BNT-based Materials Research Articles

Random field site inequality for ergodicity modification in BNT-based materials: From a phenomenological view

Achieving a simplified BNT-based composition with giant strain properties is challenging, since traditional chemical substitutions rely on trial-and-error experiments and complicates the composition. Herein, we comprehend the strain regulation mechanism from a view of relaxors and propose strain regulation rules based on classical relaxor theory. In BNT-based systems, site inequality of the random field for the ergodicity modification is observed, arising from the discrepant random field-driven (e.g., intensity and action position) structural distortions. From a phenomenological view, B-site substitution is effective in regulating relaxation behaviors due to the stronger lattice softening effect. This work points out the essence of regulating strain properties in BNT-based systems, and the proposed phenomenological relaxation regulation rules are believed to advance the development of actuator applications.

Random fields-triggered strain enhancement in BNT-based materials

Bi0.5Na0.5TiO3 is regarded a potential alternative to lead-based piezoelectric materials due to its large electro-strain. Generally, the improved electro-strain is considered to arise from a relaxor-ferroelectric transition, and the random fields play a critical role. In this study, to investigate the effect of random electric field or strain field on the strain value, the dopant Sb5+, Ta5+, Zr4+, and Hf4+ are introduced into the Bi0.5(Na0.8K0.2)0.5Ti1-xBxO3 matrix, respectively. The 1 % Ta5+ doped sample has the largest unipolar electro-strain, up to 0.49 %, followed by the 3 % Zr4+ doped sample (∼0.448 %). The electro-strain of 1 % Sb5+ and 3 % Hf4+ samples are 0.404 % and 0.377 %, respectively. The results show the random electric field or strain field can effectively regulate the relaxor-ferroelectric transition temperature, which enhances strain, but the strain value cannot be determined directly. The strain value is determined by the lattice structure before and after the electric field is applied.

Modification of both Pr and Td in ZnO-doped 0.97(Bi0.5Na0.5)TiO3-0.03BiAlO3 ferroelectric ceramics

Bi0.5Na0.5TiO3(BNT)-based ferroelectric ceramics demonstrate significant potential for application in high-power ferroelectric generators. However, their practical utilization is hindered by poor thermal stability resulting from a low depolarization temperature (Td). In this work, we simultaneously improve the remnant polarization (Pr) and Td of 0.97BNT-0.03BiAlO3 ceramics via moderate ZnO doping. The effects of ZnO doping on the sintering temperature, ferroelectricity and depolarization behavior of 0.97BNT-0.03BiAlO3 ceramics are systematically studied. The optimum sintering temperature of 0.97BNT-0.03BiAlO3 ceramics decreases from 1140 °C to 1000 °C by approximately 140 °C and the density becomes higher after ZnO doping. At x = 0.01, the phase structure remains R3c rhombohedral phase, and Zn2+ diffuses into the crystal lattices. However, when x ≥ 0.02, the secondary phases ZnO and ZnAl2O4 appear. As the ZnO content increases from x = 0 to x = 0.04, the Pr value shows a continuous increase from 31.13 µC/cm2 to 39.84 µC/cm2, while the Td value initially rises by 32 °C from 156 °C to 188 °C and then decreases to 163 °C. Notably, the highest Td is achieved at x = 0.02, owing to the combined effects of Zn2+ substitution and localized stress between the matrix and the composite. These findings highlight doping ZnO as an effective approach to optimize both Td and Pr of BNT-based materials.

