Search

Flexible piezoelectric thin films are raising interest in energy harvesting and wearable electronics, although their direct fabrication is challenging in the selection of substrates and thermal processing. In this work, we developed direct fabrication of flexible lead-free (K, Na)NbO3 (KNN)-based piezoelectric films on commercially available metallic foils by sol-gel processing. Stainless steel and platinum foils are selected as flexible substrates because of their good thermal stability, robust flexibility, and cost-efficiency. The sol-gel-processed KNN-based thin films on both of the metallic foils show good flexibility, with the bending radii reaching ±3 mm. The flexible thin films grown on stainless steel and platinum foils present high breakdown electric fields that reach 1760 and 2530 kV/cm, respectively, resulting from the fine-grained dense structure, limited leakage current density, and suppressed mobility of charged carriers. Improved effective piezoelectric coefficient d33, eff* (75.4 pm/V) with a slight decrease after bending was obtained in the flexible thin films on Pt when compared to their rigid counterparts. The flexible lead-free piezoelectric thin films with combined high breakdown electric fields and piezoelectric and energy storage properties may pave the way for integrating KNN-based multifunctional thin films into flexible electronics.

Piezoelectrics exhibit mechanical strain in response to electrical stimuli and vice versa. A high level of electric-field-induced strain with minimal hysteresis is desired for piezoelectric materials when used as actuators. The past two decades have seen extensive research into lead-free piezoelectrics to replace Pb(Zr,Ti)O3 and compositional engineering has been demonstrated to be an effective method to tailor their functional properties. Doped (K,Na)NbO3 (KNN) compositions with elaborate compositional tuning can exhibit enhanced electromechanical properties. However, a balance between enhanced properties and non-toxicity of the dopants should be considered. In this work, we propose to use microstructural engineering to enhance the properties. Based on phase-field simulations, we propose to take advantage of depolarization energies generated by polar-nonpolar interfaces, to increase the contribution of domain wall motion to electric-field-induced strain. Heterogeneous ferroelectric-paraelectric microstructures were introduced into a KNN ceramic via a two-step sintering process. Their presence was characterized by high-resolution transmission electron microscopy. Enhanced reversible domain wall motion was verified by in situ high-energy X-ray diffraction. Electric-field-induced strain is enhanced by 62% and 200% at 25 °C and 150 °C, respectively. Considering lead-free piezoelectrics also represent an emerging class of biomaterials for medical technology, the non-toxicity and biocompatibility of the investigated compositions are examined by in vitro cell viability assays. Our results demonstrate that microstructural engineering is a promising alternative approach to enhance the electric-field-induced strain of lead-free piezoelectrics while maintaining biocompatibility

BiFeO3-BaTiO3 is a promising high-temperature piezoelectric ceramic that possesses both good electromechanical properties and a Curie temperature (TC). Here, the piezoelectric charge constants (d33) and strain coefficients (d*33) of (1 - x)BiFeO3-xBaTiO3 (BF-xBT; 0.20 ≤ x ≤ 0.50) lead-free piezoelectrics were investigated at room temperature. The results showed a maximum d33 of 225 pC/N in the BF-0.30BT ceramic and a maximum d*33 of 405 pm/V in the BF-0.35BT ceramic, with TCs of 503 and 415 °C, respectively. To better understand the performance enhancement mechanisms, a phase diagram was established using the results of XRD, piezoresponse force microscopy, TEM, and electrical property measurements. The superb d33 of the BF-0.30BT ceramic arose because of its location in the optimum point in the morphotropic phase boundary, low oxygen vacancy (VO··) concentration, and domain heterogeneity. The superior d*33 of the BF-0.35BT ceramic was attributed to a weak relaxor behavior between coexisting macrodomains and polar nanoregions. The presented strategy provides guidelines for designing high-temperature BF-BT ceramics for different applications.

Ferroelectric films can sustain large strain induced by substrate constraints, which may additionally modulate their crystal structures and electrical properties. This study revealed the strain engineering effect on the piezoelectricity of (100-x)%Bi1/2Na1/2TiO3-x%BaTiO3 (BNT-BTx) epitaxial films grown on Nb:SrTiO3100 (NSTO100) substrates in association with the crystal structure affected by film thickness. Several compositions in the BNT-BTx system, i.e. BNT-BT2, BNT-BT6 and BNT-BT15, were studied as typical representatives of ferroelectric rhombohedral, relaxor ferroelectric pseudo-cubic (near morphotropic phase boundary) and ferroelectric tetragonal structures, respectively. It was found that a monoclinic–rhombohedral phase transition appeared in BNT-BT2 films when we changed the film thickness, which was caused by the coherent strain imposed by the substrates. The 3-layer BNT-BT2 film (∼150 nm) exhibited a phase coexistence of monoclinic and rhombohedral phases, so that the effective piezoelectric constant was enhanced by at least 260% compared to films with pure monoclinic or rhombohedral phases. The present work may offer a feasible way to improve the piezoelectric properties of other perovskite piezoelectric films.

