Flexoelectricity in oxide thin films

Flexoelectric effect describes the electromechanical coupling between the strain gradient and its internal polarization in all dielectrics.Despite this universality,the resulting flexoelectric field remains small at the macroscopic level.However,in nanosystems,the size-dependent effect of flexoelectricity becomes increasingly significant,leading to a notable flexoelectric field that can strongly influence the material’s physical properties.This review aims to explore the flexoelectric effect specifically at the nanoscale.We achieve this by examining strain gradients generated through two distinct methods:internal inhomogeneous strain and external stimulation.In addition,advanced synthesis techniques are utilized to enhance the properties and functionalities associated with flexoelectricity.Furthermore,we delve into other coupled phenomena observed in thin films,including the coupling and utilization of flexomagnetic and flexophotovoltaic effects.This review presents the latest advancements in these areas and highlights their role in driving further breakthroughs in the field of flexoelectricity.

Energy storage optimization of ferroelectric ceramics during phase-transition process of amorphous/nanocrystalline and polycrystalline by using a phaseffeld model for dielectric breakdown

Ferroelectric ceramics have the potential to be widely applied in the modern industry and military power systems due to their ultrafast charging/discharging speed and high energy density.Considering the structural design and electrical properties of ferroelectric capacitor,it is still a challenge to ffnd out the optimal energy storage of ferroelectric ceramics during the phase-transition process of amorphous/nanocrystalline and polycrystalline.In this work,a ffnite element model suitable for the multiphase ceramic system is constructed based on the phase ffeld breakdown theory.The nonlinear coupling relationship of multiple physical ffelds between multiphase ceramics was taken into account in this model.The basic structures of multiphase ceramics are generated by using the Voronoi diagram construction method.The speciffed structure of multiphase ceramics in the phase-transition process of amorphous/nanocrystalline and polycrystalline was further obtained through the grain boundary diffusion equation.The simulation results show that the multiphase ceramics have an optimal energy storage in the process of amorphous polycrystalline transformation,and the energy storage density reaches the maximum when the crystallinity is 13.96%and the volume fraction of grain is 2.08%.It provides a research plan and idea for revealing the correlation between microstructure and breakdown characteristics of multiphase ceramics.This simulation model realizes the nonlinear coupling of the multiphase ceramic mesoscopic structure and the phase ffeld breakdown.It provides a reference scheme for the structural design and performance optimization of ferroelectric ceramics.

Dielectric spectroscopy characterization of Naþ ion-conducting polymer nanocomposite system PEO–PVP–NaIO_(4)–TiO_(2)

We studied the effect of titanium dioxide(TiO_(2))nanoparticles(NPs)on dielectric behavior of Naþion-conducting salt-complexed polymer nanocomposite system formed from a binary polymer blend of poly(ethylene oxide)(PEO)and polyvinyl pyrrolidone(PVP),with the addition of both sodium metaperiodate(NaIO_(4))at concentration 10 wt.%and TiO_(2) NPs of size10 nm,at concentrations 1,2,3,4 and 5 wt.%.Free standing nanocomposite PEO/PVP/NaIO_(4)/TiO_(2) films(150m)were characterized at room-temperature by analyzing their complex electrical impedance and dielectric spectra in the range 1 Hz–1 MHz.At the concentration of 3 wt.%of TiO_(2) NPs,both ion conductivity and dielectric permittivity of the PEO/PVP/NaIO_(4)/TiO_(2) ion-conducting dielectrics reach an enhancement by more than one order of magnitude as compared to nanoadditive-free case.

Structure and dielectric properties of (1-x)Na_(0.5)Bi_(0.5)TiO_(3-x)Na_(0.5)K_(0.5)NbO_(3)

The solid solutions of the(1-x)Na_(0.5)Bi_(0.5)TiO_(3-x)Na_(0.5)K_(0.5)NbO_(3) system were produced by the conventional ceramic technology using mechanical activation of the synthesized product.It was found that in the(1-x)Na_(0.5)Bi_(0.5)TiO_(3-x)Na_(0.5)K_(0.5)NbO_(3) system at room temperature,a number of morphotropic phase transitions occur:rhombohedral!cubic!tetragonal!monoclinic phases.The introduction of a small amount of Na_(0.5)K_(0.5)NbO_(3) leads to an increase in the temperature stability of the dielectric properties of ceramics.A change in the relaxor properties of the solid solutions of the(1-x)Na_(0.5)Bi_(0.5)TiO_(3-x)Na_(0.5)K_(0.5)NbO_(3) system was shown.The increase in energy density and energy efficiency was found at additive 10 mol.%of Na_(0.5)K_(0.5)NbO_(3).

