Lead-free Piezoelectrics Research Articles

Stabilizing oxygen vacancies and promoting electrostrain in lead-free potassium niobate-based piezoelectrics over wide temperature ranges

Stabilizing oxygen vacancies and promoting electrostrain in lead-free potassium niobate-based piezoelectrics over wide temperature ranges

Superior Piezo-/Ferro-Electricity in Antiferroelectric AgxNbO3-δ Thin Films by Nanopillar Local Structure Design.

Antiferroelectrics are fundamental mother compounds critical in developing innovative lead-free piezoelectrics and ferroelectrics and hold great promise for wide-ranging applications in energy conversion and electronic devices. However, harnessing their superior properties presents a significant challenge due to the delicate balance required between their various states. In this study, through the unique design of nanopillar structures to alleviate the local polar heterogeneity, we have achieved significantly improved piezo-/ferro-electricity in classic lead-free antiferroelectric AgxNbO3-δ (x = 1, 0.9, and 0.8) epitaxial thin films. The effective piezoelectric coefficient reaches 440 pm V-1, 1 order of magnitude larger than the stoichiometric AgNbO3, rivaling classic lead zirconate titanate piezoelectrics. Atomic-scale electron microscopy investigations unravel the underlying mechanisms. The nanopillars, characterized by antisite occupancy of both Ag and Nb atoms and forming out-of-phase boundaries with the matrix, reduce the local crystal symmetry via interphase strain. This leads to the creation of flexible multinanodomain structures that significantly facilitate polarization rotation, thus substantially enhancing the piezoelectric performance. This study demonstrates the feasibility of engineering local heterogeneity through nanopillar design, offering a generally applicable method for property improvement of a wide range of antiferroelectrics.

Water-mediated production of K0.5Na0.5NbO3 piezoelectric ceramics with cold-sintering assisted route: Enhanced dielectric properties

Cold sintering is an attractive method to densify ceramics, providing the opportunity to produce ferroelectrics efficiently. This is particularly important for cases where ceramics like perovskites are prone to high A-site element volatilization at high temperatures. The resulting uncontrolled creation of defects is often detrimental to the ferroelectric properties. In this work, cold-sintering assisted production of KNN ceramics with water (no corrosive transient liquid) leads to a homogenous, high-density pure KNN with high dielectric permittivity and lower dielectric losses as compared to high-temperature sintering techniques. Our work could, therefore, open a new venue for further modification of defect chemistry in lead-free piezoelectrics.

Effects of B- and A/B-sites Codoping on lattice distortion and defect concentration for high electromechanical response of lead-free piezoelectrics

Effects of different thermal conditions on lead-free (Bi0.65Ba0.35)(Fe0.65Ti0.35)O3 ceramics were studied to reduce secondary phase formation, regulate bismuth vacancies and resulting oxygen vacancies, and Fe ions valence transition during the heat treatment process. The optimal thermal conditions led to a higher d33 = 200 pC/N and a d33* = 420 pm/V. A/B-sites ions replacement in BF35BT, BF35BT-Zr, and BF35BTSZ ceramics were prepared to reveal the effects of different Zr or Sr-Zr-contents. A maximum tetragonality (cT/aT) was estimated for the single doping (Zr) and co-doping (Sr and Zr) cases. For Zr = 0.02, the electric field-induced strain reached its maximum value of d33* = 562 pm/V. Similarly, a strain of d33* = 701 pm/V was observed for Sr-Zr = 0.02. Additionally, the d33* improved to 870 pm/V (90 °C) in Zr = 0.02 and 1130 pm/V (120 °C) in Sr-Zr=0.02. The presence of local structural heterogeneity causes the domain size to decrease as it moves from the micro to the nanoscale. The results demonstrate that improved performance can be associated with reduced concentration of oxygen vacancies, nanodomains by local structural heterogeneity, high lattice anisotropy, higher relative density, and appropriate grain size.

Ultrahigh Electrobending Deformation in Lead-Free Piezoelectric Ceramics via Defect Concentration Gradient Design.

