Losses in Ferroelectric Materials

Giant energy storage and power density negative capacitance superlattices

Dielectric electrostatic capacitors1, because of their ultrafast charge-discharge, are desirable for high-power energy storage applications. Along with ultrafast operation, on-chip integration can enable miniaturized energy storage devices for emerging autonomous microelectronics and microsystems2-5. Moreover, state-of-the-art miniaturized electrochemical energy storage systems-microsupercapacitors and microbatteries-currently face safety, packaging, materials and microfabrication challenges preventing on-chip technological readiness2,3,6, leaving an opportunity for electrostatic microcapacitors. Here we report record-high electrostatic energy storage density (ESD) and power density, to our knowledge, in HfO2-ZrO2-based thin film microcapacitors integrated into silicon, through a three-pronged approach. First, to increase intrinsic energy storage, atomic-layer-deposited antiferroelectric HfO2-ZrO2 films are engineered near a field-driven ferroelectric phase transition to exhibit amplified charge storage by the negative capacitance effect7-12, which enhances volumetric ESD beyond the best-known back-end-of-the-line-compatible dielectrics (115 J cm-3) (ref. 13). Second, to increase total energy storage, antiferroelectric superlattice engineering14 scales the energy storage performance beyond the conventional thickness limitations of HfO2-ZrO2-based (anti)ferroelectricity15 (100-nm regime). Third, to increase the storage per footprint, the superlattices are conformally integrated into three-dimensional capacitors, which boosts the areal ESD nine times and the areal power density 170 times that of the best-known electrostatic capacitors: 80 mJ cm-2 and 300 kW cm-2, respectively. This simultaneous demonstration of ultrahigh energy density and power density overcomes the traditional capacity-speed trade-off across the electrostatic-electrochemical energy storage hierarchy1,16. Furthermore, the integration of ultrahigh-density and ultrafast-charging thin films within a back-end-of-the-line-compatible process enables monolithic integration of on-chip microcapacitors5, which can unlock substantial energy storage and power delivery performance for electronic microsystems17-19.

High Energy Density in All-Organic Polyimide-Based Composite Film by Doping of Polyvinylidene Fluoride-Based Relaxor Ferroelectrics

Recently, due to the advantages of superior compatibility, fewer interface defects, and a high electric breakdown field, all-organic dielectric composites have attracted significant research interest. In this investigation, we produced all-organic P(VDF-TrFE-CFE) terpolymer/PI (terp/PI) composite films by incorporating a small amount of terpolymer into PI substrates for high energy density capacitor applications. The resulting terp/PI-5 (5% terpolymer) composite films exhibit a permittivity of 3.81 at 1 kHz, which is 18.7% greater than that of pristine PI (3.21). Furthermore, the terp/PI-5 film exhibited the highest energy density (9.67 J/cm3) and a relatively high charge-discharge efficiency (84.7%) among the terp/PI composite films. The energy density of the terp/PI-5 film was increased by 59.8% compared to that of the pristine PI film. The TSDC results and band structure analysis revealed the presence of deeper traps in the terp/PI composites, contributing to the suppression of leakage current and improved charge-discharge efficiency. Furthermore, durability tests confirm the stability of the composite films under extended high-temperature exposure and cycling, establishing their viability for practical applications.

Chemical Vapor Deposition Synthesis of Intrinsic High-Temperature Ferroelectric 2D CuCrSe2

Ultrathin 2D ferroelectrics with high Curie temperature are critical for multifunctional ferroelectric devices. However, the ferroelectric spontaneous polarization is consistently broken by the strong thermal fluctuations at high temperature, resulting in the rare discovery of high-temperature ferroelectricity in 2D materials. Here, a chemical vapor deposition method is reported to synthesize 2D CuCrSe2 nanosheets. The crystal structure is confirmed by scanning transmission electron microscopy characterization. The measured ferroelectric phase transition temperature of ultrathin CuCrSe2 is about ≈800 K. Significantly, the switchable ferroelectric polarization is observed in ≈5.2 nm nanosheet. Moreover, the in-plane and out-of-plane ferroelectric response are modulated by different maximum bias voltage. This work provides a new insight into the construction of 2D ferroelectrics with high Curie temperature.

Investigation and optimization of In-Vitro behaviour of Perovskite barium titanate as a scaffold and protective coatings

The piezoelectric effect is widely known to have a significant physiological function in bone development, remodeling, and fracture repair. As a well-known piezoelectric material, barium titanate is particularly appealing as a scaffold layer to improve bone tissue engineering applications. Currently, the chemical bath deposition method is used to prepare green synthesized barium titanate coatings to improve mechanical and biological characteristics. Molarity of the solutions, an essential parameter in chemical synthesis, is changed at room temperature (0.1-1.2 Molar) to prepare coatings. The XRD spectra for as deposited coatings indicate amorphous behavior, while polycrystalline nature of coatings is observed after annealing (300 °C). Coatings prepared with solutions of relatively low molarities, i.e. from 0.1 to 0.8 M, exhibit mixed tetragonal - cubic phases. However, the tetragonal phase of Perovskite barium titanate is observed using solution molarities of 1.0 M and 1.2 M. Relatively high value of transmission, i.e. ∼80%, is observed for the coatings prepared with high molarities. Band gap of annealed coatings varies between 3.47 and 3.70 eV. For 1.2 M sample, the maximum spontaneous polarization (Ps) is 0.327x10-3 (μC/cm2) and the residual polarization (Pr) is 0.072x10-3 (μC/cm2). For 1.2M solution, a high hardness value (1510 HV) is recorded, with a fracture toughness of 28.80 MPam-1/2. Low values of weight loss, after dipping the coatings in simulated body fluid, is observed. The antibacterial activity of BaTiO3 is tested against E. coli and Bacillus subtilis. Drug encapsulation capability is also tested for different time intervals. As a result, CBD-based coatings are a promising nominee for use as scaffold and protective coatings.

