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Electric field-induced high strain with small hysteresis and good temperature stability is necessary for piezoactuators devices. Most of previous works focus on the giant strain achieved in Bi0.5Na0.5TiO3 (BNT)-based ceramics through electric field-induced transition from nonergodic relaxor to ferroelectric state, while the high remnant strain or/and large hysteresis are always accompanying. In this work, through enhancing the local polarization by rare earth ions doping (e.g., La3+), we propose to improve the strain properties of BNT-based lead-free ceramics with ergodic relaxor state which displays the near zero remnant strain, small hysteresis, high temperature stability but the low strain value. The addition of La3+ would not change the ergodic relaxor state, but can improve the short-range correlation of local polar nanoregions, cut down the critical electric field of transition between relaxor and ferroelectric states, and elevate the electrostrictive effect of ceramics. Finally, high strain of ∼0.3 % with low hysteresis of ∼8−30 % was obtained in the broad temperature range from room temperature to 100 °C, which is superior to previous BNT-based ceramics and other lead-based/lead-free ceramics. This work affords a paradigm to regulate the ergodic relaxor state to optimize the strain properties, which give the significant guide for strain developments.

In this work, for CuO-modified perovskite KNbO3 (KN-xCuO), by selecting the appropriate composition and external field conditions (including direct/alternating E-field, time, temperature and bipolar cycles), we report the superior piezoelectric responses (incipient large-field piezoelectric responses d33* = 1994 pm/V, Smax>0.3 %). This work indicates that the unusual ferroelectric and electro-strain behaviors may be attributed to the discrepancies of characteristic frequency for defect dipoles transformation (CuNb-Vab1 and CuNb-Vab2) and ferroelectric domains switching. Different external fields affect the transformation process of defect dipoles, leading to the different constructing and restoring behaviors during the E-field loading and unloading process. After selecting the appropriate external field conditions, the large and stable electro-strain behaviors (maximum electro-strain Smax>0.3 % exhibits 2.6 % decline after 105 cycles at room temperature) are observed for the d.c.-poled and one-year-aged x = 0.015 samples. This research may provide a paradigm for designing the high-performance KNbO3-based perovskite ferroelectrics by exploring the greatest potential of dynamic defect dipoles.

Lead‐free ferroelectric electrocaloric ceramics that could convert electrical energy into heat are the promising candidate for environment‐friendly cooling devices. For refrigeration devices, a large temperature change (ΔT) and good temperature stability are required, which are highly related to the phase structure and the applied electric field. In this work, a diffused ferroelectric–paraelectric (FP) phase transition is formed in (K, Na)NbO3 (KNN) by using appropriate composition engineering. The relaxor ferroelectrics in this work present both a large ΔT of 1.24 K and a high ΔT/ΔE of 0.19 K mm/kV. In addition, a wide temperature span exceeds 55 °C at the high electrocaloric effect (ECE) criterion (ΔT ≥ 0.5 K) could also be observed. This work not only opens a new strategy for obtaining high‐performance ceramics for refrigeration devices but also extends the application area of the KNN‐based lead‐free ferroelectrics from sensors, actuators and energy harvesting to solid‐state cooling applications.

The piezoelectric constant (d33) and converse piezoelectric coefficient (d33*) of a piezoelectric material are critically important parameters for sensors and actuators. Here, we simultaneously achieve a high d33 of 595 ± 10 pC/N and a large d33* of ∼776 ± 20 pm/V in (K,Na)NbO3 (KNN)-based ceramics, which exceed those of PZT5H and PZT4 ceramics, presenting good potential for practical piezoelectric applications. Moreover, the ceramic exhibits a relaxor-like and diffuse dielectric behavior due to the occurrence of local heterogeneity. According to the experiments and atomic resolution polarization mapping by Z-contrast imaging, hierarchical architecture of nanodomains and even smaller polar nanoregions with multiphase coexistence caused by compositional modification is the structural origin of the enhanced piezoelectric properties in this work. This work would pave a practical way to future applications of lead-free KNN-based ceramics.

The development of an efficient electrocaloric (EC) material for refrigeration is of extreme importance for microelectronic and integrated electronic areas. No-lead-footprint 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 (BNT–BT) accompanied with a depolarization temperature (Td) and impressive ferroelectric property emerges as a potential material for EC cooling. However, its long-term room-temperature EC behavior has been restricted by its relatively low breakdown electric field (Eb) and high Td. This study reports that simultaneous Zr addition and AlN doping in BNT–BT (BNT–BZxT: yAlN) leads to a significant decrease of Td and enhancement of Eb. Satisfying EC performances (temperature change ΔT of 3.2 K and EC strength ΔT/ΔE of 0.32 K mm kV−1) close to the room temperature are realized with the composition of x = 0.2 and y = 0.2 wt% (BNT–BZ0.2T: 0.2 wt%). Such excellent EC behaviors surpass most reported no-lead ceramics, which are even superior to 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 ceramics. Most important of all, the present study offers an avenue for developing no-lead ceramics with an excellent room temperature ΔT towards solid-state cooling.

