BNT-based Ceramics Research Articles

Effects of (K0.5Na0.5)(Nb0.96Sb0.04)O3 on microstructure and electrical properties of BNT-based piezoelectric ceramics

In this work, (1-x)Bi0.5Na0.5TiO3-x(K0.5Na0.5)(Nb0.96Sb0.04)O3 [abbreviated as (1-x)BNT-xKNNS] lead-free piezoelectric ceramics were successfully prepared by the conventional solid-state reaction method, and effects of (K0.5Na0.5)(Nb0.96Sb0.04)O3 [abbreviated as KNNS] on microstructure structure and electrical properties of the ceramics were systematically discussed. Results of phase characterizations showed that KNNS led to phase transition from orthorhombic (O) phase to monoclinic (M)-O phase coexistence. However, KNNS had small effects on grain growth of the ceramics according to scanning electron microscope (SEM) analysis. The existence of KNNS resulted in unique dielectric relaxor behavior of such ceramics, i.e. vanished depolarization temperature (Td) and insensitive relative dielectric constants (εr) to frequencies when T < Ts. When T ≥ Ts, obvious frequency dispersion of εr resulted from thermally-activated B-site disorder. Furthermore, electrical properties and energy storage properties were studied and the ceramics with x = 0.02 possessed optimized dielectric properties of εr ~ 773 (@100 kHz) and dielectric loss (tanδ) ~ 0.02 over wide temperature range.

Simultaneous enhancement of recoverable energy density and efficiency of lead-free relaxor-ferroelectric BNT-based ceramics

Given the remarkable performances of Ruddlesden-Popper (RP) type oxides in electronic devices, the study of their intrinsic dielectric, magnetic and electromagnetic microwave (EMW) absorption properties are still missing. Herein, two kinds of Lead-free relaxor-ferroelectric BNT-based ceramic hierarchical polycrystalline alloys (La3Ni2O7/LaNiO3, La2NiO4/La2O3) were prepared via a facile solvothermal and high-temperature annealing technique as high-performance EMW absorption materials (MAMs). By a means of adjustment of solvothermal time in the precursor systems, Nickel salt layers precipitation declined gradually. The phase and crystal state of the powders were characterized by X-ray diffraction (XRD) with refinement treatment (Rietveld analysis). XPS was utilized on the determination of interstitial O2– quantitatively in La3Ni2O7+δ (δ = 1.95) and La2NiO4+δ (δ = 0.75). Responsible to the synergistic effect of dielectric and magnetic response and well-matched impedance, the as-fabricated MAMs performed excellent wave absorbability, especially in low frequency (S-band). In details, La3Ni2O7/LaNiO3 achieved optimal effective absorption band (fE) 3.36 GHz at 9.52–12.80 GHz and a minimum reflection loss (RLmin) −43.30 dB with a thickness of 2.2 mm. Moreover, La3Ni2O7/LaNiO3 and La2NiO4/La2O3 overcame the challenge of absorbers’ thickness in S-band, of which the RL reached –22.63 and −29.90 dB at 0.7 mm, respectively. Noteworthily, La3Ni2O7/LaNiO3 obtained RL value of −50.06 dB at 10.72 GHz with a device thickness of 2.8 mm. It is emphasized that this report has interpreted the EMW absorption capacity and mechanism exhaustively, which will initiate simultaneous enhancement of recoverable energy density and efficiency application of RPs.

Significantly enhanced energy storage density in sodium bismuth titanate-based ferroelectrics under low electric fields