Large electrostrictive response via tailoring ergodic relaxor state in Bi1/2Na1/2TiO3-based ceramics with Bi(Mn1/2Ce1/2)O3 end-member

Lead-free Bi1/2Na1/2TiO3 (BNT)-based relaxor ferroelectric ceramics with superior electrostrictive coefficient features have recently gained a lot of attention due to their application in high-precision displacement actuators. In this work, we propose an effective approach to enhance the electrostrictive effect via tuning the phase transition temperature around ambient temperature in BNT-based ceramics. Herein, a novel Bi(Mn1/2Ce1/2)O3 (BMnCe) modifier was adopted as an activator into 0.935Bi1/2Na1/2TiO3-0.065BaTiO3 (0.935BNT-0.065BT) for modifying the phase boundary (TF-R) around the ambient temperature. The compositional and temperature-dependent dielectric, ferroelectric, and electrostrain were systematically investigated. All samples exhibit a pure perovskite structure. Rietveld refinement revealed that the 0.935BNT-0.065BT ceramic presents a coexistence of rhombohedral (R3c) and tetragonal (P4bm) phases. As the BMnCe content increases, the tetragonal (P4bm) phase becomes more prevalent and eventually shifts into a pseudocubic phase. It was found that the addition of BMnCe can effectively tune the TF-R below ambient temperature and enhance the relaxor behavior. The polarization and electrostrain analysis revealed that the ferroelectric long-range order decreases with increasing BMnCe; specifically, a unique region emerges where remnant polarization, quasistatic d33, and negative strain abruptly drop. As a result, a high electrostrain (S) of 0.43 % with a normalized strain (Smax/Emax) of 716 pm/V was achieved at the critical composition (0.01 M concentration of BMnCe). More importantly, (0.935-x)BNT-0.065BT-xBMnCe (x = 0.01) shows a large electrostrictive coefficient (Q33) of 0.032 m4/C2 with a giant normalized electrostrictive coefficient (Q33/E) of 5.33ｘ10−9 m5/C2V at room temperature. Furthermore, the Q33 and Q33/E values show temperature insensitivity up to 120 °C, rendering the compound suitable for actuation-based devices. These results offer an effective avenue for designing high-performance BNT-based materials with giant electrostrictive coefficients.

Significantly enhancing piezoelectric temperature stability of BNT-based ceramics by constructing the successive ferroelectric-relaxor phase transition

Bi0.5Na0.5TiO3 (BNT)-based piezoceramics with the morphotropic phase boundary MPB are confronted with some critical challenges for practical applications, such as the low depolarization temperature Td, the poor thermal shock resistance and the high fluctuation of the real-time piezoelectric coefficient d33. Herein, an ingenious strategy that integrating quenching process and successive ferroelectric-relaxor phase transition is proposed to enhance piezoelectric temperature stability of BNT-based ceramics. The multilayer ceramic composites (0.93Bi0.5Na0.5TiO3-0.07BaTiO3/0.89Bi0.5Na0.5TiO3-0.11BaTiO3/0.85Bi0.5Na0.5TiO3-0.15BaTiO3-0.2 g/0.25 g/0.15 g-Q, abbreviated as BNT-7/11/15BT-0.2 g/0.25 g/0.15 g-Q, the Q refer the quenching process and the 0.2 g/0.25 g/0.15 g refers the mass of each layer) feature the relatively large d33 (130pC/N) and near the Curie point Td (244℃), which jumps out of the d33-Td trade-off boundary of BNT-based ceramics. Besides, the BNT-7/11/15BT-0.2 g/0.25 g/0.15 g-Q ceramic composite also shows the superior thermal shock resistance and decent temperature stability of real-time d33 over the temperature region from 25℃ to 235℃. Enhanced piezoelectric temperature stability should be attributed to the development of tetragonal P4mm and P4bm phase gradients caused by the successive ferroelectric-relaxor phase transition and the development of bamboo-shaped domain based on Ba2+ diffusion layer. In addition, the relatively large d33 stems from the coexistence of rhombohedral R3c phase and tetragonal P4bm and P4mm phases and the formation of lamellar domain. Therefore, this strategy provides a good design methodology for the practical application of BNT-based materials.

