SPECIAL TOPIC—Centennial ferroelectricity: New materials and applications
百岁铁电: 新材料、新应用专题编者按
DOI: 10.7498/aps.69.210101
铁电材料(ferroelectrics) 因其电极化(P) 与电场强度(E) 间存在与铁磁性材料(ferromagnetics)中磁化强度(M) 与磁场强度(H) 间相似的滞回关系而得名. 1920 年法国科学家Joseph Valasek 首次在罗息盐(酒石酸钾钠, C4O6H4KNa) 中观察到铁电现象. Valasek 不仅明确认识了铁电现象的主要特征: 永久自发电极化及极化翻转中的滞回现象, 更对相变或居里点、可逆电极化和畴结构等铁电材料的本质因素开展了系统的研究. 然而由于罗息盐机械强度较低且易吸水潮解, 铁电现象仅具有科学意义, 铁电材料及其技术应用发展缓慢. 直至20 世纪50 年代人们在具有钙钛矿结构的钛酸钡(BaTiO3) 中发现了室温强铁电性才真正开启了铁电材料大规模应用的时代. 对钙钛矿结构氧化物的深入研究不仅继而发现了锆钛酸铅(PbZrO3-PbTiO3) 这一迄今为止最重要的压电材料, 更带动了铁电理论的发展. A. F. Devonshire 基于钙钛矿氧化物铁电材料提出的“双势阱”唯像理论至今仍是理解铁电材料电极化响应的重要理论工具. 经过一个世纪的发展, 铁电材料早已成为现代信息技术不可或缺的基石之一. 以钛酸钡等高介电常数铁电材料为介质层的多层陶瓷电容器, 以锆钛酸铅压电陶瓷为基本功能单元的微机电换能器、驱动器等已经成为对国家信息产业发展具有战略意义的关键基础电子元器件.
近年来, 原子尺度材料制备及结构性能表征手段的进步推动了铁电材料的又一轮蓬勃发展. 对铁电材料中电荷、自旋、轨道与晶格自由度关联耦合作用的深入研究带来了极性涡旋畴、极性斯格明子等新极性拓扑物态的发现, 极大地丰富了铁电材料的物理内涵. 纳米畴工程、缔合缺陷诱导等铁电材料性能优化新范式的提出与发展, 大幅提高了铁电材料的介电常数、压电系数、放电能量密度等关键性能指标, 为新一代高性能电子元器件奠定了坚实的基础. 二维铁电体、柔性无机铁电材料、分子铁电体等新的铁电材料体系层出不穷, 为新一代半导体器件及柔性电子技术发展提供了有力支撑. 铁电材料的应用领域日益拓展, 在超高功率静电储能电容器及全固体电卡制冷器件等新领域显示出优 越的性能和良好的应用前景. 百年铁电, 风华正茂.
受《物理学报》编辑部委托, 我邀请了国内若干位活跃于铁电材料研究前沿的中青年学者撰文, 对近年来铁电材料领域的部分热点进行总结回顾. 其中既包括对二维铁电体、柔性无机铁电材料、新型铁电拓扑结构、储能电介质材料等领域的短篇综述, 也包括报道弛豫铁电、压电及电卡效应的研究短文. 组稿期间恰逢新冠肺炎疫情肆虐, 全体作者均如约交稿, 殊为不易. 受水平及时间所限, 本专题所反映的铁电材料研究现状难免挂一漏万, 错失之处恳请各位同仁不吝指正. 希望本专题能对国内铁电材料研究的学术交流做一点贡献.
2020, 69 (21): 216801.
doi: 10.7498/aps.69.20201718
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In this paper, the recent progress of ferroelectric topologies is briefly reviewed with the emphasis on the important role of state-of-the-art aberration-corrected transmission electron microscopy in revealing the topological features in nanoscale ferroelectric materials. By identifying the ion displacement at a sub-angström level, the corresponding polarization distribution can be determined which uncovers the characteristics of topological structures. The formation mechanisms of ferroelectric topological structures and their evolutions under external fields are summarized from the perspective of strain, screening, and external fields for two prototypical ferroelectric materials, PbTiO3 and BiFeO3. For the PbTiO3, its topological structures such as flux-closures, vortices, bubbles, skyrmions, and merons can be well demonstrated in a thickness-strain-screening phase diagram, which could be a guideline for better understanding the topological structures and also for the future exploration. For BiFeO3, its topological structures reported are classified as two categories: one is the unscreened topological structure such as vortices and the other is the screened topological structure (center-type domains). Finally, we present the prospects for the future development of the ferroelectric topologies.
