Lead-free Relaxor Ferroelectrics Research Articles

Large Room-Temperature Electrocaloric Effect in Lead-Free Relaxor Ferroelectric Ceramics with Wide Operation Temperature Range

In order to obtain large room-temperature electrocaloric effect (ECE) and wide operation temperature range simultaneously in lead-free ceramics, we proposed designing a relaxor ferroelectric with a Tm (the temperature at which the maximum dielectric permittivity is achieved) near-room temperature and glass addition. Based on this strategy, we designed and fabricated lead-free 0.76NaNbO3–0.24BaTiO3 (NN-24BT) ceramics with 1wt.% BaO–B2O3–SiO2 glass addition, which showed distinct relaxor ferroelectric characteristics with strongly diffused phase transition and a Tm near-room temperature. Based on a direct measurement method, a large ΔT (adiabatic temperature change) of 1.3 K was obtained at room temperature under a high field of 11.0 kV mm−1. Additionally, large ECE can be maintained (>0.6 K@6.1 kV mm−1) over a broad temperature range from 23 °C to 69 °C. Moreover, the ECE displayed excellent cyclic stability with a variation in ΔT below ±7% within 100 test cycles. The comprehensive ECE performance is significantly better than other lead-free ceramics. Our work provides a general and effective approach to designing lead-free, high-performance ECE ceramics, and the approach possesses the potential to be utilized to improve the ECE performance of other lead-free ferroelectric ceramic systems.

Ultrahigh Electrostrictive Effect in Lead-Free Sodium Bismuth Titanate-Based Relaxor Ferroelectric Thick Film.

In recent years, the development of environmentally friendly, lead-free ferroelectric films with prominent electrostrictive effects have been a key area of focus due to their potential applications in micro-actuators, sensors, and transducers for advanced microelectromechanical systems (MEMS). This work investigated the enhanced electrostrictive effect in lead-free sodium bismuth titanate-based relaxor ferroelectric films. The films, composed of (Bi0.5Na0.5)0.8-xBaxSr0.2TiO3 (BNBST, x = 0.02, 0.06, and 0.11), with thickness around 1 μm, were prepared using a sol-gel method on Pt/TiO2/SiO2/Si substrates. By varying the Ba2+ content, the crystal structure, morphology, and electrical properties, including dielectric, ferroelectric, strain, and electromechanical performance, were investigated. The films exhibited a single pseudocubic structure without preferred orientation. A remarkable strain response (S > 0.24%) was obtained in the films (x = 0.02, 0.06) with the coexistence of nonergodic and ergodic relaxor phases. Further, in the x = 0.11 thick films with an ergodic relaxor state, an ultrahigh electrostrictive coefficient Q of 0.32 m4/C2 was achieved. These findings highlight the potential of BNBST films as high-performance, environmentally friendly electrostrictive films for advanced microelectromechanical systems (MEMS) and electronic devices.

Polymorphic Heterogeneous Polar Structure Enabled Superior Capacitive Energy Storage in Lead-Free Relaxor Ferroelectrics at Low Electric Field.

High-performance energy storage dielectrics capable of low/moderate field operation are vital in advanced electrical and electronic systems. However, in contrast to achievements in enhancing recoverable energy density (Wrec), the active realization of superior Wrec and energy efficiency (η) with giant energy-storage coefficient (Wrec/E) in low/moderate electric field (E) regions is much more challenging for dielectric materials. Herein, lead-free relaxor ferroelectrics are reported with giant Wrec/E designed with polymorphic heterogeneous polar structure. Following the guidance of Landau phenomenological theory and rational composition construction, the conceived (Bi0.5Na0.5)TiO3-based ternary solid solution that delivers giant Wrec/E of ≈0.0168 µCcm-2, high Wrec of ≈4.71 J cm-3 and high η of ≈93% under low E of 280kVcm-1, accompanied by great stabilities against temperature/frequency/cycling number and excellent charging-discharging properties, which is ahead of most currently reported lead-free energy storage bulk ceramics measured at same E range. Atomistic observations reveal that the correlated coexisting local rhombohedral-tetragonal polar nanoregions embedded in the cubic matrix are constructed, which enables high polarization, minimized hysteresis, and significantly delayed polarization saturation concurrently, endowing giant Wrec/E along with high Wrec and η. These findings advance the superiority and feasibility of polymorphic nanodomains in designing highly efficient capacitors for low/moderate field-region practical applications.