Polarization switching in lead-free (K0.40Na0.60)NbO3 (KNN) single crystals was studied by switching spectroscopy piezoresponse force microscopy (SS-PFM). Acquisition of multiple hysteresis loops on a closely spaced square grid enables polarization switching parameters to be mapped in real space. Piezoresponse amplitude and phase hysteresis loops show collective symmetric/asymmetric characteristics, affording information regarding the switching behavior of different domains. As such, the out-of-plane polarization states of the domains, including amplitudes and phases can be determined. Our results could contribute to a further understanding of the relationships between polarization switching and polarization vectors at the nanoscale, and provide a feasible method to correlate the polarization hysteresis loops in a domain under an electric field with the polarization vector states.

Piezoelectric materials are an important functional material in the modern world because they are capable of converting electrical energy into mechanical energy and vice versa. Lead-free potassium sodium niobate (KNN)-based piezoelectric materials have been intensively researched in the past decade for only one purpose: to replace the less environmentally friendly lead-based piezoelectric materials in commercial products. In this review, the historical research evolution of KNN-based piezoelectric ceramics is highlighted. Meanwhile, to evaluate the technology transfer status, we emphasize on the analysis of the patents applied within the past five years, as well as certain practical applications of KNN-based materials. This paper concludes with the prediction of remaining challenges and opportunities for adopting the lead-free KNN-based piezoelectric ceramics in practical applications.

0.8(Bi0.5,Na0.5)TiO3-0.2SrTiO3 (BNT-0.2ST) thin films, with thicknesses ranging from 90 to 364 nm, were fabricated on platinized silicon substrates by sol-gel method. These films were investigated by switching spectroscopy piezoresponse force microscope (SS-PFM) as a function of frequency at room temperature, revealing the enhanced ferroelectric response in ∼ 210 nm film at all frequencies (0.1 Hz - 1.5 Hz). This enhancement was ascribed to the largest thermally-activated stress at such thicknesses generated during film fabrications. As the temperature of the investigated films increases from room temperature to 200 oC, the piezoelectric parameters were obtained from SS-PFM, such as switching polarization (Rs), coercive bias (V0), work of switching (As), maximum strain (Smax), and negative strain (Sneg), indicating an occurrence of phase transition from ferroelectrics to relaxors. This work revealed that thickness plays a crucial role for ferroelectric response and temperature-dependent phase transition in BNT-0.2ST films, since it affects the stress state and switching behavior.

The fracture toughness of unpoled and electrically poled lead-free KNN-based piezoelectric ceramics with the composition of 0.92KNN-0.02Bi0.5Li0.5TiO3-0.06BaZrO3 was investigated. Results reveal that at room temperature, the intrinsic fracture toughness (KI0) of the unpoled samples, evaluated by the near-tip crack opening displacement (COD) technique, is the lowest with a value of 0.70 MPa ⋅ m0.5; the long (through-thickness) crack fracture toughness (KIvnb), obtained by the single edge V-notch beam (SEVNB) technique, is the highest, with a value of 0.95 MPa ⋅ m0.5; intermediate short surface crack fracture toughness (KIsc) of 0.86 MPa ⋅ m0.5 was determined by the surface crack in flexure (SCF) technique. These results were rationalized by the toughening behavior of the material combined with the crack geometry-dependent stress intensity evolution during crack propagation. With increasing temperature, KIvnb and KIsc decrease, and become nearly identical at 350 °C, suggesting an absence of toughening. For electrically poled samples, their room temperature fracture toughness was characterized by both SCF and SEVNB techniques, with values of 0.88 MPa ⋅ m0.5 and 0.99 MPa ⋅ m0.5, respectively, slightly larger than the values measured for unpoled samples. Nonlinear electric field-strain and stress-strain analysis of the material was also employed during electric field loading, mechanical compression and four-point bending in order to quantify crack tip shielding by domain switching and the actual stress at the point of instable crack propagation.

Functional polymers such as polyvinylidene fluoride (PVDF) and its copolymers, which exhibit room-temperature piezoelectricity and ferroelectricity in two-dimensional (2D) limit, are promising candidates to substitute hazardous lead-based piezoceramics for flexible nanoelectronic and electromechanical energy-harvesting applications. However, realization of many polymers including PVDF in ultrathin 2D nanostructures with desired crystal phases and tunable properties remains challenging due to ineffective conventional synthesis methods. Consequently, it has remained elusive to obtain optimized piezoelectric performance of PVDF particularly in sub-10 nm regimes. Taking advantage of its high flexibility and easy processing, we fabricate ultrathin PVDF nanoflakes with thicknesses down to 7 nm by using a hot-pressing method. This thermo-mechanical strategy simultaneously induces robust thermodynamic α to electroactive β-phase transformation, with β fraction as high as 92.8% in sub-10 nm flakes. Subsequently, piezoelectric studies performed by using piezoresponse force microscopy reveal an excellent piezoelectric strain of 0.7% in 7 nm film and the highest piezoelectric coefficient ( d

(K, Na)NbO3 (KNN)-based solid solutions are outstanding lead-free piezoelectric materials, yet stable reproduction of these materials has always been an obstacle. In the present study, significant ferroelectric phase coexistence was observed in the polycrystalline ceramic sample after sintering. The ferroelectric phase coexistence can be well explained by the chemical heterogeneity in the calcined powder, which is a consequence of using monoclinic Nb2O5 as the precursor. It was demonstrated that the ferroelectric phase coexistence, as well as the chemical heterogeneity, could be a barrier of the reproducibility of performances. Therefore, we suggest that extra attention should be paid on the processing of KNN-based materials, from the very beginning, that is, the choice of precursors.