Electrocaloric refrigeration emerges as a newly developing technology with potential to be the next generation of coolers. However, the combination of large adiabatic temperature change (ΔT) and good temperature stability remains a long-term issue in lead-free ceramics for developing practical electrocaloric refrigeration devices. Herein, no-lead-footprint (0.9 − x)NaNbO3–0.1BaTiO3–xBaZrO3 (abbreviated here as NN–BT–xBZ) ceramics are optimized to select a special crossover state between ferroelectric and relaxor states. NN–BT–0.04BZ ceramic located at the crossover region benefits from multiple aspects involving large polarization, low-temperature ferro–paraelectric transition as well as the relaxor feature. Thus, a desired ΔT of 1.14 K and superior temperature stability (52 K, within ±5% variation in maximal ΔT) were achieved in the vicinity of room temperature. Such an excellent ΔT is almost two times larger than those of other niobate-based ceramics. Our work not only provides a promising electrocaloric material but expands NN-based materials to the electrocaloric refrigeration area, and offers a feasible design strategy for searching practical electrocaloric coolers in other systems.

Relaxor behavior is proved to be responsible for high piezoelectricity in piezoelectric materials due to the promoted polarization rotation, and an ultrahigh piezoelectricity can be also induced in potassium sodium niobate [(K,Na)NbO3, KNN]-based ceramics by dielectric relaxation at the multiphase coexistence region. However, it is still absent of the association study between domain evolution and relaxor behavior created by nanoscale multiphase coexistence. Herein, the frequency-dependent domain response, dwell-time dependence of domain radius and voltage-dependent domain switching are characterized in KNN-based ceramics with relaxor R–T phase boundary. These novel domain evolutions further illustrate the contribution of relaxor behavior on high piezoelectricity, yielding to easy polarization rotation based on low energy barrier of polar nanoregions merging into long-range ordering states. And an improved temperature stability is observed at 30−60 °C for relaxor R–T due to the unusual domain evolution. This study affords a new perspective in the mechanism of high-performance lead-free piezoelectrics.

Electrocaloric effect represents an environmentally friendly cooling alternative to replace the currently ubiquitous vapor-compression-based refrigeration. Previous studies have shown that large electrocaloric response is associated with the polarization change, which peaks around the phase transition temperature. However, there is only little focus on the effect of the detailed phase structure on the electrocaloric properties, particularly on the electrocaloric strength (ΔT/ΔE). For the first time, the correlation between ΔT/ΔE and the phase structure of perovskite ferroelectric materials was investigated, and a new frame to obtain optimized ΔT/ΔE related to the phase structure regulation was revealed. Given the simple and legible phase structure, BaTiO3-based ceramics are selected as a platform to reveal this relationship by employing both indirect and direct measurements. A striking variation of ΔT/ΔE and entropy change coefficient versus the electric field was first noticed at different phase structures: decreasing at the phase boundaries, whereas increasing in the pure phase regions, which could attribute to distinct entropy change mechanisms stemming from the discrepant energy profile at different phase structures. Beyond that, the maximum electrocaloric temperature change of ∼1 K and a large ΔT/ΔE of 0.48–0.25 × 10−6 K m V−1 were achieved under 5–40 kV cm−1. Also, a broad temperature span of 58 K with high ΔT/ΔE was maintained under 40 kV cm−1, and a desired electric field-stable ΔT/ΔE could be acquired. Our research provides a new paradigm for further designing and optimizing the electrocaloric performance of lead-free perovskite ferroelectrics.

(Bi1/2Na1/2)TiO3-based materials have received much attention due to large electro-strain and high piezoelectric constant (d33), but the tough issue is that the existence of inherent depolarization temperature (Td) limits the temperature stability and application temperature range. Previously, reports about the formation of BNT/oxide (i.e., ZnO, Al2O3) composites thought that Td can be deferred to a higher temperature and then thermal depolarization improves. However, the deferred Td of BNT/oxide composites is limited, accompanied by a low d33. Here, we design the {[Bi0.5(Na0.8K0.2)0.5]1-xPbx}TiO3 ceramics, leading to a big shift of Td from 77 ℃ to 390 ℃. Large d33 (140 pC/N) and high Td (∼263 ℃) can be simultaneously achieved for the sample with Pb=0.05, and Td could be further deferred higher (390 ℃) for Pb=0.20. The off-centre displacement of Pb induced by Pb-O hybridization in the PbO12 polyhedron and ferroelectric order stabilized by the addition of Pb can provide the driving force to strengthen the ferroelectric order, and then promote the thermal stability.

Polarization fatigue of bismuth ferrite-based ceramics was studied and opposite physical behaviors (polarization enhancement and degradation) during field-cycling process can be respectively observed by composition modification. Especially, partially recoverable behavior caused by unstable polarization attributed from defect dynamics can be evidently observed by designing discontinuous field-cycling. Different from well-known domain wall pinning effect and microstructure damage, a qualitative model including field assisted depinning and domain wall pinning was proposed to interpret the origin of the unusual characteristics in terms of defect dynamics during field-cycling process, which involves the migration of charged defects and polarization rotation under field-cycling.

Dr. Lv and Prof. Zhang thank the support from King Abdullah University of Science and Technology (KAUST). Prof. Wu thanks the support from the National Natural Science Foundation of China (Grant no. 51722208) and the Key Technologies Research and Development Program of Sichuan Province (Grant no. 2018JY0007).