Excellent energy-storage performance in Bi0.5Na0.5TiO3-based lead-free composite ceramics via introducing pyrochlore phase Sm2Ti2O7

The comprehensive performance of ferroelectric ceramic materials is a significant factor limiting the practical application. In this work, a novel strategy of constructing diphase compounds is proposed to significantly enhance the energy storage properties of Bi0.5Na0.5TiO3-based ceramics. A composite ceramic of pyrochlore phase Sm2Ti2O7 modified perovskite phase Bi0.5Na0.5TiO3 is successfully prepared and systematically studied. As the concentration of Sm2Ti2O7 increases, the dielectric breakdown strength is greatly strengthened because of expanded band gap and shrunk grain size, and the relaxor behavior is enhanced due to the effect of linear character pyrochlore. As a result, the optimized composition demonstrates excellent energy storage properties including high recoverable energy storage density (Wrec) of 6.387 J/cm3 at 402 kV/cm, superior stability of temperature (<6% variation in − 120 ∼ 120 °C), frequency (∼4% variation in 5 ∼ 500 Hz) and cycling (∼2% variation within 106 cycles). Besides, fast discharge behavior of t0.9 ∼ 29.4 ns and high power density of 80.49 MW/cm3 at 200 kV/cm are also achieved. This study presents a novel strategy to enhance the performance of BNT-based materials through forming composite ceramic, which is anticipated to be a general method for other material systems as regards the design of advanced energy storage ceramics.

Large electrostrain and strong photoluminescence in rare-earth-modified (Na0.5Bi0.5)TiO3-based lead-free piezoelectric ceramics

Rare-earth (RE)-doped ferroelectric materials have received much attention due to their good performance with combined excellent luminescence and electrical properties. In this work, we report multifunctional features with large electrostrain and strong photoluminescence in (Na0.5Bi0.5)TiO3-based lead-free piezoelectric ceramics. By introducing RE3+ (RE = Sm, Pr, Eu) ions as activators into the host material with the composition of (Bi0.5Na0.5)0.94Ba0.06Ti0.98(Fe0.5Nb0.5)0.02O3 (BNBT-FN), the ceramics gradually adopts a higher structural symmetry, leading to the destruction of the long-range ferroelectric order, which is conducive to the electrostrain response. When the RE3+ content is 0.6 mol.%, the ceramic samples exhibited large unipolar strains of 0.34–0.39% and high electrostrictive coefficients Q33 of 0.021–0.029 m4/C2 under an electric field of 80 kV/cm. Furthermore, due to the luminescence activity of the RE3+ ions, the samples exhibited strong red or reddish-orange emission and simultaneously showed good temperature sensing behavior. These results suggest that the RE3+-doped BNT-based materials have great application potential in multifunctional devices.

A Brief Review of Sodium Bismuth Titanate-Based Lead-Free Materials for Energy Storage: Solid Solution Modification, Metal/metallic Oxide Doping, Defect Engineering and Process Optimizing

With the ever-increasing demand for energy, research on energy storage materials is imperative. Thereinto, dielectric materials are regarded as one of the potential candidates for application in advanced pulsed capacitors by reason of their ultrahigh energy-storage density, low energy loss, and good thermal stability. Among the numerous dielectric materials for energy storage, sodium bismuth titanate (Bi0.5Na0.5TiO3, BNT) with high saturation polarization, as one of the successful alternatives to lead-based materials, has been extensively studied. However, degraded dielectric and ferroelectric properties as a consequence of chemical alterations usually produced by inhomogeneity in microstructure and composition due to the ion volatilization during preparing, thus affecting performance of devices. Hence, this review served to encompass the current state and progress on the optimization of energy storage performance in lead-free BNT-based materials over the past few years, including ceramics, multilayer ceramics, thin films, and thick films, involved in solid solution modification, metal/metallic oxide doping, process optimization and other related aspects to optimize energy storage performance. Furthermore, some prospective approach in the improvement of energy storage performance for BNT-based materials were also provided in this work according to the existing theoretical and experimental results, to impel their practical application.

Achieving giant electrostrain of above 1% in (Bi,Na)TiO3-based lead-free piezoelectrics via introducing oxygen-defect composition.