Achieving ultrahigh energy density and efficiency simultaneously in (Na, K)NbO3-based lead-free relaxor ferroelectrics via a collaborative design

Relaxor ferroelectrics have garnered enormous attention for their great application potential in pulsed power energy-storage capacitors, while simultaneously achieving large recoverable energy density (Wrec) and high efficiency (η) remains a formidable challenge. Through collaboratively controlling multiscale structure via a combination of composition design and processing improvement, polar nanodomains with multiple local symmetries were engineered in ultrafine grains, enabling significantly reduced polarization hysteresis, delayed saturation polarization, maintained high polarization, and enhanced breakdown strength. As a result, an excellent comprehensive energy-storage performance of Wrec ∼11.8 J/cm3 and η ∼88.7 % is achieved in the (Na, K)NbO3-based spark-plasma-sintered lead-free ceramics, together with stable charge-discharge properties of power density ∼176.3 MW/cm3, discharge energy density ∼3.2 J/cm3, and discharge time ∼43 ns over a wide temperature range of 20–140 °C. The studied ceramic demonstrates great advantages in advanced capacitive energy-storage applications.

Achieving High Energy Storage Performance in Lead-Free Relaxor Ferroelectric Ceramics of (1-x)Bi0.88Nd0.10Sm0.02FeO3-xBaTiO3

BiFeO3-based lead-free ferroelectrics have attracted great interest in energy storage applications due to their high spontaneous polarisation strength, however, the high residual polarisation strength (Pr) has become a serious obstacle to their practical application. In this work, (1-x)Bi0.88Nd0.10Sm0.02FeO3-xBaTiO3+0.1 wt% MnO2 ceramics were designed by ion doping and solid solution modification strategies, where Sm3+ and Nd3+ promote ionic disorder and reduce leakage current, and the introduction of nano-BaTiO3 induces the crushing of long-range ferroelectric structure into polar nano-regions with enhanced relaxation. As a result, the ceramic is able to maintain a high maximum polarisation intensity while possessing a low residual polarisation.The recoverable energy density (Wrec) of the 0.50BSNF-0.50BT ceramic is as high as 3.61 J/cm3 with an efficiency of 70.11%. In addition, the 0.50BSNF-0.50BT has good thermal stability (Wrec variation <7%) at 25-120°C. In the charge/discharge test, the Wdis was 2.58 J/cm3 and the discharge time was 273 ns. These results indicate that the 0.50BSNF-0.50BT ceramic is an ideal candidate as a material for energy storage applications.

Improved relaxor ferroelectrics and energy storage performance of polycrystalline Ba0.95Sr0.05Ti2O5 ceramics modified with La dopants

Lead-free relaxor ferroelectrics are useful in energy storage applications. In particular, BaTi2O5-based dielectric materials may have great potential for such applications. In this study, polycrystalline Ba0.95Sr0.05Ti2O5-xLa (BST2-xLa) ceramics were synthesized using a standard solid-state reaction method with as-prepared nanoparticles prepared via a modified Pechini polymeric precursor method. The structural and electrical properties of the BST2-xLa ceramics were systematically investigated using a series of measurements. The phase analysis results confirm that La3+ is a donor dopant that replaced Ba2+ sites in the structure, which results in a decrease in oxygen vacancies to maintain charge neutrality. The intrinsic electronic conduction mechanism for electrical conductivity was evaluated from the fitted activation energy based on the Arrhenius equation. The relaxor ferroelectric behaviour and energy storage density and efficiency are improved by the addition of a small amount of lanthanum (<3.6 mol%) because of the charge and ionic radius mismatch in the structure. These results suggest that lanthanum can effectively optimise the relaxor ferroelectrics of BST2-xLa ceramics, which have promising potential as energy storage materials.

Extremely consistent dielectric capacitors with excellent comprehensive dielectric permittivity characteristics using (Ba, Sr) (Ti, Zr) O3 lead-free ceramic-based relaxor ferroelectrics