2020, 69 (21): 217501.
doi: 10.7498/aps.69.20201063
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Exotic ferroelectric topological states (such as vortex state) have received intensive attention in the past decade, creating a new area for exploring the emerging physical phenomena and functionalities, as well as new applications (such as memory). In recent years, a series of discoveries in novel topological states, such as vortex, central domain, skyrmion and meron states, has inspired an upsurge of research interests. Moreover, the effort to manipulate such a topological domain structure hints the possibilities for the local, deterministic control of order parameters so that the static interface conductivity can be successfully controlled at topologically protected domain walls. These encouraging discoveries create a new avenue to the fertile emerging physic phenomena, and offer new possibilities for developing potential high-performance materials and new nano-electronic devices based on these exotic states. In the past decade, this field has developed rapidly and become a hot research topic in ferroelectrics. In this paper, we review the recent progress in the field of exotic topological state in nanoferroelectrics, and discuss some existing problems and potential directions.
2020, 69 (21): 217502.
doi: 10.7498/aps.69.20201193
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Magnetoelectricity is an emerging topic and a frontier issue of the field of ferroelectricity. Multiferroics containing more than one ferroic order is an ideal system to pursuit intrinsic and robust magnetoelectric coupling, which holds rich physics and great potential applications. As a branch of the correlated electron family, multiferroic also has multiple degrees of freedom, including the charge, spin, orbital, and lattice. Among them, the charge degree of freedom has been mostly overlooked in the past researches and actually it may play an important role in magnetoelectricity. In this topical review, the charge-mediated magnetoelectricity is introduced, including the ferroelectric field effect in heterostructures and the charge ordering in single-phase multiferroics. The physical mechanisms will be revealed, together with several examples we given in recent years. We hope that this topical review can provide a reference for the researches in this vigorous filed.
2020, 69 (21): 217701.
doi: 10.7498/aps.69.20201006
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Dielectric capacitors are widely used in modern electronic systems and power systems because of their advantages of fast charge discharge speed and high-power density. Nowadays, the new products related to renewable energy, such as hybrid electric vehicles, grid connected photovoltaic power generation and wind turbines, downhole oil, gas exploration, etc., put forward higher requirements for the energy storage capabilities of dielectric capacitors in elevated-temperature. In this review, the research progress of the polymer-based dielectrics for high-temperature capacitor energy storage in recent years is systematically reviewed to offer benefits for further study. Firstly, the physical mechanism of energy storage of dielectric materials is introduced, and several conduction mechanisms of dielectric materials are summarized and analyzed; then, several strategies to improve the high-temperature energy storage performance of polymer dielectrics are presented, including the nanocomposite modification and design of layer-structured polymer composites, and the molecular structure design and chemical crosslinking treatment of dielectric polymer. Finally the scientific and technological problems in the application of dielectric polymer and their nanocomposites for high-temperature capacitor energy storage are discussed, and a possible research direction in the future is prospected.
2020, 69 (21): 217702.
doi: 10.7498/aps.69.20201031
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With the development of power electronic device equipment towards miniaturization and high performance, the dielectric materials with high energy storage density, high charge and discharge efficiency, easy processing and molding, and stable performance are urgently needed. At present, Barium titanate-based dielectric ceramics have a high dielectric constant, but low breakdown field strength and poor flexibility. Polymer-based dielectric materials have ultra-high functional density, ultra-fast charge and discharge response time, good flexibility, high breakdown field strength, light weight and other advantages, but low dielectric constant and low polarization strength. Their energy storage density is low, which limits the power capacitor component size and application scope. In order to obtain material with high energy storage performance, it was proposed to add high dielectric constant inorganic ceramic fillers to the polymer through a composite method to improve the energy storage performance of the material. The interface plays a vital role in the performance of the composite material. In this article, we review the latest research advance in the interface design and control of barium titanate/polyvinylidene fluoride composite dielectric materials. The effects of interface modification methods such as organic surface modification, inorganic functionalization and organic-inorganic synergistic modification on the polarization and energy storage performance of composite materials are summarized. The existing interface models and theoretical research methods are discussed, and the existing challenges and practical limitations, and the future research directions are prospected.
