The mechanism for the enhanced piezoelectricity in multi-elements doped (K,Na)NbO3 ceramics

Contribution of irreversible non-180° domain to performance for multiphase coexisted potassium sodium niobate ceramics

Despite the dominance of lead-based piezoelectric materials with ultrahigh electric-field-induced strain in actuating applications, seeking eco-friendly substitutes with an equivalent performance remains an urgent demand. Here, a strategy of regulating the irreversible non-180° domain via phase engineering is introduced to optimize the available strain (the difference between the maximum strain and the remnant strain in a unipolar strain curve) in the lead-free potassium-sodium niobate-based piezoelectric ceramics. In situ synchrotron X-ray diffraction and Rayleigh analysis reveal the contribution of the non-180° domain to available strain in the tetragonal-orthorhombic-rhombohedral phase boundary. The reducing orthorhombic phase and increasing rhombohedral/tetragonal phase accompanied by the reduced irreversible non-180° domain are obtained with increasing doping of Sb5+, resulting in an enlarged available strain due to the significantly lowered remnant strain. This optimization is mainly attributed to the reduced irreversible non-180° domain wall motion and the increased lattice distortion, which are beneficial to decrease extrinsic contribution and enhance intrinsic contribution. The mesoscopic structure of miniaturized nanosized domain with facilitated domain switching also contributes to the enhancement of available strain due to the improved random field and decreased energy barrier. The study will shed light on the design of lead-free high-performance piezoelectric ceramics for actuator applications.

Novel Sandwich-Structured Flexible Composite Films with Enhanced Piezoelectric Performance

Piezoelectric poly(vinylidene fluoride) (PVDF) and its copolymers have been widely investigated for applications in wearable electric devices and sensing systems, owing to their intrinsic piezoelectricity and superior flexibility. However, their weak piezoelectricity poses major challenges for practical applications. To overcome these challenges, we propose a two-step synthesis approach to fabricate sandwich-structured piezoelectric films (BaTiO3@PDA/PVDF/BaTiO3@PDA) with significantly enhanced ferroelectric and piezoelectric properties. As compared to pristine PVDF films or conventional 0-3 composite films, a maximum polarization (Pmax) of 11.24 μC/cm2, a remanent polarization (Pr) of 5.83 μC/cm2, and an enhanced piezoelectric coefficient (d33 ∼ 14.6 pC/N) were achieved. Simulation and experimental results have demonstrated that the sandwich structure enhances the ability of composite films to withstand higher poling electric fields in comparison with 0-3 composites. The sandwich-structured piezoelectric films are further integrated into a wireless sensor system with a high force sensitivity of 288 mV/N, demonstrating great potential for movement monitoring applications. This facile approach shows great promise for the large-scale production of composite films with remarkable flexibility, ferroelectricity, and piezoelectricity for wearable sensing devices.

Stretchable polymer composites with ultrahigh piezoelectric performance

Flexible piezoelectric materials capable of withstanding large deformation play key roles in flexible electronics. Ferroelectric ceramics with a high piezoelectric coefficient are inherently brittle, whereas polar polymers exhibit a low piezoelectric coefficient. Here we report a highly stretchable/compressible piezoelectric composite composed of ferroelectric ceramic skeleton, elastomer matrix and relaxor ferroelectric-based hybrid at the ceramic/matrix interface as dielectric transition layers, exhibiting a giant piezoelectric coefficient of 250 picometers per volt, high electromechanical coupling factor k_eff of 65%, ultralow acoustic impedance of 3MRyl and high cyclic stability under 50% compression strain. The superior flexibility and piezoelectric properties are attributed to the electric polarization and mechanical load transfer paths formed by the ceramic skeleton, and dielectric mismatch mitigation between ceramic fillers and elastomer matrix by the dielectric transition layer. The synergistic fusion of ultrahigh piezoelectric properties and superior flexibility in these polymer composites is expected to drive emerging applications in flexible smart electronics.