Dual-frequency piezoelectric micromachined ultrasound transducer based on polarization switching in ferroelectric thin films

Due to its additional frequency response, dual-frequency ultrasound has advantages over conventional ultrasound, which operates at a specific frequency band. Moreover, a tunable frequency from a single transducer enables sonographers to achieve ultrasound images with a large detection area and high resolution. This facilitates the availability of more advanced techniques that simultaneously require low- and high-frequency ultrasounds, such as harmonic imaging and image-guided therapy. In this study, we present a novel method for dual-frequency ultrasound generation from a ferroelectric piezoelectric micromachined ultrasound transducer (PMUT). Uniformly designed transducer arrays can be used for both deep low-resolution imaging and shallow high-resolution imaging. To switch the ultrasound frequency, the only requirement is to tune a DC bias to control the polarization state of the ferroelectric film. Flextensional vibration of the PMUT membrane strongly depends on the polarization state, producing low- and high-frequency ultrasounds from a single excitation frequency. This strategy for dual-frequency ultrasounds meets the requirement for either multielectrode configurations or heterodesigned elements, which are integrated into an array. Consequently, this technique significantly reduces the design complexity of transducer arrays and their associated driving circuits.

Lead zirconate titanate ceramics with aligned crystallite grains

The piezoelectric properties of lead zirconate titanate [Pb(Zr,Ti)O₃ or PZT] ceramics could be enhanced by fabricating textured ceramics that would align the crystal grains along specific orientations. We present a seed-passivated texturing process to fabricate textured PZT ceramics by using newly developed Ba(Zr,Ti)O₃ microplatelet templates. This process not only ensures the template-induced grain growth in titanium-rich PZT layers but also facilitates desired composition through interlayer diffusion of zirconium and titanium. We successfully prepared textured PZT ceramics with outstanding properties, including Curie temperatures of 360°C, piezoelectric coefficients d₃₃ of 760 picocoulombs per newton and g₃₃ of 100 millivolt meters per newton, and electromechanical couplings k₃₃ of 0.85. This study addresses the challenge of fabricating textured rhombohedral PZT ceramics by suppressing the otherwise severe chemical reaction between PZT powder and titanate templates.

Enhancing Electromechanical Properties of PZT-Based Piezoelectric Ceramics by High-Temperature Poling for High-Power Applications

Defect engineering is a proven method to tune the properties of perovskite oxides. In demanding high-power piezoelectric ceramic applications, acceptor doping is the most effective method to harden ceramics, but it inevitably degrades the ceramics' electromechanical properties. Herein, a poling method based on acceptor doping, namely, high-temperature poling, is implemented by applying an electric field above the Curie temperature for poling to achieve a balance of the properties of piezoelectric coefficient d₃₃ and mechanical quality factor Q_m. After high-temperature poling, the piezoelectric property of 0.6 mol % Mn-doped Pb_0.92Sr_0.08(Zr_0.533Ti_0.443Nb_0.024)O₃ is d₃₃ = 483 pC/N and Q_m = 448. Compared with the traditional poling, the piezoelectric coefficient d₃₃ of the high-temperature poling ceramics increased by approximately 40%, and Q_m also increased by nearly 18%. Therefore, high d₃₃ and Q_m were exhibited by our PZT piezoelectric ceramics. Rayleigh's law analysis, XRD, and transmission electron microscopy analysis show that, after high-temperature poling, the considerably increased d₃₃ is related to the large increase in the reversible domain wall motion in the intrinsic effect, while the slightly increased Q_m is related to the inhibited irreversible domain wall motion in the extrinsic effect. This study reports a method for high-temperature poling and provides insights into the design of high-power piezoelectric ceramics with high d₃₃ and Q_m.

THz Thin Film Varactor Based on Integrated Ferroelectric HfZrO2

In this paper, we present a broadband microwave characterization of ferroelectric hafnium zirconium oxide (Hf_0.5Zr_0.5O₂) metal-ferroelectric-metal (MFM) thin film varactor from 1 kHz up to 0.11 THz. The varactor is integrated into the back-end-of-line (BEoL) of 180 nm CMOS technology as a shunting capacitor for the coplanar waveguide (CPW) transmission line. At low frequencies, the varactor shows a slight imprint behavior, with a maximum tunability of 15% after the wake-up. In the radio- and mmWave frequency range, the varactor's maximum tunability decreases slightly from 13% at 30 MHz to 10% at 110 GHz. Ferroelectric varactors were known for their frequency-independent, linear tunability as well as low loss. However, this potential was never fully realized due to limitations in integration. Here, we show that ferroelectric HfO₂ thin films with good back-end-of-line compatibility support very large scale integration. This opens up a broad range of possible applications in the mmWave and THz frequency range such as 6G communications, imaging radar, or THz imaging.

Determining Full Matrix Constants of Piezoelectric Crystal From a Single Sample Using Partial Electrode Electromechanical Impedance Spectroscopy

To determine all the material constants of a piezoelectric crystal using the IEEE resonance method or ultrasonic method, several samples with specific geometries and orientations are required, and the obtained results may be somewhat self-contradictory. The resonant ultrasound spectroscopy (RUS) can determine full material constants of a single piezoelectric sample by matching the numerically computed eigenfrequencies to the measured resonance spectrum. However, typically the usage of PZT transducers for excitation and reception makes the testing complicated and may add additional mass and damping. In this work, we propose a new method called partial electrode electromechanical impedance spectroscopy (PE-EMIS), which can obtain all the elastic, piezoelectric constants, and related internal frictions of a piezoelectric crystal by just using a single sample without additional transducers. In PE-EMIS, two small PEs are fabricated on one corner of a piezoelectric sample, and the sample's resonance frequencies, along with the internal frictions of the k th eigenmode, can be accurately obtained from the conductance spectrum measured using an impedance analyzer. The PE-EMIS experiment is carried out on a rectangular parallelepiped PZT-4 piezoelectric ceramic, and the extracted material constants are highly repeatable. Three of the extracted elastic constants ( c₁₁^E , c₃₃^D , and c₆₆^E ) are validated by the traditional wave propagation method. Due to its simplicity, convenience, and accuracy, the proposed PE-EMIS is expected to be widely used for characterization of piezoelectric materials in near future.