Piezoelectric ceramics have been extensively used in actuators, where the magnitude of electrostrain is key indicator for large-stroke actuation applications. Here, we propose an innovative strategy based on defect chemistry to form a defect-engineered morphotropic phase boundary and achieve a giant strain of 1.12% in lead-free Bi0.5Na0.5TiO3 (BNT)-based ceramics. The incorporation of the hypothetical perovskite BaAlO2.5 with nominal oxygen defect into BNT will form strongly polarized directional defect dipoles, leading to a strong pinning effect after aging. The large asymmetrical strain is mainly attributed to two factors: The defect dipoles along crystallographic [001] direction destroy the long-range ordering of the ferroelectric and activate a reversible phase transition while promoting polarization rotation when the dipoles are aligned along the applied electric field. Our results not only demonstrate the potential application of BNT-based materials in low-frequency, large-stroke actuators but also provide a general methodology to achieve large strain.

Unraveling the discrepancy of temperature- and composition-dependent strain regulation in Bi0.5Na0.5TiO3 ceramics

For Bi0.5Na0.5TiO3 (BNT)-based materials, regulating temperature and composition could both induce giant electro-strain under the critical condition. Nevertheless, only the temperature-dependent regulation method achieved low hysteresis and maintained a high strain under high ergodic condition simultaneously. Herein, we investigated the origin of this discrepancy by means of matrix with close strain level. These two regulation methods exhibited different regulation mechanisms, especially for the microscopic structure (i.e., the discrepant lattice structure and polar entities). The A-site and BO6 octahedral-dependent vibration modes exhibited obvious discrepancies under the highly ergodic condition, while the shift was relatively small around relaxor/ferroelectric crossover. Additionally, polar entities also exhibited discrepant morphology (e.g., composition-regulated one exhibited striped domains, and temperature-regulated one possessed fuzzy signals with partial nanosized domains under the critical condition) and kinetic behaviors (e.g., under highly ergodic condition, temperature-regulated polar entities rebounded slowly at the initial unloading stage). In a word, relatively small structural discrepancies leaded to similar strain performance under the critical condition, while the increasing ergodicity accompanied by increasing structural discrepancies, which finally induced different strain performance under the high ergodic condition. This insight for designing the BNT-based materials with giant electro-strain and low hysteresis was useful to accelerate the industrialization of eco-friendly actuators.

New strategy for simultaneously achieving enhanced piezoelectricity and deferred Td in Bi0.5Na0.5TiO3-based relaxor ferroelectrics

The inverse relationship between the piezoelectricity and depolarization temperature Td impedes the development of Bi0.5Na0.5TiO3 (BNT)-based ceramics. To realize the goal of enhancing the piezoelectricity together with a deferred Td, the intrinsic formation mechanism of Td should be well understood. In this work, considering the role A-site cations play in manipulating the relaxor behavior of BNT, Pb2+, Ba2+, Sr2+, and Ca2+ (with distinguished ferroelectricity and polarity) are selected to investigate the formation mechanism of Td. Td reflects the stability of polarizations, which could be manipulated through modifying the polarization field and local electric and strain fields. The introduction of Pb2+ and Ba2+ increases the long-range correlated ferroelectric P4mm phase, which strengthens the polarization field and stabilizes polarizations, while the introduction of Sr2+ and Ca2+ increases the short-range correlated ferroelectric P4bm phase and the non-ferroelectric phase, which weakens the polarization field and destabilizes polarizations. Domain structures captured by a piezoresponse force microscope corroborate the effect of Pb2+ and Ba2+ in stabilizing polarizations and Sr2+ and Ca2+ in destabilizing polarizations. Therefore, by introducing the ferroelectric component that exhibits a different local symmetry to the BNT-matrix and can also provide a strong polarization field, the simultaneously enhanced piezoelectricity together with a deferred Td could be realized, as validated in the designed BNT-xPbTiO3 system. This work investigates the formation mechanism of Td and guides the design of high-performance systems in BNT-based materials, benefiting the understanding of BNT-based relaxor ferroelectrics.

