High-Entropy Strategy for Improved Mechanical and Energy Storage Properties in BaTiO3-BiFeO3-Based Ceramics.

Dielectric capacitors are employed extensively due to their exceptional performance, including a rapid charge-discharge speed and superior power density. However, their practical implementation is hindered by constraints in energy-storage density (ESD), efficiency (ESE), and thermal stability. To achieve domain engineering and improved relaxor behavior in 0.67BiFeO3-0.33BaTiO3-based Pb-free ceramics, the concerns have been addressed here by employing a synergistic high-entropy strategy involving the design of the composition of Sr(Mg1/6Zn1/6Ta1/3Nb1/3)O3 with B-site multielement coexistence and high configuration entropy. Remarkably, in (0.67-x)BiFeO3-0.33BaTiO3-xSr(Mg1/6Zn1/6Ta1/3Nb1/3)O3 ceramics with x = 0.08, a good ESE (η) of 75% and a recoverable ESD (Wrec) of 2.4 J/cm3 at 190 kV/cm were attained together with an ultrahigh hardness of ∼7.2 GPa. The high-entropy strategy, which is tailored by an increase in configuration entropy, can be attributed to the superior mechanical and ES properties. It also explains the enhanced random field and relaxation behavior, the structural coexistence of ferroelectric rhombohedral (R3c) and nonpolar pseudocubic (Pm-3m) symmetries, the decreased domain size, and evenly distributed polar nanoregions (PNRs). Moreover, improved thermal stability and outstanding frequency stability are also obtained. By boosting the configuration entropy, BiFeO3-BaTiO3 materials dramatically improved their complete energy storage performance. This suggests that designing high-performance dielectrics with high entropy can be a convenient yet effective technique, leading to the development of advanced capacitors.

High Energy Density Achieved in Novel Lead-Free BiFeO3-CaTiO3 Ferroelectric Ceramics for High-Temperature Energy Storage Applications.

The development of high-performance electrostatic energy storage dielectrics is essential for various applications such as pulsed-power technologies, electric vehicles (EVs), electronic devices, and the high-temperature aviation sector. However, the usage of lead as a crucial component in conventional high-performance dielectric materials has raised severe environmental concerns. As a result of this, there is an urgent need to explore lead-free alternatives. Ferroelectric ceramics offer high energy density but lack stability at high temperatures. Here we present a lead-free (1 - x)BiFeO3-xCaTiO3 (x = 0.6, 0.7, and 0.8; BFO-CTO) ceramic capacitor with low dielectric loss, high thermal stability, and high energy density up to ∼200 °C. The introduction of CTO (x = 0.7) to the BFO matrix triggers a transition from the normal ferroelectrics to the relaxor ferroelectrics state, resulting in a high recoverable energy density of 1.18 J cm-3 at 190 °C with an ultrafast dielectric relaxation time of 44 µs. These results offer a promising, environmentally friendly, high-capacity ceramic capacitor material for high-frequency and high-temperature applications.

Anomalies of Brillouin Light Scattering in Selected Perovskite Relaxor Ferroelectric Crystals.

Compositionally disordered perovskite compounds have been one of the exotic topics of research during the past several years. Colossal piezoelectric and electrostrictive effects have been observed in disordered perovskite ferroelectric materials. The key ingredient in the physical behavior of disordered perovskites is the nucleation and growth of the local dipolar regions called polar nanoregions (PNRs). PNRs begin to nucleate far above the temperature of the dielectric maximum Tm and exhibit varied relaxation behavior with temperature. The evidence for the existence of various stages in the relaxation dynamics of PNRs was revealed through the study of the temperature evolution of optical phonons by Raman scattering. The quasi-static regime of PNRs is characterized by the strong coupling between the local polarization and strain with the local structural phase transition and the critical slowing of the relaxation time. Strong anomalies in the frequency and the width of the acoustic phonons, and emergence of the central peak in the quasi-static region of the relaxation dynamics of PNRs have been observed through Brillouin scattering studies. In this review, we discuss the anomalies observed in Brillouin scattering in selected disordered perovskite ferroelectrics crystals such as Pb(Mg1/3Ta2/3)O3, Pb(Sc1/2Ta1/2)O3, 0.65PIN-0.35PT and Sr0.97Ca0.03TiO3 to understand dynamical behavior of PNRs.

Brillouin Scattering Study of Electro-Optic KTa1-xNbxO3 Crystals.

The functionality enhancement of ferroelectrics by local polar clusters called polar nanoregions (PNRs) is one of the current interests in materials science. KTa1-xNbxO3 (KTN) with perovskite structure is a well-known electro-optic crystal with a large Kerr effect. The existence of PNRs in relaxor-like ferroelectric Nb-rich KTN with homovalent B-site cations is controversial. This paper reviews recent progress in understanding precursor dynamics in Nb-rich KTN crystals studied using Brillouin scattering. The intense central peak (CP) and significant softening of sound velocity are observed above the Curie temperature (TC) due to the polarization fluctuations in PNRs. The effects of Li-doping, defects, and electric fields on the growth and/or creation of PNRs are found using changes in acoustic properties. The electric-field-induced TC, which is shifted to higher values with increases in applied voltage, including critical endpoint (CEP) and field gradient by trapped electrons, are discussed as well. This new knowledge may give new insight into advanced functionality in perovskite ferroelectrics.