The BiAlO3 (BA) and (Bi0.5Na0.5)ZrO3 (BNZ) are selected to form a solid solution with (K0.48Na0.52)NbO3 via traditional solid state technique to optimize the electrical performance and temperature stability of KNN-based lead-free ceramics, simultaneously. Here we show that doped BA has a great influence on phase structure, morphologies, and electrical properties. The XRD patterns and dielectric constant versus temperature curves reveal that an increase in the BA content results in a transform of phase structures from a coexistence state of rhombohedral, orthorhombic and tetragonal phases to pseudocubic phase. Owing to the construction of R-O-T phase boundary, optimized performances (TC ∼ 336 °C, d33 ∼ 306 pC/N, kp = 0.48) are obtained in 0.962(K0.48Na0.52)NbO3-0.003BiAlO3-0.035(Bi0.5Na0.5)ZrO3 (KNN-3) ceramics. Based on the sintered KNN-3 ceramic samples, high-frequency ultrasound imaging transducers are designed and fabricated, which exhibits a high center frequency of 24.5 MHz, a broad −6 dB bandwidth of 97% and a high-sensitivity. Finally, the imaging characteristic of the lead-free transducers is demonstrated via ex vivo imaging of biological tissue structure. As environment friendly materials, the excellent electrical and acoustic performance of developed KNN-based ceramics has great potential for practical applications.

The electrocaloric effect (ECE) is significantly critical for environmentally friendly cooling technologies to replace vapor compression-based refrigeration. Lead-free bismuth sodium titanate-based...

This work reveals the role of Bi3+ in the controlling ability of (Bi0.5Ag0.5)ZrO3 to the phase structure of potassium sodium niobate based ceramics by introducing Ba2+. The substitution on (Bi0.5Ag0.5)2+ with Ba2+ results in the reduced degree of phase coexistence, accompanying the phase transition from coexisting orthorhombic-tetragonal (O–T) phases to an orthorhombic (O) phase, which is demonstrated by the systematic phase structure analysis and variations of electrical properties. The analysis reveals that the high electronegativity of Bi3+ results in the increased covalent bond character and then increases the stabilization of the T phase, leading to a decrease in orthorhombic-tetragonal phase transition temperature (T O-T). Therefore, this work not only compares the controlling ability of Ba2+ and (Bi0.5Ag0.5)2+ to T O-T but also reveals which and how elements cause the decreased T O-T, promoting the understanding of the effect of additives on the phase structure.

The development of lead-free ceramics with high recoverable energy density (Wrec) and high energy storage efficiency (η) is of great significance to the current energy situation. In this work, a new scheme was proposed to improve the Wrec and η of potassium sodium niobate ((K, Na)NbO3, abbreviated as KNN) lead-free ceramics. Doping Bi elements in KNN ceramics to form a second phase (K2BiNb5O15) can reduce the grain size and form Schottky-like contact. Meanwhile, the hybridization between the Bi 6p and O 2p orbitals can enhance the maximum polarization (Pmax). So the K1-3xBixNa0.5NbO3-1 mol%CuO ceramics was proposed to improve the Wrec of lead-free ceramics. A large Wrec (2.89 J/cm3) and dielectric breakdown strength (DBS) (300 kV/cm with a thickness of 0.2 mm) were achieved for K0.14Bi0.12Na0.5NbO3-1 mol%CuO ceramics. The high Wrec was supposed to benefit from low remnant polarization (Pr), high Ps and DBS of ceramics. In addition, a large η (80%) was simultaneously achieved for K1-3xBixNa0.5NbO3-1 mol%CuO ceramics, which is superior to mostly reported lead-free bulk ceramics. The results show that K1-3xBixNa0.5NbO3-1 mol%CuO ceramics have a good application prospect in the field of energy storage, and provide a new scheme for the preparation of lead-free ceramics with high energy storage density and high conversion efficiency.

0.96(K0.48Na0.52)(Nb1−xTax)O3–0.04(Bi0.5Ag0.5)ZrO3 lead-free ceramics were fabricated by solid-state reaction method. Because of the sensitive energy state in the coexisting phase structure, the addition of Ta5+ caused a palpable phase transformation from orthorhombic to tetragonal. At the same time, doping with (Bi0.5Ag0.5)ZrO3 could have effectively promoted densification and continued to enhance the piezoelectric and dielectric properties. Enhanced dielectric/ferroelectric properties were achieved for the ceramics which has a composition of (x = 0.15), and the ceramics expressed a high d33 of ∼ 373 pC/N with a TC of ∼ 255 °C. We believe that to design a binary phase system is a valid method to attain both large d33 and high TC in (K,Na)NbO3 (KNN)-based lead-free ceramics.

Lead-free relaxor ferroelectrics (RFEs) exhibit a broader variety of phenomena in comparison with the “canonical” lead-containing compositions, rendering them attractive for newly multifunctional m...

Potassium-sodium niobate (K1-xNaxNbO3, referred to as KNN) solid solutions, which are an important type of lead-free piezoelectric materials possessing environmentally friendly features, good piezoelectric response and high Curie temperature, have attracted considerable attention in replacing lead-based ceramics. In order to promote the application of KNN-based ceramics in piezoelectric devices, we characterized a complete set of material constants of a high performance KNN-based ceramic, that is 0.965(K0.48Na0.52) (Nb0.96Sb0.04)O3-0.035Bi0.5Na0.5Zr0.15Hf0.75O3 (KNNS-BNZH), whose Curie temperature is 235 °C, piezoelectric coefficient d33 is 380 pC/N and electromechanical coupling factor k33 is 70%. These results will benefit the design of piezoelectric transducers and actuators using lead-free piezoelectric ceramics.

