Polycrystalline Ferroelectrics Research Articles

Significant Enhancement of Electrocaloric Effect in Ferroelectric Polycrystalline Ceramics Through Grain Boundary Barrier Engineering

AbstractA key challenge currently for the new ferroelectric refrigeration with high efficiency and environmental friendliness lies in the urgent demand for ferroelectric materials with huge electrocaloric effects (ECE). Ferroelectric polycrystalline ceramics with high ECE stand out as one of the most promising candidates for electrocaloric cooling applications. However, the grain boundary network, as a barrier for the cross‐transmission of charged carriers, widely exists in electrocaloric polycrystalline ceramics and is often neglected in favor of focusing more on composition regulation and structural design. Herein, a grain boundary barrier engineering is proposed that regulates the Schottky barrier at the grain boundary network in the Ba0.8Zr0.2TiO3 ceramics by a maneuverable annealing process and clarifies its critical role in enhancing the ECE of polycrystalline ceramics. As a result, a substantial enhancement of the EC performance (from 0.68 to 1.63 K at 50 °C and 80 kV cm−1, ≈2.4 times) has been achieved in the annealed Ba0.8Zr0.2TiO3 ceramics with a lower Schottky barrier. The microstructural and electrical characterization reveals that the lower Schottky barrier in the grain boundary network facilitates the domain switching and electronic transition, hence resulting in enhanced polarization response and EC performance.

Ferroelectric polycrystalline Bi2SiO5 thin films on Pt-covered Si substrates prepared by pulsed laser deposition combined with post-annealing

In this study, we report on the fabrication of polycrystalline thin film Bi2SiO5, a notable non-perovskite ferroelectric material. The thin films were deposited on Pt-covered Si substrates by pulsed laser deposition (PLD) at room temperature, followed by a post-annealing process in air. The as-deposited thin film was of an amorphous phase of Bi-Si-O, and turned to a polycrystalline phase of Bi2SiO5 by the post-annealing above 550°C. The influences of the post-annealing temperature on the phase preference, surface morphology, and dielectric properties of the thin films were investigated by Raman spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), and dielectric measurements. As a result, the optimal post-annealing temperature was determined to be 600°C, yielding a good ferroelectric polycrystalline Bi2SiO5 thin film with a relative dielectric constant of 143 and a remanent polarization (2Pr) of 7.2 μC cm−2.

An Extended Kolmogorov-Avrami-Ishibashi (EKAI) Model to Simulate Dynamic Characteristics of Polycrystalline-Ferroelectric-Gate Field-Effect Transistors.

A physics-based model on polarization switching in ferroelectric polycrystalline films is proposed. The calculation results by the model agree well with experimental results regarding dynamic operations of ferroelectric-gate field-effect transistors (FeFETs). In the model, an angle θ for each grain in the ferroelectric polycrystal is defined, where θ is the angle between the spontaneous polarization and the film normal direction. Under a constant electric field for a single-crystal film with θ = 0, phenomena regarding polarization domain nucleation and wall propagation are well described by the Kolmogorov-Avrami-Ishibashi theory. Since the electric fields are time-dependent in FeFET operations and the θ values are distributed in the polycrystalline film, the model in this paper forms an extended Kolmogorov-Avrami-Ishibashi (EKAI) model. Under a low electric field, the nucleation and domain propagation proceed according to thermally activated processes, meaning that switching the time scale of a grain with the angle θ is proportional to an exponential form as exp(const./Ezcosθ) [Ez: the film-normal electric field]. Wide θ distribution makes the time response quite broad even on the logarithmic scale, which relates well with the broad switching time experimentally shown by FeFETs. The EKAI model is physics based and need not assume non-physical distribution functions in it.