Lead-Free Relaxor Ferroelectric Ceramics with Ultrahigh Energy Storage Densities via Polymorphic Polar Nanoregions Design.

One of the long-standing challenges of current lead-free energy storage ceramics for capacitors is how to improve their comprehensive energy storage properties effectively, that is, to achieve a synergistic improvement in the breakdown strength (Eb ) and the difference between maximum polarization (Pmax ) and remnant polarization (Pr ), making them comparable to those of lead-based capacitor materials. Here, a polymorphic polar nanoregions (PNRs) structural design by first introducing 0.06 mol BaTiO3 into Bi0.5 Na0.5 TiO3 is proposed to construct the morphotropic phase boundary with coexisting structures of micrometer-size domains and polymorphic nanodomains, enhance the electric field-induced polarization response (increase Pmax ). Then Sr(Al0.5 Ta0.5 )O3 (SAT)-doped 0.94 Bi0.5 Na0.5 TiO3 -0.06BaTiO3 (BNBT) energy storage ceramics with polymorphic PNRs structures are synthesized following the guidance of phase-field simulation and rational composition design (decrease Pr ). Finally, a large recoverable energy density (Wrec ) of 8.33 J cm-3 and a high energy efficiency (η) of 90.8% under 555 kV cm-1 are obtained in the 0.85BNBT-0.15SAT ceramic prepared by repeated rolling process method (enhance Eb ), superior to most practical lead-free competitors increased consideration of the stability of temperature (a variation <±6.2%) and frequency (Wrec > 5.0  cm-3 , η > 90%) at 400 kV cm-1 . This strategy provides a new conception for the design of other-based multifunctional energy storage dielectrics.

High-Performance Electrostrictive Relaxors with Dispersive Endotaxial Nanoprecipitations.

Ultrahigh-precision manufacturing and detection have highlighted the importance of investigating electrostrictive materials with a weak stimulated extrinsic electric field and a simultaneous large hysteresis-free strain. In this study, a new type of electrostrictive relaxor ferroelectric is designed by constructing a complex inhomogeneous local structure to realize excellent electrostrictive properties. A remarkably large electrostrictive coefficient, M33 (8 × 10-16 m2 V-2 ) is achieved. Through a combined atomic-scale scanning transmission electron microscopy and advanced in situ high-energy synchrotron X-ray diffraction analysis, it is observed that such superior electrostrictive properties can be ascribed to a special domain structure that consists of endotaxial nanoprecipitations embedded in a polar matrix at the phase boundary of the rhombohedral/tetragonal/cubic phases. The matrix contributes to the high strain response under the weak extrinsic electric field because of the highly flexible polarization and randomly dispersed endotaxial nanoprecipitations with a nonpolar central region, which provide a strong restoring force that reduces the strain hysteresis. The approach developed in this study is widely applicable to numerous relaxor ferroelectrics, as well as other dielectrics, for further enhancing their electrical properties, such as electrostriction and energy-storage capacity.

Ultrahigh Energy-Storage Performances in Lead-free Na0.5Bi0.5TiO3-Based Relaxor Antiferroelectric Ceramics through a Synergistic Design Strategy.

Dielectric ceramics with outstanding energy-storage performances are nowadays in great demand for pulsed power electronic systems. Here, we propose a synergistic design strategy to significantly enhance the energy-storage properties of (1 - x)(0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-xCaTi0.75Ta0.2O3 solid solution ceramics through introducing polar nanoregions, shifting rhombohedral to tetragonal phase transition below room temperature (stable antiferroelectric characteristic), as well as increasing the band gap in the system. Ultrahigh energy-storage properties with a record value of recoverable energy-storage density Wrec ∼ 9.55 J/cm3 and a high efficiency η ∼ 88% are achieved in Na0.5Bi0.5TiO3-based bulk ceramics with x = 0.24. Moreover, high Wrec (>3.4 J/cm3) and η (>90%) with a variation of less than 6% can be observed in a wide frequency and temperature frequency range of 5-200 Hz and 25-140 °C. Our research result not only indicates the great possibility of Na0.5Bi0.5TiO3-based lead-free compositions to replace lead-based energy-storage ceramics but also gives an effective strategy to design ultrahigh energy-storage performances for eco-friendly ceramics.

Explaining the Frequency Dependence of the DC-Biased Dielectric Response of Polar Nanoregions by Field-Enhanced Correlation Length.