Bi0.5Na0.5TiO3 (BNT)-based ferroelectrics have received more and more attention due to their environment-friendliness and large maximum polarization. Herein, the enhanced energy-storage properties of BNT-based ceramics were successfully prepared by introducing multi-ions (La3+, K+, Al3+, Nb5+, Zr4+) to improve the breakdown strength and simultaneously suppress the remnant polarization. An excellent discharge energy-storage density (Wd) of 3.24 J cm−3 and a high energy-storage efficiency (η) of 82 % under 200 kV cm-1 have been recorded for Bi0.44La0.06(Na0.82K0.18)0.5Ti0.90(Al0.5Nb0.5)0.08Zr0.02O3 ceramic. Meanwhile, the outstanding thermal stability with Wd of 1.84–1.96 J·cm−3 were also achieved in 25−125 °C at 140 kV cm-1. More importantly, piezoresponse force microscopy reveals that the threshold voltage for inducing long range order enhances while the stability of polar nanoregions (PNRs) on the nanoscale decreased with the increase of La doping amount, leading to more linear polarization behavior and higher energy-storage properties. These results promote the practical applications of BNT-based ferroelectrics in advanced pulsed power systems.

Influence of MoO3 on electrical properties and thermal depolarization of Bi0.5Na0.5TiO3-BaTiO3 lead-free piezoceramics

In this work, electric properties of undoped and MoO3-doped 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 ceramics (BNT-BT: x mol. % MoO3) are systematically studied. At room temperature, the optimum piezoelectric property (d33 = 177.0 pC/N) was observed in BNT-BT: 0.10Mo ceramics, with remanent polarization (Pr = 32.15 μC/cm2), electromechanical coupling coefficient (kt = 0.578, kp = 0.169, k31 = 0.180), and mechanical quality factor (Qm = 90.3). Mo doping was found to have a positive effect on the depolarization temperature Td, and k31 showed excellent thermal stability at BNT-BT: 0.05Mo ceramics. These findings confirm that Mo doping is an effective method to improve BNT-based lead-free ceramics.

Significantly enhanced energy storage density and efficiency of BNT-based perovskite ceramics via A-site defect engineering

In recent years, sodium bismuth titanate (Bi0.5Na0.5TiO3, BNT) -based relaxor ferroelectrics have attracted more and more attention for energy storage applications owing to their high power density, large saturated polarization (PS)/maximum polarization (Pmax) as well as meeting the needs of environment-friendly society. However, the recoverable energy storage density (Wrec) and energy storage efficiency (η) of most BNT-based relaxor ferroelectric ceramics are lower than 3.5 ​J ​cm−3 and/or 80%, respectively, in recently. In this work, the relaxor ferroelectric ceramics of 0.75Bi(0.5+x)Na(0.5-x)TiO3-0.25SrTiO3 (BNST-x) were constructed via A-site defect engineering and prepared by tape-casting method. It is worth noting that an ultrahigh Wrec of 5.63 ​J ​cm−3 together with outstanding η of 94% can be achieved simultaneously at a relative high electric field of 535 ​kV ​cm−1 with the composition of BNST-0.08, Meanwhile, for BNST-0.08 ceramic, the η is higher than 90% and the variation of Wrec is less than ±2% and ±5% within the frequency range of 1–100 ​Hz and temperature range of 30–130 ​°C, respectively. The Wrec is always higher than 3 ​J ​cm−3 and did not deteriorate significantly after 104 fatigue cycles. In addition, the BNST-0.08 ceramic also possesses ultrafast discharge speed (t0.9, less than 125 ​ns) and ultrahigh power density (PD, higher than 147 ​MW ​cm−3) within the temperature range of 30–130 ​°C at 300 ​kV ​cm−1. Therefore, the BNST-0.08 ceramic is promising candidate environment-friendly materials for advanced pulsed power capacitor applications and the energy storage properties of BNT-based relaxor ferroelectrics can be enhanced significantly via A-site defect engineering.

