Enhancement of piezoelectric catalysis of Na0.5Bi0.5TiO3 with electric poling for dye decomposition

In this work, the performance of solid-phase sintered Na_0.5Bi_0.5TiO₃ ceramic powder catalyst is investigated for Rhodamine B (RhB) dye decomposition under different electric poling fields. As a result of the vibration excitation, the piezoelectric catalytic decomposition ratio of the poled Na_0.5Bi_0.5TiO₃ for RhB dye wastewater increases from 10.84% to 51.32% as the electric poling field increases from 0 to 0.5 kV/mm. The improved piezoelectric catalytic activity could be attributed to a rise in the piezoelectric performance of Na_0.5Bi_0.5TiO₃ after electric poling. The addition of the different types of radical scavengers in piezoelectric catalysis further proves that a large amount of superoxide-free radicals and a small number of hydroxyl radicals participate in the catalytic reaction. Our results demonstrate that electric poling, as a simple and effective method of improving piezoelectric catalysis, has potential applications in dye-containing wastewater treatment.     

Structural evolution and ferroelectric properties in lead-free (1−x)(Bi0.5Na0.4K0.1)TiO3-xSrTiO3 solid solutions

Bi_0.5Na_0.5TiO₃ (BNT)-based dielectric ceramics have received a lot of attention due to the increased demand for pulse ceramic capacitors. However, comprehensive study on the relationship between their internal phase structure, dielectric characteristics, and ferroelectric properties is still lacking. The phase evolution and its impact on dielectric and ferroelectric properties of an important BNT-based solid solution, Bi_0.5Na_0.4K_0.1TiO₃-xSrTiO₃ (x = 0, 0.1, 0.2, 0.3 and 0.4), were investigated systematically in this work using structural, dielectric, and ferroelectric characterization techniques. X-ray diffraction indicated the coexistence of rhombohedral and tetragonal phases. The frequency- and temperature-dielectric characterization was then used to derive the characteristic temperatures T_B, T_m, T_s, and T_d, and a phase diagram was developed. Furthermore, the temperature-dependent current against electric field curves and polarization versus electric field loops were used to derive the characteristic temperatures connected to high electric field features. This study not only explains the phase evolution of the Bi_0.5Na_0.4K_0.1TiO₃-xSrTiO₃ solid solution, but it also correlates microscopic domains and polar nanoregions to macroscopic dielectric and ferroelectric properties.     

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.     

Effect of SrZrO3 on phase structure and electrical properties of 0.974(K0.5Na0.5)NbO3–0.026Bi0.5K0.5TiO3 lead-free ceramics

(0.974−x)(K_0.5Na_0.5)NbO₃–0.026Bi_0.5K_0.5TiO₃–xSrZrO₃ lead-free piezoelectric ceramics have been prepared by the conventional solid state sintering method. Systematic investigation on the microstructure, crystalline structures as well as electrical properties of the ceramics was carried out. With the addition of SrZrO₃, the rhombohedral–orthorhombic phase transition temperature of the ceramics increases. Both the rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions of the ceramics were modified to be around room temperature when x~0.05, and as a result remarkably strong piezoelectricity has been obtained in 0.924(K_0.5Na_0.5)NbO₃–0.026Bi_0.5K_0.5TiO₃–0.05SrZrO₃ ternary system, whose piezoelectric parameters were d₃₃=324 pC/N and k_p=41%.     

Normal-relaxor ferroelectric phase transition induced morphotropic phase boundary accompanied by enhanced piezoelectric and electrostrain properties in strontium modulated Bi0.5K0.5TiO3 lead-free ceramics