It is an urgency to find capable lead-free piezoelectric ceramics to show effort towards environmental protection. Bi0.5Na0.5TiO3 (BNT)-based piezoelectric ceramics is one of the most promising lead-free systems, for which compositional modification is an efficient way to enhance the piezoelectric performance. In this work, lead-free 0.82(Bi0.5Na0.5)TiO3-(0.18-x)BaTiO3-x(Bi0.5Na0.5)(Mn1/3Nb2/3)O3 ceramics were synthesized by a conventional solid state reaction method. Polarization and strain hysteresis loops implies a phase transition from FE (ferroelectric phase) to ER (ergodic relaxor). An electric field induced high strain of 0.28% occurred at x = 0.0275, which was attributed to the coexistence of FE and ER phases. It is considered that our work can help to understand the complex phenomenon observed in BNT-based lead-free system.

Lead-free (K,Na)NbO3 (KNN) nanorods (NRs), which have a relatively high piezoelectric response and good environmental compatibility, are expected to be used in the application fields of sensors, actuators, etc. In the present study, flexible composite devices with piezoelectric KNN nanostructures were successfully fabricated as a flexible and cost-effective strain sensor. The composition-controlled KNN nanorods (NRs) were synthesized via the molten salt reaction, while piezoresponse force microscopy (PFM) was utilized to characterize the three-dimensional (3-D) morphology as well as the piezoelectric properties of single crystalline KNN nanorods. Then, the KNN NR-polydimethylsiloxane (PDMS) films were fabricated through the tape casting process, and assembled into the self-powered sensor with Cu-coated polyethylene terephthalate (PET) substrates. The as-fabricated KNN-PDMS sensor was affected by the pressing and releasing activities of the human body, and the output voltage was measured concurrently. As a result, the strain sensor obtains an output signal of ∼0.5 V with KNN NR fillers of 0.5 vol% in PDMS, which implies that KNN NRs are promising in the application of lead-free flexible sensor devices.

Piezoelectric materials convert mechanical energy into electric energy, and vice versa. They have been applied in many critical fields, such as motor vehicles, medical devices, the military and aerospace. Recently, the development of lead-free piezoelectrics due to environmental concerns has attracted enormous attention in both scientific and industrial fields. This review summarises the effect of chemical modification on the enhancement of potassium sodium niobate lead-free piezoelectric materials. The origin of the high piezoelectricity is attributed to the construction of phase coexistence and local heterogeneities. The choice of dopants is discussed from the aspects of ionic radius, valency, electronegativity and hybridisation, as well as their influence on thermal stability and fatigue behaviour. An assessment of heterogeneity at different length scales on the piezoelectric performance is provided.

A morphotropic phase boundary (MPB) between rhombohedral (R) and tetragonal (T) phases was identified in a few (0.9-x)NaNbO3-0.1BaTiO3-xABO3 (x = 0–0.05) lead-free systems. Critical roles of R-phase inducers were specially evaluated in terms of phase boundary position, microstructure and piezoelectric responses. The results indicate not only the tolerance factor of the ABO3 additive but also its ferroelectricity and corresponding volume change would influence the formation of phase boundary and further determine dielectric and ferroelectric responses. The piezoelectric coefficient d33 of MPB compositions was compared with theoretically-calculated d33-cal according to d33 = 2Pr·e33·Q33, demonstrating that the piezoelectric response of these systems should be determined by combined effects of the phase coexistence, nano-scale domains and particularly enhanced dielectric responses. The largest d33 ∼305 pC/N, the highest e33T/eo ∼2815 and the lowest Pr ∼14.7 μC/cm2 were achieved in the MPB composition with 3.75% SrZrO3. These experimental results provide a valuable reference for designing new NaNbO3-based lead-free piezoelectric materials.

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...

Due to growing concern for the environment and human health, searching for high-performance lead-free piezoceramics has been a hot topic of scientific and industrial research. Despite the significant progress achieved toward enhancing piezoelectricity, further efforts should be devoted to the synergistic improvement of piezoelectricity and its thermal stability. This study provides new insight into these topics. A new KNN-based lead-free ceramic material is presented, which features a large piezoelectric coefficient (d33) exceeding 500 pC/N and a high Curie temperature (Tc) of ∼200°C. The superior piezoelectric response strongly relies on the increased composition-induced structural flexibility due to lattice softening and decreased unit cell distortion. In contrast to piezoelectricity anomalies induced via polymorphic transition, this piezoelectricity enhancement is effective within a broad temperature range rather than a specific small range. In particular, a hierarchical domain architecture composed of nano-sized domains along the submicron domains was detected in this material system, which further contributes to the high piezoelectricity.