Scale transition and residual fields in modeling of polycrystalline ferroelectrics based on the internal energy potential and a Voigt–Reuss approximation

A multiscale constitutive model, implemented within a semi-analytical computational framework, is presented. Accounting for domain switching in a tetragonal lattice and electromechanical intergranular interactions, it aims to accurately reproduce experimental data available from butterfly and dielectric hysteresis loops. For the sake of validation, these have been collected from various sources providing ranges of values for characteristic quantities. Adopting the fundamental ideas of a previously published model, a different homogenization approach is employed for the transition from grain to polycrystalline domain, providing both residual stress and electric fields. Furthermore, the thermodynamic driving force and constitutive equations on the grain level are based on a different thermodynamical potential. While the modeling approach inherently does not include any tuning parameters, the introduction of one and two, respectively, additional parameters is investigated, although computational results are basically in good agreement with experimental ranges.

Effect of sub-micron grains and defect-dipole interactions on dielectric properties of iron, cobalt, and copper doped barium titanate ceramics.

Introduction: Dilutely doped ferroelectric materials are of interest, as engineering these materials by introducing point defects via doping often leads to unique behavior not otherwise achievable in the undoped material. For example, B-site doping with transition metals in barium titanate (BaTiO3, or BTO) creates defect dipoles via oxygen vacancies leading enhanced polarization, strain, and the ability to tune dielectric properties. Though defect dipoles should lead to dielectric property enhancements, the effect of grain size in polycrystalline ferroelectrics such as BTO plays a significant role in those properties as well. Methods: Herein, doped BTO with 1.0% copper (Cu), iron (Fe), or cobalt (Co) was synthesized using traditional solid-state processing to observe the contribution of both defect-dipole formation and grain size on the ferroelectric and dielectric properties. Results and discussion: 1.0% Cu doped BTO showed the highest polarization and strain (9.3μC/cm2 and 0.1%, respectively) of the three doped BTO samples. While some results, such as the aforementioned electrical properties of the 1.0% Cu doped BTO can be explained by the strong chemical driving force of the Cu atoms to form defect dipoles with oxygen vacancies and copper's consistent +2 valency leading to stable defect-dipole formation (versus the readily mixed valency states of Fe and Co at +2/+3), other properties cannot. For instance, all three Tc values should fall below that of undoped BTO (typically 120°C-135°C), but the Tc of 1.0% Cu BTO actually exceeds that range (139.4°C). Data presented on the average grain size and distribution of grain sizes provides insight allowing us to decouple the effect of defect dipoles and the effect of grain size on properties such as Tc, where the 1.0% Cu BTO was shown to possess the largest overall grains, leading to its increase in Tc. Conclusion/future work: Overall, the 1% Cu BTO possessed the highest polarization, strain, and Tc and is a promising dopant for engineering the performance of the material. This work emphasizes the challenge of extricating one effect (such as defect-dipole formation) from another (grain size modification) inherent to doping polycrystalline BTO.

Ferroelectric domain switching pathways—From grain boundary to grain body

Grain boundaries (GBs) are one of the main factors influencing the polar domain evolution of polycrystalline ferroelectrics. However, domain switching from GBs to grains remains an unsolved aspect. Previous microscopic GB assumptions hinder such theoretical investigations, assuming that the structure and properties of GB are independent of the misorientation of adjacent grains. This work investigates the competition between the energy densities and domain-switching pathways based on the formation mechanism of the GB model. It is found that the domain-switching pathways in polycrystalline ferroelectrics follow three rules: (1) domain switching occurs near low-energy-density GBs; (2) the development of domain-switching pathway originates near the low-energy-density GBs. This pathway ultimately influences the overall domain-switching process, which follows the energy minimization principle; and (3) the domain-switching trend expands to both sides of the pathways after complete formation. The domain evolution rules for polycrystalline ferroelectric materials proposed in this work are conducive to improving the performance of ferroelectric ceramics via GB engineering.

Light-induced strain and its correlation with the optical absorption at charged domain walls in polycrystalline ferroelectrics

Photostrictive materials have a growing interest because of their great potential as light-driven actuators, among other optomechanical applications. In this context, the optical control of macroscopic strain in ferroelectrics has recently attracted remarkable attention as an effective alternative to the conventional electric control of strain. Here, a clear correlation between optical absorption and light-induced strain in polycrystalline BaTiO3 is shown. Specifically, the grain size and the sample thickness dependence of optical absorption when the material is irradiated with energy photons lower than the band gap evidence that light absorption at charged domain walls is the core of the observed photo-response in ferroelectrics. The photoinduced electronic reconstruction phenomenon is proposed as the primary physical mechanism for light absorption at charged domain walls. Results open a new pathway to designing ferroelectric-based devices with new functionalities like thickness gradient-based photo-controlled nanoactuators.