Recent breakthroughs in defect-engineered lead-free piezoelectric ceramics have reported remarkable electrostrain values, surpassing the limit of lattice distortion. This has aroused wide concern on bending deformation and the associated underlying mechanism. Herein, via designing lead-free piezoelectric ceramics with varying volatilization characteristics, it is uncovered that the ultrahigh electrobending deformation is primarily attributed to a large strain gradient induced by unevenly distributed defect dipoles. In 0.5 mm thick Sr/Sn co-doped potassium sodium niobate ceramics featuring volatile K/Na elements, the inherent bipolar electrostrain value can reach 0.3% at 20kV cm-1 due to the existence of defect dipoles, while the gradient distribution of defect dipole generates significant bending displacement, amplifying apparent electrostrain value to 1.1%. Notably, nonvolatile Ba0.99TiO2.99 ceramic with homogeneous defect dipole distribution does not present electrobending. Of particular interest is that the electrobending phenomenon can be observed through introducing a defect dipole gradient into barium titanate ceramic. A monolayer ceramic with defect dipole gradient can generate large tip displacement (±1.5mm) in cantilever structure, demonstrating its promising potential in precise positioning. This study delves into the underlying mechanism driving electrobending deformation and its impact on the apparent electrostrain measurement in defect-engineered piezoelectric ceramics, providing fresh perspectives for the development of piezoelectric bending actuators.

Simultaneous increase in piezoelectric response and Curie point in BaTiO3 based Pb-free piezoceramic

Piezoelectric ceramics are used in wide-ranging applications as actuators, transducers, and sensors. For applications at ambient environmental conditions, it is desirable that the piezoelectric material exhibit large piezoelectric coefficient (d33) and possess Curie point well above room temperature. Among the lead-free piezoelectrics, the classical ferroelectric BaTiO3 exhibits Curie point and d33 ∼ 120 °C and 190 pC/N, respectively. Most attempts at improving the d33 of BaTiO3 via chemical modifications significantly reduces the Curie point (Tc). Here we show that it is possible to improve both the d33 and Tc of BaTiO3. We found this possibility while investigating a solid solution of BaTiO3 with BaAlO2.5. An optimum composition, 0.97 BaTiO3-0.03BaAlO2.5 of this solid solution exhibit d33 ∼356 pC/N, Curie point 125 °C, and good thermal stability of the piezo response.

Establishing a Relationship between the Piezoelectric Response and Oxygen Vacancies in Lead-Free Piezoelectrics.

BiFeO3-BaTiO3 (BF-BT)-based lead-free piezoceramics are desired materials for high-temperature applications of piezoelectric sensors with a high Curie temperature and good piezoelectric properties. Recent studies have shown that oxygen vacancies have a significant effect on electrostrain and piezoelectric properties. Interestingly, two different phenomena exist, i.e., the increase in piezoelectric properties is often associated with a decrease in the concentration of oxygen vacancies, while the increase in electrostrain is often associated with an increase in the concentration of oxygen vacancies. Especially, for BF-based ceramics, the physical mechanisms related to property differences caused by oxygen vacancies are rarely reported, which needs further exploration. Here, two ceramics with differences in their oxygen vacancy concentrations are designed. Based on Rayleigh analysis, thermal/electric field-induced domain response (ferroelectric scaling), and macro-microstructural characterization, we can conclude that the transient piezoelectric response and the aging process are significantly affected by the oxygen vacancy concentration. In other words, the increasing concentration of oxygen vacancies in BF-BT ceramics enhances the reversible piezoelectric response contributed by lattice distortion and strengthens the response of domain switching and domain wall motion to electric and thermal fields but deteriorates their aging behavior, which leads to the degradation of piezoelectric performance. Besides, polarization saturation and defect pegging significantly improve the temperature stability of the strain.

New High-Performance Piezoelectric: Ferroelectric Carbon-Boron Clathrate.

High-performance piezoelectrics have been extensively reported with a typical perovskite structure, in which a huge breakthrough in piezoelectric constants is found to be more and more difficult. Hence, the development of materials beyond perovskite is a potential means of achieving lead-free and high piezoelectricity in next-generation piezoelectrics. Here, we demonstrate the possibility of developing high piezoelectricity in the nonperovskite carbon-boron clathrate with the composition of ScB_{3}C_{3} using first-principles calculations. The robust and highly symmetric B-C cage with mobilizable Sc atom constructs a flat potential valley to connect the ferroelectric orthorhombic and rhombohedral structures, which allows an easy, continuous, and strong polarization rotation. By manipulating the cell parameter b, the potential energy surface can be further flattened to produce an extra-high shear piezoelectric constant d_{15} of 9424 pC/N. Our calculations also confirm the effectiveness of the partial chemical replacement of Sc by Y to form a morphotropic phase boundary in the clathrate. The significance of large polarization and high symmetric polyhedron structure is demonstrated for realizing strong polarization rotation, offering the universal physical principles to aid the search for new high-performance piezoelectrics. This work takes ScB_{3}C_{3} as an example to exhibit the great potential for realizing high piezoelectricity in clathrate structure, which opens the door to developing next-generation lead-free piezoelectric applications.

