains. This demonstrates that the stabilized ferroelectric state upon quenching is associated with the conversion of polar units on the nanometer scale into long-range domain structures.     

Enhancing giant dielectric properties of Ta5+-doped Na1/2Y1/2Cu3Ti4O12 ceramics by engineering grain and grain boundary

Various strategies to improve the dielectric properties of ACu₃Ti₄O₁₂ (A = Sr, Ca, Ba, Cd, and Na_1/2Bi_1/2) ceramics have widely been investigated. However, the reduction in the loss tangent (tanδ) is usually accompanied by the decreased dielectric permittivity (ε′), or vice versa. Herein, we report a route to considerably increase ε′ with a simultaneous reduction in tanδ in Ta⁵⁺–doped Na_1/2Y_1/2Cu₃Ti₄O₁₂ (NYCTO) ceramics. Dense microstructures with segregation of Cu– and Ta–rich phases along the grain boundaries (GBs) and slightly increased mean grain size were observed. The samples prepared via solid-state reaction displayed an increase in ε′ by more than a factor of 3, whereas tanδ was significantly reduced by an order of magnitude. The GB–conduction activation energy and resistance raised due to the segregation of Cu/Ta–rich phases along the GBs, resulting in a decreased tanδ. Concurrently, the grain–conduction activation energy and grain resistance of the NYCTO ceramics were reduced by Ta⁵⁺ doping ions owing to the increased Cu⁺/Cu²⁺, Cu³⁺/Cu²⁺, and Ti³⁺/Ti⁴⁺ ratios, resulting in enhanced interfacial polarization and ε′. The effects of Ta⁵⁺ dopant on the giant dielectric response and electrical properties of the grain and GBs were described based on the Maxwell–Wagner polarization at the insulating GB interface, following the internal barrier layer capacitor model.     

An effective optimization strategy and analysis for BaTiO3–Na0.5Bi0.5TiO3 system with colossal permittivity

Colossal permittivity (CP, ε＞10⁴) behavior in BaTiO₃–Na_0.5Bi_0.5TiO₃ (BT-NBT) ceramics has been studied, which showed extremely high permittivity up to ~10⁵. Dielectric properties of samples showed Debye-like relaxations in the frequency range 20 Hz–30 MHz. Two different polarizations located in grain boundaries and grains respectively are responsible for the CP behavior and the models of defect charge compensation achieved by niobium doping are proposed to explain the phenomenon of abnormal variation of dielectric constant.By using defect engineering, a Nb-doped BaTiO₃ ceramics with stable colossal permittivity (ε_r =1.3×10⁴ at 1 kHz and room temperature),high bulk resistivity (>10¹⁰ Ω·cm) as well as relative low dielectric loss (tanδ~0.06) has been obtained over a wide temperature range of −55–150 °C, satisfying IEA X8R specification, which has a potential application prospect in high capacity solid supercapacitor.     

Bi0.5Na0.5TiO3-BaTiO3-CaZrO3:ZnO high-temperature dielectric composites with wide operational range

0.82[0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃]-0.18CaZrO₃:xZnO (BNT-BT-CZ:xZnO, x = 0–0.40 with interval of 0.10) high temperature dielectric composites were prepared and the structural and electrical properties were investigated. Significantly improved temperature-insensitive permittivity spectra have been observed in the composites: the temperature range for low variance in permittivity (Δε_r/ε_r,150?°C < 10%) is 70–190?°C for x?=?0, whereas it is extended at least to 30–250?°C for the optimal x?=?0.10 at 1?kHz. Especially, for this optimal composite, the variance of permittivity is less than 4.0% in the temperature range of 30–400?°C with the suitable permittivity value of ~ 600 at 10?kHz. By comparatively investigating the properties of unpoled and poled samples, the improved temperature-insensitive permittivity is rationalized by the ZnO-induced local electric field that can suppress the evolution of polar nanoregions and thus enhance the temperature-insensitivity of permittivity.