Enhanced electromechanical properties of SrTiO3-BiFeO3-BaTiO3 ceramics via relaxor behavior and phase boundary design

High temperature lead-free piezoceramics of 0.06SrTiO₃-0.94[(1-x)BaTiO₃- xBiFeO₃] were fabricated by the traditional solid state reaction method. The ceramics possess the pure perovskite structures, and a morphotropic phase boundary (MPB) with the pseudo-cubic and rhombohedral phases is observed at x = 0.55-0.71. The ceramics have the obvious relaxor behaviors as confirmed by the temperature and frequency dependent dielectric curves. The largest positive electrostrain (0.25%) under 65 kV/cm is achieved in the ceramics with x = 0.63. The ceramics with x = 0.63 also exhibit a large d₃₃* (425 pm/V) with a high curie temperature (T_C =282 °C) and a low hysteresis (43%). The improved electromechanical properties are attributed to the existence of relaxor behavior at BiFeO₃-BaTiO₃ phase boundary after doping SrTiO₃. These results demonstrate that the design of a ternary system like ST-BF-BT based ceramics with relaxor behavior and phase boundary composition provides an effective way to optimize the electrostrain behavior of high temperature lead-free ceramics.     

Simultaneously achieving high energy storage density and efficiency under low electric field in BiFeO3-based lead-free relaxor ferroelectric ceramics

BiFeO₃-BaTiO₃-based relaxor ferroelectric ceramic has attracted increasing attention for energy storage applications. However, simultaneously achieving high recoverable energy storage density (W_rec) and efficiency (η) under low electric field has been a longstanding drawback for their practical applications. Herein, a novel relaxor ferroelectric material was designed by introducing (Sr_0.7Bi_0.2)TiO₃ (SBT) into the composition 0.67BiFeO₃-0.33BaTiO₃ (BF-BT-xSBT). A large W_rec of ∼2.40 J/cm³ and a high η of ∼90.4 % were simultaneously realized under a low electric field of 180 kV/cm, which is superior to that of most previously reported lead-free ceramics. Moreover, moderate temperature endurance and excellent frequency stability were also obtained. More importantly, this ceramic has a large discharge current density (∼289.18 A/cm²), a discharge power density (∼14.46 MW/cm³) and short discharge time (<0.25 μs). These results not only demonstrate superior potential in BF-BT-SBT ceramics, but also offer a new design to tune the energy storage performance of lead-free relaxor ferroelectric ceramics.     

Enhanced piezoelectric and ferroelectric properties of BiFeO3-BaTiO3 lead-free ceramics by optimizing the sintering temperature and dwell time

0.7BiFeO₃-0.3BaTiO₃ (BFO-0.3BT) ceramics were prepared to uncover the impacts of sintering temperature (T_S) and dwell time (t_d) on the microstructure and electrical properties. With increasing the T_S or t_d, the grain sizes increase along with the porosity decreases, which is in favor of the alignment of dipole. However, excess T_S or t_d are inclined to cause the volatilization of Bi₂O₃, which deteriorates piezoelectric properties. Because of the R-T two-phase coexistence, low defect ions concentration and porosity, as well as appropriate grain size, the excellent d₃₃?=?208?pC/N and k_p?=?35.46% as well as P_r?=?28.52?μC/cm² were achieved in BFO-0.3BT ceramics at T_S?=?1000?°C and t_d?=?6?h. In addition,large unipolar strain 0.13% and d₃₃*?=?256.2?pm/V also were obtained in BFO-0.3BT ceramics at T_S?=?1000?°C and t_d?=?6?h. This research indicates that the porosity and defect ion concentration as well as grain size also play an important role in piezoelectric properties in BFO-BT ceramics.     

Large energy storage density in BiFeO3-BaTiO3-AgNbO3 lead-free relaxor ceramics

Lead-free (0.70-x)BiFeO₃-0.30BaTiO₃-xAgNbO₃+5‰mol CuO (abbreviated as BF-BT-xAN) ceramics were fabricated using a modified thermal quenching technique. BF-BT-xAN ceramics are of a perovskite structure with morphotropic phase boundary (MPB) and show strong relaxor properties. Remarkably, the high recoverable energy storage density of 2.11 J/cm³ is obtained for BF-BT-xAN with x = 0.14. For the x = 0.14 ceramics, its energy storage efficiency is as high as 84 % at relative low field of 195 kV/cm, together with an outstanding thermal stability in a broad temperature range from 25 °C to 150 °C. In addition, this ceramic maintains superior energy storage performance even after 8 × 10⁴ electrical cycles due to its high densification after doping Ag₂O and Nb₂O₅. The result suggests that lead-free BF-BT-xAN ceramics may be promising candidate for dielectric energy storage application.     