Enhancement of up-conversion emission and field-induced strain in BNT-based multifunctional ceramics doping with LiNbO3

Enhancement of up-conversion emission and field-induced strain in BNT-based multifunctional ceramics doping with LiNbO3

Low temperature synthesis of plate-like Na0.5Bi0.5TiO3via molten salt method

In this study, plate-like Na0.5Bi0.5TiO3 (BNT) templates with perovskite structure were obtained by two-step molten salt synthesis (MSS) method at a low temperature. Firstly, Bi4Ti3O12 precursors were synthesized at 1030 °C in NaCl–KCl molten salt. Secondly, plate-like Na0.5Bi0.5TiO3 particles with perovskite structure were obtained from plate-like layer-structured ferroelectric ceramic of Bi4Ti3O12 by topochemical microcrystal conversion method. Result showed that excessive Na2CO3 was beneficial to facilitate the low temperature synthesis. In the case of an excess of 30 mol% Na2CO3, plate-like BNT particles could be obtained by synthesis at temperatures ranging from 760 °C to 800 °C, which indicated a flexible processing route. Also, it has been observed that plate-like BNT particles show a high aspect ratio with 1 μm in thickness and 10–20 μm in length. These Na0.5Bi0.5TiO3 plate-like particles can be good candidates for the preparation of lead-free BNT-based piezoelectric ceramics with oriented grain microstructure.

A structural phase boundary due to oxygen octahedral tilt–untilt transition in Bi0.5Na0.5TiO3-based piezoelectric ceramics

The investigation of fine structural evolutions around the morphotropic phase boundary (MPB) is important to understand the origin of the high piezoelectricity in ferroelectric ceramics. Here, we report a new phase boundary associated with oxygen octahedral tilt–untilt transition by exploring average and local structural evolutions in the vicinity of MPB in Bi0.5Na0.5TiO3–PbTiO3 (BNT–PT) ceramics. It is found that the local structural change of the oxygen octahedral tilt–untilt transition is decoupled with the average structural change of the rhombohedral-to-tetragonal phase transition at MPB in BNT–PT. The slight octahedral tilting distortion at MPB is responsible for the large high-field-induced electrostrain and small low-field-induced piezoelectricity in BNT–PT. The crucial difference of MPB between BNT–PT and the classical piezoelectric Pb(Zr,Ti)O3 is discussed from their local structural evolutions of octahedral tilting to the performance changes at MPB. It helps us to figure out the origin of large high-field-induced electrostrain but small piezoelectricity in BNT-based ceramics.

Superior electrostrictive coefficient of 6BNT-4SCxBT lead-free relaxor ferroelectrics with Ca2+ substitution

In this study, lead-free 0.6(Bi0.5Na0.5)TiO3-0.4(Sr0.7-xCaxBi0.2)TiO3 ceramics (abbreviated as 6BNT-4SCxBT, x=0, 0.05, 0.10, 0.15 and 0.20 mol) were prepared by a traditional solid-state method. The effects of Ca2+ on phase structure, state transformation, ferroelectric and energy storage properties, and electrostrictive performance of 6BNT-4SCBT-x ceramics were comprehensively investigated. Co-occupied Bi3+, Na+, Sr2+ and Ca2+ ions induced the inhomogeneous distribution of A-site ions and charge, which resulted in polar nano-regions (PNRs) for relaxor ferroelectrics. 6BNT-4SCxBT ceramics present the ergodic relaxor state of polar rhombohedral PNRs and weakly polar tetragonal PNRs at room temperature. When the Ca2+ content was increased, 6BNT-4SC0.01BT ceramics showed the properties of weakly polar tetragonal PNRs (with lowest Tp~71 °C). 6BNT-4SCxBT ceramics exhibit electrostrictive effect Q33 (~0.045 m4/C2) and high energy storage efficiency. Moreover, large Q33 (>0.0421 m4/C2) indicates good electrical field stability. This result is a significant advancement in the area of lead-free electrostrictive materials, which has a great prospect in BNT-based ceramics.

Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics

In this work, the effects of rare earth (RE) dopants on the phase structure, microstructure, and electrical properties of lead-free bismuth sodium titanate (BNT)-based ceramics were studied. Rare earth elements (i.e., Sm, Gd, Ho, Pr, Y, and Yb) were selected as the dopants to replace the Bi site of Bi0.5(Na0.8K0.2)0.5TiO3 ceramics. It was found that the addition of moderate RE ions reduced the temperature of ferroelectric-relaxor transition (Tf-r) and the dielectric maximum at lower temperature, while the slight influence on the dielectric maximum at higher temperature and microstructure were observed. Hereby, the three-stage evolution of phase structure after introducing RE ions was confirmed. A high piezoelectric coefficient d33 of ∼190 pC/N was achieved due to the improved dielectric constant in the state with stable electric field-induced ferroelectric phase. The high strain property and converse piezoelectric coefficient d33∗ of ∼340 pm/V were obtained in the state with the coexistence of nonergodic and ergodic relaxor phases when Tf-r approached room temperature. The degenerated overall electromechanical properties exhibited in the state with complete ergodic relaxor phase. This study affords a significant guidance for optimizing the piezoelectricity of BNT-based ceramics utilizing rare earth dopants.

High temperature lead-free BNT-based ceramics with stable energy storage and dielectric properties

The designed 0.8BNTSZ–0.2NN ceramic demonstrates superb temperature stability with a capacitance variation &lt;±15 from −55 °C to 545 °C.

Antiferroelectric behavior and giant strain in BNKT ceramics complex Cs2Nb4O11 tungsten bronze phase

BNT-based ceramics are very attractive for the electrical energy storage and electric field-induced strain. However, the reported BNT-based ceramics are usually the perovskite solid solution. In this work, the super adjustability of Cs2Nb4O11 antiferroelectric phase for lead-free (1-x)Bi0.5(Na0.82K0.18)0.5TiO3-xCs2Nb4O11 ceramics with complex-phase are carefully studied. The Bi0.5(Na0.82K0.18)0.5TiO3 ceramics modified by a trace of Cs2Nb4O11 component suddenly transform to antiferroelectric phase from typical ferroelectric. A giant strain of 0.48% and a d33* of 600 pm/V are obtained originating from a reversible transition between antiferroelectric and ferroelectric states. And a high energy density of 1.0 J/cm3 is also achieved by reversible phase transitions. Therefore, the composite ceramics by designing tungsten bronze and perovskite complex phase illustrate great potential for actuator or energy storage applications.

Tailoring electromechanical performance in BiScO3-modified Bi0.5Na0.5TiO3-based lead-free piezoceramics

The strain response of lead-free ternary solid solution (1 − x) (0.94 Bi0.5Na0.5TiO3–0.06BaTiO3)–xBiScO3 (BNT–BT–BS) have been tailored by controlling the phase transition temperature. The effects of BiScO3 on the phase structure, dielectric, ferroelectric, piezoelectric properties, and electric field-induced strain are systematically investigated. A schematic phase diagram has also been established to clarify the relationship between strain behavior and structure evolution. The field-induced strain increases progressively, and a large strain of 0.35% with Smax/Emax = 471 pm/V is achieved at the critical composition x = 0.03, owing to the shift of the ferroelectric-to-relaxor transition temperature TF–R to ambient temperature. Intriguingly, the established correlation between strain response and TF–R demonstrates that the optimization of strain behavior of BNT-based ceramics can be realized though the regulation of TF–R to room temperature. Furthermore, a high and thermally stable electrostrictive coefficient Q33 of 0.018 m4/C2 can also be attained at the composition x = 0.06. In this study, it is believed that the modulation of phase transition temperature is feasible to achieve the large strain response in BNT-based ceramics.

Ba and Mg co-doping to suppress high-temperature dielectric loss in lead-free Na0.5Bi0.5TiO3-based systems

In this study, Ba, Mg co-doped BNT-based ferroelectric ceramic (Ba0.2Na0.3Bi0.5)Ti0.9Mg0.1O3 is fabricated by conventional solid-state reaction to obtain high-temperature dielectric performance. The temperature dependent dielectric constant and dielectric loss are investigated to understand the high-temperature dielectric behavior of the ceramic. The results show that Ba, Mg co-doped BNT ceramic has a very low dielectric loss (about 0.006–0.023) in a wide temperature range of 200–400 °C with dielectric constant about 3200–3800. The complex impedance plots, temperature dependent conductivity and first principle results reveal that the low dielectric loss at high-temperature is mainly due to the Ba doping increases the migration barrier energy of oxygen ions in BNT. The oxygen ion conduction of BNT-based ceramics is restrained, so that the dielectric loss reduces significantly. The study provides a new BNT-based material with low dielectric loss and temperature- stable dielectric constant in a wide temperature range which has great value of high-temperature applications.