Polarity Modulation Induced High Electrostrain Performance with Near-Zero Hysteresis in a (Sr0.7Bi0.2□0.1)TiO3-Based System

High-precision piezo actuators necessitate dielectrics with high electrostrain performance with low hysteresis. Polarity-modulated (Sr_0.7Bi_0.2□0.1)TiO₃-based ceramics exhibit extraordinarily discrete multiphase coexistence regions: (i) the relaxor phase coexistence (RPC) region with local weakly polar tetragonal (T) and pseudocubic (Pc) short-range polar nanodomains and (ii) the ferroelectric phase coexistence (FPC) region with T long-range domains and Pc nanodomains. The RPC composition features a specially high and pure electrostrain performance with near-zero hysteresis (S ∼ 0.185%, Q₃₃ ∼ 0.038 m⁴·C^-2), which is double those of conventional Pb(Mg_1/3Nb_2/3)O₃-based ceramics. Particular interest is paid to the RPC and FPC with multiscale characterization to unravel local structure-performance relationships. Guided by piezoelectric force microscopy, scanning transmission electron microscopy, and phase-field simulations, the RPC composition with multiphase low-angle weakly polar nanodomains shows local structural heterogeneity and contributes to a flat local free energy profile and thus to nanodomain switching and superior electrostrain performance, in contrast to the FPC composition with a macroscopic domain that shows stark hysteresis. This work provides a paradigm to design high-precision actuator materials with large electrostrain and ultralow hysteresis, extending our knowledge of multiphase coexistence species in ferroelectrics.

Piezoelectric Materials and Sensors for Structural Health Monitoring: Fundamental Aspects, Current Status, and Future Perspectives

Structural health monitoring technology can assess the status and integrity of structures in real time by advanced sensors, evaluate the remaining life of structure, and make the maintenance decisions on the structures. Piezoelectric materials, which can yield electrical output in response to mechanical strain/stress, are at the heart of structural health monitoring. Here, we present an overview of the recent progress in piezoelectric materials and sensors for structural health monitoring. The article commences with a brief introduction of the fundamental physical science of piezoelectric effect. Emphases are placed on the piezoelectric materials engineered by various strategies and the applications of piezoelectric sensors for structural health monitoring. Finally, challenges along with opportunities for future research and development of high-performance piezoelectric materials and sensors for structural health monitoring are highlighted.

Water Quenched and Acceptor-Doped Textured Piezoelectric Ceramics for Off-Resonance and On-Resonance Devices

Piezoelectric materials should simultaneously possess the soft properties (high piezoelectric coefficient, d₃₃ ; high voltage coefficient, g₃₃ ; high electromechanical coupling factor, k) and hard properties (high mechanical quality factor, Q_m ; low dielectric loss, tan δ) along with wide operation temperature (e.g., high rhombohedral-tetragonal phase transition temperature T_r-t ) for covering off-resonance (figure of merit (FOM), d₃₃  × g₃₃ ) and on-resonance (FOM, Q_m  × k² ) applications. However, achieving hard and soft piezoelectric properties simultaneously along with high transition temperature is quite challenging since these properties are inversely related to each other. Here, through a synergistic design strategy of combining composition/phase selection, crystallographic texturing, defect engineering, and water quenching technique, <001> textured 2 mol% MnO₂ doped 0.19PIN-0.445PSN-0.365PT ceramics exhibiting giant FOM values of Q_m  × k 31 2 $k_{31}^2$ (227-261) along with high d₃₃  × g₃₃ (28-35 × 10^-12 m² N^-1 ), low tan δ (0.3-0.39%) and high T_r-t of 140-190 °C, which is far beyond the performance of the state-of-the-art piezoelectric materials, are fabricated. Further, a novel water quenching (WQ) room temperature poling technique, which results in enhanced piezoelectricity of textured MnO₂ doped PIN-PSN-PT ceramics, is reported. Based upon the experiments and phase-field modeling, the enhanced piezoelectricity is explained in terms of the quenching-induced rhombohedral phase formation. These findings will have tremendous impact on development of high performance off-resonance and on-resonance piezoelectric devices with high stability.