Domain switching model embedded on scaled boundary finite element method for nonlinear hysteretic behavior of ferroelectrics

The complex nonlinear hysteretic behavior of polycrystalline ferroelectrics for higher electromechanical loads stems from the underlying microscopic domain switching. A micromechanical model embedded onto an advanced numerical framework, the scaled boundary finite element method, as applied to Voronoi-based discretization of two-dimensional ferroelectric plane is presented in this work. Each ferroelectric grain is represented by a naturally evolving random Voronoi polygon with all possible domain variants during the numerical implementation. The butterfly strain responses and the hysteresis polarization responses of ferroelectrics under various electromechanical loading conditions are simulated and compared with the experimental observations, in addition to complex polarization rotations tests.

Electromechanical grain boundary model with formation mechanism in polycrystalline ferroelectrics

Grain boundaries (GBs) are transitional, defective, and anisotropic interfaces between adjacent grains with different orientations. However, most models assume that the GB is an isotropic dielectric determined by itself and lacks formation information; these assumptions hinder the theoretical investigation of the effect GBs have on polycrystalline ferroelectrics at the mesoscopic scale. Here, a novel GB model based on the formation mechanism is established for ferroelectric polycrystals. It has been found that the Curie–Weiss temperature range, elastic coefficient, and permittivity of GBs are related to the orientation of adjacent grains and the polarization state. The shielding effect, polarization enhancement, domain continuity, and spontaneous polarization on the GBs are obtained in mesoscopic simulations based on this model. In addition, the proportion of GBs can significantly affect the electric field distribution in grains. It provides a mechanistic explanation for the relationship between the coercive electric field and the proportion of GBs in the previous experiment. By achieving a better mesoscopic description of GBs, the GB model proposed in this work provides an effective investigation tool for electromechanical, electrocaloric, and energy storage of polycrystalline functional materials.

Polarization Switching Kinetics in Thin Ferroelectric HZO Films.

Ferroelectric polycrystalline HfO2 thin films are the most promising material for the implementation of novel non-volatile ferroelectric memories because of their attractive properties, such as compatibility with modern Si technology, perfect scalability, low power consumption and high endurance. However, for the commercialization of ferroelectric memory, some crucial aspects of its operation should be addressed, including the polarization switching mechanism that determines the switching speed. Although several reports on polarization switching kinetics in HfO2-based layers already exist, the physical origin of retardation behavior of polarization switching at the low and medium switching fields remains unclear. In this work, we examine several models of switching kinetics that can potentially explain or describe retardation behavior observed in experimental switching kinetics for Hf0.5Zr0.5O2 (HZO)-based capacitors and propose a new model. The proposed model is based on a statistical model of switching kinetics, which has been significantly extended to take into account the specific properties of HZO. The model includes contributions of the depolarization field and the built-in internal field originating from the charge injection into the functional HZO layer during the read procedure as well as in-plane inhomogeneity of the total electric field in ferroelectric. The general model of switching kinetics shows excellent agreement with the experimental results.

Medium electric field-induced ultrahigh polarization response and boosted energy-storage characteristics in BNT-based relaxor ferroelectric polycrystalline ceramics