For decades, lead-free eco-friendly BaTiO3 ferroelectrics and their derivate have been established to be exceptional candidates for maintainable development and piezoelectric devices. In this work, the lead-free Ba0.8Sr0.2Ti0.75Zr0.25O3 system is prepared using a conventional solid-state reaction method. The incorporation of the Sr and Zr elements into the BaTiO3 matrix has broken the long-range ferroelectric polarization to announce the formation of Polar Nano-Regions (PNRs). The structural results have confirmed that the Ba0.8Sr0.2Ti0.75Zr0.25O3 reveals a single phase with a P4mm space group. The Raman analysis indicates the presence of active vibration Raman modes ҐR3c, Raman = 9A1 + 9E. Moreover, the X-ray diffraction (XRD) study and Raman results approve the absence of secondary phases. The temperature-dependent permittivity shows the presence of broad and frequency-dependent maximum dielectric pics. At 100 Hz, the maximum value of the relative permittivity ε′ ≈ 7000 is higher than that for conventional BaTiO3 perovskites (ε′ = 3000–6000). These results are due to the existence of domain boundary effects (PNRs) in Ba0.8Sr0.2Ti0.75Zr0.25O3 and the lattice distortion effects in high entropy systems. In addition, the observed elevated dielectric permittivity could be attributed to the dense microstructure of the ceramic with a large grain size of approximately 1.1 μm. The evolution of the characteristic frequency versus the maximum temperature is discussed in terms of the Vogel-Fulcher formula. Likewise, the characteristic diffuseness of the dielectric peak is described based on an empirical Lorenz-type relation. The deviation from the Curie-Weiss law (γ = 1.623) indicates that the lead-free Ba0.8Sr0.2Ti0.75Zr0.25O3 ferroelectric compound reveals a relaxor behavior. The encouraging dielectric properties of our solid solution (high relative permittivity of ε′ ≈ 7000 and very low dielectric losses of tan (δ) = 0.1) allow us to classify our Ba0.8Sr0.2Ti0.75Zr0.25O3 sample among the dielectric materials intended for energy storage in capacitors used in power electronic devices.

Enhanced electrocaloric and energy storage performances of lead-free BZT-based relaxor ferroelectrics

As electronic devices continue to miniaturize and integrate, the demand for multifunctional materials has become increasingly urgent. In this study, composition-dependent electrocaloric and energy storage properties for 0.72Ba(Zr0.8Ti0.2)O3-0.18(Ba0.65Sr0.35)TiO3-0.1BaTiO3-x wt.% MgO (0.72BZT-0.18BST-0.1BT-x wt.% MgO, x = 0.1, 0.2, 0.5, 1) relaxor ferroelectrics are investigated. 0.72BZT-0.18BST-0.1BT-0.5 wt% MgO ceramics exhibit high energy storage density (Wdis) of 1.15 J/cm3 and high energy storage efficiency (η) of 92.5 %. They also show excellent stability in energy density with temperature (ΔW/W23°C ≤ ±5.43 %) and frequency (ΔW/W1 Hz ≤ ±3.88 %). Notably, the 0.72BZT-0.18BST-0.1BT-0.2 wt% MgO ceramics display the highest electrocaloric effect with adiabatic temperature change (ΔT) of 0.4 K at room temperature, boasting an electrocaloric strength of 0.1 K·m/MV and an operating temperature span (Tspan) greater than 35 °C. Our study provides an effective method to design high-performance ceramics for application in energy storage and electrocaloric refrigeration.

Enhancement of energy storage properties of BNST-based lead-free relaxor ferroelectrics via optimization strategy

In the context of achieving carbon neutrality, dielectric capacitors have garnered significant attention due to their inherent advantages, including rapid charge and discharge capabilities, high power density, and extended service life. Nevertheless, contemporary lead-free dielectric capacitors encounter challenges such as insufficient energy storage density, suboptimal energy storage efficiency, and inadequate temperature stability. To address these issues, we have chosen the lead-free (Bi0.5Na0.5)0.65Sr0.35TiO3 ceramic as the foundational system. Through the addition of Bi(Zn0.5Sn0.5)O3 for doping modification, this study focuses on optimizing the energy storage characteristics. The results demonstrate an achieved energy storage density of 4.34 J/cm3 and an energy storage efficiency of 84.1% at 358 kV/cm. These findings offer valuable insights for the advancement of lead-free ceramics in the realm of dielectric energy storage.

Collaboratively improved energy density and efficiency in NaNbO3-based lead-free relaxor ferroelectrics via enhancing antiferrodistortion

A novel strategy for enhancing antiferrodistortion is proposed to effectively achieve simultaneously high energy density and efficiency in lead-free relaxor ferroelectric capacitors.