In this work, (Bi_0.5K_0.5)_1-xSr_xTiO₃ compositions (x = 0.03∼0.18) are designed to clarify the role of normal-relaxor ferroelectric phase transition and morphotropic phase boundary on dielectric, piezoelectric and electrostrain properties. With increasing strontium content, tetragonal distortion decreases and tetragonal and pseudocubic phases coexist in 0.09 ≤ x ≤ 0.15 compositions; the spontaneous phase-transition temperature and curie temperature decrease, as certified by phase-structure, dielectric properties and Raman spectra analysis. Optimized piezoelectric constant ∼106 pC/N and electrostrain ∼0.17 % are obtained for (Bi_0.5K_0.5)_0.88Sr_0.12TiO₃ composition. Piezoelectric force microscopic technique is exploited to clarify the origin of enhancement in macroscopic performances. Increase in temperature enhances ferroelectric performance and a large strain value ∼0.25 % with low hysteresis ∼27 % are obtained at 140 °C for the optimized composition, which are believed to originate from electric-field induced relaxor-to-ferroelectric phase transition with thermally-activated reduced energy barriers. This work clearly demonstrates that lead-free Bi_0.5K_0.5TiO₃-based ceramics are another promising bismuth-based species in applications of piezoelectric sensors and actuators.     

Piezoelectric properties and microstructures of ZnO-doped Bi0.5Na0.5TiO3 ceramics

This article studies the microstructure and piezoelectric properties of a ceramic lead-free NBT under different amount of ZnO doping. X-ray diffraction shows that Zn²⁺ diffuses into the lattice of (Bi_0.5Na_0.5)TiO₃ to form a solid solution with a pure perovskite structure. By modifying the zinc oxide content, the sintering behavior of (Bi_0.5Na_0.5)TiO₃ ceramics was significantly improved and the grain size was increased. The piezoelectric coefficient d₃₃ for the 1.0 wt.% ZnO-doped (Bi_0.5Na_0.5)TiO₃ ceramics sintered at 1050 °C was found to be 95 pC/N, and the electromechanical coupling factor k_p = 0.13. However, the piezoelectric coefficient d₃₃ for the 0.5 wt.% ZnO-doped (Bi_0.5Na_0.5)TiO₃ ceramics sintered at 1140 °C was found to be 110 pC/N, and the electromechanical coupling factor k_p = 0.17.     

Rapid preparation of Bi0.5Na0.5TiO3 ceramics by reactive flash sintering of Bi2O3-NaCO3-TiO2 mixed powders

Lead-free Bi_0.5Na_0.5TiO₃ piezoelectric ceramics were successfully prepared by reactive flash sintering of Bi₂O₃-NaCO₃-TiO₂ mixed powders, where phase transformation and densification occurred simultaneously. The influence of electric field strength, current density and holding time at constant current state on the phase transformation and densification were investigated. The current density had a significant influence on the extent of phase transformation and densification. The holding time had no influence on the phase transformation, but had an important effect on crystallinity of sample. The sintered bulks exhibited the maximum polarization P_m of 16.8 μC/cm², remanent polarization P_r of 9.6 μC/cm², coercive field E_c of 29 kV/cmm, maximum electric-field-induced strain of 0.053 %, and piezoelectric coefficient d₃₃ of 85 pC/N. The reactive flash sintering can prepare the dense and single-phase ceramics from multiphase precursor powders in one step of flash, providing a new way for rapid production of ceramic materials.     

Stress driven high electrostrain at low field in incipient piezoelectrics

We use a strategy to reduce the driving electric field of the relaxor 0.72Bi_1/2Na_1/2TiO₃–0.28SrTiO₃ (BNT–28ST) by introducing ferroelectric plate–type Bi_1/2(Na_0.78K_0.22)_1/2TiO₃ (BNKT) particles. Consequently, 10 vol.% BNKT added BNT–28ST/BNKT (relaxor/ferroelectric) composite has delivered large normalized strain (S_max/E_max = 650 pm/V) under relatively low applied field of 2.5 kV/mm with the reduction in the poling field value to 36% compared to that of pure BNT–28ST ceramics. We demonstrated that such behavior comes from the stress induced effect at the boundary of the ferroelectric and relaxor materials due to their different nature (relaxor/ferroelectric). The generation of stress is expected from the ferroelectric (BNKT) to the relaxor (BNT–28ST) at the boundary. Thereby, field–dependent stress at relaxor/ferroelectric phase boundary is monitored proposing the reduction of the poling field due to the strain coupling mechanism. Our findings provide a pathway for reducing the poling field in BNT–based incipient piezoelectric ceramics.     