Superior Thermal Stability of High Energy Density and Power Density in Domain-Engineered Bi0.5Na0.5TiO3-NaTaO3 Relaxor Ferroelectrics.

Thermal-stable dielectric capacitors with high energy density and power density have attracted increasing attention in recent years. In this work, (1 - x)Bi0.5Na0.5TiO3-xNaTaO3 [(1 - x)BNT-xNT, x = 0-0.30] lead-free relaxor ferroelectric ceramics are developed for capacitor applications. The x = 0.20 ceramic exhibits superior thermal stability of discharged energy density (WD) with a variation of less than 10% in an ultrawide temperature range of -50 to 300 °C, showing a significant advantage compared with the previously reported ceramic systems. The WD reaches 4.21 J/cm3 under 38 kV/mm at room temperature. Besides, a record high of power density (PD ≈ 89.5 MW/cm3) in BNT-based ceramics is also achieved in x = 0.20 ceramic with an excellent temperature insensitivity within 25-160 °C. The x = 0.20 ceramic is indicated to be an ergodic relaxor ferroelectric with coexisted R3c nanodomains and P4bm polar nanoregions at room temperature, greatly inducing large maximum polarization, maintaining low remnant polarization, and thus achieving high WD and PD. Furthermore, the diffuse phase transition from R3c to P4bm phase on heating is considered to be responsible for the superior thermal stability of the high WD and PD. These results imply the large potential of the 0.80BNT-0.20NT ceramic in temperature-stable dielectric capacitor applications.

Electrocaloric effect in BNT-based lead-free ceramics by local-structure and phase-boundary evolution

This work directly tests electrocaloric effect (ECE) of (0.94-x)Bi0.5Na0.5TiO3–0.06BaTiO3–xSrZrO3 (BNBTSZx) with x = 0 (SZ0), x = 0.01 (SZ0.01), x = 0.02 (SZ0.02) and x = 0.03 (SZ0.03) at different temperature points. The Sr2+ and Zr4+ are introduced into 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 to convert nonergodic relaxor phase into ergodic relaxor phase and form new morphotropic phase boundary (MPBⅡ) at room temperature. A temperature change of 0.37 K and electrocaloric strength of 6.17 × 10−3 K cm/kV are obtained at room temperature. According to the temperature dependence of ΔT and the electric-field dependence of electrocaloric strength, this work focus on the mechanism about higher electrocaloric effect at over-MPBⅡ temperature point. Electrical performances, microstructure and piezoelectric properties are investigated in detail to analyze the local-area phase transformation causing the high electrocaloric effect. The electrocaloric strength of ergodic relaxor phase shows stronger dependence on the electric field than that of nonergodic relaxor phase.

Electric field-temperature phase diagram of Bi1/2(Na0.8K0.2)1/2TiO3 relaxor ferroelectrics with Fe doping

