Tuning Piezoelectricity via Thermal Annealing at a Freestanding Ferroelectric Membrane.

Tuning the ferroelectric domain structure by a combination of elastic and electrostatic engineering provides an effective route for enhanced piezoelectricity. However, for epitaxial thin films, the clamping effect imposed by the substrate does not allow aftergrowth tuning and also limits the electromechanical response. In contrast, freestanding membranes, which are free of substrate constraints, enable the tuning of a subtle balance between elastic and electrostatic energies, giving new platforms for enhanced and tunable functionalities. Here, highly tunable piezoelectricity is demonstrated in freestanding PbTiO3 membranes, by varying the ferroelectric domain structures from c-dominated to c/a and a domains via aftergrowth thermal treatment. Significantly, the piezoelectric coefficient of the c/a domain structure is enhanced by a factor of 2.5 compared with typical c domain PbTiO3. This work presents a new strategy to manipulate the piezoelectricity in ferroelectric membranes, highlighting their great potential for nano actuators, transducers, sensors and other NEMS device applications.

Improved Electric Breakdown Strength and Energy Storage Performances in La(Mg2/3Nb1/3)O3 and MnO2-Modified BiFeO3-SrTiO3 Ceramics.

Dielectric capacitors have become an important component in current pulsed power devices and thus have attracted great research interest in recent years. Among all kinds of dielectric materials, the bismuth ferrite (BiFeO3)-based ceramic capacitors show possible applications in dielectric energy storage because of their large polarization. However, the relatively high conductivity badly limits the improvement of electric breakdown strength, thus leading to low energy density. Herein, the perovskite end-member La(Mg2/3Nb1/3)O3 and sintering aid MnO2 were simultaneously introduced into BiFeO3-SrTiO3 solid solutions to improve the relaxation features and electric breakdown strength. Accordingly, a high recoverable energy density of 6.3 J/cm3 and an acceptable efficiency of 74.3% were realized under 450 kV/cm. In addition, the good frequency/thermal stability and superior charge-discharge performances were also realized. This work provides feasible approaches to modify the capacitive energy storage of BiFeO3-based relaxor ferroelectric ceramics.

Modeling the Converse Magnetoelectric Effect in the Low-Frequency Range.

This article is devoted to the theory of the converse magnetoelectric (CME) effect for the longitudinal, bending, longitudinal-shear, and torsional resonance modes and its quasi-static regime. In contrast to the direct ME effect (DME), these issues have not been studied in sufficient detail in the literature. However, in a number of cases, in particular in the study of low-frequency ME antennas, the results obtained are of interest. Detailed calculations with examples were carried out for the longitudinal mode on the symmetric and asymmetric structures based on Metglas/PZT (LN); the bending mode was considered for the asymmetric free structure and structure with rigidly fixed left-end Metglas/PZT (LN); the longitudinal-shear and torsional modes were investigated for the symmetric and asymmetric free structures based on Metglas/GaAs. For the identification of the torsion mode, it was suggested to perform an experiment on the ME structure based on Metglas/bimorphic LN. All calculation results are presented in the form of graphs for the CME coefficients.

Strain-Manipulated Photovoltaic and Photoelectric Effects of the MAPbBr3 Single Crystal.

Lead halide perovskite materials, such as MAPbBr3 and MAPbI3, show excellent semiconductor properties, and thus, they have attracted a lot of attention for applications in solar cells, photodetectors, etc. Here, a periodic strain can dynamically manipulate the build-in electric field (Ebi) of the depletion region with piezoelectricity at the Au/MAPbBr3 interface. As a result, the photovoltaic short-circuit current density (Jsc) and the open-circuit voltage (Voc) are increased by 670 and 82%, respectively, by applying an external strain upon an asymmetric solar-cell-like Au/MAPbBr3/Ga structure. Furthermore, the equivalent piezoelectric d33 values of ∼3.5 pC/N are confirmed in the Au/MAPbBr3/Au structure with both the sinusoidal strain and the 405 nm light illumination with 220 mW/cm2 upon one semitransparent Au electrode. This study not only proves that pressure can effectively enhance the energy conversion efficiency of the halide perovskite-based solar cells and light detectors but also supposes a multifunctional sensor, which can detect light intensity, sense dynamic pressure, explore accelerated speed, etc. simultaneously.

Significantly Enhanced Energy Storage Performance of Lead-Free BiFeO3-Based Ceramics via Synergic Optimization Strategy.

Owing to the merits of giant power density and ultrafast charge-discharge time, dielectric capacitors including ceramics and films have inspired increasing interest lately. Nevertheless, the energy storage density of dielectric ceramics should be further optimized to cater to the boosting demand for the compact and portable electronic devices. Herein, an ultrahigh recoverable energy storage density Wrec of 13.44 J/cm3 and a high efficiency η of 90.14% are simultaneously realized in BiFeO3-BaTiO3-NaTaO3 relaxor ferroelectric ceramics with high polarization Pmax, reduced remanent polarization Pr, and optimized electric breakdown strength Eb. High Pmax originates from the genes of BiFeO3-based ceramics, and reduced Pr is induced by enhanced relaxor behavior. Particularly, a large Eb is achieved by the synergic contributions from complicated internal and external factors, such as decreased grain size and improved resistivity and electrical homogeneity. Furthermore, the ceramics also exhibit satisfactory frequency, cycling and thermal reliability, and decent charge-discharge property. This work not only indicates that the BiFeO3-based relaxor ferroelectric materials are promising choices for the next-generation electrostatic capacitors but also paves a potential approach to exploit novel high-performance dielectric ceramics.