2020, 69 (21): 217703.
doi: 10.7498/aps.69.20200980
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As an important member of the dielectric family, ferroelectric materials are known for their various physical properties and have been attracted considerable attention from both scientific respect and technology respect. Piezoelectricity is one of the most important properties of ferroelectrics, which has been widely used in many devices for the conversion between electric energy and mechanical energy. For example, the main piezoelectric elements of underwater acoustic transducers, medical imaging systems, piezoelectric actuators, etc., are all ferroelectrics. In this paper, the history of the piezoelectric effect in lead-based perovskite ferroelectrics is introduced, and the relationship among the microstructure, the state of polarization and the piezoelectricity is discussed. In addition, we introduce some important factors for optimizing the piezoelectricity of ferroelectrics, such as morphotropic phase boundary, soft doping, polarization rotation, local structural heterogeneity, etc. It is expected that this paper could shed light on the future design of ferroelectric materials with various functionalities.
2020, 69 (21): 217704.
doi: 10.7498/aps.69.20201079
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Piezoelectric ceramics is a versatile functional material that can realize interconversion between electrical energy and mechanical energy. As the electrical properties of piezoelectric ceramics are extremely sensitive to the grain size variation, the investigation of grain size effect has attracted much attention. In this paper, the recent research progress of the grain size effect on perovskite piezoelectric ceramics, including barium titanate (BT), lead zirconate titanate (PZT), potassium sodium niobate (KNN), and sodium bismuth titanate (BNT), is comprehensively reviewed. We especially focus on topics including feasible ways of fabricating piezoelectric ceramics with the desired grain sizes, the influence of the grain size effect on piezoelectric properties, and the corresponding physical mechanisms. This review would be beneficial to understanding the influence of the grain size effect on piezoelectric properties. The review concludes with the prediction of the further investigation on the grain size effect.
2020, 69 (21): 217706.
doi: 10.7498/aps.69.20201209
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Electrostatic capacitors based on dielectrics delivering an ultrahigh power density, low loss and high operating voltage, are widely used in energy storage devices for modern electronic and electrical systems. Dielectric polymers, especially ferroelectric polymers, are preferable for an energy storage medium in film capacitors due to their superiority in ultrahigh breakdown strength, low mass density, flexibility, and easy fabrication process. Ferroelectric polymer nanocomposites combining the advantageous properties of ferroelectric polymer matrix and high dielectric constant of ceramic fillers, show great potential applications in achieving superior energy storage performances and have aroused substantial academic interest. This review focuses on the recent research progress of high-energy-density ferroelectric polymer nanocomposites. First, the synthesis and properties of PVDF-based ferroelectric polymers are introduced. Second, the effects of nanofillers, composite structures and interfaces on the dielectric and energy storage properties of ferroelectric polymer nanocomposites are summarized. Third, the underline mechanism of dielectric and energy storage behaviors in ferroelectric nanocomposites are discussed in the aspect of phase-field simulation. Last, the existing challenges and future directions of ferroelectric polymer nanocomposites with high energy storage density are summarized and prospected.
2020, 69 (21): 217707.
doi: 10.7498/aps.69.20201432
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In recent years, the existence of ferroelectricity in a series of two-dimensional van der Waals materials has been experimentally confirmed, in which the ferroelectricity induced by interlayer sliding is an important type. This mechanism is not available in traditional ferroelectrics but can be applied to many two-dimensional materials. In this paper we review the relevant researches and introduce the origin of this type of ferroelectricity: in many two-dimensional van der Waals bilayers, the upper layer is not equivalent to the lower layer, thus giving rise to a net interlayer charge transfer and the inducing vertical polarization to be switchable via interlayer sliding. This unique sliding ferroelectricity can widely exist in many van der Waals bilayers, multilayers and even bulk structures. The interlayer sliding barrier is several orders of magnitude lower than that of traditional ferroelectric, which may greatly save the energy required by ferroelectric switching. At present, this type of interlayer sliding ferroelectricity has been experimentally confirmed in WTe2 and β-InSe bilayer/multilayer systems, and more systems predicted to be with much stronger interlayer sliding ferroelectricity (like BN) may be realized in near future.