Perovskite Piezoelectric-Based Flexible Energy Harvesters for Self-Powered Implantable and Wearable IoT Devices

In the ongoing fourth industrial revolution, the internet of things (IoT) will play a crucial role in collecting and analyzing information related to human healthcare, public safety, environmental monitoring and home/industrial automation. Even though conventional batteries are widely used to operate IoT devices as a power source, these batteries have a drawback of limited capacity, which impedes broad commercialization of the IoT. In this regard, piezoelectric energy harvesting technology has attracted a great deal of attention because piezoelectric materials can convert electricity from mechanical and vibrational movements in the ambient environment. In particular, piezoelectric-based flexible energy harvesters can precisely harvest tiny mechanical movements of muscles and internal organs from the human body to produce electricity. These inherent properties of flexible piezoelectric harvesters make it possible to eliminate conventional batteries for lifetime extension of implantable and wearable IoTs. This paper describes the progress of piezoelectric perovskite material-based flexible energy harvesters for self-powered IoT devices for biomedical/wearable electronics over the last decade.

Deciphering the atomic-scale structural origin for large dynamic electromechanical response in lead-free Bi(0.5)Na(0.5)TiO(3)-based relaxor ferroelectrics

Despite the extraordinary electromechanical properties of relaxor ferroelectrics, correlating their properties to underlying atomic-scale structures remains a decisive challenge for these "mess" systems. Here, taking the lead-free relaxor ferroelectric Bi_0.5Na_0.5TiO₃-based system as an example, we decipher the atomic-scale structure and its relationship to the polar structure evolution and large dynamic electromechanical response, using the direct atomic-scale point-by-point correlation analysis. With judicious chemical modification, we demonstrate the increased defect concentration is the main driving force for deviating polarizations with high-angle walls, leading to the increased random field. Meanwhile, the main driving force for deviating polarizations with low-angle walls changes from the anti-phase oxygen octahedral tilting to the multidirectional A-O displacement, leading to the decreased anisotropy field. Benefiting from the competitive and synergetic equilibrium of anisotropic field versus random field, the facilitated polarization rotation and extension versus facilitated domain switching are identified to be responsible for the giant electromechanical response. These observations lay a foundation for understanding the "composition-structure-property" relationships in relaxor ferroelectric systems, guiding the design of functional materials for electromechanical applications.

Expedient secondary functions of flexible piezoelectrics for biomedical energy harvesting

Flexible piezoelectrics realise the conversion between mechanical movements and electrical power by conformally attaching onto curvilinear surfaces, which are promising for energy harvesting of biomedical devices due to their sustainable body movements and/or deformations. Developing secondary functions of flexible piezoelectric energy harvesters is becoming increasingly significant in recent years via aiming at issues that cannot be addressed or mitigated by merely increasing piezoelectric efficiencies. These issues include loose interfacial contact and pucker generation by stretching, power shortage or instability induced by inadequate mechanical energy, and premature function degeneration or failure caused by fatigue fracture after cyclic deformations. Herein, the expedient secondary functions of flexible piezoelectrics to mitigate above issues are reviewed, including stretchability, hybrid energy harvesting, and self-healing. Efforts have been devoted to understanding the state-of-the-art strategies and their mechanisms of achieving secondary functions based on piezoelectric fundamentals. The link between structural characteristic and function performance is unravelled by providing insights into carefully selected progresses. The remaining challenges of developing secondary functions are proposed in the end with corresponding outlooks. The current work hopes to help and inspire future research in this promising field focusing on developing the secondary functions of flexible piezoelectric energy harvesters.

Molten-Salt Processed Potassium Sodium Niobate Single-Crystal Microcuboids with Dislocation-Induced Nanodomain Structures and Relaxor Ferroelectric Behavior