Enhanced energy storage properties in La(Mg1/2Ti1/2)O3-modified BiFeO3-BaTiO3 lead-free relaxor ferroelectric ceramics within a wide temperature range

A new ternary lead-free (0.67-x)BiFeO₃-0.33BaTiO₃-xLa(Mg_1/2Ti_1/2)O₃ ferroelectric ceramic exhibited an obvious evolution of dielectric relaxation behavior. A significantly enhanced energy-storage property was observed at room temperature, showing a good energy-storage density of 1.66 J/cm³ at 13 kV/mm and a relatively high energy-storage efficiency of 82% at x = 0.06. This was basically ascribed to the formation of a slim polarization-electric field hysteresis loop, in which a high saturated polarization P_max and a rather small remnant polarization P_r were simultaneously obtained. Particularly, its energy storage properties were found to depend weakly on frequency (0.2 Hz–100 Hz), and also to exhibit a good stability against temperature (25 °C–180 °C). The achievement of these characteristics was attributed to both a rapid response of the electric field induced reversible ergodic relaxor to long-range ferroelectric phase transition and a typical diffuse phase transformation process in the dielectric maxima.     

Enhanced electromechanical properties of CaZrO3-modified (K0.5Na0.5)NbO3-based lead-free ceramics

Pairing of large strain response and high d₃₃ with high T_c in (K_0.5Na_0.5)NbO₃-based materials is of high significance in practical applications for piezoelectric actuators. Here, we report remarkable enhancement in the electromechanical properties for (1-x)(K_0.52Na_0.48) (Nb_0.95Sb_0.05)O₃-xCaZrO₃ (KNNS-xCZ) lead-free ceramics through the construction of a rhombohedral (R)-tetragonal (T) phase boundary. We investigated the correlation between the composition-driven phase boundary and resulting ferroelectric, piezoelectric, and strain properties in KNNS-xCZ ceramics. The KNNS-xCZ ceramics with x=0.02 exhibited a large strain response of 0.23% while keeping a relatively large d₃₃ of 237pC/N, which was mainly ascribed to the coexistence of R and T phases confirmed by the XRD and dielectric results. It was found that pairing of large strain response and high d₃₃ in KNN-based materials was achieved. As a consequence, we believe that this study opens the possibility to achieve high-performance lead-free electromechanical compounds for piezoelectric actuators applications.     

Temperature dependent,large electromechanical strain in Nd-doped BiFeO3-BaTiO3 lead-free ceramics

Lead-free piezoceramics with the composition 0.7(Bi_1-xNd_x)FeO₃-0.3BaTiO₃+0.1 wt% MnO₂ (BNxF-BT) were prepared using a conventional solid state route. X-ray diffraction and temperature dependent permittivity measurements indicated a transition from a composition lying at a morphotropic phase boundary (MPB) to a pseudocubic phase as a function of Nd concentration. The highest maximum strain (S_max ∼ 0.2% at 60 kV/cm) and effective piezoelectric coefficient (d₃₃* = 333 pm/V) were obtained at room temperature for the composition BN0.02F-BT. The decrease in remanent polarization (P_r) and Berlincourt d₃₃ with increase in Nd concentration can be attributed to the coexistence of ferroelectric and relaxor phases. In-situ polarisation and strain measurements revealed an increase in P_r and d₃₃* with temperature and a reduction in the coercive field E_C. Presumably this behavior is due to a combination of thermally activated domain wall motion and lowering of the activation energy for a field induced relaxor-ferroelectric transition, as the Curie maximum is approached.     