The temperature sensitivity of enhanced piezoelectricity in (K,Na)NbO3 perovskite-based lead-free piezoelectrics is one of the technical bottlenecks impeding practical applications. This work found a recipe that possesses a piezoelectric strain coefficient d33* as high as 600 pm V−1 over a wide temperature range up to 100 °C, which is additionally insensitive against the frequency of an applied electrical field. Electrical property measurement and nanostructure observation revealed that the high performance combining high piezoelectricity and excellent stability benefits from a diffused phase transition (DPT), which is closely associated with the transition from ferroelectric to relaxor behavior induced by the local structural disorder due to the complicated compositional modification. The high-resolution transmission electron microscopy images of Moire fringes imply the existence of polar nanoregions (PNRs) that are responsible for the relaxor-like electrical behavior in the present materials. The findings and understanding obtained in this work should be valuable for pushing lead-free piezoelectric ceramics towards practical applications.

An easy-to-use domain strategy by utilizing phase boundary design, chemical modification, and electric-field driving was employed to optimize strain and temperature stability of a polycrystalline ceramic. First, the effects of phase boundary, composition, and electric field on the domain were investigated in detail. Distinct spontaneous polarization rotation appears in various phase transition processes, inducing different evolution of domain structure and piezoresponse at elevated temperatures. Measurements reveal that the domain at the rhombohedral–tetragonal (R–T) phase boundary shows giant piezoresponse and high temperature stability. The structure origin is assigned to the slow degradation of spontaneous polarization, and the highly steady domain contribution stimulates excellent strain stability at the R–T phase boundary. Additionally, doped components significantly influenced domain stability, and boosted electric fields stabilized the piezoresponse of the domain. Second, modulating domain stability and its piezoresponse through tuning the phase boundary and driving field, as well as chemical modification, realized a large and temperature-insensitive strain ( fluctuation less than 3% at 23–80 °C) in (K, Na)NbO3 (KNN)-based ceramics. This value is largely superior to that obtained from current lead-free piezoelectric materials and some soft lead-based counterparts, indicating a tremendous application potential for (K, Na)NbO3 (KNN)-based ceramics. This study affords a significant guidance for designing lead-free piezoelectric materials with high piezoelectricity and temperature stability.

Here we have, for the first time, reported a giant unipolar strain of ∼0.45% and a temperature-insensitive piezoelectric effect (d33 ∼ 520 pC N−1) in (1 − y)[xBiScO3–(1 − x)PbTiO3]–yBi(Zn1/2Ti1/2)O3 (BS–PT–BZT) ceramics with a high Curie temperature of TC ∼ 425 °C. The morphotropic phase boundary (MPB) of rhombohedral–tetragonal (R–T) phase coexistence can be driven in samples with x = 0.36 and y = 0.01, which is mainly responsible for the enhancement of electrical properties and temperature stability. PFM test results indicate that the addition of BZT can improve the local piezoelectric response of domains, which may lead to the enhancement of piezoelectricity and strain. In addition, ceramics with BZT exhibit good temperature stability of electrical properties. We believe that these materials can be well applied in the field of high-temperature piezoelectric actuators.

(K,Na)NbO3 is a leading lead-free piezoelectric system, but its thermal instability of high piezoelectricity is still one of its shortcomings compared to its lead-containing counterparts. This work found that incorporating perovskite compounds with Zr ions at the B-site can lead to the simultaneous enhancement of piezoelectricity and thermal stability. The piezoelectric coefficient d33 increased to ∼450 pC N−1, and such a high d33 value can be maintained within 10% change over a wide temperature range of 22–80 °C. The piezoelectric strain also reached 0.13% strain with a small variation (

High-quality (1 - x)(Bi0.5Na0.5)TiO3-xSrTiO3 lead-free piezoelectric thin films (x = 0, 0.1, and 0.25) on Pt(111)/Ti/SiO2/Si(100) substrates were prepared by a sol-gel method. The microstructures of the thin films as a function of SrTiO3 doping level and temperature were investigated by X-ray diffraction and Raman spectroscopy. Their temperature- and frequency-dependent piezoelectric properties were studied on the nanoscale using switching spectroscopy piezoresponse force microscopy (SS-PFM). A rhombohedral ferroelectric to pseudocubic relaxor phase transition was observed when either ST content or temperature increased. The significant frequency dependence of both ferroelectric and piezoelectric properties was also disclosed by analyzing polarization hysteresis loops on the macroscopic scale and local switching dynamics at various frequencies. It was determined that the short-range order clusters came out through the long-range ferroelectric order, thus the nanoscale approaches are consistent with macroscopic data at elevated temperatures and various frequency ranges.

The temperature-dependence behaviors of ferroelectric, piezoelectric, kp and electrical-field-induced strain were carefully evaluated for high-performance BiFeO3–0.3BaTiO3 (BF–0.3BT) ceramics. There results indicate, combined with Rayleigh analysis and temperature-dependence XRD and PFM, that the increase of strain and large signal with increasing the temperature from room temperature to 180 °C is related to the joint effect of intrinsic contribution (lattice expansion) and extrinsic contribution (domain switching). With further increasing the temperature to 300 °C, the large signal d33 and electrical-field-induced strain mildly decrease because of the increase of conductivity for BF–0.3BT ceramics. However, different from strain and large signal the small signal d33(E0) and kp exhibit excellent temperature stability behavior as the temperature increases from room temperature to 300 °C.