We herein report a facile molten-salt synthetic strategy to prepare transparent and uniform Li, Ba-doped (K,Na)NbO₃ (KNN) single-crystal microcuboids (∼80 μm). By controlling the degree of supersaturation, different growth modes were found and the single-crystal microcuboids were synthesized via island-like oriented attachment of KNN particles onto the growing surface. The distinct relaxor ferroelectric (RFE) properties were achieved in the single-crystal microcuboids, which were different from the normal ferroelectric (FE) properties found in their KNN ceramic counterparts prepared through a solid-state reaction using the same initial precursors. The RFE properties were realized by dislocation-induced nanodomain formation during oriented attachment growth of single-crystal microcuboids, which is different from the current strategies to derive the nanodomains by the local compositional inhomogeneity or the application of an electric field. The dislocations served as nucleation sites for ferroelectric domain walls and block the growth of domains. The KNN single-crystal microcuboids exhibited a higher effective piezoelectric coefficient (∼459 pm/V) compared to that of the bulk KNN ceramic counterpart (∼90 pm/V) and showed the broad diffuse maxima in the temperature dependence dielectric permittivity. The high maximum polarization (69.6 μC/cm²) at a relatively low electric field (30 kV/cm) was beneficial for energy storage applications. Furthermore, the KNN-based transparent, flexible pressure sensor directly monitored the mechanical motion of human activity without any external electric power. This study provides insights and synthetic strategies of single-crystal RFE microcuboids for other different perovskites, in which nanodomain structures are primarily imposed by their chemical composition.

Controllable Piezo-flexoelectric Effect in Ferroelectric Ba0.7Sr0.3TiO3 Materials for Harvesting Vibration Energy

The rapid development of the automotive and aerospace industries has led to an increasingly urgent need for electromechanical coupling materials and devices. Here, we have demonstrated the tunable piezo-flexoelectric effect in ferroelectric Ba_0.7Sr_0.3TiO₃ materials for scavenging vibration energy. The positive peak output current of an ITO/Ba_0.7Sr_0.3TiO₃/Ag cantilever device based on the flexoelectric effect is only 45 nA at room temperature, which is promoted to 90 nA by the piezo-flexoelectric effect. In addition, the piezo-flexoelectric current of the device can be further boosted to 270 nA by increasing the working temperature to 41.0 °C with a corresponding enhancement ratio of 348.28%. The significantly improved piezo-flexoelectric current is ascribed to the ultrahigh dielectric constant, which is related to the tetragonal-cubic phase transition of the Ba_0.7Sr_0.3TiO₃ materials. This work reveals the temperature-modulated piezo-flexoelectric effect in ferroelectric Ba_0.7Sr_0.3TiO₃ materials, providing a convenient route for scavenging and sensing of vibration energy.

Domain Engineering in Bulk Ferroelectric Ceramics via Mesoscopic Chemical Inhomogeneity

Domain engineering in ferroelectrics endows flexibility for different functional applications. Whereas the domain engineering strategy for single crystals and thin films is diverse, there is only a limited number of strategies for bulk ceramics. Here, a domain engineering strategy for achieving a compact domain architecture with increased domain-wall density in (K,Na)NbO₃ (KNN)-based ferroelectric ceramics via mesoscopic chemical inhomogeneity (MCI) is developed. The MCI-induced interfaces can effectively hinder domain continuity and modify the domain configuration. Besides, the MCI effect also results in diffused phase transitions, which is beneficial for achieving enhanced thermal stability. Modulation of chemical inhomogeneity demonstrates great potential for engineering desirable domain configuration and properties in ferroelectric ceramics. Additionally, the MCI can be easily controlled by regulating the processing condition during solid-state synthesis, which is advantageous to industrial production.

Microstructure and Intrinsic Strain of Nanocrystals in Ferroelectric (Na,K)NbO3 Nanofibers

Densely woven highly crystallized biocompatible sodium-potassium niobate Na_0.35K_0.65NbO₃ fibers with an average diameter of 100-200 nm and several hundreds of microns in length were sintered by the sol-gel calcination-assisted electrospinning technique. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM) confirmed preferential cube-on-cube [001] orientation of nanocrystals within the fiber's body, separated by a low angle grain boundary. The Williamson-Hall method was employed to analyze the broadening of XRD reflections and to accurately determine the size and intrinsic strain of nanocrystal fiber aggregates. The main objective of this article is to test the potential capacity of direct XRD analysis to noninvasively control crystallite size and lattice distortion in core-shell coaxial nanofibers.