Achieving ultrahigh energy storage performance in BiFeO3-BaTiO3 based lead free relaxors via a composition optimization strategy

The 0.63(1-x)Bi_1.02FeO₃-0.37BaTiO₃-xBi(Zn_2/3(Nb_0.85Ta_0.15)_1/3)O₃ (abbreviated BF-BT-xBZNT) high temperature dielectric ceramics were prepared via a two-step sintering (TTS) method. The appropriate medium permittivity achieved in the BF-BT-0.13BZNT ceramic is conducive to mitigating the polarization saturation and improving the breakdown field strength. The domain evolution behavior from piezoresponse force microscopy (PFM) reveals that the introduction of BZNT promotes the formation and switching of more nanodomains of BF-BT ceramics, facilitating the enhancement of energy storage efficiency. The excellent energy storage performance of total energy storage density (W_tot) of 6.06 J/cm³, recoverable energy storage density (W_rec) of 4.85 J/cm³ and a high energy storage efficiency (η) of 80% are simultaneously obtained under 410 kV/cm in the BF-BT-0.13BZNT ceramic. Meanwhile, the ceramic exhibits excellent thermal endurance (10–130 ℃), frequency (1–100 Hz) and fatigue (10⁵ cycles) stability. The current work provides a promising strategy for designing high-performance dielectric energy storage materials which operate in harsh environments.     

Enhanced piezoelectric response and high-temperature sensitivity by site-selected doping of BiFeO3-BaTiO3 ceramics

Effect of Zn site-selected doping on electrical properties, high-temperature stability and sensitivity of piezoelectric response for BiFeO₃-BaTiO₃ ceramics was investigated. The results revealed that the addition of Zn leaded to an evident modification of the microstructure. The B-site selected doping was a more effective approach in improving piezoelectric properties as well as their thermal stability than those of A-site selected doping. Moreover, the enhanced piezoelectric properties accompanying by excellent high-temperature stability and sensitivity in B-site selected doping ceramics were obtained. The microstructure, domain switching behavior and temperature-dependent piezoelectric response in Zn site-selected doping ceramics were investigated, and their relationships with improving piezoelectric properties and high-temperature stability were explored. These results showed that the B-site selected doping ceramics had excellent piezoelectric properties (d₃₃ = 192pC/N) along with a high-temperature stability (T_d = 450 °C).     

Synthesis and properties of ultra-small BiFeO3 nanoparticles doped with cobalt

Bismuth ferrite (BiFeO₃, BFO) is the most promising material in the field of multiferroics. Additionally, it has excellent potential in photocatalysis applications due to its relatively small bandgap, stable structure, and low cost. Doping this material and enhancing its particle size showed promising results in many applications, especially photocatalysis, but the effect of both parameters has never been combined and studied before.In the present work, we report on the effect of cobalt doping of bismuth ferrite Bi Fe_1-xCo_xO₃ x (with x = 0, 0.02, 0.04, 0.06 and 0.08) synthesized by hydrothermal route. The crystalline phase was obtained at a temperature as low as 200 °C. Rietveld's refinement of X-ray diffraction (XRD) confirmed the rhombohedral structure of the crystalline powders. Transmission Electron Microscopy (TEM) alongside High-Resolution Transmission Electron Microscopy (HRTEM) was conducted to probe microstructural features and showed a remarkable decrease in particle size from 12.5 nm to only 4.5 nm, when increasing the cobalt concentration to 8%. The material's optical properties were also studied and showed an increase in band gap values from 2.12 to 2.33eV.The application of the samples to the photodecomposition of methylene blue (MB) under direct sunlight irradiation was reported and have shown unusual results that were explained by the unique sizes of the samples and the phenomena that can occur at that order of magnitude.     

High recoverable energy storage density in nominal (0.67-x)BiFeO3-0.33BaTiO3-xBaBi2Nb2O9 lead-free composite ceramics

A series of novel lead-free energy storage ceramics, (0.67-x)BiFeO₃-0.33BaTiO₃-xBaBi₂Nb₂O₉ (BF-BT-xBBN), were fabricated by traditional solid-state reaction, where bismuth layer-structured BaBiNb₂O₉ was incorporated into perovskite-structured BiFeO₃–BaTiO₃ ceramic as an additive. The addition of BaBi₂Nb₂O₉ increased the relaxor behavior and breakdown strength of BF-BT ceramics due to the formation of polar nanoregionals (PNRs), inducing enhanced energy storage performance. The composite ceramics, with x = 0.08, showed a large recoverable energy density (W_rec) of 3.08 J/cm³ and an outstanding energy storage efficiency (η) of 85.57% under an applied electric field of 230 kV/cm. Moreover, the composite ceramics exhibited excellent thermal stability and high stability toward different frequencies in a temperature range of 20–100 °C and a frequency range of 0.1–1500 Hz. These results demonstrate great potential of novel BF-BT-xBBN composite ceramics for next-generation energy storage applications.     

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.     