Synergistic enhancement of piezoelectricity and thermal stability in AlN-doped Bi0.5Na0.5TiO3-based ceramics

The inverse relationship between piezoelectric coefficient (d₃₃) and depolarization temperature (T_d) in Bi_0.5Na_0.5TiO₃-based ceramics is a longstanding obstacle for their applications. In this work, synergistically enhanced d₃₃ and T_d is achieved in AlN-modified 0.84Bi_0.5Na_0.5TiO₃-0.11Bi_0.5K_0.5TiO₃-0.05BaTiO₃ ceramics. Addition of 1 mol% AlN, increases both d₃₃ from 165 to 234 pC/N and T_d by ～50 °C. Rietveld analysis of X-ray powder diffraction (XRD) data reveals an increase in the proportion of the tetragonal phase at 1 mol% AlN incorporation. Moreover, at this composition the modified ceramics exhibit larger grains and high-density lamellar nanodomains with sizes of 30–50 nm. Polarization reversal and domain mobility are thus significantly enhanced, contributing to the large d₃₃. Temperature-dependent dielectric and XRD data revealed that the delayed thermal depolarization is attributed to the improved and poling-field stabilized tetragonality in the modified ceramics.     

Pairing high piezoelectric properties and enhanced thermal stability in grain-oriented BNT-based lead-free piezoceramics

(Bi_0.5Na_0.5)TiO₃ (BNT)-based piezoceramics usually exhibit enhanced piezoelectric coefficient d₃₃ together with the deterioration of depolarization temperature T_d, which is the common drawback limiting their use in practical application. Here, we demonstrate that harnessing the microstructure in BNT-based ceramics will be an efficient way to resolve this obstacle. <00l> oriented piezoelectric ceramics 0.94(Bi_0.5Na_0.5)TiO₃ ?0.06BaTiO₃ was engineered by templated grain growth (TGG) using NaNbO₃ (NN) as templates. The manufactured textured ceramics with the optimized microstructure was characterized by not only approximately 200% enhancement in the magnitude of piezoelectric response (d₃₃~297pC/N) but also improved thermal stability (T_d~57?°C) in comparison to its randomly oriented counterparts (d₃₃~151pC/N and T_d~32?°C). Moreover, the enhanced piezoelectricity in grain oriented specimens primarily originated from a high degree of non-180° domain switching as compared to the randomly axed ones. The current study opens the door to pair high piezoelectric properties and enhanced thermal stability in BNT-related materials though texture technique.     

Microstructures,piezoelectric properties and energy harvesting performance of undoped (K0.5Na0.5)NbO3 lead-free ceramics fabricated via two-step sintering

Piezoelectric energy harvesters have become increasingly popular in the field of green energy because of the ability to convert low-frequency environmental vibrations into usable electricity. To fabricate high-performance energy harvesters, the key requirements are piezoelectric ceramics with a small grain size, of near-full density, the intended stoichiometric ratio and a high transduction coefficient. In this work, the effects of two-step sintering on the sinterability, microstructure, piezoelectric properties and energy harvesting performance of (K_0.5Na_0.5)NbO₃ were systematically investigated. Compared with conventional single-step sintering, two-step sintering samples were of higher density, increasing from 91 % to 95 % of theoretical, reduced mean grain size, down from 17 μm to 7.5 μm, and decreased evaporation of the alkali metals. This translated into an improved piezoelectric performance (d₃₃ ∼122 pC/N, k_p ∼36 % and Q_m ∼76), a higher transduction coefficient and energy conversion efficiency as well as a higher open-circuit voltage and power density. This demonstrates the potential of two-step sintering as a high through-put sintering technique for moderate-performance, pure KNN ceramics.     

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

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

Enhanced piezoelectric property and promoted depolarization temperature in Fe doped Bi1/2(Na0.8K0.2)1/2TiO3 lead-free ceramics