Enhanced Piezoelectricity and Thermal Stability of Electrostrain Performance in BiFeO3-Based Lead-Free Ceramics

BiFeO3-based ceramics possess an advantage over large spontaneous polarization and high Curie temperature, and are thus widely explored in the field of high-temperature lead-free piezoelectrics and actuators. However, poor piezoelectricity/resistivity and thermal stability of electrostrain make them less competitive. To address this problem, (1 - x) (0.65BiFeO3-0.35BaTiO3)-xLa0.5Na0.5TiO3 (BF-BT-xLNT) systems are designed in this work. It is found that piezoelectricity is significantly improved with LNT addition, which is contributed by the phase boundary effect of rhombohedral and pseudocubic phase coexistence. The small-signal and large-signal piezoelectric coefficient (d33 and d33*) peaks at x = 0.02 with 97 pC/N and 303 pm/V, respectively. The relaxor property and resistivity are enhanced as well. This is verified by Rietveld refinement, dielectric/impedance spectroscopy and piezoelectric force microscopy (PFM) technique. Interestingly, a good thermal stability of electrostrain is obtained at x = 0.04 composition with fluctuation η = 31% (Smax'-SRTSRT×100%), in a wide temperature range of 25-180 °C, which is considered as a compromise of negative temperature dependent electrostrain for relaxors and the positive one for ferroelectric matrix. This work provides an implication for designing high-temperature piezoelectrics and stable electrostrain materials.

Electrode-dependent asymmetric conduction mechanisms in K0.5Na0.5NbO3 micro-capacitors

The ultimate performance of devices employing lead-free piezoelectrics is determined not only by the intrinsic properties of the piezo, but also by processes and materials employed to create the electric contacts. In this paper, we investigate the impact of different metallic electrodes with increasing chemical reactivity (Pt, Ni, Ti, Cr), on the asymmetric behavior of the leakage current in M/K0.5Na0.5NbO3/Pt(111) micro-capacitors, where M stands for the top metallic electrode. For all electrodes we found a marked leakage asymmetry that we ascribed to the presence of a Schottky-like rectifying junction at the M/K0·5Na0·5NbO3/Pt(111) bottom interface, while the corresponding junction at the top interface is deeply affected by the creation of oxygen vacancies due to oxygen scavenging during the growth of the top metallic electrodes, leading to an almost ohmic top contact. The leakage increases with the reactivity of the electrodes, while the asymmetry decreases, thus suggesting that the creation of the top metal/K0·5Na0·5NbO3 interface generates oxygen vacancies diffusing down to the bottom interface and impacting on the rectifying behavior of the Schottky-like junction. Noteworthy, this asymmetric conduction can reflect in an asymmetric piezoelectric and ferroelectric behavior, as a sizable portion of the applied voltage drops across the rectifying junction in reverse bias, thus hampering symmetric bipolar operation, especially in leaky materials.

Electric field-induced two-step phase transformation and its contribution to the electromechanical strain in lead-free relaxor-based ceramics

Sodium bismuth titanate-based solid solutions are important lead-free piezoelectrics with potential applications. Large electromechanical strain is of particular interest, which can be realized via the structural change in the relaxor states under an electric field. In this work, a polycrystalline ceramic 0.85Na1/2Bi1/2TiO3–0.15BaZr0.2Ti0.8O3, in which a long-range ordered ferroelectric phase and a relaxor state coexist, has been investigated to unveil the origin of its electromechanical strain using various experimental techniques. An ergodic-relaxor to nonergodic-relaxor transition is first observed under a relatively weak electric field, and a more stable long-range ferroelectric phase is induced under a larger electric field. This two-step phase transformation is accompanied with the process of local polarization freezing as well as ferroelectric domain growth. The domain formation during the reversible phase transition is found to be the main contribution to the macroscopic strain. Our investigation provides an in-depth understanding of the origin of reversible electromechanical strain in the NBT-based relaxor-ferroelectric system.