Coherent Precipitates with Strong Domain Wall Pinning in Alkaline Niobate Ferroelectrics

High-power piezoelectric applications are predicted to share approximately one-third of the lead-free piezoelectric ceramic market in 2024 with alkaline niobates as the primary competitor. To suppress self-heating in high-power devices due to mechanical loss when driven by large electric fields, piezoelectric hardening to restrict domain wall motion is required. In the present work, highly effective piezoelectric hardening via coherent plate-like precipitates in a model system of the (Li,Na)NbO₃ (LNN) solid solution delivers a reduction in losses, quantified as an electromechanical quality factor, by a factor of ten. Various thermal aging schemes are demonstrated to control the average size, number density, and location of the precipitates. The established properties are correlated with a detailed determination of short- and long-range atomic structure by X-ray diffraction and pair distribution function analysis, respectively, as well as microstructure determined by transmission electron microscopy. The impact of microstructure with precipitates on both small- and large-field properties is also established. These results pave the way to implement precipitate hardening in piezoelectric materials, analogous to precipitate hardening in metals, broadening their use cases in applications.

Structural, Morphologic, and Ferroelectric Properties of PZT Films Deposited through Layer-by-Layer Reactive DC Magnetron Sputtering

Lead zirconate titanate (PZT) is a widely used material with applications ranging from piezoelectric sensors to developing non-volatile memory devices. Pb(ZrxTi1−x)O3 films were deposited by DC reactive magnetron sputtering at a temperature range of (500–600) °C. X-ray diffraction (XRD) indicated the perovskite phase formation in samples synthesized at 550 °C, which agrees with Raman data analysis. Scanning electron microscopy (SEM) measurements supplemented XRD data and showed the formation of dense PZT microstructures. Further X-ray photoelectron spectroscopy (XPS) analysis confirmed that the Zr/Ti ratio corresponds to the Pb(Zr0.58Ti0.42)O3 content. Dielectric measurement of the same sample indicated dielectric permittivity to be around 150 at room temperature, possibly due to the defects in the structure. P-E measurements show ferroelectric behavior at a temperature range of (50–180) °C. It was found that the remnant polarization increased with temperature, and at the same time, coercive field values decreased. Such behavior can be attributed to energetically deep defects.

Simultaneously achieving giant piezoelectricity and record coercive field enhancement in relaxor-based ferroelectric crystals

A large coercive field (E_C) and ultrahigh piezoelectricity are essential for ferroelectrics used in high-drive electromechanical applications. The discovery of relaxor-PbTiO₃ crystals is a recent breakthrough; they currently afford the highest piezoelectricity, but usually with a low E_C. Such performance deterioration occurs because high piezoelectricity is interlinked with an easy polarization rotation, subsequently favoring a dipole switch under small fields. Therefore, the search for ferroelectrics with both a large E_C and ultrahigh piezoelectricity has become an imminent challenge. Herein, ternary Pb(Sc_1/2Nb_1/2)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ crystals are reported, wherein the dispersed local heterogeneity comprises abundant tetragonal phases, affording a E_C of 8.2 kV/cm (greater than that of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ by a factor of three) and ultrahigh piezoelectricity (d₃₃ = 2630 pC/N; d₁₅ = 490 pC/N). The observed E_C enhancement is the largest reported for ultrahigh-piezoelectric materials, providing a simple, practical, and universal route for improving functionalities in ferroelectrics with an atomic-level understanding.

High-drive electromechanical applications require ferroelectrics accounting for a large coercive field and high piezoelectricity simultaneously but it is still a challenge. Here, the authors demonstrate it in a relaxor-based ferroelectric crystal.

Ultrahigh Piezoelectric Performance through Synergistic Compositional and Microstructural Engineering

Piezoelectric materials enable the conversion of mechanical energy into electrical energy and vice-versa. Ultrahigh piezoelectricity has been only observed in single crystals. Realization of piezoelectric ceramics with longitudinal piezoelectric constant (d₃₃ ) close to 2000 pC N^-1 , which combines single crystal-like high properties and ceramic-like cost effectiveness, large-scale manufacturing, and machinability will be a milestone in advancement of piezoelectric ceramic materials. Here, guided by phenomenological models and phase-field simulations that provide conditions for flattening the energy landscape of polarization, a synergistic design strategy is demonstrated that exploits compositionally driven local structural heterogeneity and microstructural grain orientation/texturing to provide record piezoelectricity in ceramics. This strategy is demonstrated on [001]_PC -textured and Eu³⁺ -doped Pb(Mg_1/3 Nb_2/3 )O₃ -PbTiO₃ (PMN-PT) ceramics that exhibit the highest piezoelectric coefficient (small-signal d₃₃ of up to 1950 pC N^-1 and large-signal d₃₃ * of ≈2100 pm V^-1 ) among all the reported piezoelectric ceramics. Extensive characterization conducted using high-resolution microscopy and diffraction techniques in conjunction with the computational models reveals the underlying mechanisms governing the piezoelectric performance. Further, the impact of losses on the electromechanical coupling is identified, which plays major role in suppressing the percentage of piezoelectricity enhancement, and the fundamental understanding of loss in this study sheds light on further enhancement of piezoelectricity. These results on cost-effective and record performance piezoelectric ceramics will launch a new generation of piezoelectric applications.

Nonlinear electromechanical impedance spectroscopy: A powerful tool for studying amplitude dependent internal frictions of solids

Strain amplitude dependent effects of materials/structures are very important in the field of material science and engineering and have been found to be extremely sensitive to defects or damage. In this work, a nonlinear electromechanical impedance spectroscopy (N-EMIS) technique is proposed to characterize the amplitude dependent internal frictions (ADIFs) and modulus defects (or resonance shift) of materials. First, a new experimental scheme called the on/off parallel resistor capacitor circuit is proposed to measure the N-EMIS of a piezoelectric transducer (PZT)-specimen composite system under high driving levels. Second, based on the N-EMIS, the formulas for calculating the ADIF are derived and validated by vibration measurement using a laser vibrometer. To further enlarge the strain amplitude, a PZT-stepped horn-specimen three-component system is then introduced, with which the maximum strain amplitude can reach 10^-3. Finally, ADIF tests are conducted on polycrystalline pure copper and 1045-steel. The results show that at high strain levels, the internal frictions of both materials can reach several times than those at low driving levels, while the modulus drops only slightly. The proposed N-EMIS technique can effectively assess the strain amplitude dependent properties of materials and is expected to be widely used in the near future for evaluation of plasticity, fatigue, and damage.