Achieving ultrahigh energy storage performance over a broad temperature range in (Bi0.5Na0.5)TiO3-based eco-friendly relaxor ferroelectric ceramics via multiple engineering processes

The development of ABO3 perovskite-structured dielectric materials with high recoverable energy storage density (Wrec) and power density (PD) is crucial for the downsizing of pulsed power devices. Despite several research efforts, achieving a high Wrec over a wide working temperature range in an environmentally benign system remains a difficulty. A synergistic design strategy is given here, which includes concurrently doping at the A- and B-site to achieve a spread and depressed dielectric response, adding sintering aids, and employing advanced viscous polymer rolling technology for dense and ultra-thin ceramic samples, respectively. Finally, at a relatively low electric field of 380 kV/cm, an ultrahigh Wrec of 6.57 J/cm3 is realized in (Bi0.5Na0.5)0.93Ca0.07Ti0.85Zr0.15O3-0.5 wt% Li2CO3 component, which benefits from gentle polarization saturating and improved breakdown strength. The Wrec can be maintained above 6 J/cm3 while maintaining strong thermal stability (variation ≤ ± 3%) over a temperature range of 30–150 °C. Because BNT-based materials have such high energy storage performance and temperature stability, they are not only a promising candidate for replacing lead-based dielectrics, but also a valuable guide for developing new high-performance ferroelectric materials for future energy storage devices in the pulsed power system.

Structural origin for the high piezoelectric performance of (Na0.5Bi0.5)TiO3-BaTiO3-BiAlO3 lead-free ceramics

Achieving high piezoelectric performance in the representative lead-free materials of Bi1/2Na1/2TiO3-BaTiO3 (BNT-BT) is of great interest. However, it remains a challenge in these materials, as they feature structural phases with strong octahedral tilting at morphotropic phase boundary (MPB). Herein, we obtain good piezoelectric performance with relatively large d33 value (219 pC/N), small coercive field (1.5 kV/mm), and good thermal stability in the (1-x)(Na0.5Bi0.5)TiO3-x(0.9BaTiO3-0.1BiAlO3) ternary system by crystal structure design. It is found that the incorporation of BiAlO3 with large tolerance factor can suppress the octahedral tilting and reduce the structural distortion of R3c and P4bm at MPB, and thus allows the electric field induced an extensive and reversible phase transformation. By using in-situ electric field synchrotron X-ray diffraction, it is revealed that the crystal lattice of the R3c phase with a smaller structural distortion responds more flexible to the electric field than that of the P4bm phase with a larger structural distortion. The piezoelectric strain analysis through profiles fitting indicates that the enhanced piezoelectric response attributed to both the activated domain switching and enhanced lattice strain benefited from the subtle phase structure. These findings provide further insights into the dynamic structural response of BNT-based materials, and will be helpful in designing high-performance piezoelectric materials beyond the MPB.

Enhanced room temperature energy storage density of Bi(Li1/3Ti2/3)O3 substituted Bi0.5Na0.5TiO3–BaTiO3 ceramics

For high power electronics applications, relaxor ferroelectrics are promising materials due to their superior energy storage properties. In this study, we investigate the energy storage properties of novel lead free relaxor ferroelectric ceramics (1−x)(0.92Bi0.5Na0.5TiO3–0.08BaTiO3)–xBi(Li1/3Ti2/3)O3 (abbreviated as BNT–8BT–xBLT). BNT–8BT composition which is close to morphotropic phase boundary was chosen as the base due to its large maximum polarization (P m) and higher ratio of weakly polar tetragonal phase which is expected to facilitate ergodic relaxor behavior and improve energy storage density. The substitution of BLT to the BNT–8BT strongly disrupts the correlations between the polar nanoregions and the transition from nonergodic to ergodic relaxor state occurs already at x = 0.02 BLT at room temperature. Largest energy density (W rec) at 61 kV cm−1 was obtained for x = 0.02 sample (0.656 J cm−3), followed by x = 0.03 (W rec = 0.614 J cm−3) and x = 0.05 (W rec= 0.559 J cm−3). The x = 0.02 sample keeps its energy storage density at high temperatures (i.e. W rec= 0.88 J cm−3, η = 97%, E m= 65 kV cm−1 at 125 °C), while larger electric field (up to 89 kV cm−1) could be applied to the x = 0.05 sample with the smallest grain size and energy density of 1.03 J cm−3 was reached at room temperature. Energy storage density values of BLT substituted materials normalized per unit applied electric field are promising among BNT-based materials.