Electrical and nonlinear current-voltage characteristics of La-doped BiFeO3 ceramics

Multiferroic Bi_1-xLa_xFeO₃(x=0, 0.05, 0.1, 0.2, and 0.3) ceramics with particle sizes of ~67–19 nm were prepared by a simple co-precipitation method. The effects of La dopant on the microstructure, giant dielectric response, and electrical properties were investigated. The grain size of Bi_1-xLa_xFeO₃ ceramics significantly decreased with increasing La doping ions. The Bi_0.95La_0.05FeO₃ ceramic exhibited the highest leakage current density value. Interestingly, it strongly decreased as the concentration of La increased. The nonlinear coefficient of La doped BFO slightly decreased with increasing La. This shows a space-charge-limited conduction mechanism, which is involved in low electric field regions for all samples investigated. La substitution significantly enhanced the breakdown field. It was found that the potential barrier height at the grain boundary was slightly reduced from 0.3 to 0.16 eV by substitution of La ions. Using impedance spectroscopy analysis, except for the Bi_0.7La_0.3FeO₃ ceramic, the grain boundary resistance at room temperature was affected by dc bias, whereas the grain resistance of all samples was independent of dc bias. This result was well consistent with the variation in low-frequency dielectric constant and loss tangent value due to the effect of dc bias. These results were closely related to the existence of the interfacial polarization at the grain boundary.     

Improving the photostriction of BiFeO3-based ceramics by bandgap and domain size engineering

The photostrictive properties of (1?x)BiFe_0.96Mn_0.04O₃-xBaTiO₃ (0.23 ≤ x ≤ 0.38) ceramics were investigated using the solid-state synthesis method. Appropriate addition of manganese significantly reduces the bandgap, while the introduction of BaTiO₃ changes the phase structure from rhombohedral to pseudo-cubic and significantly optimizes the ferroelectric domain size. The photostriction was observed in the visible light wavelength range with a response time of around 45 s. Specifically, both enhanced photo-induced deformation around 1.27×10^?3 and high photostrictive efficiency of 8.40×10^?12 m³ W^?1 were obtained for the 0.67BiFe_0.96Mn_0.04O₃-0.33BaTiO₃ ceramics. The significantly narrow bandgap (～1.89 eV) and the increased domain wall density due to reduction in ferroelectric domain size enhance the separation and motion of photo-generated carriers, and consequently improve the photostrictive performance. Besides, the prominent Raman peak redshift with the increasing of Raman power reveals the enhanced FeO₆ octahedral distortion and stretching vibration of Fe–O bond, which indicates the lattice expansion caused by the photoexcited charge carriers.     

Improved ferroelectric properties of BiFeO3-based piezoelectric ceramics through morphotropic phase boundary construction

The ceramic (1-x)BiFe_0.985Sc_0.015O₃-xBaZr_0.2Ti_0.8O₃ + 1mol% MnO₂ (x = 0.20, 0.25, 0.30, 0.35) (BFS-xBZT) was synthesized using the traditional ceramic sintering method. The components of the ceramic were determined by constructing a morphotropic phase boundary (MPB) which consists of rhombohedral (R) and tetragonal (T) phases (0.24≤ x ≤ 0.26). With the accumulation of BZT content, relax behaviors were observed by dielectric properties measurements. At the MPB, the crystal structures of the R phase and T phase change abruptly. The distortion degree of the R phase increases, and the differences between a and c of the T phase decrease. An enhanced ferroelectricity Pr of ~27.8 μC/cm² and apex piezoelectric coefficient d₃₃ of 131 ± 4 pC/N are obtained near the MPB (x = 0.25), due to the R-T phase coexistence near room temperature. The results show that BFS-xBZT ceramics could be a candidate for lead-free piezoelectric ceramics at high operating temperatures.     

NaNbO3 templates-induced phase evolution and enhancement of electromechanical properties in <00l> grain oriented lead-free BNT-based piezoelectric materials