Excellent energy storage properties with ultrahigh Wrec in lead-free relaxor ferroelectrics of ternary Bi0.5Na0.5TiO3-SrTiO3-Bi0.5Li0.5TiO3 via multiple synergistic optimization

Advanced energy storage capacitors play important roles in modern power systems and electronic devices. Next-generation high/pulsed power capacitors will rely heavily on eco-friendly dielectric ceramics with high energy storage density (Wrec), high efficiency (η), wide work temperature range and stable charge-discharge ability, etc. Lead-free Bi0.5Na0.5TiO3 (BNT) based relaxor ferroelectric (RFE) ceramics are considered as one of the most promising candidates for energy storage capacitors. However, the application fields of them are greatly limited by their relatively low Wrec (generally <5 J/cm3). In this paper, excellent energy storage properties characterized by a great breakthrough in Wrec are achieved in a novel BNT system, (1-x)BNT-x(0.7SrTiO3-0.3Bi0.5Li0.5TiO3)+0.5 at.%Nb2O5 (x = 0, 0.1, 0.2, 0.3, 0.4), by synergistically constructing highly dynamic polar nanoregions (PNRs) and nanodomains, ultrafine grains (submicron size) and intrinsic conduction. Of great importance, the x = 0.4 ceramic exhibits an ultrahigh Wrec of 8.63 J/cm3 as well as a high efficiency (ƞ) of 89.6 % under a giant electric field of 520 kV/cm, due to coexistence of ultrahigh polarization difference (ΔP=Pmax −Pr) and high dielectric breakdown electric strength (Eb). Furthermore, excellent temperature stability (20−180 °C), frequency stability (1 − 500 Hz), and cycling stability (1 − 105 times) with the variation of Wrec < ±4 % and η < ±3 % are also found in the x = 0.4 ceramic. These results demonstrate that it is a very promising lead-free dielectric capacitor with enormous energy storage applications.

Creep in 0.91(Na1/2Bi1/2)TiO3-0.06BaTiO3-0.03K0.5Na0.5NbO3 lead-free piezoceramic under constant electric field loads

This work explores the creep behavior of polycrystalline 0.91(Na1/2Bi1/2)TiO3-0.06BaTiO3-0.03K0.5Na0.5NbO3 under constant electric fields. It reveals intriguing time-dependent variations in both polarization and strain response, which can be attributed to a transformation from the relaxor state to a long-range ferroelectric order. Meanwhile, bulk volume resistivity values are obtained to eliminate the influence of leakage current on the polarization assessment. The findings provide valuable insights into the creep behavior of lead-free relaxor ferroelectrics, laying a solid foundation for enhancing the performance and reliability of piezoactuators.

Boosting capacitive performance of lead-free relaxor ferroelectrics by introducing a linear dielectric

As an important core component used in high-power pulse electronic systems, dielectric ceramic capacitors have gained great interest because of their ultrahigh powder density (PD) and fast discharge speed (t0.9). However, inferior energy storage performance (ESP) greatly limits the development of energy storage devices with the requirement of miniaturization and integration. Herein, we propose a feasible approach to introduce linear dielectric CaTiO3 (CT) into (Bi0.5Na0.5)TiO3 (BNT) for the design of engineered relaxor ferroelectrics to tune ESP at the grain, domain, and macroscopic scales. The results demonstrate that introducing CT generates enhanced relaxor behavior with highly dynamic polar nanoregions, refines grain size, and suppresses dielectric loss, synergistically yielding increased breakdown strength, negligible remnant polarization, and a nearly linear polarization hysteresis loop. Consequently, superior ESP is achieved in BNT-0.3CT relaxor ferroelectric ceramics with a large energy density (∼5.65 J/cm3) and a high efficiency (∼88.1 %). Based on CT-induced merits, the excellent temperature/frequency/cycling stabilities and remarkable charge-discharge properties featuring a high PD of 200.4 MW/cm3, high discharge energy density of 2.27 J/cm3 and ultrashort t0.9 of 58 ns of the designed ceramics broaden the actual application prospects. This work demonstrates that introducing linear dielectric CT holds great promise for developing high-performance energy storage dielectrics.