One of the key remaining challenges for the most promising Pb-free (K,Na)NbO3 (KNN) piezoelectrics is how to improve the temperature stability of piezoelectric voltage coefficients, which is vital to promoting their practical applications. This study proposes an effective approach to develop highly piezoelectric KNN-based ceramics with temperature-insensitive piezoelectric voltage coefficients (d33), which is also called direct piezo-coefficient, by broadening the boundaries from tetragonal to orthorhombic and then to rhombohedral phases. The Li-doped KNN ceramics modified with BaZrO3 and (Bi,Zr)TiO3 exhibit a high d33 up to 330 pC N−1, whose variation is limited to ±10% in a common application temperature range (25–100 °C), when their composition is optimized for strengthening their temperature stability. A high d33 up to 380 pC N−1 with a larger variation ( 260 °C as well as high strength associated with high relative density. Piezoelectric force microscopy (PFM) studies reveal that the good piezoelectric response could be related to the easier domain reorientation and domain wall motion. The present results validated that our approach based on phase transition boundary engineering should be an effective strategy for further development of high-performance lead-free piezoceramics towards the demand for industrial applications.

(K,Na)NbO3 (KNN) based lead-free materials have gained great progress and drawn much attention recently, due to their attractive piezoelectric performance and the demand for environmental protection. As is known, domain motion is essential for piezoelectric output; however, there is limited understanding on this topic for KNN. In the present study, the domain growth process of the KNN thin film is studied by piezoresponse force microscopy (PFM). The domain size shows a linear dependence on the pulse voltage and a complex dependence, unlike the commonly accepted logarithmic relationship, on the pulse duration. Meanwhile, a power decay relationship between the domain wall velocity and the domain size is observed. The extremely low dynamical exponent proves that our sample is influenced by extremely strong pinning effects caused by defects, resulting in the uncommon pulse duration dependence of domain size.

Piezoelectric actuators are typically preloaded with a modest mechanical compressive stress during actuation to reduce cracking and allow for operation in the dynamic range. In addition, actuators are required to carry out mechanical work during operation, resulting in a nonlinear relationship between stress and actuation voltage. In fact, mechanical loading can significantly impact the electromechanical performance of lead-free piezoelectrics. Herein, we report the dependence of electromechanical properties of CaZrO3 modified (K,Na)NbO3-based lead-free piezoceramics on uniaxial compressive stress, comparing to their lead-based counterparts. It is demonstrated that increased non-180° domain switching enhances the strain output at a moderate stress of approximately -50 MPa from room temperature to 150°C. Larger uniaxial stress, however, is found to suppress ferroelectric domain switching, resulting in the continuous strain and polarization decrease.

Various strain contributions of (Na 0.52 K 0.48 − x )(Nb 0.92 − x Sb 0.08 )O 3 − xLiTaO 3 ceramics in the proximity of orthorhombic (O) and tetragonal (T) polymorphic phase boundary (PPB) were quantitatively resolved by means of synchrotron x-ray diffraction together with macroscopic strain measurements. Compared with O-rich compositions with a governing mechanism of intrinsic lattice strains, T-rich compositions exhibited a dominant strain mechanism from reversible domain switching. Quantitative analysis of diffraction data suggested that extrinsic strain contributions should depend on not only the lattice distortion δ, but also the poling texture Δf, phase content (for PPB compositions) and domain types. Smaller lattice distortion and higher poling texture tended to enhance the number of irreversible domain switching in O-rich compositions, thus leading to larger fraction of intrinsic lattice strain contribution. The calculated results demonstrated that the product of two parameters Δf and δ would give a reliable estimation of domain-switching strains for T-phase compositions but an overestimation for O-phase compositions.

(K,Na)NbO3 (KNN) is considered as one of the most promising candidates for lead-free piezoelectric ceramics, due to its large piezoelectric response as well as high Curie temperature. In the present study, the effect of various poling conditions on CaZrO3-modified KNN-based lead-free piezoceramics was investigated. Featured by a polymorphic phase transition around ambient temperature, the samples differ from conventional Pb(Zr,Ti)O3-based (PZT) ceramics in phase constitution, which inevitably influences the optimal poling conditions. It is found that the KNN ceramics can reach a saturated polarization state under reduced poling field and at short times, compared with PZT. Besides, poling at a relatively high temperature of 120 °C can yield an enhanced piezoelectric constant d33 of up to 345 pC N−1. The poling behavior of CaZrO3-modified KNN-based ceramics is rationalized by the competition between domain reorientation and space charge accumulation.

A feasible replacement for lead-based piezoelectric ceramics is desperately required for increasing environmental concerns. Benefiting from the construction of a rhombohedral-tetragonal (R-T) phase boundary, a potential lead-free candidate (Na 0.5 K 0.5 )NbO 3 -(Bi 0.5 Li 0.5 )TiO 3 -BaZrO 3 is presented in this study, with a large piezoelectric coefficient d 33 of 330 pC/N and a unipolar strain of 0.16% at 4 kV/mm. More importantly, the piezoresponse is extremely thermally stable that the strain can maintain a considerable value of 0.12% at the temperature as high as 190 °C, rendering the current system very promising for piezoelectric actuator applications. The temperature dependent phase structure evolution as well as piezoelectric response is investigated systematically for interpretation of underlying mechanisms.