Recent Developments on Relaxor-PbTiO3 Ferroelectric Crystals

Numerous investigations on the development of the relaxor-PbTiO3 ferroelectric crystals have been carried out since their extraordinary properties were revealed. Recent developments on these crystals have offered further advances in electromechanical applications. In this review, recent developments on relaxor-PbTiO3 crystals and their practical applications are reviewed. The single crystal growth methods are first discussed. Two different strategies, poling and doping, for piezoelectric improvement are surveyed in the following section. After this, the anisotropic features of the single crystals are discussed. Application perspectives arising from the property improvements for electromechanical devices are finally reviewed.

2D ferroelectric devices: working principles and research progress

Two-dimensional (2D) ferroelectric materials are promising for use in high-performance nanoelectronic devices due to the non-volatility, high storage density, low energy cost and short response time originating from their bistable and switchable polarization states. In this mini review, we first discuss the mechanism and operation principles of ferroelectric devices to facilitate understanding of these novel nanoelectronics and then summarize the latest research progress of electronic devices based on 2D ferroelectrics. Finally, the perspectives for future research and development directions in various fields are provided. We expect this will provide an overview regarding the application of 2D ferroelectrics in electronic appliances.

Ultrahigh energy storage in superparaelectric relaxor ferroelectrics

[Figure: see text].

Optimizing the Piezoelectric and Dielectric Properties of Pb(In1/2Nb1/2)O₃-PbTiO₃ Ferroelectric Crystals via Alternating Current Poling Waveform

The alternating current poling (ACP) method has been attracting significant attention because of the enhanced piezoelectric and dielectric properties of relaxor-based ferroelectric (FE) crystals with advantages of high-efficiency, time-savings, and ease of operation. The most commonly used poling waveform is the bipolar triangle. Other waveforms have been seldom applied in ACP. However, the waveform plays a significant influence on the fatigue performance of FE materials. Therefore, optimizing the ACP waveform to gain high piezoelectric performance and decrease dielectric loss factors is technologically essential. Here, the effects of ACP waveforms on the piezoelectric and dielectric properties of [001]_c-oriented 0.66 PIN-0.34 PT FE crystals were studied. The highest piezoelectric coefficient d₃₃ (1450 pC/N) and dielectric permittivity ε^T₃₃/ε₀ (3180) were obtained using the sesquipolar triangle waveform while the lowest dielectric loss factors tan δ (0.99%) was obtained using the unipolar triangle waveform. FE hysteresis loop measurements revealed that the lower coercive field and current density are beneficial for higher piezoelectric performance. Domain analysis showed that ACP samples exhibit a regular domain structure and high density 109° domain walls, which are the critical reasons for enhanced piezoelectric performance. This work provides guidance in designing an ACP waveform to achieve higher performance.

A Virtual Instrument for Measuring the Piezoelectric Coefficients of a Thin Disc in Radial Resonant Mode

In this paper, we describe and present a Virtual Instrument, a tool that allows the determination of the electromechanical, dielectric, and elastic coefficients in polarised ferroelectric ceramic discs (piezoceramics) in the linear range, including all of the losses when the piezoceramics are vibrating in radial mode. There is no evidence in the recent scientific literature of any automatic system conceived and implemented as a Virtual Instrument based on an iterative algorithm issued as an alternative to solve the limitations of the ANSI IEEE 176 standard for the characterisation of piezoelectric coefficients of thin discs in resonant mode. The characterisation of these coefficients is needed for the design of ultrasonic sensors and generators. In 1995, two of the authors of this work, together with other authors, published an iterative procedure that allowed for the automatic determination of the complex constants for lossy piezoelectric materials in radial mode. As described in this work, the procedures involved in using a Virtual Instrument have been improved: the response time for the characterisation of a piezoelectric sample is shorter (approximately 5 s); the accuracy in measurement and, therefore, in the estimates of the coefficients has been increased; the calculation speed has been increased; an intuitive, simple, and friendly user interface has been designed, and tools have been provided for exporting and inspecting the measured and processed data. No Virtual Instrument has been found in the recent scientific literature that has improved on the iterative procedure designed in 1995. This Virtual Instrument is based on the measurement of a unique magnitude, the electrical admittance (Y = G + iB) in the frequency range of interest. After measuring the electrical admittance, estimates of the set of piezoelectric coefficients of the device are obtained. The programming language used in the construction of the Virtual Instrument is LabVIEW 2019^®.

Structural and Electric Properties of Epitaxial Na0.5Bi0.5TiO3-Based Thin Films

Substantial efforts are dedicated worldwide to use lead-free materials for environmentally friendly processes in electrocaloric cooling. Whereas investigations on bulk materials showed that Na0.5Bi0.5TiO3 (NBT)-based compounds might be suitable for such applications, our aim is to clarify the feasibility of epitaxial NBT-based thin films for more detailed investigations on the correlation between the composition, microstructure, and functional properties. Therefore, NBT-based thin films were grown by pulsed laser deposition on different single crystalline substrates using a thin epitaxial La0.5Sr0.5CoO3 layer as the bottom electrode for subsequent electric measurements. Structural characterization revealed an undisturbed epitaxial growth of NBT on lattice-matching substrates with a columnar microstructure, but high roughness and increasing grain size with larger film thickness. Dielectric measurements indicate a shift of the phase transition to lower temperatures compared to bulk samples as well as a reduced permittivity and increased losses at higher temperatures. Whereas polarization loops taken at −100 °C revealed a distinct ferroelectric behavior, room temperature data showed a significant resistive contribution in these measurements. Leakage current studies confirmed a non-negligible conductivity between the electrodes, thus preventing an indirect characterization of the electrocaloric properties of these films.