Different lanthanides (La, Ce, Pr, Nd, Sm, Ho, Er and Yb) doped potassium sodium niobate [(K,Na)NbO3, KNN] based ceramics are fabricated through solid-state method to study the influences and connections of lanthanides on phase structures, microstructures, electrical properties of ceramics. The coexistence of rhombohedral and tetragonal phases is possessed in the ceramics. The fractions of tetragonal phase increase firstly and decrease with decreasing the dopant ionic radius. Higher tetragonal phase fraction and large tetragonality can be beneficial to properties. The Nd3+ doped ceramics present good combination properties (TC = 223 °C, d33 = 400 pC/N, d 33 * = 530 pm/V, kp = 0.46).

Since the positive influences of defects on the performance of electroceramics were discovered, investigations concerning on defects and aliovalent doping routes have grown rapidly in the fields of inorganic chemistry and condensed matter physics. In this article, we summarized the types of defects in electroceramics as well as characterization tools of defects and highlighted the effects of intrinsic and extrinsic defects on the material performances with the emphasis on dielectric, ferroelectric, and piezoelectric properties. We mainly introduced defect related theoretical simulation and experimental results in several typical incipient ferroelectrics, ferroelectrics, and antiferroelectrics. Hence, the influences of defects on the crystal lattice were summed up, and then the main physical mechanisms were highlighted. Particularly, the performance enhancements of aliovalently doped electroceramics were also evaluated and reviewed. Finally, the outlook and challenges were discussed on the basis of their current developments. This article covers not only an overview of the state-of-the-art advances of defects and aliovalent doping routes in electroceramics but also the future prospects that may open another window to tune the electrical performance of electroceramics via intentionally introducing certain defects.

Reducing or even prohibiting the use of toxic lead in electronic devices has become one of the most cutting-edge topics in various disciplines. The recently proposed phase boundary engineering endows lead-free piezoceramics with comparable performances to that of some lead-based piezoceramics. However, the enhancement in performance hinges on the coexistence of multi-phases and complex domain structures, particularly the occurrence of nano-domains and polar nanoregions (PNRs). Although nano-domains have been significantly studied in lead-based piezoceramics, understanding the nano-domains and PNRs in lead-free piezoceramics is in its infancy and needs a systematic summary and in-depth analysis. Herein, we summarize the nano-domains and PNRs in three representative lead-free piezoceramics (i.e., potassium sodium niobate, barium titanate, and sodium bismuth titanate), focusing on their effects on macro performance. First, we introduce the foundation and tools for the observation of the domains. Then, we summarize the variations in the nano-domains with phase structure, electric field, and temperature, and their effects on performance including piezoelectricity, strain, temperature stability, aging, and fatigue. Finally, we present our perspectives on the future of nano-domains, concentrating on nano-domain engineering. Therefore, this review can help better understand the nano-domains and PNRs in lead-free piezoceramics, and be used for the further development of high-performance lead-free piezoceramics.

A combination of large adiabatic temperature change (ΔT) and wide temperature span (Tspan) in lead-free ceramics is highly desired to develop eco-friendly and highly efficient solid-state refrigeration based on electrocaloric effects. However, the lack of success in synergistically optimizing both large ΔT and wide Tspan in one material impedes the practical applications of electrocaloric refrigeration technology. Here, a relaxor multiphase boundary (RMPB) is structured in (1 − x)(K0.49Na0.49Li0.02)(Nb0.8Ta0.2)O3–xCaZrO3 (KNLNT–xCZ) ceramics through intentional composition engineering, successfully achieving a region of multiphase coexistence accompanied by diffuse phase transition. A moderate ΔT (0.63 K under 80 kV cm−1) with a wide Tspan (43 K within 90% ΔT attenuation) is obtained in a relaxor KNLNT–5CZ ceramic with the coexistence of an R–O–T phase. The results indicate that a coupling effect originally from multi-directional polarization and polar nano-regions (PNRs) in RMPB contributes to the excellent performance combining large ΔT and wide Tspan. This work not only expands the applications of KNN-based ceramics from electric energy storage and piezoelectric applications to the refrigeration field, but also opens up a new design avenue for the development of high performance electrocaloric materials with broad temperature ranges.

For Bi0.5Na0.5TiO3-based complex oxides, a critical bottleneck is that a higher depolarization temperature Td is often obtained at the cost of sacrificing their piezoelectricity d33 (d33 ∝ 1/Td), which severely restricts the further development of this kind of promising material. Here, by combining compositional materials selection with rapid quenching in liquid nitrogen from ultra-high temperatures, we report an advance in simultaneously achieving a large d33 (232 ± 5 pC N−1) and a high Td (188 °C) in Bi0.5Na0.5TiO3-based complex oxides. In situ characterization indicates that the superior performance (232 pC N−1 < d33 < 281 pC N−1) can be maintained in a wide working temperature range (25 °C < T < 185 °C). A series of quenching treatments and multi-scale structural analyses (ion environment, lattice distortion and domain evolution) reveal that the increased coherence length (the size of unit cells sharing the same ferroelectric symmetry) of the ferroelectric lattice enhances the ferroelectric ordering and leads to the deferred Td. Theoretical simulations and experimental results validate the quenching-induced redistribution of A-site ions. And the resulting enhanced ferroelectric polarization compensates the degenerated dielectric response, accounting for the effectively increased Td without decreasing the superior piezoelectric response. This research provides a paradigm for designing high-performance Bi0.5Na0.5TiO3-based electromechanical materials in a wide working temperature range.