Lead-based piezoelectric materials are currently facing global restrictions due to their lead toxicity. Thus it is urgent to develop lead-free substitutes with high piezoelectricity and temperature stability, among which, potassium-sodium niobate [(K,Na)NbO3, KNN] has the most potential. It is very difficult to simultaneously achieve high piezoelectric performance and reliable stability in KNN-based systems. In particular, the structural/physical origin for their high piezoelectricity is still unclear, which hinders property optimization. Here we report the achievement of high temperature stability (less than 10% variation for electric field-induced strain from 27 °C to 80 °C), good fatigue properties (stable up to 106 cycles) as well as an enhanced piezoelectric coefficient (d33) of 525 pC N−1 in (1 − x)(K1−yNay)(Nb1−zSbz)O3–xBi0.5(Na1−wKw)0.5HfO3 (KNNS–BNKH) ceramics through manipulating the rhombohedral–tetragonal (R–T) phase boundary. The structural origin of their high piezoelectric performance can be attributed to a hierarchical nanodomain architecture, where the local structure inside nanodomains comprises R and T nanotwins. The physical origin can be attributed to low domain wall energy and nearly vanishing polarization anisotropy, facilitating easy polarization rotation among different states. We believe that the new breakthrough will open a window for the practical applications of KNN-based ceramics.

Extensive studies on (Bi0.5Na0.5)TiO3–BaTiO3 piezoceramics have aroused growing interest in developing high-performance piezoelectric films based on this promising lead-free piezoelectric system. High quality (1 − x%) (Bi0.5Na0.5)TiO3–x% BaTiO3 (x = 0–15) thin films were synthesized on Pt(111)/Ti/SiO2/Si(100) substrates via a sol–gel method. The phase transitions as functions of composition and temperature were investigated by X-ray diffraction (XRD) and temperature-dependent Raman spectra analysis. A morphotropic phase boundary (MPB) located at 5 ≤ x ≤ 10 was found in the films, being identical to that in bulk materials. Additionally, complicated structural changes with temperature in a wide composition range were observed, which correspond to the evolution from a low-symmetry ferroelectric phase or a relaxor phase to high-symmetry relaxor phases. The ferroelectricity and piezoelectricity were also systematically studied with an emphasis on the influence of composition and phase structure. A phase diagram focusing on the above two kinds of phase transitions has been proposed in this work.

The electrical properties and microstructures of (1 − x ) Bi 1/2 Na 1/2 TiO 3 – x SrTiO 3 (BNT-ST100 x , 0.20 ≤ x ≤ 0.30) piezoceramics with different grain size distributions were investigated. The critical region separating the ferroelectric from ergodic relaxor is located around x = 0.26. Strains up to 0.2% could be achieved under a low driving fields ( E S max /E max >1000 pm/V. The electrostrain was largely depended on the grain size and an increment of strain up to ∼38% can be realized by increasing the grain size from 5 μm to 13 μm in the BNT-ST26 samples. The results indicate that the field-induced strain performance of BNT-based incipient piezoelectric ceramics can be tailored via microstructure modifications, an alternative strategy to enhance the electromechanical properties.

An ultrahigh electric field–induced strain of 0.9% (under 1 kV mm−1) is achieved in lead-free ferroelectric crystals., Electromechanical coupling in piezoelectric materials allows direct conversion of electrical energy into mechanical energy and vice versa. Here, we demonstrate lead-free (KxNa1−x)NbO3 single crystals with an ultrahigh large-signal piezoelectric coefficient d33* of 9000 pm V−1, which is superior to the highest value reported in state-of-the-art lead-based single crystals (~2500 pm V−1). The enhanced electromechanical properties in our crystals are realized by an engineered compositional gradient in the as-grown crystal, allowing notable reversible non-180° domain wall motion. Moreover, our crystals exhibit temperature-insensitive strain performance within the temperature range of 25°C to 125°C. The enhanced temperature stability of the response also allows the materials to be used in a wider range of applications that exceed the temperature limits of current lead-based piezoelectric crystals.

Distinctive structure-property relationships are revealed in the relaxor ferroelectric ceramic solid solution, (1-x)K0.5Bi0.5TiO3 - xBiMg0.5Ti0.5O3: 0.02 < x < 0.08. The constructed phase diagram and results of in-situ synchrotron X-ray diffraction provide explanations for temperature and electric field dependent anomalies in dielectric, ferroelectric and electromechanical properties. At room temperature a mixed phase tetragonal and pseudocubic phase field occurs for compositions 0 > x ≤ 0.07. As temperature rises to ≥ 150 °C, the ferroelectric tetragonal relaxor phase changes to a pseudocubic ergodic relaxor phase; this change in length scale of polar order is responsible for an inflection in relative permittivity - temperature plots. The transition is reversed by a sufficient electric field, thereby explaining the constricted form of polarisation-electric field loops measured at >150°C. It is also responsible for a change in slope of the strain-electric field (S-E) plots which are relatively linear in the ferroelectric regime i.e. at temperatures up to 150 °C, giving unipolar strains of 0.11 % at 20 °C and 0.14% at 150 °C (50 kV cm-1 field). The additional contribution from the effect of the field-induced pseudocubic to tetragonal transition, generates strains of ∼ 0.2 % at 185 °C. Unusual for a piezoelectric solid solution, the maximum strains and charge coefficients (d33 =150 pC N-1, 20 °C) do not coincide with a morphotropic or polymorphic phase boundary.