Multifunctional Cantilevers as Working Elements in Solid-State Cooling Devices

Despite the challenges of practical implementation, electrocaloric (EC) cooling remains a promising technology because of its good scalability and high efficiency. Here, we investigate the feasibility of an EC cooling device that couples the EC and electromechanical (EM) responses of a highly functionally, efficient, lead magnesium niobate ceramic material. We fabricated multifunctional cantilevers from this material and characterized their electrical, EM and EC properties. Two active cantilevers were stacked in a cascade structure, forming a proof-of-concept device, which was then analyzed in detail. The cooling effect was lower than the EC effect of the material itself, mainly due to the poor solid-to-solid heat transfer. However, we show that the use of ethylene glycol in the thermal contact area can significantly reduce the contact resistance, thereby improving the heat transfer. Although this solution is most likely impractical from the design point of view, the results clearly show that in this and similar cooling devices, a non-destructive, surface-modification method, with the same effectiveness as that of ethylene glycol, will have to be developed to reduce the thermal contact resistance. We hope this study will motivate the further development of multifunctional cooling devices.

Strategies to Improve the Energy Storage Properties of Perovskite Lead-Free Relaxor Ferroelectrics: A Review

Electrical energy storage systems (EESSs) with high energy density and power density are essential for the effective miniaturization of future electronic devices. Among different EESSs available in the market, dielectric capacitors relying on swift electronic and ionic polarization-based mechanisms to store and deliver energy already demonstrate high power densities. However, different intrinsic and extrinsic contributions to energy dissipations prevent ceramic-based dielectric capacitors from reaching high recoverable energy density levels. Interestingly, relaxor ferroelectric-based dielectric capacitors, because of their low remnant polarization, show relatively high energy density and thus display great potential for applications requiring high energy density properties. In this study, some of the main strategies to improve the energy density properties of perovskite lead-free relaxor systems are reviewed, including (i) chemical modification at different crystallographic sites, (ii) chemical additives that do not target lattice sites, and (iii) novel processing approaches dedicated to bulk ceramics, thick and thin films, respectively. Recent advancements are summarized concerning the search for relaxor materials with superior energy density properties and the appropriate choice of both composition and processing routes to match various applications' needs. Finally, future trends in computationally-aided materials design are presented.

Processing-Structure-Property Relations for Radiation Tolerance of Relaxor-Ferroelectric Thin Films

This work investigates the role of microstructure on the radiation tolerance of relaxor-ferroelectric, lead magnesium niobate-lead titanate, thin films for piezoelectric microelectromechanical system (MEMS) applications. Thin films comprised of 0.7Pb[Mg_1/3Nb_2/3]O₃-0.3PbTiO₃ were fabricated via chemical solution deposition on platinized silicon wafers. Processing parameters, i.e., pyrolysis and annealing temperatures and durations, were varied to change the microstructure of the films. The functional response of the films was characterized before and after exposure to gamma radiation [up to 10 Mrad(Si)]. Within the total ionization dose studied, all films showed a <5% change in dielectric response and polarization and <15% change in piezoelectric response, after irradiation. While all films showed substantial radiation tolerance, those with large columnar grains showed the highest dielectric and piezoelectric response and, therefore, might offer the best approach for enabling piezoelectric MEMS devices for applications in radiative environments.

Liquid metal nanocomposites

Liquid metal (LM) has attracted tremendous interest over the past decade for its enabling combination of high electrical and thermal conductivity and low mechanical compliance and viscosity. Efforts to harness LM in electronics, robotics, and biomedical applications have largely involved methods to encapsulate the liquid so that it can support functionality without leaking or smearing. In recent years, there has been increasing interest in LM "nanocomposites" in which either liquid metal is mixed with metallic nanoparticles or nanoscale droplets of liquid metal are suspended within a soft polymer matrix. Both of these material systems represent an important step towards utilizing liquid metal for breakthrough applications. In this minireview, we present a brief overview of recent progress over the past few years in methods to synthesize LM nanomaterials and utilize them as transducers for sensing, actuation, and energy harvesting. In particular, we focus on techniques for stable synthesis of LM nanodroplets, suspension of nanodroplets within various matrix materials, and methods for incorporating metallic nanoparticles within an LM matrix.

Ultrahigh-energy density lead-free dielectric films via polymorphic nanodomain design

Dielectric capacitors with ultrahigh power densities are fundamental energy storage components in electrical and electronic systems. However, a long-standing challenge is improving their energy densities. We report dielectrics with ultrahigh energy densities designed with polymorphic nanodomains. Guided by phase-field simulations, we conceived and synthesized lead-free BiFeO₃-BaTiO₃-SrTiO₃ solid-solution films to realize the coexistence of rhombohedral and tetragonal nanodomains embedded in a cubic matrix. We obtained minimized hysteresis while maintaining high polarization and achieved a high energy density of 112 joules per cubic centimeter with a high energy efficiency of ~80%. This approach should be generalizable for designing high-performance dielectrics and other functional materials that benefit from nanoscale domain structure manipulation.

High-Performance Sm-Doped Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3-Based Piezoceramics

High-performance piezoelectric materials are pivotal to many electromechanical applications including piezoelectric actuators, sensors, and transducers. However, the general approach to achieve high piezoelectric properties by establishing morphotropic phase boundary (MPB) has limitation due to the weak anisotropy of the Gibbs free energy profile at the MPB region. Here, aliovalent Sm³⁺-doped 0.4Pb(Mg_1/3Nb_2/3)O₃-(0.6-x)PbZrO₃-xPbTiO₃ piezoelectric ceramics were fabricated by a solid-state method, where the optimized piezoelectric coefficient d₃₃ = 910 pC/N, dielectric constant ε_r = 4090, and Curie temperature T_C = 184 °C were obtained at x = 0.352, being attributed to the synergistic contributions from the MPB and enhanced local structural heterogeneity. Rayleigh analysis was adopted to study the intrinsic and extrinsic contributions in Sm-doped PMN-PZ-PT ceramics, where the extrinsic contribution was found to be on the order of 25-67% at 4 kV/cm. Of particular significance is that a large signal d₃₃* = 820 pm/V (at 20 kV/cm) with a minimal strain variation of 5% was achieved for a composition of x = 0.372 over the temperature range of 20-160 °C, being superior to those previously reported piezoelectric ceramic materials. This work offers a good paradigm to simultaneously achieve high piezoelectric properties with good temperature stability in ferroelectric ceramics, which have great potential for piezoelectric application at elevated temperatures.