Recently developed Bi_0.5Na_0.5TiO₃(BNT)-based piezoceramics face two urgent obstacles: high driving field required to trigger large strain and poor temperature stability. Highly oriented (1-x)(0.83Bi_0.5Na_0.5TiO₃-0.17Bi_0.5K_0.5TiO₃)-xNaNbO₃ (BNT-BKT-xNN) piezoceramics were synthesized using NN templates to resolve both obstacles. Measurements of polarization and strain hysteresis loops as well as phase transition temperature revealed a phase evolution from ergodic relaxor to ferroelectric phases, generating a high strain of 0.43% and large S_max/E_max = 720pm/V for textured BNT-BKT-4NN ceramics. The field-dependent strain was largely depended on the degree of texturing. Most intriguingly, grain-oriented specimens provided excellent actuating performance characterized by both large S_max/E_max = 693 pm/V at a low driving field of 45 kV/cm and enhanced temperature stability with S_max/E_max = 537pm/V at 120 °C. This was basically ascribed to the facilitated switching between ergodic relaxor and ferroelectric phases owing to the grain-oriented structure. As a consequence, design of <00l> oriented microstructure opens the possibility to produce efficient BNT-based piezoceramics for transferral into real-world applications.     

Effect of different ionic valence state doping on the structure and characteristic of BiFeO3-BaTiO3-based ceramics

BiFeO₃-BaTiO₃-based ceramics are promising multiferroic materials for a wide range of applications. In this paper, the effects of different ionic valence state((Ga_0.5Ta_0.5)⁴⁺, Ta⁵⁺) doping on the structure and properties of 0.73BiFeO₃-0.27BaTi_1-y(Ga_1-xTa_x)_yO₃ ceramics were investigated. X-ray diffraction analysis showed that all samples were determined by rhombohedral R3c phase and tetragonal P4mm phase. A frequency-independent dielectric anomaly with obvious thermal hysteresis was detected in all samples, indicating the first-order ferroelectric phase transition, and the Curie temperatures were all above 600°C. The ferroelectric analysis found that the doping of donor element Ta reduced the coercive field, with a slight decrease of remanent polarization, which can be attributed to the soft effect of Ta element. Meanwhile, the magnetic properties were enhanced for all samples, with a maximum in 15T (x = 1, y = .015) sample, where M_r = .13 emu/g. According to relevant analysis, high-valence element substitution can enhance the magnetic properties and reduce the coercive field of BiFeO₃, while low-valence element substitution can reduce dielectric loss. This result points out a way for us to choose appropriate doping elements to improve the performance of BiFeO₃ ceramics.     

Effects of Hf4+ substitute on the enhanced electrostrain properties of 0.7BiFeO3-0.3BaTiO3-based lead-free piezoelectric ceramics

The 0.7Bi_1.05FeO₃-0.3BaHf_xTi_(1?x)O₃ (BF-BHT-x) ceramics are prepared by using conventional method. The mechanism of large electric-induced strain is studied in detail through Rietveld refinement, transmission electron microscopy (TEM), and piezoelectric force microscopy (PFM). The BF-BHT-0.05 ceramics, which possess the coexistence of cubic (Pm-3m) and rhombohedral (R3c) phase proved by Rietveld refinement and TEM measurement, have the maximum electric-induced strain of S_max = 0.58% at 130 kV/cm (1 Hz). The reduced potential barrier is beneficial to the switching and reorientation of domain under an external electric field due to the coexistence of cubic and rhombohedral phase. A mass of nanodomains embedded into the microdomains, which is conducive to domain switching, are observed obviously through PFM. Therefore, the findings show that the large strain is because the external electric field drives the conversion mixed phase into the major rhombohedral phase, the reduced potential barrier, and the nanodomain in macrodomain.     

Electric field induced phase transition and accompanying giant poling strain in lead-free NaNbO3-BaZrO3 ceramics

A series of phase transitions in (1-x)NaNbO₃-xBaZrO₃ ((1-x)NN-xBZ) ceramics was observed from antiferroelectric orthorhombic phase to ferroelectric orthorhombic phase and finally into ferroelectric rhombohedral phase with increasing x. An electric field induced irreversible phase transition was found in different compositions, irrespective of their virgin phase structures. Particularly, an antiferroelectric orthorhombic phase is irreversibly transformed into a ferroelectric monoclinic phase within 0.02?≤?x?≤?0.05, leading to a giant poling strain of ～0.58%. This is much larger than that observed in ferroelectric orthorhombic (0.06?≤?x?≤?0.07) and rhombohedral phases (0.08?≤?x?≤?0.11) suffering from an irreversible ferroelectric-ferroelectric (monoclinic) phase transition. The synchrotron x-ray diffraction and the measurement of longitudinal and transverse strains suggest that this irreversible phase transition should involve not only a distinct volume expansion, but also an obvious lattice elongation. The present study demonstrates a unique nature of the composition and field dependent phase stability and an underlying mechanism of giant poling strains in NN-BZ ceramics.