Electrically and thermally induced transitions between ferroelectric and relaxor states are of great importance for Bi1/2Na1/2TiO3 (BNT)-based materials because of their close relevance to electromechanical properties. In this study, the electric field-temperature (E-T) phase diagrams of Fe doped Bi1/2(Na0.8K0.2)1/2TiO3 (BNKT) ceramics are constructed via an experiment and theory combined approach. A novel phenomenological model based on the Landau-Devonshire theory and the Preisach model is proposed to describe the electric field induced phase transitions. Using this model, an approximate free-energy landscape is obtained by fitting the experimental double polarization-electric field loop, and then the electric field representing ferroelectric and relaxor two-phase equilibrium is calculated from the free-energy landscape for the construction of an E-T phase diagram. The constructed E-T phase diagrams meet basic thermodynamics requirements, for example, the Clausius-Clapeyron relation, and, therefore, overcome some of the shortcomings of previously reported E-T phase diagrams for BNT-based ceramics. The relationship between the E-T phase diagram and electromechanical properties is also established. From the E-T phase diagrams, it is predicted that Fe doping could lower the threshold field of triggering giant strains of BNKT ceramics at a high temperature. This prediction is successfully verified by experimental measurement of the electric field induced strain. At the optimized temperature for strain property, the threshold field of triggering giant strain is estimated to be about 2.6 kV/mm for a 3.0% Fe doped sample, significantly lower than 3.5 kV/mm for the undoped sample. This shows that the E-T phase diagram can provide valuable guidance for the improvement of electromechanical properties of BNT-based ceramics.

Fine-grained BNT-based lead-free composite ceramics with high energy-storage density

The low breakdown strength of BNT-based dielectric ceramics limits the increase in energy-storage density. In this study, we successfully reduced the sintering temperature of BNT-ST-5AN relaxor ferroelectric ceramics from 1150 to 980 °C by two-phase compounding with nano-SiO2. Meanwhile, the average grain size of the composite ceramics is also greatly reduced from 4.45 μm to 0.37 μm. Thus, a large recoverable energy-storage density (3.22 J/cm3) is achieved under the ultrahigh breakdown strength (316 kV/cm). Moreover, good temperature (25–150 °C) and frequency (10–200 Hz) stabilities are simultaneously achieved. The excellent energy-storage properties suggest that BNT-ST-based ceramics composited with SiO2 form a promising low-temperature sintered dielectric material for pulsed power multilayer ceramic capacitors.

Large electro-strain signal of the BNT–BT–KNN lead-free piezoelectric ceramics with CuO doping

This paper investigates a system of 0.93[Formula: see text][Formula: see text]TiO3–0.06BaTiO3–0.01[Formula: see text][Formula: see text]NbO3–[Formula: see text]CuO (BNT–BT–KNN–[Formula: see text]CuO, [Formula: see text][Formula: see text]mol.%) ceramics, which were fabricated by the conventional solid-state process through the granulation of vacuum freeze drier. The results show that the CuO doping made a significant enhancement on the piezoelectric properties of the BNT–BT–KNN ceramics. With the doping of CuO, the transition temperature between ferroelectric phase and ergodic relaxor state is reduced to near room temperature, resulting in pinched [Formula: see text]–[Formula: see text] loops and “sprout” shape [Formula: see text]–[Formula: see text] curves. For the composition with [Formula: see text], a high unipolar strain of 0.39% under 5[Formula: see text]kV/mm contributes a large [Formula: see text][Formula: see text]pm/V at room temperature, which is competitive with the other BNT-based ceramics.

Effect of the processing temperature on the electrical properties of lead-free 0.965Bi0.5Na0.5TiO3–0.035BaTiO3 piezoelectric ceramics synthesized by sol–gel method

In this work, lead-free 0.965Bi0.5Na0.5TiO3–0.035BaTiO3 (BNT–BT) piezoelectric ceramics were synthesized by a sol–gel process. X-ray diffraction pattern of the BNT–BT samples revealed that powder calcined at 600 °C, exhibit a pure perovskite structure. Scanning electron microscopy (SEM) was used to find the surface morphology and grain size of the investigated ceramics. At an optimized sintering temperature of 1150 °C, well-saturated ferroelectric (P–E) hysteresis loops with high remnant polarization (Pr = 34.8 µC/cm2) and low corrosive field (Ec = 23 kV/cm) were obtained. Electric field-induced strain of 0.137% with a dynamic piezoelectric coefficient (d33* = 196 pm/V) was observed under 75 kV/cm drawing field. In addition, high static piezoelectric constant (d33 = 138 pC/N) was recorded for the sample sintered at 1150 °C. This value of d33 is comparable to that of BNT-based bulk piezoelectric ceramics.