Energy Storage Performance of Sandwich Structured Pb(Zr0.4Ti0.6)O3/BaZr0.2Ti0.8O3/Pb(Zr0.4Ti0.6)O3 Films

We reported a sandwich structured Pb(Zr0.4Ti0.6)O3/BaZr0.2Ti0.8O3/Pb(Zr0.4Ti0.6)O3 (PZT/BZT/PZT) film fabricated by using the sol–gel method, which was dense and uniform with a unique perovskite structure. The PZT/BZT/PZT films displayed high dielectric constants up to 1722.45 at the frequency of 10 kHz. Additionally, the enhanced energy storage density of 39.27 J·cm−3 was achieved at room temperature and 2.00 MV/cm, which was higher than that of the individual BaZr0.2Ti0.8O3 film (21.28 J·cm−3). Furthermore, the energy storage density and efficiency of PZT/BZT/PZT film increased slightly with the increasing temperature from −140 °C to 200 °C. This work proves the feasibility and effectiveness of a sandwich structure in improving dielectric, leakage, and energy storage performances, providing a new paradigm for high-energy–density dielectrics applications.

Ferroelectricity-Enhanced Piezo-Phototronic Effect in 2D V-Doped ZnO Nanosheets

Emerging 2D electronic materials have shown great potential for regulating and controlling optoelectronic processes. A 2D ferroelectric semiconductor coupled with the piezo-phototronic effect may bring unprecedented functional characteristics. Here, a heterojunction photodetector made of p-Si/V-doped-ferroelectric-ZnO 2D nanosheets (FESZ-PD) is fabricated, and the ferroelectricity-enhanced piezo-phototronic effect on the photoresponse behavior of the FESZ-PD is carefully investigated. By introducing the ferroelectricity and the piezo-phototronic effect, improved current rectification performance is achieved and the photoresponse performance of the heterojunction is enhanced in a broad spectral range. The applied voltage bias during measurement naturally causes ferroelectric spontaneous polarizations to align, resulting in a change in band structure near the interface and the local piezo-phototronic effect. The modulated energy band promotes the generation, separation, and transportation efficiency of photogenerated carriers greatly. Compared with the Si/ZnO 2D nanosheets photodetector without ferroelectricity under strain-free conditions, the photoresponsivity R of the FESZ-PD increases by 2.4 times when applying a -0.20‰ compressive strain at +1 V forward bias. These results confirm the feasibility of coupling the ferroelectricity with the piezo-phototronic effect in 2D ferroelectric materials to enhance the photoresponse behavior, which provides a good way to enable the development of high-performance electronic and optoelectronic devices.

The challenge of distinguishing mechanical, electrical and piezoelectric losses

Understanding the energy loss in piezoelectric materials is of significant importance for manufacturers of acoustic transducers. The contributions to the power dissipation due to nonzero phase angles of the mechanical, electrical, and piezoelectric constants can be separated in the expression for power dissipation density. However, this division into separate contributions depends on the piezoelectric constitutive equation form used. Thus, it is problematic to identify any of the three terms with a specific physical domain, electric or mechanical, or to a coupling as is common in the discussion of loss in piezoelectric materials. Therefore, assumptions on the phase of the material constants based on this distinction could be erroneous and lead to incorrect piezoelectric models. This study demonstrates the challenge of distinguishing mechanical, electrical, and piezoelectric losses by investigating the power dissipation density and its contributions in a piezoelectric rod for all four piezoelectric constitutive equation forms.

Hi-Fi Stake Piezo Single Crystal Actuator

High fidelity (Hi-Fi) piezoelectric single crystal stake actuators are presented in this work. They are made of multiple rectangular d32 mode lead-based relaxor ferroelectric (notably Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) and Pb(In0.5Nb0.5)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT)) single crystals bonded along their long edges with the aid of compliant polymeric edge guides into a square or polygonal pipe-like construction. Due to the highly stable engineered domain structure and high piezoelectricity of single crystal active materials, the actuators exhibit large linear displacement responses with negligible (<1%) hysteresis. Prototypes of square-pipe stake actuators were first fabricated and their phase transformation curves under different applied voltages, axial compressive loads and temperatures were established. Based on the information obtained, a range of Hi-Fi stake actuators with external square cross-sections of 5 × 5 mm2, 7.5 × 7.5 mm2 and 10 × 10 mm2, each of 4 different overall lengths of 15, 28, 41 and 54 mm, were further designed and fabricated using either PZN-PT or PIN-PMN-PT single crystals (both with TRO ≈ 110–125 °C) of 0.4 mm in crystal thickness. The stroke for the longest stake actuator fabricated (L = 54 mm) reaches −58 µm at 240 V. The working conditions, over which these Hi-Fi stake actuators remain linear with negligible hysteresis, were established for a total load of up to 10 kg and use temperature of up to 40 °C.

