Journal of Advanced Dielectrics

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.

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 (La₃Ga₅SiO${}_{14}$, LGS), as a high-temperature piezoelectric crystal with stable electro-elastic performance, is used as core element, and air and porous Al₂O₃ are selected as backing layers respectively to prepare two kinds of high-temperature acoustic emission (AE) sensors. The detection sensitivities at 25–500^∘C are analyzed by the ball falling test and Hsu–Nielsen experiment. Under the condition of 25–500^∘C, the received amplitude signals by both sensors are maintained above 90dB stimulated by the ZrO₂ ceramic ball dropping. In the Hsu–Nielsen experiment, as the temperature rising from 25^∘C to 500^∘C, the signal amplitude of sensor with air backing layer decays from 447mV to 365mV, while the signal amplitude varies from 270mV to 203mV for the sensor with porous Al₂O₃ backing layer. Significantly, compared with the bandwidth of the air-backing sensor (37–183kHz), the sensor with porous Al₂O₃ backing layer broadens bandwidth to 28–273kHz. These results show that both these AE sensors have strong and stable response ability to AE signals at high-temperature of 500^∘C. Therefore, piezoelectric AE sensor based on LGS has great potential application in the field of high-temperature structural health monitoring.

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 find out the optimal energy storage of ferroelectric ceramics during the phase-transition process of amorphous/nanocrystalline and polycrystalline. In this work, a finite element model suitable for the multiphase ceramic system is constructed based on the phase field breakdown theory. The nonlinear coupling relationship of multiple physical fields 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 specified 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 field breakdown. It provides a reference scheme for the structural design and performance optimization of ferroelectric ceramics.

Thin nanocomposite films based on tin dioxide with a low content of zinc oxide (0.5–5mol.%) were obtained by the sol–gel method. The synthesized films are 300–600nm thick and contains pore sizes of 19–29nm. The resulting ZnO–SnO₂ films 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 470nm were also studied. The study of the photosensitivity kinetics of ZnO–SnO₂ films showed that the film 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–SnO₂ films showed that these parameters have maxima at ZnO concentrations of 0.5mol.% and 1mol.%. Films with 1mol.% ZnO exhibit high response values when exposed to 5–50ppm of nitrogen dioxide at operating temperatures of $200^{\circ }$C and $250^{\circ }$C.

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 finite element simulation software COMSOL Multiphysics, the performance of ultrasound transducer (UT) with AFM at different design parameters in Airy distributions $(r_0,\omega )$ and the bottom thickness (d) of AFM are simulated and analyzed. Based on the simulation data, the artificial 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 field modulation underwater, acoustic tweezers, biomedical imaging, industrial nondestructive testing and neural regulation.

We studied the effect of titanium dioxide (TiO₂) nanoparticles (NPs) on dielectric behavior of Na${}^{+}$ ion-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 (NaIO₄) at concentration 10wt.% and TiO₂ NPs of size ∼10nm, at concentrations 1, 2, 3, 4 and 5wt.%. Free standing nanocomposite PEO/PVP/NaIO₄/TiO₂ films (150$\mu$m) were characterized at room-temperature by analyzing their complex electrical impedance and dielectric spectra in the range 1Hz–1MHz. At the concentration of 3wt.% of TiO₂ NPs, both ion conductivity and dielectric permittivity of the PEO/PVP/NaIO₄/TiO₂ ion-conducting dielectrics reach an enhancement by more than one order of magnitude as compared to nanoadditive-free case.

PbZrO₃-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${}_{0.09}$]O₃, 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 (3.22J/cm${}^3)$ are achieved in the composition of $x=0.12$ with a low sintering temperature (1075^∘C).

The solid solutions of the (1-x)Na${}_{0.5}$Bi${}_{0.5}$TiO₃-xNa${}_{0.5}$K${}_{0.5}$NbO₃ 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${}_{0.5}$TiO₃-xNa${}_{0.5}$K${}_{0.5}$NbO₃ 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${}_{0.5}$NbO₃ 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${}_{0.5}$TiO₃-xNa${}_{0.5}$K${}_{0.5}$NbO₃ system was shown. The increase in energy density and energy efficiency was found at additive 10mol.% of Na${}_{0.5}$K${}_{0.5}$NbO₃.