Lead-free dielectric ceramics with a high recoverable energy-storage density (Wrec) and improved efficiency (η) are crucial for the development of pulse power capacitor devices. Although Wrec has been constantly improving, mainly via an increased breakdown electric field strength (Eb), a large driving electric field (>500 kV/cm) increases security risks and consequently increases insulating technology costs. In this work, we present a new (Bi0.5Na0.5)TiO3 (BNT)-based relaxor ferroelectric system, (1−x)(0.75Bi0.5Na0.4K0.1TiO3-0.25SrTiO3)-xBi(Mg0.5Ti0.5)O3 (BS-xBMT), with x ranging from 0.05 to 0.20. BMT disrupts both the A- and B-site long-range ferroelectric order of the ABO3 perovskite-structured BS matrix, induces polar nanoregions, and simultaneously increases Wrec and η. Aided by the viscous polymer process, the Eb with x = 0.15 increased to 270 kV/cm, and a maximum polarization (Pm) of 62 μC/cm2 was attained. A boosted Wrec of 4.82 J/cm3 and a high η of 84.9% were simultaneously obtained, together with good temperature stability from 30 to 140 °C. These results show that BNT-based dielectric ceramics with superior energy-storage properties can be obtained under a medium electric field.

Dynamic scaling properties of multistep polarization response in ferroelectrics

Ferroelectrics are multifunctional smart materials finding applications in sensor technology, micromechanical actuation, digital information storage, etc. Their most fundamental property is the ability of polarization switching under an applied electric field. In particular, understanding of switching kinetics is essential for digital information storage. In this regard, scaling properties of the temporal polarization response are well-known for 180°-switching processes in ferroelectrics characterized by a unique field-dependent local switching time. Unexpectedly, these properties are now observed in multiaxial polycrystalline ferroelectrics, exhibiting a number of parallel and sequential non-180°-switching processes with distinct switching times. This behavior can be explained by a combination of the multistep stochastic mechanism and the inhomogeneous field mechanism models of polarization reversal. Scaling properties are predicted for polycrystalline ferroelectrics of tetragonal, rhombohedral, and orthorhombic symmetries and are exemplarily demonstrated by the measurements of polarization kinetics in (K,Na)NbO3-based ferroelectric ceramic over a timescale of 7 orders of magnitude. Dynamic scaling properties allow insight into the microscopic switching mechanisms, on the one hand, and into statistical material characteristics, on the other hand, thereby providing the description of temporal polarization with high accuracy. The gained deeper insight into the mechanisms of multistep polarization switching is crucial for future ultrafast and multilevel digital information storage.

Exploring domain continuity across BaTiO3 grain boundaries: Theory meets experiment

Polycrystalline ferroelectrics constitute the basis of many advanced technologies, including sensors and actuators. Their intricate domain patterns, and switching, drive the macroscopic electrical and mechanical properties of the material, where the domain switching behaviour is largely influenced by the grain-grain interaction of the domain walls. Domain wall continuity across grain boundaries is speculated to affect the domain wall – grain boundary interaction, although the true impact of this phenomenon on the ferroelectric properties, and the conditions under which continuity occurs, are not yet well understood. Whilst there are some theoretical reports, the link to experimental evidence is limited, greatly hindering the applicability and fundamental understanding of current polycrystalline based devices. In this work, we close this gap by studying several grain junctions in free-standing BaTiO3 thin films using microscopy techniques and rationalising the domain configurations with reference to martensite theory. A pleasing agreement of minimal strain and polarisation mismatch for a pair of domain variants were found in cases where domain wall continuity across grain boundaries was observed, confirming that domain continuity is related to the compatibility conditions at the grain boundary. Following this experimental validation, the mismatches for various combinations of Euler angles in bi-grain junctions were theoretically explored, offering valuable insights into specific cases where domain continuity can be expected. These results offer an advancement in the understanding of grain-grain-domain interactions and provides a template for the prediction and control of domain wall continuity in polycrystalline ferroelectrics, appealing to those working in polycrystal design and domain engineering.

The Latest Technology of Polycrystalline Ferroelectric Composite Materials in the Design of the Shovel Frame Structure of the Vibratory Excavating Shovel of the Harvester

China is a large agricultural country. With the development of industrial production, mechanized harvesting has become an inevitable development trend of agricultural material harvesting. In this paper, based on the latest technology of polycrystalline ferroelectric composite materials in the design of the shovel frame of the harvester’s vibrating excavating shovel, the relevant theories of the polycrystalline ferroelectric composite material and the shovel frame of the harvester’s vibrating excavating shovel are based on the literature. After understanding, the shovel frame structure of the vibrating excavating shovel of the harvester was designed, and the latest technology of polycrystalline ferroelectric composite material was introduced in the design of the shovel frame of the vibrating excavating shovel of the harvester to make the vibration system have better performance. The designed shovel frame structure is tested, and the test results show that the horizontal output force received by the shovel designed in this paper increases with the increase of speed and is minimized at 0.26 m/s. The vertical output force decreases first and then increases with the increase of speed. It is the smallest when the running speed is 0.39 m/s. The horizontal working resistance is always greater than the vertical working resistance, and it can be seen that the design of the shovel frame structure can be realized in actual work.