2020, 69 (21): 217708.
doi: 10.7498/aps.69.20201365
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Inorganic ferroelectric films exhibit excellent electric and optic properties, which have been widely used in dielectrics, memory, piezoelectric, photoelectric devices, etc. However, conventional synthesis strategies based on rigid single-crystal substrates severely limit their applications in flexible electronics. Realization of flexible inorganic ferroelectric films can introduce the excellent properties of inorganic ferroelectric materials into flexible devices, which is the developing trend for the next generation of electronic devices. In this review, the strategies to fabricate flexible inorganic perovskite structures’ ferroelectric films are summarized, including 1) direct growth on flexible substrates, 2) transferring ferroelectric film from a rigid substrate to a flexible one. Subsequently, the applications of flexible inorganic ferroelectric films are briefly introduced. Finally, research status, prospects and future development trend of flexible inorganic ferroelectric films are discussed.
2020, 69 (21): 217709.
doi: 10.7498/aps.69.20200884
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Over the past decades, exploration and artificial control of the surface and interfacial structure of the materials have played an important role in chemical catalyzing, energy conversion, information storage and medical field, and thus the finding of suitable materials with controllable surface/interface properties has attracted intense interest in recent years. Perovskite-type ferroelectric oxides are considered to be one of the most promising functional materials due to their intrinsic, non-volatile, reversible spontaneous polarization and controllable polar surface with high charge density. The investigating of the interaction between polarization and surface structure of perovskite-type ferroelectric oxide is very important for understanding the surface (interface) energy conversion, regulating the adsorption and desorption on the surface, controlling interfacial chemical reaction, and designing stable low-power electronic devices. In this paper, we summarize the theoretical mechanism and potential applications of the surface structures and functionality in perovskite-type ferroelectric oxide from three aspects. Firstly, we describe the inseparable relationship between the stabilized ferroelectric phase and surface structure of ferroelectric material, and illustrate the formation mechanism of complex surface structure of perovskite-type ferroelectric oxide. In order to reduce the surface energy to stabilize the polar surface of the material, perovskite-type ferroelectric oxide always needs to absorb foreign charged particles, change the stoichiometry and conduct electron orbital hybridization or surface relaxation, etc., which will cause the complexity of the surface structure of ferroelectric. Secondly, we outline the influence of ferroelectric polarization on the surface structure of ferroelectric and the behavior of changing ferroelectric polarization by controlling surface structure through adjusting the external environment, which provides an important basis for the subsequent regulation of the surface performance and functionality of perovskite-type ferroelectric oxide. Finally, we introduce the utilization of the controllable physical and chemical properties of ferroelectric surface (interface) into large area and into nanoscale (nanodomain), which has bright application prospects in many frontier fields, including non-volatile memory system, cell proliferation, microfluidic control system, catalysis, optical device and photodetector and so on. Furthermore, considering the limitations of current scientific research about the ferroelectric surface, we put forward the prospects for the future development of the ferroelectric material in the areas of information storage, controllable chemical reactions and new energy conversion.
Recent research progress of two-dimensional intrinsic ferroelectrics and their multiferroic coupling
2020, 69 (21): 217710.