High-k perovskite gate oxide for modulation beyond 1014 cm-2

Scaling down of semiconductor devices requires high-k dielectric materials to continue lowering the operating voltage of field-effect transistors (FETs) and storing sufficient charge on a smaller area. Here, we investigate the dielectric properties of epitaxial BaHf_0.6Ti_0.4O₃ (BHTO), an alloy of perovskite oxide barium hafnate (BaHfO₃) and barium titanate (BaTiO₃). We found the dielectric constant, the breakdown field, and the leakage current to be 150, 5.0 megavolts per centimeter (MV cm^-1), and 10^-4 amperes per square centimeter at 2 MV cm^-1, respectively. The results suggest that two-dimensional (2D) carrier density of more than n_2D = 10¹⁴ per square centimeter (cm^-2) could be modulated by the BHTO gate oxide. We demonstrate an n-type accumulation mode FET and direct suppression of more than n_2D = 10¹⁴ cm^-2 via an n-type depletion-mode FET. We attribute the large dielectric constant, high breakdown field, and low leakage current of BHTO to the nanometer scale stoichiometric modulation of hafnium and titanium.

Compositionally Graded KNN-Based Multilayer Composite with Excellent Piezoelectric Temperature Stability

The inherent disadvantage of lead-free potassium sodium niobate (KNN)-based ceramics is the severe temperature instability of piezoelectric charge coefficient (d₃₃ ) caused by the polymorphic phase boundary. Herein, a new concept of structural gradient is proposed by designing compositionally graded multilayer composites with multiple successive phase transitions, to solve the challenge of the inferior temperature stability. The structural gradient ceramics exhibit a superior temperature reliability (d₃₃ remains almost unchanged in the temperature range of 25-100 °C), far outperforming the previously reported KNN counterparts with d₃₃ variation above 27% over the same temperature range. The synergistic contribution of the continuous phase transition, the strain gradient, and the complementary effect of each constituent layer leads to the excellent temperature stability, which is also confirmed by phase-field simulation. These findings are expected to provide a new paradigm for functional material design with outstanding temperature stability.

Comparison of contribution to phase boundary from A-site aliovalent dopants in high-performance KNN-based ceramics

Different influence of A-site aliovalent dopants, Bi3+ and Ca2+, on phase boundary and electrical properties in KNN-based ceramics.

Optimization of Ferroelectric Ordering and Thermal Stability in Na1/2Bi1/2TiO3-Based Lead-Free Single Crystal through Defect Engineering

Environmentally friendly lead-free piezoelectric materials have been attracting significant attention in recent years. Na_1/2Bi_1/2TiO₃-based relaxor ferroelectrics have found acceptance for application in promising lead-free transducers in high-power ultrasonic devices. However, their low thermal stability, i.e., their relatively low ferroelectric-relaxor transition temperature (T_F-R), hinders their practical application. Herein, a thermal-quenching approach is applied on a Na_1/2Bi_1/2TiO₃ (NBT)-based single crystal, which yields a large increase in T_F-R and dramatic enhancement of its ferroelectric ordering, leading to excellent thermal stability of its dielectric, ferroelectric, and piezoelectric properties. This behavior is mainly attributed to quenching-induced domain evolution as well as its octahedral tilt, which is linked to the increased oxygen vacancies. The substitution of long-range ordered ferroelectric domains for short-range polar nanodomains contributes to its increased coherence length and, consequently, enhancement of T_F-R. This work provides an approach to the optimization of the ferroelectric ordering and thermal stability of NBT as well as an in-depth understanding of the quenching effect on the local structure, which could be applied to other relaxor-based ferroelectrics for optimization of their macroscopic properties.