Piezoelectric sensors and energy harvesters require piezoelectric materials with large piezoelectric responses and good thermal stability. However, a commonly accepted concept is that the promotion of depolarization temperature of Bi_1/2Na_1/2TiO₃-based lead-free ceramics is usually companied by deterioration of piezoelectric properties. In the present study, the effects of acceptor-Fe doping on piezoelectric property and thermal depolarization behavior of Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ ceramics are investigated. Fe doping at an appropriate level (≤ 3.0%) improves piezoelectric property and thermal stability simultaneously, due to the stabilization of long-range ferroelectric order. Piezoelectric constant d₃₃ increases from 125 pC/N to 148 pC/N with Fe amount of 3.0%, and then decreases. The depolarization temperature T_d is promoted continuously with Fe addition, from 76 °C for the undoped sample to 118 °C for the sample with Fe amount of 5.0%. It is proposed that the piezoelectric property and thermal stability can be simultaneously improved by stabilizing the long-range ferroelectric order in Bi_1/2Na_1/2TiO₃-based systems with obvious relaxor character. This work provides a new insight into the improvement of Bi_1/2Na_1/2TiO₃-based lead-free piezoelectric ceramics.     

Construction of multi-domain coexistence enhanced piezoelectric properties of Bi0.5Na0.5TiO3-based thin films

Although the multi-phase coexistence makes Bi_0.5Na_0.5TiO₃-based piezoelectric thin films possess stronger piezoelectric properties and more spacious application prospects in electronic devices, the domain reversal mechanism of Bi_0.5Na_0.5TiO₃-based thin films cannot be accurately understood due to the size effect. In this study, the relationship between domain structure and piezoelectric properties of the (0.94-x)Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-xBi(Fe_0.95Mn_0.03Ti_0.02)O₃ thin films are studied by using visualization technology PFM, structure and electrical properties characterizations. The results show that the addition of Bi(Fe_0.95Mn_0.03Ti_0.02)O₃ creates a long-range ordered/short-range disordered nanodomain coexisting structure. This kind of coexisting domain structure can realize the long-range reversal driven by disordered nanodomains under the external electric field, reduce the potential barrier and the hysteresis, and significantly enhance the piezoelectric properties of the thin films. Under the same conditions, the piezoelectric properties of the 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ thin films are enhanced nearly 2.3 times. This provides a reference for exploring the physical mechanism of high performance lead-free piezoelectric thin films.     

Selection of Lead‐Free Piezoelectric Ceramics

Lead‐free piezoelectric ceramics are extensively investigated for the alternatives of lead‐based piezoceramics. (K,Na)NbO₃ (KNN), (Bi_0.5Na_0.5)TiO₃ (BNT), and (Bi_0.5K_0.5)TiO₃ (BKT)‐based ceramics are reported as promising piezoelectric material families. Several researchers have reported solid solution of these ceramics using various chemical and physical routes. In this study, we have rank these materials using multiple attribute decision making techniques. KNN‐LT‐LS and 0.7BNT‐0.2BKT‐0.1(Bi_0.5Li_0.5)TiO₃ are found to be top rank in all the materials of respective families under study. We have also reported Pareto‐optimal (nondominated) lead‐free piezoelectric ceramics for d₃₃ and T_c parameters.     

Large strain under low driving field in lead-free relaxor/ferroelectric composite ceramics

0.75(Na_0.5Bi_0.5)TiO₃–0.25SrTiO₃ lead-free incipient piezoceramic is a promising candidate for actuator applications due to their large reversible electromechanical strains at the relatively low driving field of 40 kV/cm. In order to further reduce the driving field of 0.75(Na_0.5Bi_0.5)TiO–0.25SrTiO₃ relaxor ceramic to meet the requirements for real actuators application, the relaxor/ferroelectric (RE/FE) 0-3 composite ceramics method was employed. The polarization and strain behaviors were examined as a function of the weight ratio of the relaxor/ferroelectric phases. It was found that 90 wt% 0.75(Na_0.5Bi_0.5)TiO₃–0.25SrTiO₃/10 wt% 0.96(Na_0.84K_0.16)_1/2Bi_1/2TiO₃–0.04SrTiO₃ RE/FE 0-3 type composite samples provided a high unipolar strain of 0.25% and the corresponding large-signal piezoelectric coefficient, d^*₃₃ of 833 pm/V at 30 kV/cm, which are 32% higher than the values of the pure 0.75(Na_0.5Bi_0.5)TiO₃–0.25SrTiO₃. The enhanced electric-field-induced strain at relatively lower field was attributed primarily to the reduction in the RE-FE phase transition electric field. It was also found that the RE/FE composite ceramics exhibited significantly reduced frequency dependence in the unipolar strain behavior at room temperature.     