Energy storage performance of Bi0.5Na0.5TiO3-based relaxor ferroelectric ceramics with superior temperature stability under low electric fields

Dielectric ceramics are highly favorable for pulsed power applications owing to their ultra-fast charge-discharge speed and excellent reliability. However, their low energy density under low electric fields remains as a bottleneck for them to be employed in integrated electronic devices with shrinking dimensions. In this work, we have designed a comprehensive strategy to synthesize lead-free (Bi1/2Na1/2)1−xSrxTi0.98(Fe1/2Nb1/2)0.02O3 (BNT-xST-2FN, x = 0.30, 0.35, 0.40 and 0.45) ceramics via traditional solid-state method. On one hand, the hybridizations of Bi3+ orbitals and the displacement of B-site cations under the applied electric field lead to a high polarization. On the other hand, the (Fe1/2Nb1/2)4+ complex-ion and SrTiO3 were introduced to improve the relaxor behavior of the system, resulting in a high energy-storage efficiency. In this way, an excellent energy density of 3.36 J/cm3 and a high energy efficiency of 81% are simultaneously achieved in the BNT-0.40ST-0.02FN composition under a low electric field of 170 kV/cm, which is superior to other BNT-based materials under similar electric fields. Moreover, the ceramic exhibits a superior thermal stability (25–200 °C). This work provides a promising approach for designing high-performance lead-free energy storage ceramics under low electric fields.

BaTiO3 nanowires-induced phase transition and thermally stable strain in (Bi0.5Na0.5)TiO3 piezoelectric ceramics

Environment-friendly lead-free piezoceramics with high strain response and extremely excellent stability in a wide operating temperature range are critically important in practical actuator applications. Here, we develop a new strategy to tune the electrostrictive strain behavior in Bi 0.5 Na 0.5 TiO 3 (BNT)-based ceramics via using high aspect ratio BaTiO 3 nanowires (BT NWs) as a modifier. The addition of BT NWs generates a crossover from a typical ferroelectric (BT conventional spherical particles) to a complete ergodic relaxor (ER) phase at ambient temperature, accompanied by a large electrostrictive strain of ∼0.17% with d 33 *( S max / E max ) = 284 pm/V. Such a high electrostrictive strain is extremely thermally stable with only <7% fluctuation from 27 °C to 120 °C. In addition, the BT NWs-modified ceramics also exhibit acceptable fatigue endurance (<30% up to 10 5 cycles) and frequency dependence (<20% at 10Hz–100Hz). These achieved exceptional performances can be ascribed to the BT NWs-driven complete ER phase at room temperature. The findings of this study can inspire enhanced interest in nanowires as a viable modifier to BNT-based materials due to promising potential for practical actuator applications in a wide temperature range.

Structure evolution and ferroelectric properties in stoichiometric Bi0.5+xNa0.5−xTi1−0.5xO3

The influence of B-site deficiency on the stability of electrically induced long-range ferroelectric order of the stoichiometric Bi0.5+xNa0.5−xTi1−0.5x□0.5xO3 (BNT-xVTi) (“□” denotes vacancies) ceramics is studied. The depolarization and ferroelectric to relaxor transition are identified as separate and discrete processes in BNT-based materials. For BNT-0.02VTi, the resonance and anti-resonance peaks on dielectric permittivity-frequency curves indicate dominating ferroelectric phase at room temperature. The depolarization temperature, determined by thermally stimulated depolarization current, is ~ 65 °C. However, the ferroelectric to relaxor transition temperature is absent, as no distinct frequency-independent anomalies for the dielectric permittivity exist. This depolarization process can be ascribed to nanoscale ferroelectric domain at room temperature for BNT-0.02VTi, which is induced by chemical disorder and strong random field as VTi generated. Hence, the results imply that the B-site deficiency in BNT is a very effective route to tailor the stability of electrically induced long-range ferroelectric order.