Investigation of electrophysical, photo- and gas-sensitive properties of ZnO–SnO_(2) sol–gel fflms

Thin nanocomposite fflms based on tin dioxide with a low content of zinc oxide(0.5–5 mol.%)were obtained by the sol–gel method.The synthesized fflms are 300–600 nm thick and contains pore sizes of 19–29 nm.The resulting ZnO–SnO_(2) fflms were comprehensively studied by atomic force and Kelvin probe force microscopy,X-ray diffraction,scanning electron microscopy,and high-resolution X-ray photoelectron spectroscopy spectra.The photoconductivity parameters on exposure to light with a wavelength of 470 nm were also studied.The study of the photosensitivity kinetics of ZnO–SnO_(2) fflms showed that the fflm with the Zn:Sn ratio equal to 0.5:99.5 has the minimum value of the charge carrier generation time constant.Measurements of the activation energy of the conductivity,potential barrier,and surface potential of ZnO–SnO_(2) fflms showed that these parameters have maxima at ZnO concentrations of 0.5 mol.%and 1 mol.%.Films with 1 mol.%ZnO exhibit high response values when exposed to 5–50 ppm of nitrogen dioxide at operating temperatures of 200℃ and 250℃.

Preparation and acoustic properties of high-temperature acoustic emission sensor based on La_(3)Ga_(5)SiO_(14) crystal

With the rapid development of modern industries,the high-temperature piezoelectric sensors that can work in extreme environments are in great demand.In this work,langasite(La_(3)Ga_(5)SiO_(14),LGS),as a high-temperature piezoelectric crystal with stable electro-elastic performance,is used as core element,and air and porous Al_(2)O_(3) are selected as backing layers respectively to prepare two kinds of high-temperature acoustic emission(AE)sensors.The detection sensitivities at 25–500℃ are analyzed by the ball falling test and Hsu–Nielsen experiment.Under the condition of 25–500℃,the received amplitude signals by both sensors are maintained above 90 dB stimulated by the ZrO_(2) ceramic ball dropping.In the Hsu–Nielsen experiment,as the temperature rising from 25℃ to 500℃,the signal amplitude of sensor with air backing layer decays from 447 mV to 365 mV,while the signal amplitude varies from 270 mV to 203 mV for the sensor with porous Al_(2)O_(3) backing layer.Signiffcantly,compared with the bandwidth of the air-backing sensor(37–183 kHz),the sensor with porous Al_(2)O_(3) backing layer broadens bandwidth to 28–273 kHz.These results show that both these AE sensors have strong and stable response ability to AE signals at high-temperature of 500℃.Therefore,piezoelectric AE sensor based on LGS has great potential application in the ffeld of high-temperature structural health monitoring.

Substitution of Pb with (Li_(1/2)Bi_(1/2)) in PbZrO_(3)-based antiferroelectric ceramics

PbZrO_(3)-based antiferroelectric(AFE)ceramics are promising dielectrics for high-energy-density capacitors due to their reversible phase transitions during charge-discharge cycles.In this work,a new composition series,[Pb_(0.93-x)La_(0.02)(Li_(1/2)Bi_(1/2))_(x)Sr_(0.04)][Zr_(0.57)Sn_(0.34)Ti_(0.09)]O_(3),with Li^(+)and Bi^(3+)substitution of Pb^(2+)at x=0,0.04,0.08,0.12,0.16 is investigated for the microstructure evolution,ferroelectric(FE)and dielectric properties.It is found that Li^(+) and Bi^(3+) substitution can significantly reduce the sintering temperature and simultaneously enhance the dielectric breakdown strength.An ultrahigh energy efficiency(94.0%)and a large energy density(3.22 J/cm^(3))are achieved in the composition of x=0.12 with a low sintering temperature(1075℃).

Optimization design of autofocusing metasurface for ultrasound wave application

In this paper,two optimized autofocusing metasurfaces(AFMs)with different desired focal distances are designed by using particle swarm optimization(PSO)algorithm.Based on the ffnite element simulation software COMSOL Multiphysics,the performance of ultrasound transducer(UT)with AFM at different design parameters in Airy distributions(r0,ω)and the bottom thickness(d)of AFM are simulated and analyzed.Based on the simulation data,the artiffcial neural network model is trained to describe the complex relationship between the design parameters of AFM and the performance parameters of UT.Then,the multiobjective optimization function for AFM is determined according to the desired performance parameters of UT,including focal position,lateral resolution,longitudinal resolution and absolute sound pressure.In order to obtain AFMs with the desired performance,PSO algorithm is adopted to optimize the design parameters of AFM according to the multiobjective optimization function,and two AFMs are optimized and fabricated.The experimental results well agree with the simulation and optimization results,and the optimized AFMs can achieve the desired performance.The fabricated AFM can be easily integrated with UT,which has great potential applications in wave ffeld modulation underwater,acoustic tweezers,biomedical imaging,industrial nondestructive testing and neural regulation.