Electrostrictive materials fabricated with relaxor ferroelectrics have attracted much attention because of their excellent features (e.g., high precision displacement). In this study, we realized the dielectric relaxation behavior of bismuth ferrite (BiFeO3, BFO)-based ferroelectrics by designing polar nanostructures. Accompanied by the strengthened random field and the degenerated octahedral distortion, the grain inhomogeneity featuring the coexistence of strong piezo-response polar cluster grains and weak piezo-response polar entity grains contribute to the relaxation behavior. Finally, such a dielectric relaxation behavior results in a large electrostrictive strain (S = 0.18%–0.27% @ T = 20–100 °C) together with the composition/electric field/temperature independence of Q33. We believe that our research provides a new paradigm to study the dielectric relaxation behavior and optimize electrostrictive properties of perovskite materials.

In view of unsolved key scientific problems of (K,Na)NbO3 (KNN)-based ceramics, this work focuses on intrinsic structure elucidation and physical mechanism of temperature stability in both piezoelectricity d33 and strain property d33* through domain strategy and crystal structure engineering. The introduction of BiFeO3 into KNN-BNH can stabilize multiphase coexistence structure and then results in high performance (d33=400–450 pC/N, TC=280–320 °C, and Suni=0.16–0.18%) with composition in-sensitivity (x = 0–0.6%). Different from the viewpoint of electric field induced diffuse phase transition, we first verified that electric field compensation effect can greatly offset the negative effects induced by polycrystalline phase transition and thermal depolarization, leading to much better temperature stability of d33* than that of d33. We believe that this work has clarified the origin of the difference in temperature stability between d33 and d33*, which can provide some useful clues for further improving the stability of KNN-based ceramics, especially d33 temperature stability for sensor and transducer applications.

Excellent piezoelectric properties have been achieved in (K,Na)NbO3 (KNN)-based ceramics in the past few years. However, temperature-sensitive strain restricts the use of these ceramics in practical applications. In this work, we report improved piezoelectricity and temperature-insensitive strain in KNN-based ceramics. A broad temperature range for phase boundaries is formed by composition optimization, accounting for the observation of a highly improved strain stability. Interestingly, the strain for a ceramic with x = 0.025 varies by less than 5% and 9%, even if the temperature reaches 150 °C and 170 °C, respectively, which is comparable to that found for textured-LF4T ceramics. Furthermore, the stability of the original domain configuration with increasing temperature is tested to understand the underlying mechanism for the temperature-insensitive strain. The huge number of original domains retaining a piezoresponse at high temperature due to the extended phase boundary contributes to the temperature-insensitive strain. The present results open a new window for KNN-based ceramics to be used in applications demanding temperature-insensitive piezoelectric properties.

Due to the presence of pores and low density, a high recoverable energy density (Wrec) value is usually obtained at the cost of energy storage efficiency (η) in lead-free potassium sodium niobate [(K, Na)NbO3, KNN] based ceramics, which also affects the hardness of ceramics, finally limiting the further development of practical applications. A high Wrec (∼3.60 J/cm3 ) and a high η (∼74.2%) are obtained in 0.975K0.5Na0.5NbO3-0.025LaBiO3 (0.975KNN-0.025LB) ceramics simultaneously under a high dielectric breakdown strength (DBS) of 340 kV/cm, together with a fast discharge rate (t0.9 ∼ 46 ns) and high power density (PD ∼ 49.4 MW/cm3). Further analysis of the intrinsic electronic structure is carried out via the first-principles calculation based on the density functional theory (DFT). An ultrahigh hardness (H) of 6.63 GPa can be accordingly obtained. This work combines excellent energy storage properties and ultrahigh hardness, which provides significant guidelines for applications in pulsed-power systems.

In order to achieve the long-term objective of high-performance KNN-based ceramics, BiMnO3-modified lead-free KNN-based ceramics were fabricated by conventional solid-state method in this work. The R–O–T multi-phases coexistence and the absence of Sb doping lead to a large piezoelectricity (d33 = 440 pC/N), a high Curie temperature (TC = 310 °C), and a good in situ temperature stability (d33 is more than 300 pC/N below 100 °C). Based on the high performance, the effects of phase structure and microstructure on electrical properties were carried out by different sintering methods. All the results indicate that phase structure plays a major role on the excellent performance. And the sample with bimodal grain size distribution has a better electrical response than the one with uniform coarse grain size distribution, which can be elucidated by the inter-granular coupling effect. For the sample with bimodal grain size distribution, the large grains endure weaker inter-granular constraints from the surrounding refined grains, while much stronger constraints exist in the sample with uniformly coarse grains.