Influence of structural evolution on electrocaloric effect in Bi0.5Na0.5TiO3-SrTiO3 ferroelectric ceramics

The electrocaloric effect (ECE) in lead-free relaxor ferroelectrics has gained significant interest over the past decades. However, certain aspects of the ECE in relaxor ferroelectrics, such as Bi0.5Na0.5TiO3, remain poorly understood. In this work, we investigate the ECE by considering Bi0.5Na0.5TiO3-SrTiO3 (BNT-ST) as an example. The results show that, for BNT-0.25ST ceramics, the directly-measured ECE is optimal when the freezing temperature is tailored to be about room temperature. For this material, ΔT = 0.51 K under an electric field of 6 kV/mm and the ECE has excellent thermal stability (the instability η ≤ 20% in the range 30–120 °C). The addition of strontium increases the cubic-phase fraction, enhances the local random field, and changes the local structure, as clarified by in situ Raman spectroscopy and piezoelectric force morphology. In addition, we discuss in detail the correlation between the ECE and local structure. This work thus improves our understanding of the ECE in BNT-based materials for EC cooling technologies.

Tristate ferroelectric memory and strain memory in Bi1/2Na1/2TiO3-based relaxor ferroelectrics

Tristate ferroelectric memory and strain memory of Bi1/2Na1/2TiO3 (BNT)-based relaxor ferroelectrics are proposed. These memory effects can be realized in BNT-based materials with double-like P-E hysteresis loops and obvious non-zero remanent polarization. An underlying triple-well free-energy landscape, in which the relaxor state serves as an intermediate stable state between two ferroelectric remanent states, is thought to be responsible for such a ferroelectric behavior and provides the basis for the tristate ferroelectric memory effect. Besides, the strain memory effect utilizes the inherent strain difference between relaxor and ferroelectric states. Experimental verifications on Bi1/2(Na0.8K0.2)1/2(Ti0.955Fe0.030Nb0.015)O3 ceramics show that the tristate ferroelectric memory and the strain memory can be operated as proposed, and the programmability and the retention ability of both effects are fairly good. The present study provides a facile approach to the multistate ferroelectric memory and shape memory piezoelectric actuator applications.

Thermal depolarization regulation by oxides selection in lead-free BNT/oxides piezoelectric composites

For the bismuth sodium titanate (BNT)-based materials, the thermal depolarization temperature (Td) is always an obstacle for practical applications. Recently, BNT/ZnO composite has provided one method to resist the thermal depolarization, and however, Td values just increase to a small extent and the conclusions derive from ZnO merely. A universal selection principle for the oxides will be helpful for us to choose the suitable oxides to effectively resist the thermal depolarization, which is desperately demanded but still lacks in BNT/oxide composites. Here, we report that the deferred thermal depolarization can be also obtained in piezoelectric Bi0.5(Na0.8K0.2)0.5TiO3: Al2O3(BNKT:Al2O3) composites. Td is deferred to the higher temperatures (from 116 °C to 227 °C) with increasing Al2O3 contents, as evidenced by the temperature dependence of dielectric, ferroelectric and piezoelectric properties. In addition, the piezoelectricity of BNKT:0.15Al2O3 remains stable at a high temperature (∼210 °C). And, the thermal deviatoric stress from the coefficients of thermal expansion (CTE) discrepancies between Al2O3 and BNKT matrix provides a stronger stabilization force than the ions diffusion-induced destabilization force, resulting in the ultimate deferred thermal depolarization and the significantly increased Td values. In particular, according to the results from the representative BNT/oxides (i.e., ZnO, Al2O3, ZrO2, HfO2) composite, the oxide selection principle (regulating several competing factors) is given to form the appropriate thermal resistant BNT/oxide composite, which may further open the door for piezoelectric BNT-based materials from the research and application scope.