doi: 10.7498/aps.69.20201433
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Ferroelectric materials have become a research focus of condensed matter physics because of their electric polarization state which can be regulated by external field and has potential applications in sensors, optoelectronic devices and information memory devices. With the rapid development of microelectronic integration technology, electronic devices are becoming more and more miniaturized, integrated and multifunctional. Due to the size effect and interface effect, the traditional bulk ferroelectric materials are difficult to meet the requirements for this development. Therefore, low-dimensional ferroelectric materials have received extensive attention of the academic circle. In recent years, stable room temperature intrinsic two-dimensional ferroelectric materials have been successfully prepared. The prediction and design of new materials in theoretical method such as first principles calculation also promote the development of two-dimensional ferroelectric materials. At the same time, the multiferroic coupling effect of two-dimensional ferroelectricity, ferrovalley and magnetism can be used to realize the electronic valley polarization, electronic magnetic control and other regulatory mechanisms. The coupling of multiple degrees of freedom will produce strange physical properties such as optical selectivity of circular (linear) polarization between energy valleys and quantum spin Hall effect, which is of great significance for developing spintronics, valley electronics and optics. In this paper, the recent progress of theoretical and experimental research of new two-dimensional ferroelectric materials is introduced, and the applications of two-dimensional ferroelectric materials in two-dimensional ferroelectric devices such as ferroelectric tunnel junctions and ferroelectric diodes are presented. Secondly, the multiferroic coupling effect of two-dimensional electrically controlled ferroelectric valley and electronically controlled magnetism and their derived new physical phenomena and mechanisms are described. Finally, the rich physical connotation and broad application prospects of coupling two-dimensional ferroelectric materials with other physical properties are analyzed and discussed.
2020, 69 (21): 217705.
doi: 10.7498/aps.69.20200540
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Piezoelectric ceramics, as a kind of functional material, can realize the mutual transformation between mechanical energy and electrical energy, and has been widely used in civil and military fields. With the improvement of people's awareness of environment protection and self-health care, the study of lead-free piezoelectric ceramics with excellent performance and environmental friendliness has become an urgent task. Among several kinds of lead-free piezoelectric materials, potassium sodium niobate [(K, Na)NbO3, KNN]-based ceramics has attracted much attention due to its good comprehensive properties, but there have been carried out few studies focusing on the utilization of phase boundary to regulate the properties of high piezoelectric and electrocaloric effect simultaneously. In this work, lead-free 0.944K0.48Na0.52Nb0.95Sb0.05O3 -0.04Bi0.5(Na0.82K0.18)0.5ZrO3-1.6%(AgxNa1–x)SbO3-0.4%Fe2O3 ceramics is prepared via the conventional solid-state method, and the effect of AS/NS ratio on phase structure, electrical properties, and electrocaloric effect are studied. The obtained results show that the ceramics has a multiphase coexistence with “rhombohedral-orthorhombic-tetragonal” (R-O-T) in all compositions. With the increase of AS content, the piezoelectric and ferroelectric properties of the ceramics fluctuate (d33 = 518–563 pC/N, kp = 0.45–0.56; Pmax = 21–23 μC/cm2, Pr = 14–17 μC/cm2). In addition, the electrocaloric effect (ECE) for each of the samples is studied by the indirect method. Broadening temperature span (～90 K) of electrocaloric effect is obtained in the vicinity of O-T phase transition region, while a low ECE value is observed. A stronger ECE peak (ΔTmax > 0.6 K) can be observed when the measurement temperature reaches near the Curie temperature. Consequently, both large piezoelectric property and high electrocaloric performance can be realized in KNN-based ceramics by new phase boundary construction.
2020, 69 (21): 217801.
doi: 10.7498/aps.69.20201195
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The electrocaloric effects in various types of materials, including inorganic perovskites, organic perovskites, organic polymers, molecular ferroelectrics and two-dimensional ferroelectric materials, possess great potential in realizing solid-state cooling devices due to the advantages of low-cost, high-efficiency and environmental friendly. Different ferroelectric materials have distinct characteristics in terms of phase transition and electrocaloric response. The mechanism for enhancing the electrocaloric effect currently remains elusive. Here, typical inorganic perovskite BaTiO3, PbTiO3 and BiFeO3, organic perovskite [MDABCO](NH4)I3, organic polymer P(VDF-TrFE), molecular ferroelectric ImClO4 and two-dimensional ferroelectric CuInP2S6 are selected to analyze the origins of their electrocaloric effects based on the Landau-Devonshire theory. The temperature-dependent pyroelectric coefficients and electrocaloric performances of different ferroelectric materials indicate that the first-order phase transition material MDABCO and the second-order phase transition material ImClO4 have excellent performances for electrocaloric refrigeration. The predicted results also strongly suggest that near the phase transition point of the ferroelectric material, the variation rate of free energy barrier height with temperature contributes to the polarizability change with temperature, resulting in enhanced electrocaloric effect. This present work provides a theoretical basis and a new insight into the further development of ferroelectric materials with high electrocaloric response.