High recoverable energy storage density of Na_(0.5)Bi_(0.5)TiO_(3) lead-free ceramics modified by Bi(Mg_(0.5)Hf_(0.5))O_(3)

Enhancing the availability and reliability of dielectric ceramic energy storage devices is of great importance.In this work,(1-x)Na_(0.5)Bi_(0.5)TiO_(3)-xBi(Mg_(0.5)Hf_(0.5))O_(3)(NBT-xBMH)lead-free ceramics were created utilizing a solid-state reaction technique.All NBT-xBMH ceramics have a single perovskite structure.With increasing BMH doping,the grain size shrinks drastically,which greatly enhances the breakdown electric field(310 kV/cm at x=0.25).Additionally,the relaxation behaviors of NBT-xBMH ceramics with high BMH content are more remarkable.Among all designed components,the NBT-0.25BMH ceramic exhibits the best energy storage performance with a high Wrec of 4.63 J/cm^(3) and anηof 75.1%at 310 kV/cm.The NBT-0.25BMH ceramic has exceptional resistance to fluctuations in both frequency(5-500 Hz)and temperature(30-100°C).Charge-discharge test shows that the NBT-0.25BMH ceramic has a quick discharge rate(t0.9<110 ns).With these properties,the NBT-0.25BMH ceramic may have applications in microdevices as well as in ultra-high power electronic systems.

Impedance spectra of different capacitor technologies

This paper reviews the interpretation of impedance and capacitance spectra for different capacitor technologies and discusses how basic electrical characteristics can be inferred from them.The basis of the interpretation is the equivalent circuit for capacitors.It is demonstrated how the model parameters,such as capacitance and equivalent series resistance,can be extracted from the measured spectra.The aspects of measurement accuracy are exemplarily discussed on the measured spectra.

Preparation of elastomeric nanocomposites using nanocellulose and recycled alum sludge for flexible dielectric materials

Flexible dielectric materials with environmental-friendly,low-cost and high-energy density characteristics are in increasing demand as the world steps into the new Industrial 4.0 era.In this work,an elastomeric nanocomposite was developed by incor­porating two components:cellulose nanofibrils(CNFs)and recycled alum sludge,as the reinforcement phase and to improve the dielectric properties,in a bio-elastomer matrix.CNF and alum sludge were produced by processing waste materials that would otherwise be disposed to landfills.A biodegradable elastomer polydimethylsiloxane was used as the matrix and the nanocompos­ites were processed by casting the materials in Petri dishes.Nanocellulose extraction and heat treatment of alum sludge were con­ducted and characterized using various techniques including scanning electron microscopy(SEM),thermogravimetric analysis/derivative thermogravimetric(TGA/DTG)and X-ray diffraction(XRD)analysis.When preparing the nanocomposite samples,various amount of alum sludge was added to examine their impact on the mechanical,thermal and electrical properties.Results have shown that it could be a sustainable practice of reusing such wastes in preparing flexible,lightweight and miniature dielectric materials that can be used for energy storage applications.

Stable large-area monodomain in as-grown bulk ferroelectric single crystal Sn_(2)P_(2)S_(6)

Driven by the minimization of total energy,the multi-domain morphology is preferred in as-grown ferroelectrics to reduce the depolarization and strain energy during the paraelectric to ferroelectric phase transition.However,the complicated multi-domain is not desirable for certain high-performance ferroelectric electro-optic devices.In this work,we achieve a reproducible and stable large-area monodomain in as-grown bulk ferroelectric single crystal Sn_(2)P_(2)S_(6).The monodomain dominates the entire single crystal,which is attributed to the internal charge carriers from the photoexcited disproportionation reaction of Sn ions.The charge carriers effectively screen the depolarization field and therefore decrease the depolarization energy and facilitate the formation of monodomain.This work offers a potential approach for engineering bulk ferroelectrics with a stable monodomain,which is desirable for the high-performance ferroelectric electro-optic devices.