This work investigated the effect of MnO2 addition on the phase structure, microstructure, and electrical properties of AgSbO3-modified (Li,K,Na)(Nb,Ta)O3 (abbreviated as LKNNT-AS) lead-free piezoelectric ceramics with an optimized composition endowed with a state of two-phase coexistence. A small amount (0.1 wt%) of MnO2 can significantly further enhance the piezoelectric property of LKNNT-AS ceramics, whose piezoelectric constant d33 and converse piezoelectric coefficient d33* as well as planar electromechanical coupling factor kp reach 363 pC/N, 543 pm/V, and 0.49, respectively. The temperature stability of piezoelectricity in MnO2-modified LKNNT-AS samples also improved, which is associated with the more uniform and better thermally stable ferroelectric domains that were revealed by piezoresponse force microscopy.

Piezoelectric composites with high-aspect-ratio lead-free piezoelectric microrods embedded in a polymer matrix were fabricated by applying the modified dice-and-fill method to a (K,Na)NbO 3 -based piezoelectric ceramic, 0.9475[Li 0.02 (K 0.48 Na 0.53 ) 0.98 ](Nb 0.80 Ta 0.20 )O 3 –0.0525AgSbO 3 –0.5 wt% MnO 2 (abbreviated as LKNNT–AS–M) with enhanced piezoelectricity but low mechanical strength. The Pb-free LKNNT–AS–M/polymer composite specimens containing ∼10–23 vol% ceramic pillars with sectional width ∼50 μm, separation ∼55–110 μm and high aspect ratio ∼11, were prepared and investigated with an emphasis on their electrical properties. The developed composites possess a higher thickness factor k t ∼63–67%, lower planar factor k p ∼27–34% and enhanced k t / k p ratio ∼2.3. The composites also possess favorable properties which includes low electrical (tan δ ∼0.029) and mechanical ( Q m ∼6) losses, low acoustic impedance Z ∼4.2–7.8 Mrayl and ∼5 times higher voltage coefficient g 33 compared to LKNNT–AS–M piezoceramic. The promising results indicate that the LKNNT–AS–M/polymer composites have the potential to be used as active elements in high performance ultrasonic transducers.

In recent years, one-dimensional piezoelectric nanomaterials have become a research topic of interest because of their special morphology and excellent piezoelectric properties. This article presents a short review on one dimensional perovskite piezoelectric materials in different systems including Pb(Zr,Ti)O3, BaTiO3 and (K,Na)NbO3 (KNN). We emphasize KNN as a promising lead-free piezoelectric compound with a high Curie temperature and high piezoelectric properties and describe its synthesis and characterization. In particular, details are presented for nanoscale piezoelectricity characterization of a single KNN nanocrystal by piezoresponse force microscopy. Finally, this review describes recent progress in applications based on one dimensional piezoelectric nanostructures with a focus on energy harvesting composite materials.

High piezoelectricity of (K,Na)NbO3 (KNN) lead-free materials benefits from a polymorphic phase transition (PPT) around room temperature, but its temperature sensitivity has been a bottleneck impeding their applications. We find that good thermal stability can be achieved in CaZrO3-modified KNN lead-free piezoceramics, in which the normalized strain d33* almost keeps constant from room temperature up to 140 oC. In situ synchrotron X-ray diffraction experiments combined with permitivity measurements disclose the occurrence of a new phase transformation under an electrical field, which extends the transition range between tetragonal and orthorhombic phases. It is revealed that such an electrically-enhanced diffused polymorphic phase transition (EED-PPT) contributed to the boosted thermal stability of KNN based lead-free piezoceramics with high piezoelectricity. The present approach based on phase engineering should also be effective in endowing other lead-free piezoelectrics with high piezoelectricity and good temperature stability.

Morphotropic phase boundary (MPB) in Nb-doped Pb(Zr,Ti)O3 thin films grown epitaxially on Nb:SrTiO3 (111) surfaces was studied by investigating their local piezoelectric response and domain structure using piezoresponse force microscopy. A sharp peak of piezoelectric coefficients was observed at Zr/Ti = 40/60, supporting the fact that the substrate constraint shifted the MPB in [111]-epitaxial Pb(Zr,Ti)O3 thin films toward the PbTiO3 side of composition from Zr/Ti = 52/48 for bulk materials. The domains at MPB also show a distinguishing structure with shrinking size and remarkably larger vertical piezoresponse compared to the single-phase samples. This work provided a deeper understanding about the effect of substrate constraint on the phase structures and electrical properties of epitaxial ferroelectric films.

Piezoelectric ceramics is a versatile functional material that can realize interconversion between electrical energy and mechanical energy. As the electrical properties of piezoelectric ceramics are extremely sensitive to the grain size variation, the investigation of grain size effect has attracted much attention. In this paper, the recent research progress of the grain size effect on perovskite piezoelectric ceramics, including barium titanate (BT), lead zirconate titanate (PZT), potassium sodium niobate (KNN), and sodium bismuth titanate (BNT), is comprehensively reviewed. We especially focus on topics including feasible ways of fabricating piezoelectric ceramics with the desired grain sizes, the influence of the grain size effect on piezoelectric properties, and the corresponding physical mechanisms. This review would be beneficial to understanding the influence of the grain size effect on piezoelectric properties. The review concludes with the prediction of the further investigation on the grain size effect.