It is difficult to realize the balanced development of piezoelectric response {piezoelectric constant (d33) and strain} and temperature stability of potassium-sodium niobate ceramics. Here, we constructed one kind of lead-free piezoceramic system of 0.965K0.45Na0.55Nb0.96Sb0.04O3-0.035(Bi1-xHox)0.5Na0.5HfO3, and rhombohedral & tetragonal (R-T) multiphase coexistence maintains in a wide compositional range of x = 0.05–0.55. Greatly improved temperature stability of piezoelectric response is realized by doping rare earth Ho. Excellent temperature stability of both strain and d33 mainly originates from the weak temperature dependence of domain structure as well as the increased R-T phase transition temperature. As a result, the addition of rare earth Ho can boost temperature stability of potassium-sodium niobate-based ceramics.

Phase boundaries and composition design were explored to achieve both high piezoelectricity and favorable temperature stability in potassium-sodium niobate ceramics, using (1-x)(K,Na)(Nb,Sb)O3-xBi(Na,K)(Zr,Sn,Hf)O3 ceramics. A rhombohedral-tetragonal (R-T) phase boundary was constructed at x=0.035–0.04 by co-doping with Sb5+ and Bi(Na,K)(Zr,Sn,Hf)O3. More importantly, a superior temperature stability was observed in the ceramics with x=0.035, accompanying with a stable unipolar strain at room temperature to 100 °C. The ceramics with x=0.035 also exhibited improved piezoelectric properties (e.g., piezoelectric coefficient d33∼465 pC/N and electromechanical coupling factor kp=0.47) and Curie temperature (Tc∼240 °C). The Rietveld refinement and in-situ temperature-dependent piezoresponse force microscopy (PFM) results indicated that the enhancement of the piezoelectric properties was caused by the easy domain switching, high tetragonal fraction, and tetragonality, while the improved temperature stability mainly originated from the stable domain structures.

Acceptor and donor co-doped titanium dioxide (TiO2) materials have a colossal permittivity (CP), a low dielectric loss, and a good temperature/frequency stability, showing an immense potential for microelectronics and high-energy storage devices. In this study, we propose to decode the role of acceptors and donors on CP dielectric properties in TiO2 by doping a series of A3+ acceptors or B5+ donors. A reduced dielectric loss of tan δ 105 is displayed in B5+-TiO2 accompanied by a high dielectric loss (>0.40). Both of them possess an obvious temperature/frequency sensitivity. Inversely, an additive effect with CP and low dielectric loss appears in A3+ and B5+ co-doped TiO2 over a broad temperature/frequency range, e.g., a giant er value (5.7–9.6 × 104) and a low tan δ value (0.05–0.096) are achieved in a paradigm system of (Gd0.5Nb0.5)xTi1−xO2. The XPS analysis indicates that introducing A3+ causes the appearance of , while the B5+ substitution can afford an extra electron to transform Ti4+ to Ti3+. The introduced cations and generated defects combine together to form an electron-pinned defect-dipole and cause a strong dipole polarization. These defects can also facilitate the internal barrier layer capacitance and surface barrier layer capacitor effects, leading to a higher CP property.

In this work, (0.99 − x)(K0.5Na0.5)(Nb0.965Sb0.035)O3–0.01SrZrO3–x(Bi0.5Na0.5)ZrO3 ceramics were selected to show how a phase engineering strategy (PES) affects the strain properties in potassium sodium niobate (KNN)-based ceramics. The application of PES resulted in the coexistence of multiple phases in KNN-based ceramics accompanied by an increased diffuseness of ferroelectricity and decreased domain size. The strain properties, including the dependencies of strain on composition, temperature and fatigue behavior, were evaluated by considering the phase structure, domain configuration and microstructure. The improved room-temperature strain properties of the KNN-based ceramics with PES originated from the converse piezoelectric response, domain switching and possible electric-induced phase transition, which resulted from the coexistence of multiple phases and complex domain configuration. The enhanced temperature stability mainly originated from the converse piezoelectric response. Endurable fatigue resistance (no degradation within 100–105 electric cycles) and a high electrostriction coefficient (Q33 = 0.035 m4 C−2) were observed in the ceramics with x = 0.03 and 0.05, respectively. This study provides a systematic analysis of the effects of PES on strain properties in KNN-based ceramics.

Rietveld refinement is used to investigate the crystal structure of prepared (0.965 – x)(K0.48Na0.52)NbO3–xBiInO3–0.035(Bi0.5Na0.5)ZrO3 (KNN-xBI-BNZ) ceramics. From refined results, the distortion degree of crystal structures in KNN-xBI-BNZ ceramics presents a rising trend with BiInO3 modification, which is in keeping with the results of diffuseness. The spontaneous polarization (Ps) is also calculated using refined structural parameters. The submicron domains are observed when x = 0.004, which presents good electrical properties (d33 = 317 pC/N, Tc = 336 °C) simultaneously. Excellent thermal stability of ceramics modified with BiInO3 is observed in a broad temperature range.

To achieve high piezoelectricity and the corresponding good temperature stability of BaTiO3 (BT) ceramics, a new system of (1-x-y) BaTiO3 – yCaTiO3 – x(BaZr1-zHfz)O3 (BT-yCT-xBZHz) was designed. The O-T phase boundary was observed at 0.065 ≤ x ≤ 0.085, 0.08 ≤ y ≤ 0.12 and 0 ≤ z ≤ 1.0 near room temperature. Due to the O-T phase boundary, the enhancement of electrical properties (d33 = 500 pC/N, kp = 0.52, Pr = 14.0 μC/cm2 and er = 2800), large strain (∼0.23%) and d33* = 1100 pm/V (10 kV/cm) can be obtained. Moreover, temperature-dependent electrical properties were investigated in detail, and a reliable performance was realized due to high TC (>100 °C). The d33* was demonstrated to be temperature-insensitive from 16 °C to 60 °C (varying less than 7%), which is superior to the reported BT-based ceramics. Besides, d33 and Pr fluctuate less than 22% in this temperature range, also showing an usable stability. We believe that this work can be beneficial to facilitate an increasing adoption of lead-free BT-based piezoceramics for practical applications.