Impact of oxygen-containing carbonyl and ether groups on dielectric properties of poly(oxa)norbornene cyclic olefins

Flexible dielectric polymers that can withstand high electric field and simultaneously have high dielectric constant are desired for high-density energy storage.Here,we systematically investigated the impact of oxygen-containing ether and carbonyl groups in the backbone structure on dielectric properties of a series of cyclic olefin.In comparison to the influence of the-CF3 pendant groups that had more impact on the dielectric constant rather than the band gap,the change of the backbone structure affected both the dielectric constant and band gaps.The one polymer with ether and carbonyl groups in the backbone has the largest band gap and highest discharge efficiency,while it has the lowest dielectric constant.The polymer without any ether groups in the backbone has the smallest band gap and lowest discharge efficiency,but it has the highest dielectric constant.Polymers that have no dipolar relaxation exhibit an inversely correlated dielectric constant and band gap.Enhancing the dipolar relaxation through rational molecular structure design can be a novel way to break through the exclusive constraint of dielectric constant and band gap for high-density energy storage.

Ferroelectricity in biological building blocks:Slipping on a banana peel?

Ferroelectricity in biological system has been anticipated both theoretically and experimentally over the past few decades.Claims of ferroelectricity in biological systems have given rise to confusion and methodological controversy.Over the years,a“loop”of induced polarization in response to a varying applied electrical field and a consequent polarization reversal has prompted many researchers to claim ferroelectricity in biological structures and their building blocks.Other observers were skeptical about the methodology adopted in generating the data and questioned the validity of the claimed ferroelectricity as such,“loop”can also be obtained from linear capacitors.In a paper with somewhat tongue-in-cheek title,Jim Scott showed that ordinary banana peels could exhibit closed loops of electrical charge which closely resemble and thus could be misinterpreted as ferroelectric hysteresis loops in barium sodium niobate,BNN paraphrasing it as“banana”.In this paper,we critically review ferroelectricity in biological system and argue that knowing the molecular and crystalline structure of biological building blocks and experimenting on such building blocks may be the way forward in revealing the“true”nature of ferroelectricity in biological systems.

Microstructure, domain structure, ferroelectric and piezoelectric properties of textured bismuth-containing ceramics

In this report,the processes of texture formation in grain-oriented ferroelectric ceramics based on layer-structured ferroelectric Bi_(4)Ti_(3)O_(12)(LSBT)prepared by hot forging method are considered.The microstructural and X-ray methods revealed the axial textured formation in ferroelectric ceramic that are used to estimate the orientation factor of ceramics.For the first time,the domain structure changes when poling the anisotropic ferroelectric ceramics are investigated.The anisotropy of electromechanical,piezoelectric and ferroelectric properties of ferroelectric ceramics due to the crystal texture existence in it is studied.The aim of this study is to study the processes of crystalline texture formation in polycrystalline BLSF and to establish the dependence of the electrophysical properties of ceramics on the degree of texturing.Ceramics were textured using the hot stamping(HS)method developed at the Research Institute of Physics.The mechanism of the method is that the workpiece is subjected to uniaxial pressure and free radial deformation occurs due to the plastic flow of the material until the workpiece fills the free volume of the mold,which is created by placing the workpiece in the mold with a gap.The study of the microstructure of ceramics showed that an increase in the firing temperature in the range 950-1050℃ causes a sharp decrease in porosity and increases the density to 7.95 g/cm^(3),which is 98%of theoretical.An X-ray analysis was performed and microstructural studies were carried out,which revealed the formation of an axial texture in ceramics.The features of the switching processes of textured ceramics are revealed.The characteristics of the polarization switching of ceramics in the directions parallel and perpendicular(⊥)of the pressure axis during hot processing were obtained from the dielectric hysteresis P(E)loops,i.e.,axis axial texture.The⊥-cut ceramics are characterized by a more complete polarization switching,which is associated with the additional orientation of the(001)crystallographic planes in the textured material,as well as the presence of a threshold switching field.In the temperature range from-196 to+600℃,the anisotropy of the electro physical properties of ceramics due to the presence of a crystalline texture in it was studied.The dielectric constant,electrical conductivity,piezoelectric and elastic coefficients were measured for sections of ceramics of different orientations relative to the axis of the texture.The anisotropy of the dielectric constant and electrical conductivity manifests itself weakly at room temperature and increases sharply when approaching the Curie temperature.In the temperature range+20-400℃,the high thermal stability of the piezoelectric module d_(33),measured by the quasistatic method,was established.