Q-Factor Spectrum of a Piezoceramic Resonator and Method for Piezoelectric Loss Factor Determination

The quality factor (Q) spectrum of a piezoceramic resonator, as a Q-factor frequency dependence for the specific resonator and its vibrational modes, was determined under baseline low-excitation level with a new approach proposed. The theoretical prediction was experimentally confirmed that the resonator Q-factor increases with frequency nearly linearly from the resonance reaching its maximum near the antiresonance. For the industrial PZT-5A piezoceramic the antiresonance-to-resonance quality factor ratio is 1.8 to 2.4 as much, depending on the type of vibration. As the theory states, this effect is directly related to the piezoelectric "losses" in piezoceramics, usually represented by the imaginary part of complex piezocoefficients, which have a unique property of lowering the total cumulative energy dissipation in a resonator in certain frequency intervals. Based on the electromechanical Q-factor (EMQ) concept, a new relatively simple method was proposed for the piezoelectric loss factor determination at just a single resonance frequency-it requires measurements of the resonance Q-factor and its frequency derivative at the resonance, or the first and second frequency derivatives of the immittance phase at the resonance. Experimentally determined is close to near 0.8 of its upper (positive) phenomenological limit in the conventional PZT-5A piezoceramic at the basic vibrational modes. The piezoelectric loss factor, theoretically reaching the upper limit, can provide extremely high value of the Q-factor (near the fundamental antiresonance) with possible an order of magnitude EMQ increase. That paradoxical fact for the piezoelectric "losses" is a novel possible way of improving the piezoceramic performance and operation.

High piezoelectric response and polymorphic phase region in the lead-free piezoelectric BaTiO3–CaTiO3–BaSnO3 ternary system

A new coexistence phase compositions between rhombohedral, orthorhombic and tetragonal phase was identified in the (Ba,Ca)(Ti,Sn)O₃ system.

Synthesis, Characterization, and Microwave Absorption Properties of Reduced Graphene Oxide/Strontium Ferrite/Polyaniline Nanocomposites

Strontium ferrite nanoparticles were prepared by a coprecipitation method, and reduced graphene oxide/strontium ferrite/polyaniline (R-GO/SF/PANI) ternary nanocomposites were prepared by in situ polymerization method. The morphology, structure, and magnetic properties of the ternary nanocomposites were investigated by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), TEM, Raman, and VSM. The microwave-absorbing properties of the composites were measured by a vector network analyzer. The XRD patterns show the single phase of strontium hexaferrite without other intermediate phases. TEM photographs reveal that strontium ferrite nanoparticles are uniformly dispersed on the surfaces of R-GO sheets. The R-GO/SF/PANI nanocomposite exhibited the best absorption property with the optimum matching thickness of 1.5 mm in the frequency of 2-18 GHz. The value of the maximum RL was -45.00 dB at 16.08 GHz with the 5.48-GHz bandwidth. The excellent absorption properties of R-GO/SF/PANI nanocomposites indicated their great potential as microwave-absorbing materials.

Damping Constant (Linewidth) and the Relaxation Time of the Brillouin LA Mode for the Ferroelectric-Paraelectric Transition in PbZr1-xTixO3

The damping constant (linewidth) of the longitudinal acoustic (LA) mode is calculated as a function of temperature using the observed Brillouin frequencies of this mode from the literature for the ferroelectric-paraelectric transition ( T_C = 657 K) in PbZr_1-xTi_xO₃ ( x = 0.45 ). For this calculation of the damping constant, the pseudospin-phonon coupled model and the energy fluctuation model are used by fitting to the observed data for the Brillouin frequencies of the LA mode in the ferroelectric ( ) and paraelectric ( T > T_C) phases of this compound ( x = 0.45 ). Values of the activation energy are deduced for both ferroelectric and paraelectric phases. The relaxation time is also obtained by means of fitting to the observed data from the literature for the inverse relaxation time at various temperatures in the paraelectric phase of PbZr_1-xTi_xO₃. The temperature dependences of the damping constant and of the relaxation time with the values of the activation energy that we have calculated indicate that the pseudospin-phonon coupled model and the energy fluctuation model are capable of describing the ferroelectric-paraelectric transition ( T_C = 657 K) in PbZr_1-xTi_xO₃ ( x = 0.45 ) adequately.

High Performance Flexible Piezoelectric Nanogenerators based on BaTiO3 Nanofibers in Different Alignment Modes

Piezoelectric nanogenerators, harvesting energy from mechanical stimuli in our living environments, hold great promise to power sustainable self-sufficient micro/nanosystems and mobile/portable electronics. BaTiO3 as a lead-free material with high piezoelectric coefficient and dielectric constant has been widely examined to realize nanogenerators, capacitors, sensors, etc. In this study, polydimethylsiloxane (PDMS)-based flexible composites including BaTiO3 nanofibers with different alignment modes were manufactured and their piezoelectric performance was examined. For the study, BaTiO3 nanofibers were prepared by an electrospinning technique utilizing a sol-gel precursor and following calcination process, and they were then aligned vertically or horizontally or randomly in PDMS matrix-based nanogenerators. The morphological structures of BaTiO3 nanofibers and their nanogenerators were analyzed by using SEM images. The crystal structures of the nanogenerators before and after poling were characterized by X-ray diffraction. The dielectric and piezoelectric properties of the nanogenerators were investigated as a function of the nanofiber alignment mode. The nanogenerator with BaTiO3 nanofibers aligned vertically in the PDMS matrix sheet achieved high piezoelectric performance of an output power of 0.1841 μW with maximum voltage of 2.67 V and current of 261.40 nA under a low mechanical stress of 0.002 MPa, in addition to a high dielectric constant of 40.23 at 100 Hz. The harvested energy could thus power a commercial LED directly or be stored into capacitors after rectification.

Switching 70Pb(Mg1/3Nb2/3)O3-0.30PbTiO3 single crystal by 3 MHz bipolar field

Polarization switching and associated electromechanical property changes at 3.0 MHz were investigated with and without a direct current (dc) bias for [001]c poled 0.70Pb(Mg1/3Nb2/3)O3-0.30PbTiO3 single crystal. The results showed that the coercive field under a bipolar pulse at 3.0 MHz is 2.75 times as large as conventional defined Ec (2.58 kV/cm at 0.1 Hz), and a dc bias can further enlarge the driving field. Our results point to an innovative transducer operating mechanism at high frequencies since one could drive the crystal under much larger fields at high frequencies to produce much stronger signals from a small array element for deeper penetration imaging.