Local Heterogeneous Polarization Enhanced Superior Low-Field Energy Storage Performance in Lead-Free Relaxor Ferroelectric Ceramics

Local Heterogeneous Polarization Enhanced Superior Low-Field Energy Storage Performance in Lead-Free Relaxor Ferroelectric Ceramics

Hidden phases in homovalent and heterovalent substituted BaTiO3

Ferroelectric materials can exhibit metastable phases when exposed to THz pulses, characterized by a polarization integration capability related to the amplitude and frequency of the pulses. These so-called ``hidden'' phases enable gradual switching of polarization that can be utilized in artificial synapses for nonconventional (neuromorphic) computing machines. In this work, we employ large-scale molecular-dynamics simulation based on an effective Hamiltonian approach, and we report on the discovery of hidden phases in Zr- and Nb-doped barium titanate ($\mathrm{BaTi}{\mathrm{O}}_{3}$). We investigate the formation and the stability of those phases at different stimuli and temperatures (20 and 200 K). Our results shed light on the compositional dependence of the properties of these phases, demonstrating the potential of lead-free relaxor ferroelectrics for near-room-temperature neuromorphic computing.

Large Energy Capacitive High-Entropy Lead-Free Ferroelectrics

HighlightsUltrahigh energy storage density of ~ 13.8 J cm−3 and large efficiency of ~ 82.4% are achieved in high-entropy lead-free relaxor ferroelectrics via high-entropy strategy, realizing nearly ten times growth.Outstanding energy storage properties are attributed to the enhanced random field and breakdown field, decreased nanodomain sizes, strong multiple local distortions coexisting in-phase and anti-phase oxygen octahedron tilts.Evolution of energy storage performance and domain structure with the increase in configuration entropy is systematically revealed for the first time.

Superior energy storage properties in lead-free Na0.5Bi0.5TiO3-based relaxor ferroelectric ceramics via compositional tailoring and bandgap engineering

Currently, lead-free ferroelectrics have been paid much attention due to their versatile applications, environmental friendliness and excellent energy storage properties (ESP). In this work, lead-free relaxor ferroelectrics (0.74-x)Na0.5Bi0.5TiO3-0.06BaTiO3-0.2SrTiO3-xBi(Mg0.5Zr0.5)O3 with superior ESP were successfully designed via compositional tailoring and bandgap engineering. A new peak was observed in the dielectric properties, corresponding to the phase transition from the R3c polar nano regions (PNRs) to the P4bm PNRs, which were consistent with the Rietveld refined XRD and TEM SAED analyses, in which superlattice spots for R3c and P4bm were observed. A recoverable energy storage density (Wrec) of 6.57 J/cm3 and an energy conversion efficiency of 88.6% were procured for ceramics with x = 0.16 at 355 kV/cm. Furthermore, a variation within 5% in Wrec was obtained over a wide temperature and frequency range. Moreover, a power density of 77.39 MW/cm3 and a discharge speed of 52.5 ns were also achieved at 180 kV/cm.

Improved energy storage performance of BST‒BNT ceramics via composition modification

Lead-free relaxor ferroelectrics are a promising material owing to their excellent energy storage performance. In this work, a new lead-free ceramic system was synthesized by introducing linear dielectric Ca0.85Bi0.1TiO3 as the component modification of 0.5(Ba0.4Sr0.6TiO3)‒0.5(Bi0.5Na0.5TiO3) ceramics. All samples optimized with Ca0.85Bi0.1TiO3 showed strong relaxor behavior. Refined grain sizes and compact microstructures were observed in this pseudoternary system. Moreover, the addition of a linear dielectric decreased the Curie temperature and enhanced the breakdown strength of BST‒BNT ceramics. A high energy storage density of 2.2 J/cm3 with a good storage efficiency of 73.2% was obtained in the 0.72(0.5(Ba0.4Sr0.6TiO3)‒0.5(Bi0.5Na0.5TiO3))‒0.28Ca0.85Bi0.1TiO3 (CBT28) ceramic. Furthermore, the CBT28 ceramic maintained good temperature and frequency stability over a wide range, a feature crucial in practical applications. Results suggest the feasibility of composition modification as an approach for improving the energy storage of ceramics.

Giant comprehensive capacitive energy storage in lead-free quasi-linear relaxor ferroelectrics via local heterogeneous polarization configuration

The local heterogeneous polarization configuration in quasi-linear RFEs delivers a large Wrec (∼7.01 J cm−3), concurrent with an ultrahigh η (∼94.3%), demonstrating giant comprehensive energy storage for cutting-edge capacitors applications.

Multi-objective collaborative design optimized highly efficient energy capacitive lead-free relaxor ferroelectrics

Highly efficient capacitive ESP with a high Wrec (6.82 J cm−3) and η (90%), concurrent with an ultrahigh Wrec/Eb of ∼0.01624 μC cm−2, as well as superior stability and charge/discharge performance are gained via multi-objective collaboration design.