Ultrasonic motors provide high torques and quick responses compared to their magnetic counterparts; therefore, they are widely used in small-scale applications such as mobile phones, microrobots, and auto-focusing modules in digital cameras. To determine the feasibility of lead-free piezoceramics for ultrasonic motor applications, we fabricated a ring-type piezoceramic with a KNN-based lead-free piezoceramic (referred to as CZ5), intended for use in an auto-focusing module of a digital camera. The vibration of the lead-free stator was observed at 45.1 kHz. It is noteworthy that the fully assembled lead-free ultrasonic motor exhibited a revolution speed of 5-7 rpm, even though impedance matching with neighboring components was not considered. This result suggests that the tested KNN-based piezoceramic has great potential for use in ultrasonic motor applications, requiring minimal modifications to existing lead-based systems.

The origins of distinctive compositional dependence of relative permittivity, ɛr, in the Pb-free piezoelectric system (1 − x) Na0.5K0.5NbO3–xLiTaO3, x ⩽ 10 mol.% modified with BiScO3 have been revealed using transmission electron microscopy with energy dispersive X-ray analysis (TEM-EDX). As the LiTaO3 content increased the Curie peak at ∼370 °C in ɛr–T plots became more diffuse, and at x = 5 mol.% an additional higher temperature peak occurred. TEM-EDX analysis showed the change in dielectric properties at x = 5 mol.% was due to a change in microstructure: micron-scale grains were replaced by submicron grains exhibiting core–shell chemical segregation. The outer shell was similar to the target solid solution composition, slightly enriched in Bi, Sc and Ta, whilst the core approximated to (Na, K, Li)NbO3 and was responsible for the additional dielectric peak. Examples of a novel three-tier metastable grain structure were observed for certain compositions.

The temperature dependence of piezoelectric properties (direct piezoelectric coefficient d33, converse piezoelectric coefficient d33(E = 0), strain S and electromechanical coupling coefficient kp) for two niobate-based lead-free piezoceramics have been contrasted. 0.92(Na0.5K0.5)NbO3–0.02(Bi1/2Li1/2)TiO3–0.06BaZrO3 (6BZ/2BLT/92NKN) has a morphotropic phase boundary (MPB) between rhombohedral and tetragonal at room temperature and 0.92(Na0.5K0.5)NbO3–0.03(Bi1/2Li1/2)TiO3–0.05BaZrO3 (5BZ/3BLT/92NKN) features an MPB engineered to be located below room temperature. At 30°C, d33, d33(E = 0), S (at 2 kV/mm), and kp are 252 pC/N, 230 pm/V, 0.069%, 0.51 for 5BZ/3BLT/92NKN; and 348 pC/N, 380 pm/V, 0.106%, 0.57 for 6BZ/2BLT/92NKN, respectively. With increasing temperature, the piezoelectric properties decrease. At 200°C, d33, d33(E = 0), S (at 2 kV/mm), and kp are 170 pC/N, 160 pm/V, 0.059%, 0.36 for 5BZ/3BLT/92NKN; and 181 pC/N, 190 pm/V, 0.061%, 0.39 for 6BZ/2BLT/92NKN. It is found that the electromechanical coupling coefficient has a better temperature stability than the piezoelectric coefficient in the studied system due to a large temperature-dependent compliance change. The results demonstrate that engineering an MPB is highly effective in tailoring temperature stability of piezoceramics.

Legislation arising from health and environmental concerns has intensified research into finding suitable alternatives to lead-based piezoceramics. Recently, solid solutions based on sodium potassium niobate (K,Na)NbO3 (KNN) have become one of the globally-important lead-free counterparts, due to their favourable dielectric and piezoelectric properties. This data article provides information on the ferroelectric properties and core–shell grain structures for the system, (1−y)[(1−x)Na0.5K0.5NbO3 – xLiTaO3] – yBiScO3 (x=0–0.1, y=0.02, abbreviated as KNN–xLT–2BS). We show elemental analysis with aid of TEM spot-EDX to identify three-type grain-types in the KNN–LT–BS ternary system. Melting behaviour has been assessed using a tube furnace with build-in camera. Details for the ferroelectric properties and core–shell chemical segregation are illustrated.

Growing environmental concerns are pushing the development of lead-free piezoceramics with both outstanding piezoelectric properties and reasonable thermal stability. Herein, we realized a large piezoelectric coefficient d33 of 430 pC N−1 in 0.96(K0.4Na0.6)(Nb0.96Sb0.04)O3–0.04Bi0.5K0.5Zr0.85Sn0.15O3 (KNNS–BKZS) polycrystals by constructing a rhombohedral–tetragonal (R–T) phase boundary. Investigations of the in situ thermal stability of the piezoelectric properties on multiple scales reveal that the micro-scale piezoelectric response is much more stable compared to the macro-scale response, indicating the significant role of extrinsic contributions from domain wall movements. These findings demonstrate the relationship between multi-scale properties and domain structures, revealing that the high piezoelectricity is attributed to nano-domains at the R–T phase boundary.