Multitudinous topological configurations spawn oases of many physical properties and phenomena in condensed-matter physics. Nano-sized ferroelectric bubble domains with various polar topologies (e.g., vortices, skyrmions) achieved in ferroelectric films present great potential for valuable physical properties. However, experimentally manipulating bubble domains has remained elusive especially in the bulk form. Here, in any bulk material, we achieve self-confined bubble domains with multiple polar topologies in bulk Bi0.5Na0.5TiO3 ferroelectrics, especially skyrmions, as validated by direct Z-contrast imaging. This phenomenon is driven by the interplay of bulk, elastic and electrostatic energies of coexisting modulated phases with strong and weak spontaneous polarizations. We demonstrate reversable and tip-voltage magnitude/time-dependent donut-like domain morphology evolution towards continuously and reversibly modulated high-density nonvolatile ferroelectric memories., Experimentally manipulating bubble domains remains elusive especially in the bulk form of ferroelectrics. Here, the authors achieve self-confined bubble domains with multiple polar topologies in bulk Bi0.5Na0.5TiO3 ferroelectrics, demonstrating reversible and donut-like domain morphology evolution.

Preferable temperature stability of ferroelectric and strain properties can be achieved in potassium sodium niobate (KNN)-based ceramics compared to that of other lead-free piezoelectric material systems, giving it greater potential for use in electronic devices once the temperature reliability issue related to the piezoelectric constant (

Electromechanical actuators with toxic lead-based electrostrictive materials dominate the market of high-precision electronics devices, so one of urgent aims becomes how to explore the new generation of ecofriendly electrostrictive materials with superior electrostriction behaviors. Herein, a strategy of modifying the active space for the B-site in lead-free relaxor ferroelectrics arouses the potential capacity of electrostriction, including K

Simultaneous improvement of piezoelectricity and temperature stability is vital for developing high-performance KNN-based lead-free materials. Here, effects of CaTiO3 on phase structure, piezoelectricity and temperature stability of (1 – x)[0.95(K0.5Na0.5)NbO3–0.05(Bi0.5Na0.5)ZrO3]–xCaTiO3–0.2%MnO2 ceramics were studied. It was found that the orthorhombic-tetragonal phase transition temperature can be gradually decreased by increasing CaTiO3 content. Particularly, the improved strain temperature stability (strain varied

The elimination of lead in piezoelectric applications remains challenging. Since the advances in the piezoelectricity were found in the perovskite family in 2000, studies into lead-free piezoelectric materials have grown exponentially in the fields of condensed matter physics and materials science. In this review, we highlighted the compelling physical properties of lead-free piezoelectric perovskite materials and summarized their state-of-the-art progress, with an emphasis on recent advances in the piezoelectric effect. We mainly introduced the unique advances in lead-free perovskites piezoelectric bulk materials, along with the descriptions of phase boundaries, domain configurations, and piezoelectric effects, and then the main physical mechanisms of high piezoelectricity were summarized. In particular, the applications of lead-free materials were also introduced and evaluated. Finally, challenge and perspective are featured on the basis of their current developments. This review provides an overview of the development of lead-free piezoelectric perovskite materials in the past fifteen years along with future prospects, which may inspire material design toward practical applications based on their unique properties.

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.

MnO2-doped 0.99(0.36BiScO3-0.64PbTi1-xCexO3)-0.01Bi(Zn0.5Ti0.5)O3 (BS-PTC-BZT-MnO2) ceramics are fabricated by the solid-state method. Here, it's firstly reported that Ce element can reduce dielectric loss (tan δ) and suppress the decrease of piezoelectric constant (d33) simultaneously. Effects of Ce contents on the structure and electrical properties of BS-PTC-BZT-MnO2 ceramics are studied. The ceramics (x = 0.02) with MPB (rhombohedral-tetragonal) possess low dielectric loss (tan δ = 1.36%, 1 kHz) and high piezoelectric constant (d33 = 360 pC/N) simultaneously, which is superior to most reported BS-PT. Besides, excellent comprehensive properties including high Curie temperature (TC = 422 °C), large dielectric constant (ɛr = 1324), and high remnant polarization (Pr = 35.1 µC/cm2) are obtained. Asymmetric S-E and P-E hysteresis loops indicate that defects and oxygen vacancies are induced by multi-valence elements (Ce and Mn), which is the origin for reducing tan δ. In addition, good thermal stability of piezoelectric and dielectric properties is observed. These results indicate that Ce and Mn co-doped BS-PTC-BZT-MnO2 ceramics can be well applied as power electronic devices under high temperature.