Unleashing the Full Sustainable Potential of Thick Films of Lead-Free Potassium Sodium Niobate (K0.5Na0.5NbO3) by Aqueous Electrophoretic Deposition

A current challenge for the fabrication of functional oxide-based devices is related with the need of environmental and sustainable materials and processes. By considering both lead-free ferroelectrics of potassium sodium niobate (K0.5Na0.5NbO3, KNN) and aqueous-based electrophoretic deposition here we demonstrate that an eco-friendly aqueous solution-based process can be used to produce KNN thick coatings with improved electromechanical performance. KNN thick films on platinum substrates with thickness varying between 10 and 15 μm have a dielectric permittivity of 495, dielectric losses of 0.08 at 1 MHz, and a piezoelectric coefficient d33 of ∼70 pC/N. At TC these films display a relative permittivity of 2166 and loss tangent of 0.11 at 1 MHz. A comparison of the physical properties between these films and their bulk ceramics counterparts demonstrates the impact of the aqueous-based electrophoretic deposition (EPD) technique for the preparation of lead-free ferroelectric thick films. This opens the door to the possible development of high-performance, lead-free piezoelectric thick films by a sustainable low-cost process, expanding the applicability of lead-free piezoelectrics.

Revisiting the Characterization of the Losses in Piezoelectric Materials from Impedance Spectroscopy at Resonance

Electronic devices using the piezoelectric effect contain piezoelectric materials: often crystals, but in many cases poled ferroelectric ceramics (piezoceramics), polymers or composites. On the one hand, these materials exhibit non-negligible losses, not only dielectric, but also mechanical and piezoelectric. In this work, we made simulations of the effect of the three types of losses in piezoelectric materials on the impedance spectrum at the resonance. We analyze independently each type of loss and show the differences among them. On the other hand, electrical and electronic engineers include piezoelectric sensors in electrical circuits to build devices and need electrical models of the sensor element. Frequently, material scientists and engineers use different languages, and the characteristic material coefficients do not have a straightforward translation to those specific electrical circuit components. To connect both fields of study, we propose the use of accurate methods of characterization from impedance measurements at electromechanical resonance that lead to determination of all types of losses, as an alternative to current standards. We introduce a simplified equivalent circuit model with electrical parameters that account for piezoceramic losses needed for the modeling and design of industrial applications.

Lead-Free Piezoceramics: Revealing the Role of the Rhombohedral-Tetragonal Phase Coexistence in Enhancement of the Piezoelectric Properties

Until now, lead zirconate titanate (PZT) based ceramics are the most widely used in piezoelectric devices. However, the use of lead is being avoided due to its toxicity and environmental risks. Indeed, the attention in piezoelectric devices has been moved to lead-free ceramics, especially on (K,Na)NbO3-based materials, due to growing environmental concerns. Here we report a systematic evaluation of the effects of the compositional modifications induced by replacement of the B-sites with Sb(5+) ions in 0.96[(K0.48Na0.52)0.95Li0.05Nb1-xSbxO3]-0.04[BaZrO3] lead-free piezoceramics. We show that this compositional design is the driving force for the development of the high piezoelectric properties. So, we find that this phenomenon can be explained by the stabilization of a Rhombohedral-Tetragonal (R-T) phase boundary close to room temperature, that facilities the polarization process of the system and exhibits a significantly high piezoelectric response with a d33 value as high as ∼400 pC/N, which is comparable to part soft PZTs. As a result, we believe that the general strategy and design principles described in this study open the possibility of obtaining (K,Na)NbO3-based lead-free ceramics with enhanced properties, expanding their application range.

Oil saturation effects in lead metaniobate porous piezoceramic: transient material characteristics

Lead metaniobate PbNb2O6 (PN) has a unique combination of high piezoelectric anisotropy; relatively low dielectric permittivity and high Curie temperature; and a low Q-factor, near 20. The very low Q-factor is the most intriguing PN property among the piezoelectric materials, and as shown in this research, this internal high dissipation and damping effect is directly related to the presence of silicon oil in the porous PN structure; consequently, it is dependent on the oil properties. To the contrary, the quality factor of PN not saturated with oil was found to be as high as nearly 400. Full sets of PN electro-mechanical constants, transient resonance and dissipation characteristics, and their temperature dependencies were determined under both conditions: PN conventionally saturated with oil and PN not saturated with oil. As was experimentally shown, at higher temperatures particularly after a 260°C soak for several days, a transition from the "with oil" state to the "no oil" state takes place in the conventional PN properties; this effect is a consequence of the phase transition in the silicon oil from liquid to solid state.

Domain evolution with electric field and delineation of extrinsic contributions in (K, Na, Li)(Nb, Ta, Sb)O3 single crystal

Extrinsic contributions play an important role in the functionalities of ferroelectric materials, while domain structure evolution is crucial for understanding the extrinsic dielectric and piezoelectric responses. In this work, domain configuration changes with an electric field applied along [001] C in the tetragonal (K, Na, Li)(Nb, Sb, Ta)O3 single crystal were studied by means of polarizing light microscopy. Results show that parts of the spontaneous polarizations in the (001) C plane are switched to [001] C direction, while others still stay in the (001) C plane due to high induced internal stresses. Single domain state cannot be achieved even under a high electric field. After being poled along [001] C , the volume fraction of domains with polarzations in the (001) C plane is still about 25.2%. The extrinsic contributions to the dielectric constant are 15.7% and 27.2% under the E field of 1 kV/cm and under 2 kV/cm, respectively, estimated by the Rayleigh analysis.

An insight into the origin of low-symmetry bridging phase and enhanced functionality in systems containing competing phases

High piezoelectric activity of ferroelectrics with morphotropic phase boundary (MPB) compositions has been the focus of numerous recent investigations. The concept of a bridging low-symmetry phase between competing phase structures of the MPB composition remains controversial due to the compositional inhomogeneity near the MPB and the lack of appropriate experimental techniques to delineate the complex crystal structures. We have studied a simple ferroelectric BaTiO₃ by employing a high resolution synchrotron-based technique, in which the formation of different symmetry regions due to chemical inhomogeneity can be ruled out. We observed two types of thermotropic phase boundaries, revealing the importance of interphase-strain in the formation of a bridging phase between competing phases and the enhancement of functionality.