Ultra-slim electrostrains with superior temperature-stability in lead-free sodium niobate-based ferroelectric perovskite

Large electrostrains with high temperature stability and low hysteresis are essential for applications in high-precision actuator devices. However, achieving simultaneously all three of the aforementioned features in ferroelectric ceramics remains a considerable challenge. In this work, we firstly report a high unipolar electrostrain (0.12% at 60 kV/cm) in (1–x)NaNbO3-x[(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3] (NN-xBCZT) ferroelectric polycrystalline ceramics with excellent thermal stability (variation less than 10% in the temperature range of 30–160 °C) and ultra-low hysteresis (<6%). Secondly, the high-field electrostrain response is dominated by the intrinsic electrostrictive effect, which may account for more than 80% of the electrostrain. Furthermore, due to the thermal stability of the polarization in the pure tetragonal phase, the large electrostrain demonstrates extraordinarily high stability from room temperature to 140 °C. Finally, in-situ piezoelectric force microscopy reveals ultra-highly stable domain structures, which also guarantee the thermal stability of the electrostrain in (NN-xBCZT ferroelectrics ceramics. This study not only clarifies the origin of thermally stable electrostrain in NN-xBCZT ferroelectric perovskite in terms of electrostrictive effect, but also provides ideas for developing applicable ferroelectric ceramic materials used in actuator devices with excellent thermal stability.

Negative Capacitance from the Inhomogenous Stray Field in a Ferroelectric–Dielectric Structure

AbstractThe phenomenological Landau–Ginzburg–Devonshire model provides a fundamental background for an understanding of the peculiar charge–voltage behavior of ferroelectric (FE) materials. However, the model cannot explain the polarization behavior of multidomain FE materials. The experimentally observed negative capacitance (NC) effect, which is interpreted as an emergence of the Landau barrier effect, involves particular conceptual difficulty. This work provides a new conceptual framework to explain the quasi‐static NC effect based on the energy formula for a stacked dielectric/ferroelectric (DE/FE) layer structure with the multidomain configuration with arbitrary shape. The presence of such domain configuration causes the energy–displacement curve of the inhomogenous Helmholtz energy term to have negative curvature. This is caused by the stray field between the neighboring domains. The model can be further extended to the DE/FE system with polycrystalline FE grains using the advanced phase‐field analysis. It is determined that the NC effect from the stray field is a universal phenomenon. These models provide quantitative explanations for the previously reported short‐pulse measurement results for various DE/FE material systems, which have lacked accurate interpretations.

Direct evidence for bulk photovoltaic charge transport in a ferroelectric polycrystalline film

The anomalous or bulk photovoltaic (BPV) effect in ferroelectrics is not subject to the Shockley-Queisser limit capping the efficiency of interface-based semiconductor photovoltaics. Therefore, it has been widely explored for applications in photovoltaic energy harvesters, non-destructive read-out mechanisms for data storage, or photorefractive thin-film devices. However, all investigations have so far been restricted to single crystals and epitaxial thin films; there is yet no direct evidence of a bulk photovoltaic charge transport in polycrystalline thin films, which would open a much larger range of economical applications. This study demonstrates the bulk nature of the photovoltaic effect in polycrystalline solution-processed BiFeO3 thin films. A light-polarization-dependent photocurrent is shown. Combined with the dependence of the photocurrent on the poling state of the film and the light intensity, as well as the scaling of open-circuit voltage with electrode distance, this clearly indicates that the charge carrier separation relies on the BPV effect.