Because of their high toxicity, lead-based materials in electronic devices must be replaced by lead-free piezoelectric materials. However, some issues remain that hinder the industrial applications of these alternative ceramics. Here, we report the construction of a 0-3-type ceramic composite (KNNS–BNKZ:xZnO), where the Sb-doped ZnO submicronic particles were randomly distributed throughout the potassium–sodium niobate-based ceramic matrix. In this (K,Na)NbO3 (KNN)-based ceramic composite, superior temperature stability, excellent piezoelectric properties, and a high Curie temperature were simultaneously achieved. The unipolar strain varied from +20 to −16% when the temperature was increased from 23 to 200 °C in KNNS–BNKZ:xZnO with x = 0.75. By increasing the ZnO content from x = 0 to x = 5.0, the Curie temperature was increased from 227 to 294 °C. More importantly, the piezoelectric coefficient remained high (d33 = 480–510 pC/N) for a wide range of compositions, x = 0.25–1.0. Transmission electron micros...

(1 − x)K0.40Na0.60Nb0.96Sb0.04O3–x(Bi0.92Nd0.08)0.5Na0.5ZrO3 (KNNS–xBNNZ) lead-free piezoceramics have been synthesized by a conventional solid state sintering method, and the effects of BNNZ content on their phase structure and electrical properties were investigated. The phase boundary of rhombohedral–orthorhombic–tetragonal (R–O–T) has been formed in the composition range of 0.030 ≤ x ≤ 0.040. An optimal comprehensive performance (e.g., d33 ~ 470 ± 10 pC/N, kp ~ 0.50 ± 0.02, TC ~ 283 °C, er ~ 1950, tanδ ~ 0.043, Strain ~ 0.135%, and d33* ~ 450 pm/V) is obtained in the ceramics with x = 0.035. Moreover, a relatively good component stability of piezoelectric properties (d33 ~ 402–470 pC/N, kp ~ 0.485–0.524, Strain ~ 0.133–0.135%, and d33* ~ 443–450 pm/V) is gained in a broad component range from 0.030 to 0.045, which is benefited from the contribution of multiphase boundary (R–O–T, R–T). Therefore, we believe that KNNS–xBNNZ ceramics opens a window for the practical applications in piezoelectric material markets.

The focus on piezoelectric ceramics based on the potassium sodium niobate system began in 2004. After years of dedicated research, these materials can be considered one of the most promising lead-free piezoceramics with comprehensive performance. While their structure–property relationships are still not completely understood, the thermal stability issue is partly resolved, which leaves further room for phase-boundary engineering. Technological advancement has recently focused on using base metals as inner electrodes for multilayer actuators, which provides cost benefits as compared to lead zirconate titanate devices. The remaining challenges, however, such as poor sinterability and weak reproducibility of functional properties, still hinder extensive applications of these materials.

To further balance the relationship between piezoelectricity and temperature stability, the (0.975 – y)NaNbO3-yBaTiO3-0.025BaZrO3 (y = 0–0.20) ceramics are developed by constructing a wide tetragonal phase region. Effects of BaTiO3 on the relationships among phase structure, electrical properties, and temperature dependence are investigated. With increasing BaTiO3 contents, the ceramics endure the structural evolutions from orthorhombic phase to tetragonal phase, and then to relaxor cubic phase. A wide tetragonal phase zone of 24–180 °C can be realized in the ceramics with y = 0.08, together with an enhanced piezoelectric coefficient d33 = 215 pC/N. Intriguingly, excellent temperature stability of unipolar strain (Suni) and piezoelectric coefficient (d33) are observed in the ceramics with y = 0.08 within 20–180 °C. This work provides an alternative way to enhance piezoelectricity and temperature stability in lead-free piezoceramics.

(1−x−y)(K0.45Na0.55)NbO3–yBi0.5Na0.5ZrO3–xBaHfO3–0.2%MnO2 ceramics without antimony have been fabricated by the conventional solid-state method, and influences of Bi0.5Na0.5ZrO3 and BaHfO3 contents on their structure and electrical properties are studied. Composition modification can result in the formation of rhombohedral–orthorhombic–tetragonal phase coexistence in the ceramics (y = 0.04 and 0.01 ≤ x ≤ 0.02 as well as x = 0.01 and 0.04 ≤ y ≤ 0.045). In the region of this phase boundary, the ceramics exhibit both enhanced piezoelectric properties (d33 ~ 385 pC/N, kp ~ 51%, S ~ 0.157%) and high Curie temperature (TC ~ 320 °C). Especially, good comprehensive properties and the absence of antimony make the material more environmentally friendly.

Although phase boundary engineering has made notable progress in improving the electrical properties of (K,Na)NbO3-based piezoceramics, lattice distortion and spontaneous polarization of multiphase coexisting systems are a few of the remaining concerns. Here, new research employing XRD Rietveld refinement was performed to explore crystal structures, phase fractions and atomic parameters of Fe2O3-added (0.995-x)K0.48Na0.52NbO3-xBi0.5Na0.5ZrO3-0.005BiScO3 ceramics. The distortion of the oxygen octahedron and the spontaneous polarization were presented. Central cation displacement provides a much larger contribution to polarization and the electric dipole moment in orthorhombic phase is much larger than that in tetragonal phase. Benefiting from tetragonal-orthorhombic phase coexistence and lattice distortion, optimized ferroelectric and piezoelectric properties (d33 ∼ 381 pC/N, Pr ∼ 20.47 μC/cm2) were obtained. The ceramic still holds a large d33 (313 pC/N) after up to 300 ℃ of thermal annealing. A series of material constants was also calculated and compared to lead-based ones.