Feasible Way to Achieve Multifunctional (K, Na)NbO3-Based Ceramics: Controlling Long-Range Ferroelectric Ordering

It is challenging to achieve highly tunable multifunctional properties in one piezoelectric ceramic system through a simple method due to the complicated relationship between the microscopic structure and macroscopic property. Here, multifunctional potassium sodium niobate [(K, Na)NbO₃ (KNN)]-based lead-free piezoceramics with tunable piezoelectric and electrostrictive properties are achieved by controlling the long-range ferroelectric ordering (LRFO) through antimony (Sb) doping. At a low Sb doping, the slightly distorted NbO₆ octahedron and the softened B-O repulsion well maintain the LRFO and induce plenty of nanoscale domains coexisting with a few polar nanoregions (PNRs). Thereby, the diffused rhombohedral-orthorhombic-tetragonal (R-O-T) multiphase coexistence with distinct dielectric jumping is constructed near room temperature, by which the nearly 2-fold increase in the piezoelectric coefficient (d₃₃ ∼ 539 pC/N) and the temperature-insensitive strain (the unipolar strain varies less than 8% at 27-120 °C) are obtained. At a high Sb doping, the LRFO is significantly destroyed, leading to predominant PNRs. Thus, a typical relaxor is obtained at the ferroelectric-paraelectric phase transition near room temperature, in which a large electrostrictive coefficient (Q₃₃ = 0.035 m⁴/C²), independent of the electric field and temperature, is obtained and comparable to that of lead-based materials. Therefore, our results prove that controlling the LRFO is a feasible way to achieve high-performance multifunctional KNN-based ceramics and is beneficial to the future composition design for KNN-based ceramics.

On-Body Piezoelectric Energy Harvesters through Innovative Designs and Conformable Structures

Recent advancements in wearable technology have improved lifestyle and medical practices, enabling personalized care ranging from fitness tracking, to real-time health monitoring, to predictive sensing. Wearable devices serve as an interface between humans and technology; however, this integration is far from seamless. These devices face various limitations such as size, biocompatibility, and battery constraints wherein batteries are bulky, are expensive, and require regular replacement. On-body energy harvesting presents a promising alternative to battery power by utilizing the human body's continuous generation of energy. This review paper begins with an investigation of contemporary energy harvesting methods, with a deep focus on piezoelectricity. We then highlight the materials, configurations, and structures of such methods for self-powered devices. Here, we propose a novel combination of thin-film composites, kirigami patterns, and auxetic structures to lay the groundwork for an integrated piezoelectric system to monitor and sense. This approach has the potential to maximize energy output by amplifying the piezoelectric effect and manipulating the strain distribution. As a departure from bulky, rigid device design, we explore compositions and microfabrication processes for conformable energy harvesters. We conclude by discussing the limitations of these harvesters and future directions that expand upon current applications for wearable technology. Further exploration of materials, configurations, and structures introduce interdisciplinary applications for such integrated systems. Considering these factors can revolutionize the production and consumption of energy as wearable technology becomes increasingly prevalent in everyday life.

Piezoelectric Nanomaterials Activated by Ultrasound: The Pathway from Discovery to Future Clinical Adoption

Electrical stimulation has shown great promise in biomedical applications, such as regenerative medicine, neuromodulation, and cancer treatment. Yet, the use of electrical end effectors such as electrodes requires connectors and batteries, which dramatically hamper the translation of electrical stimulation technologies in several scenarios. Piezoelectric nanomaterials can overcome the limitations of current electrical stimulation procedures as they can be wirelessly activated by external energy sources such as ultrasound. Wireless electrical stimulation mediated by piezoelectric nanoarchitectures constitutes an innovative paradigm enabling the induction of electrical cues within the body in a localized, wireless, and minimally invasive fashion. In this review, we highlight the fundamental mechanisms of acoustically mediated piezoelectric stimulation and its applications in the biomedical area. Yet, the adoption of this technology in a clinical practice is in its infancy, as several open issues, such as piezoelectric properties measurement, control of the ultrasound dose in vitro, modeling and measurement of the piezo effects, knowledge on the triggered bioeffects, therapy targeting, biocompatibility studies, and control of the ultrasound dose delivered in vivo, must be addressed. This article explores the current open challenges in piezoelectric stimulation and proposes strategies that may guide future research efforts in this field toward the translation of this technology to the clinical scene.

Sizing up (K1−xNax)NbO3 films: a review of synthesis routes, properties & applications

This review discusses (K,Na)NbO₃ thin films, with a focus on synthesis, chemically modifying properties, plus piezoelectric and biomedical KNN devices.