Microstructural evolution and ferroelectricity in HfO2 films

Ferroelectric (FE) materials, which typically adopt the perovskite structure with non-centrosymmetry and exhibit spontaneous polarization, are promising for applications in memory, electromechanical and energy storage devices. However, these advanced applications suffer from the intrinsic limitations of perovskite FEs, including poor complementary metal oxide semiconductor (CMOS) compatibility and environmental issues associated with lead. Hafnium oxide (HfO2), with stable bulk centrosymmetric phases, possesses robust ferroelectricity in nanoscale thin films due to the formation of non-centrosymmetric phases. Owing to its high CMOS compatibility and high scalability, HfO2 has attracted significant attention. In the last decade, significant efforts have been made to explore the origin of the ferroelectricity and factors that influence the FE properties in HfO2 films, particularly regarding the role of microstructure, which is vital in clarifying these issues. Although several comprehensive reviews of HfO2 films have been published, there is currently no review focused on the relationship between microstructure and FE properties. This review focuses on the microstructure-property relationships in FE polycrystalline and epitaxial HfO2 films. The crystallographic structures and characterization methods for HfO2 polymorphs are first discussed. For polycrystalline HfO2 films, the microstructure-FE properties relationships, driving force and kinetic pathway of phase transformations under growth parameters or external stimuli are reviewed. For epitaxial films, the lattice matching relations between HfO2 films and substrates and the corresponding impact on the FE properties are discussed. The FE properties between polycrystalline and epitaxial HfO2 films are compared based on their different microstructural characteristics. Finally, a future perspective is given for further investigating FE HfO2 films.

Investigation into the phase structures and temperature stability of BiScO3‐PbTiO3‐based piezoelectric ceramics

AbstractPhase structure has a strong influence on the temperature stability of ceramics; however, their influence on BiScO3‐xPbTiO3 has been neglected. To meet the requirements for practical applications, (0.98‐x)BiScO3‐xPbTiO3‐0.02Bi(Sn1/3Nb2/3)O3 (BS‐xPT‐BSN, 0.59 ≤ x ≤ 0.65) polycrystalline ferroelectrics with rhombohedral phase, morphotropic phase boundary (Cm and P4mm coexisting), and tetragonal phase has been prepared and studied. The relationship between the phase structures and temperature stability is established from the macro properties as well as the underlying domain behaviors. The results show that the tetragonal phase is not sensitive to temperature because of its stable domains, which is a positive motivator for improving the temperature stability of ceramics. This work provides a new strategy for the design of high‐temperature piezoelectric ceramics in the future.

Effect of Grain Size on the Electrocaloric Properties of Polycrystalline Ferroelectrics

The physical properties, including the electrocaloric effect (ECE), of polycrystalline ferroelectrics are highly dependent on grain size in the materials. Using a phase-field model based on the time-dependent Ginzburg-Landau equation, the electrocaloric properties of ferroelectric ceramics with different grain sizes are investigated. The ferroelectric hysteresis loops under different temperatures are calculated and the adiabatic temperature changes (ATCs) are obtained through the indirect method based on the Maxwell relation. Among the investigated models with different grain sizes, the model with larger grain size possesses a larger value of ATC as well as a higher temperature at which the maximum ATC exhibits. In contrast, for the model with smaller grain size, the value of ATC declines and the temperature related to the maximum ATC shifts to a lower temperature. The significant influence of grain sizes on the ECE of polycrystalline ferroelectrics is due to the different type of domain patterns formed inside the grains. Compared to the vortex domains formed in the model with smaller grain size, the 90\ifmmode^\circ\else\textdegree\fi{} domains and single domains shown in larger grain size contribute to a more pronounced change of temperature-dependent polarization, which results in the larger ECE. In addition, the shifting of corresponding temperature to a lower one in smaller grain sizes is due to the size-effect-induced decrease of transition temperature. This work suggests a degree of freedom to understand the grain-size-dependent ECE properties of polycrystalline ferroelectrics from the perspective of domain structure and its evolution with temperature changing, which can be further studied to improve the performance of solid-state cooling devices.