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

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

Enhanced piezoelectric properties and thermal stability of Bi0.5Na0.5TiO3 modified BiFeO3[sbnd]BaTiO3 ceramics with morphotropic phase boundary

Morphotropic phase boundary (MPB) plays a key role in tuning piezoelectric responses of ferroelectric ceramics. Here, Bi0·5Na0·5TiO3 modified BiFeO3BaTiO3 ternary solid solutions of 0.7BiFeO3-(0.3-x)BaTiO3-xBi0.5Na0·5TiO3 (referred to as BF-BT-xBNT, 0.00 ≤ x ≤ 0.04) were prepared for lead-free piezoelectrics. All the ceramics exhibit an MPB with coexisting rhombohedral (R) and tetragonal (T) phases, and the R/T phase ratio decreases upon increasing x. The increment of BNT promotes the grain growth, lowers the leakage current and Curie temperature (TC), and gradually drives the ferroelectric to relaxor transition. Because of the MPB with appropriate R/T phase ratio, increased grain size and density, and decreased leakage current, the well-balanced performance between d33 = 206 pC/N and TC = 488 °C is obtained in x = 0.01 case. In addition, the further enhanced in-situ d33 = 286–347 pC/N is obtained in BF-BT-xBNT ceramics along with the improved depolarization temperature Td from 280 to 312 °C, showing a potential application for lead-free piezoceramics at high temperature.

Centimeter-Sized Piezoelectric Single Crystal of Chiral Bismuth-Based Hybrid Halide with Superior Electrostrictive Coefficient.

The lead-free hybrid perovskite piezoelectrics possess advantages of easy processing, light weight, and low-toxicity over inorganic ceramics. However, the lack of understanding in structure-property relationships hinders exploration of new molecular piezoelectric crystals with excellent performances. Herein, by introducing chiral α-phenylethylammonium (α-PEA+ ) cations into bismuth-based hybrid halides, centimeter-sized (R-α-PEA)4 Bi2 I10 and (S-α-PEA)4 Bi2 I10 single crystals with a superior piezoelectric voltage coefficient g22 of 309mV m N-1 , are obtained. Structural rigidity in crystals leads to a remarkable electrostrictive coefficient Q22 of 25.8 m4 C-2 , nearly 20 times higher than that of poly(vinylidene fluoride) (PVDF), which is beneficial to improve piezoelectricity with the synergistic effect of chirality. Moreover, the as-grown crystals show outstanding phase stability from 173 K to ≈470 K. This work suggests a design strategy based on rigidity and chirality to exploit novel piezoelectrics among hybrid metal halides.

Is electrostrain >1% in oxygen deficient Na0.5Bi0.5TiO3 a composition effect?

Abstract For over two decades Na0.5Bi0.5TiO3 (NBT) -based lead-free piezoelectrics have attracted attention due to its ability to exhibit large electric-field driven strain. Compared to the popular Pb(Zr, Ti)O3 (PZT)-based piezoelectrics, which exhibit electrostrain of about 0.3%, the derivatives of NBT-based lead-free piezoelectrics at the ergodic—non ergodic relaxor crossover exhibit larger electric-field driven strain ∼0.45%. In recent years, there has been a concerted effort to increase the maximum electrostrain in lead-free piezoceramics. Recent reports suggest that oxygen deficient NBT- based piezoceramics can exhibit electrostrain ∼1%. In this paper we explore this phenomenon and show that the ultra high electric field driven strain measured is primarily a consequence of reducing the thickness of the disc dimension below 500 microns and not an exclusive effect of the composition.

Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT)/0.94(Na0.5Bi0.5)TiO3-0.06BaTiO3(NBT-BT) heterostructures for the applications of vibration sensing and energy harvester

Lead-free piezoelectrics are considered an alternative to lead-based piezo-material for various device applications such as energy harvesters, actuators, and sensors. Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) and 0.94(Na0.5Bi0.5)TiO3-0.06BaTiO3 (NBT-BT) are the lead-free piezoelectrics that are commonly studied due to their superior properties. In this study, we have fabricated thick films of NBT-BT (∼700 nm), BCZT (∼700 nm), and heterostructures comprising both BCZT/NBT-BT(∼500 nm) on commercial Pt(111)/TiO2/SiO2/Si substrates using Pulsed Laser Deposition method. The phase purity of these films was confirmed by X-ray diffraction analysis. The ferroelectric properties of these films were measured using piezo-response force microscopy. The devices were tested by subjecting them to the vibrations of varying frequencies and loads using a vibration simulator/tester for vibration sensing. The BCZT film generated a significant voltage output (∼2 mV). These thick films possess excellent sensitivity and high voltage output, making them candidates for device applications.

Development and application of screening-level risk analysis for emerging materials

Analysis of a material's impact on society is increasingly recognized as a necessary step in materials development, especially in the area of lead-free piezoelectrics. Evaluations of the environmental, health, and societal impacts that occur throughout the material's life cycle are critical for determining the viability of lead-free alternatives. Risk screening approaches, such as the screening-level Emerging Materials Risk Analysis (EMRA) proposed in this work, may help researchers compare materials or material production routes to determine more sustainable solutions. As a first demonstration of its utility in the development of lead-free piezoelectrics, the approach introduced in this paper is applied to piezoelectric HfO2 (hafnia) to compare mining and processing routes and to elucidate the more sustainable route for HfO2 production. This paper aims to exemplify how the EMRA risk screening approach incorporates perspectives on environmental, health, and societal impacts into the materials research process by providing a relative risk screening evaluation of different material processing routes and/or different materials. Results from applying EMRA to hafnia show that the major known environmental impacts of hafnia mining and processing involve ecosystem destruction and heavy use of fossil fuels and electricity; health impacts related to potentially unsafe working conditions and potential exposure to radioactive elements; and societal impacts including land disputes and supply concerns. Results also demonstrate that the more sustainable production route currently available includes commercial wet mining with land rehabilitation followed by beneficiation via wet processes with consistent personal protective equipment use and water recycling. Almost all of the previously-mentioned impacts are avoided in this life cycle route. Outcomes from this analysis identify hafnia as a potentially sustainable replacement for certain applications of PZT and therefore encourage continued development of the material. Future efforts will test EMRA on a wide variety of other materials and revise the approach accordingly.

Lead free 0.9Na1/2Bi1/2TiO3–0.1BaZr0.2Ti0.8O3 thin film with large piezoelectric electrostrain

A sodium bismuth titanate-based thin film is widely investigated lead-free piezoelectrics with potential applications for modern micro-devices such as PiezoMEMS. In this work, a 0.9Na1/2Bi1/2TiO3–0.1BaZr0.2Ti0.8O3 thin film was deposited on a Pt/Ti/SiO2/Si (001) substrate by the sol–gel spin coating method. The deposited piezoelectric film shows low dielectric loss and high remnant polarization. The measured ferroelectricity loop showed a coercive field of 110 kV/cm and a saturation polarization of 46.83 μC/cm2. The piezoelectric response of this thin film does not decrease from room temperature to around 100 °C. The fabricated piezoelectric device with bottom and top electrodes showed a large macro-scale strain value of ∼4% under the DC (30 V) and AC voltages (f = 800 kHz, Vpp = 10 V).

Call for Papers: Spotlight Issue on Lead-Free Piezoelectric Transducers

Call for Papers: Spotlight Issue on Lead-Free Piezoelectric Transducers

Heterostrain-enabled ultrahigh electrostrain in lead-free piezoelectric

Piezoelectric materials provide high strain and large driving forces in actuators and can transform electrical energy into mechanical energy. Although they were discovered over 100 years ago, scientists are still searching for alternative lead-free piezoelectrics to reduce their environmental impact. Developing high-strain piezoelectric materials has been a long-term challenge, particularly challenging for the design of high-strain polycrystalline piezoelectrics containing no toxic lead element. In this work, we report one strategy to enhance the electrostrain via designing “heterostrain” through atomic-scale defect engineering and mesoscale domain engineering. We achieve an ultrahigh electrostrain of 2.3% at high temperature (220 °C) in lead-free polycrystalline ceramics, higher than all state-of-the-art piezoelectric materials, including lead-free and lead-based ceramics and single crystals. We demonstrate practical solutions for achieving high electrostrain in low-cost environmentally piezoelectric for various applications.