Poling temperature-insensitive piezoelectric constant of high-performance potassium sodium niobate piezoceramics

Deriving from strong temperature dependence of polymorphic phase boundaries, piezoelectric constant of potassium sodium niobate (K_0.5N_0.5NbO₃)-based ceramics is sensitive to the poling temperature. Here, an effective strategy of doping rare earth (RE) elements is proposed to solve this issue. Piezoelectric constant (d₃₃ ~ 440 pC/N) can be well kept in 0.965K_0.45Na_0.55Nb_0.96Sb_0.04O₃-0.035(Bi_0.75Dy_0.25)_0.5Na_0.5HfO₃ ceramics at a wide poling temperature range (35-120°C) by the method of replacing Bi³⁺ with rare earth dysprosium, and importantly, this method can be effectively expanded to other RE elements. The high piezoelectricity roots in the multiphase coexistence with the addition of RE elements. The dull sensitivity of poling temperature is analyzed by microstructure information, that is, ferroelectric domains can be well rotated under a wide temperature range by introducing RE, which is responsible for the stabilized piezo-response against the poling temperature. Moreover, comparing with Bi³⁺, a lower electronegativity of RE³⁺ may effectively reduce the micro-stress produced by lattice distortion, also yielding to the lattice softening and the easier domain rotation.     

Large electrostrain at high temperature in bismuth sodium titanate-based piezoelectric ceramics

(1-x)[0.82Bi_0.5Na_0.5TiO₃-0.18Bi_0.5K_0.5TiO₃]-xAgNbO₃ ceramics (abbreviated as BNKT-100xAN, x = 0–0.06) are fabricated by the solid state reaction method. It is discovered that silver niobate is completely dissolved into the lattice of 0.82Bi_0.5Na_0.5TiO₃-0.18Bi_0.5K_0.5TiO₃ ceramics based on characterization. The relaxation mechanism and characteristics of BNKT-100xAN ceramics are analyzed by dielectric temperature spectra and AC impedance spectra. The content of ordered domains and PNRs of BNKT-4AN ceramics is optimum, which enables it possess excellent electrostrain of 0.51%. The electrostrain almost remain constant after the 10⁴ cycles test, suggesting that it also has great fatigue resistance. After polarization at high temperature, large electrostrain of 1.4% and inverse piezoelectric constants of 2004 pm/V are obtained at 150 °C in BNKT-4AN ceramics. The ultrahigh electrostrain is mainly promoted by the larger polarizability of the <111>-oriented and <001>-oriented defect dipoles, the same direction of defect dipoles and the spontaneous polarization direction of phase, the pinning effects of the defect dipoles on the PNRs and the collinear arrangement of the PNRs. This work indicates that lead-free piezoelectric ceramics is hopeful to be applied in piezoelectrics working at high temperature. Simultaneously, a new design paradigm also is provided for the preparation of piezoelectric ceramics with excellent performance.     

Phase structure,ferroelectric properties,and electric field-induced large strain in lead-free 0.99[(1−x)(Bi0.5Na0.5)TiO3-x(Bi0.5K0.5)TiO3]–0.01Ta piezoelectric ceramics

Lead-free 0.99[(1−x)(Bi_0.5Na_0.5)TiO₃-x(Bi_0.5K_0.5)TiO₃]–0.01Ta piezoelectric ceramics were prepared by a conventional solid-state reaction process. The ferroelectric properties, and strain behaviors were characterized. Increase of the (Bi_0.5K_0.5)TiO₃ content induces a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic phase. Accordingly, the ferroelectric order is disrupted significantly with the increase of (Bi_0.5K_0.5)TiO₃ content and the destabilization of the ferroelectric order is accompanied by an enhancement of the unipolar strain, which peaks at a value of 0.35% (corresponding to a large signal d₃₃ of 438 pm/V) in samples with 20 mol% (Bi_0.5K_0.5)TiO₃ content. Temperature dependent measurements of both polarization and strain from room temperature to 120 °C suggested that the origin of the large strain is due to a reversible field-induced nonpolar pseudocubic-to-polar ferroelectric phase transformation.     

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

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