Ferroelectric solid solutions with perovskite- and columbite-type components: From structures formation to domain and hysteresis phenomena

The paper reports results on the complex study on ferroelectric ceramics that represent solid solutions containing components with a perovskite-type or columbite-type structure.Solid solutions of a three-component(1−x−y)NaNbO_(3)−xKNbO_(3)−yCdNb_(2)O_(6) system are manufactured at x=0.05-0.20 and y=0.10.Domain structures in ceramic grains are studied.The consistency between experimental and calculated results is examined for coexisting phases split into non-180°domains(mechanical twins)in the solid solution with x=0.15.A correlation between the internal structure(crystal,domain,granular,and defect)and fundamental elec­tromechanical and polarization properties is stated for the studied three-component solid solutions.

Bubble domain evolution in well-ordered BiFeO_(3) nanocapacitors

Ferroelectric nanocapacitors have attracted intensive research interest due to their novel functionalities and potential application in nanodevices.However,due to the lack of knowledge of domain evolution in isolated nanocapacitors,precise manipulation of topological domain switching in the nanocapacitor is still a challenge.Here,we report unique bubble and cylindrical domains in the well-ordered BiFeO_(3) nanocapacitor array.The transformation of bubble,cylindrical and mono domains in isolated ferroelectric nanocapacitor has been demonstrated via scanning probe microscopy(SPM).The bubble domain can be erased to mono domain or written to cylindrical domain and mono domain by positive and negative voltage,respectively.Additionally,the domain evolution rules,which are mainly affected by the depolarization field,have been observed in the nanocapacitors with different domain structures.This work will be helpful in understanding the domain evolution in ferroelectric nanocapacitors and providing guidance on the manipulation of nanoscale topological domains.

Resonance phenomena in dielectric media: A review and comparison of acoustic and electromagnetic modes

The theory and application of resonances and vibrational modes are part of the foundation of science.In this contribution,examples of acoustical resonators are highlighted,and compared to electromagnetic modes.As an example from architecture,we describe the phenomenon of whispering galleries;such modes are nowadays known in dielectric and optical resonators.A specimen of a semicircular whispering bench in Park Sanssouci in Potsdam is acoustically investigated and demonstrated to show low losses for sound propagation.A special acoustical bug is discussed which was used for the espionage of the U.S.ambassador in Moscow.The Sovyets could interrogate this passive device by radio waves.Its working principle was based on the electromagnetic resonance of the cavity that the sound-sensitive membrane was part of.The underlying relation between excitation and resonance is compared to the sound production in flue organ pipes.A stopped flue organ pipe was investigated using a piezoelectric film sensor inside the pipe body.The results show that even-numbered modes,which are usually suppressed in the radiated sound of a stopped pipe,are still present in the vibrations inside the resonator.

Ferroelectrets:Heterogenous polymer electrets with high piezoelectric sensitivity for transducers

Nowadays,the demand for advanced functional materials in transducer technology is growing rapidly.Piezoelectric materials transform mechanical variables(displacement or force)into electrical signals(charge or voltage)and vice versa.They are interesting from both fundamental and application points of view.Ferrooelectrets(also called piezoelectrets)are a relatively young group of piezo-,pyro-and ferroelectric materials.They exhibit ferroic behavior phenomenologically undistinguishable from that of traditional ferroelectrics,although the materials per se are essentially non-polar space-charge electrets with artificial macroscopic dipoles(i.e.,internally charged cavities).A lot of work has been done on ferroelectrets and their applications up to now.In this paper,we review and discuss mostly the work done at University of Potsdam on the research and development of ferroelectrets.We will,however,also mention important results from other teams,and prospect the challenges and future progress trend of the field of ferroelectret research.

Orientation dependence on piezoresponse of lead-free piezoelectric sodium bismuth titanate epitaxial thin films

Lead-free piezoelectric sodium bismuth titanate((Bi0.5Na0.5)TiO3,BNT)thin films were epitaxially grown onto(001)-,(110)-,and(111)-oriented Nb:SrTiO3(STO)single crystal substrates prepared by sol-gel processing.Highly oriented growth in(001),(110),and(111)BNT thin films was obtained in this work benefiting from the lattice match between the BNT film and the STO substrate.The different growth models in thin films with various orientations result in various surface morphologies dependent on the film orientation.The piezoresponse of the BNT thin films was represented exhibiting a strong orientation dependence that(110)>(001)>(111).This is contributed by the various domain switching contribution related to the crystal symmetry and polarization distribution in the three oriented thin films.