Understanding the effects of polar nanoregions (PNRs) dynamics on dielectric properties is a complex question of essential importance for both fundamental studies of relaxor ferroelectrics and their applications to electro-optic devices. The frequency dependence of dielectric response to the bias electric field opens a brand new window for the study of this problem. A novel model from mesoscopic to macroscopic, revealing the relationship between the dielectric permittivity to the applied electric field, temperature, and PNRs, was established based on mean field approximation and the theory of continuum percolation, and not only validates the field-induced percolation and the relaxation time divergency at the freezing temperature, but also predicts the frequency dependence of dielectric response. Unexpectedly, the model reveals the field-enhanced correlation length results in the nonmonotonic behavior of dielectric response, and implies that the increased orientation consistency of dipolar clusters and coercive fields originated from inherent inhomogeneity slow down the relaxation time of PNR reorientation. Considering the multi-scale heterogeneity of PNRs in relaxor, we found that the increased heterogeneity degree reduces the dielectric permittivity, but changes the slope of dielectric response to the bias electric field.

Dipole ordering of water molecules in cordierite: Monte Carlo simulations.

Electric dipoles of water molecules, enclosed singly in regularly spaced nanopores of a cordierite crystal, become ordered at low temperature due to their mutual interaction and show the frequency dependence of their dielectric susceptibility, typical for relaxor ferroelectrics, according to recent experimental data. The corresponding phase transition is accompanied by anomalies in thermodynamic quantities, such as heat capacity and dielectric susceptibility, which are calculated here using the Monte Carlo method, and their agreement with the experimental data is discussed. Despite the increase in the correlation length, the partially filled dipole lattice at low temperatures, according to the calculations, does not have long-range order and corresponds to a dipole glass. This simulation gives a microscopical insight into the formation of polar nanoregions in relaxor ferroelectrics and the temperature dependence of their size.

Symmetry of the Underlying Lattice in (K,Na)NbO3-Based Relaxor Ferroelectrics with Large Electromechanical Response.

The piezoelectric constant (d33) and converse piezoelectric coefficient (d33*) of a piezoelectric material are critically important parameters for sensors and actuators. Here, we simultaneously achieve a high d33 of 595 ± 10 pC/N and a large d33* of ∼776 ± 20 pm/V in (K,Na)NbO3 (KNN)-based ceramics, which exceed those of PZT5H and PZT4 ceramics, presenting good potential for practical piezoelectric applications. Moreover, the ceramic exhibits a relaxor-like and diffuse dielectric behavior due to the occurrence of local heterogeneity. According to the experiments and atomic resolution polarization mapping by Z-contrast imaging, hierarchical architecture of nanodomains and even smaller polar nanoregions with multiphase coexistence caused by compositional modification is the structural origin of the enhanced piezoelectric properties in this work. This work would pave a practical way to future applications of lead-free KNN-based ceramics.

Superior Thermal Stability of High Energy Density and Power Density in Domain-Engineered Bi0.5Na0.5TiO3-NaTaO3 Relaxor Ferroelectrics.

Thermal-stable dielectric capacitors with high energy density and power density have attracted increasing attention in recent years. In this work, (1 - x)Bi0.5Na0.5TiO3-xNaTaO3 [(1 - x)BNT-xNT, x = 0-0.30] lead-free relaxor ferroelectric ceramics are developed for capacitor applications. The x = 0.20 ceramic exhibits superior thermal stability of discharged energy density (WD) with a variation of less than 10% in an ultrawide temperature range of -50 to 300 °C, showing a significant advantage compared with the previously reported ceramic systems. The WD reaches 4.21 J/cm3 under 38 kV/mm at room temperature. Besides, a record high of power density (PD ≈ 89.5 MW/cm3) in BNT-based ceramics is also achieved in x = 0.20 ceramic with an excellent temperature insensitivity within 25-160 °C. The x = 0.20 ceramic is indicated to be an ergodic relaxor ferroelectric with coexisted R3c nanodomains and P4bm polar nanoregions at room temperature, greatly inducing large maximum polarization, maintaining low remnant polarization, and thus achieving high WD and PD. Furthermore, the diffuse phase transition from R3c to P4bm phase on heating is considered to be responsible for the superior thermal stability of the high WD and PD. These results imply the large potential of the 0.80BNT-0.20NT ceramic in temperature-stable dielectric capacitor applications.

Structural analysis of lead magnesium niobate using synchrotron powder X-ray diffraction and the Rietveld method.

The room-temperature synchrotron powder X-ray diffraction pattern of the single phase perovskite lead magnesium niobate (PMN) has shown significant broadening in the q range ∼ 5-7 Å(-1) compared with standard LaB6 synchrotron powder X-ray diffraction data, taken under similar conditions. This broadening/asymmetry lies mainly towards the lower 2θ side of the Bragg peaks. Attempts to fit this data with the paraelectric cubic phase (Pm\bar 3m) and the local rhombohedral phase (R3m) corresponding to polar nanoregions (PNRs) are made using the Rietveld method. Rietveld refinements show that neither cubic (Pm\bar 3m) nor rhombohedral (R3m) symmetry can fit this XRD pattern satisfactorily. The two-phase refinement fits the experimental data satisfactorily and suggests that the weight percentage of the PNRs is approximately 12-16% at room temperature. The unit-cell volume of these rhombohedral PNRs is approximately 0.15% larger than that of the unit cell volume of the paraelectric cubic phase.