A very low frequency (VLF) antenna based on clamped bending-mode structure magnetoelectric laminates.

As the development of wireless communication devices tends to be highly integrated, the miniaturization of very low frequency (VLF) antenna units has always been an unresolved issue. Here, a novel VLF mechanical communication antenna using magnetoelectric (ME) laminates with bending-mode structure is realized. ME laminates combines magnetostrictive Metglas amorphous ribbons and piezoelectric 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3single crystal plates. From the simulation, we confirmed that the ME laminates can reduce the resonance peak from 18 kHz to 7.5 kHz by bending-mode structure. Experiment results show the resonance frequency can be farther reduced to 6.3 kHz by clamping one end of the ME antenna. The ME laminate exhibits a giant converse ME coefficient of 6 Oe cm V-1at 6.3 kHz. The magnetic flux density generated by the ME antenna has been tested along with distance ranging from 0 to 60 cm and it is estimated that a 1 fT flux could be detected around 100 m with an excitation power of 10 mW.

Pyro-catalysis for tooth whitening via oral temperature fluctuation.

Tooth whitening has recently become one of the most popular aesthetic dentistry procedures. Beyond classic hydrogen peroxide-based whitening agents, photo-catalysts and piezo-catalysts have been demonstrated for non-destructive on-demand tooth whitening. However, their usage has been challenged due to the relatively limited physical stimuli of light irradiation and ultrasonic mechanical vibration. To address this challenge, we report here a non-destructive and convenient tooth whitening strategy based on the pyro-catalysis effect, realized via ubiquitous oral motion-induced temperature fluctuations. Degradation of organic dyes via pyro-catalysis is performed under cooling/heating cycling to simulate natural temperature fluctuations associated with intake and speech. Teeth stained by habitual beverages and flavorings can be whitened by the pyroelectric particles-embedded hydrogel under a small surrounding temperature fluctuation. Furthermore, the pyro-catalysis-based tooth whitening procedure exhibits a therapeutic biosafety and sustainability. In view of the exemplary demonstration, the most prevalent oral temperature fluctuation will enable the pyro-catalysis-based tooth whitening strategy to have tremendous potential for practical applications.

A Biodegradable and Recyclable Piezoelectric Sensor Based on a Molecular Ferroelectric Embedded in a Bacterial Cellulose Hydrogel.

Currently, various electronic devices make our life more and more safe, healthy, and comfortable, but at the same time, they produce a large amount of nondegradable and nonrecyclable electronic waste that threatens our environment. In this work, we explore an environmentally friendly and flexible mechanical sensor that is biodegradable and recyclable. The sensor consists of a bacterial cellulose (BC) hydrogel as the matrix and imidazolium perchlorate (ImClO4) molecular ferroelectric as the functional element, the hybrid of which possesses a high sensitivity of 4 mV kPa-1 and a wide operational range from 0.2 to 31.25 kPa, outperforming those of most devices based on conventional functional biomaterials. Moreover, the BC hydrogel can be fully degraded into glucose and oligosaccharides, while ImClO4 can be recyclable and reused for the same devices, leaving no environmentally hazardous electronic waste.

Magnetoelectric Magnetic Field Sensors: A Review.

One of the new materials that have recently attracted wide attention of researchers are magnetoelectric (ME) composites. Great interest in these materials is due to their properties associated with the transformation of electric polarization/magnetization under the influence of external magnetic/electric fields and the possibility of their use to create new devices. In the proposed review, ME magnetic field sensors based on the widely used structures Terfenol-PZT/PMN-PT, Metglas-PZT/PMN-PT, and Metglas-Lithium niobate, among others, are considered as the first applications of the ME effect in technology. Estimates of the parameters of ME sensors are given, and comparative characteristics of magnetic field sensors are presented. Taking into account the high sensitivity of ME magnetic field sensors, comparable to superconducting quantum interference devices (SQUIDs), we discuss the areas of their application.

Piezoelectricity in Excess of 800 pC/N over 400 °C in BiScO3-PbTiO3-CaTiO3 Ceramics.

Ultrasonic sensors are widely applied in industries near room temperature; however, their application at high temperature is still a challenge mainly due to the lack of high-performance piezoelectric ceramics. Here, the 0.364BiScO3-0.636PbTiO3-0.005CaTiO3 ceramic exhibits excellent piezoelectric performances at 20-440 °C. Its piezoelectric coefficient d33 increases from 475 pC/N at 20 °C to 853 pC/N at 360 °C, and then it gradually decreases to 669 pC/N at 440 °C. Furthermore, the planar electromechanical coupling factor kp gradually increases from 0.59 at 20 °C to 0.67 at 200 °C, and then it remains at a stable value of 0.65-0.67 at 150-350 °C. These achievements are because the ceramic morphotropic phase boundaries have a flat Gibbs free energy versus polarization curve and a wide temperature range. Since the piezoelectric ceramic shows satisfactory piezoelectric properties at 20-440 °C, the corresponding ultrasonic sensors can in situ monitor many high-temperature devices, such as engines, wheels, drills, boilers, etc.

Giant Bulk Photostriction of Lead Halide Perovskite Single Crystals.

It is well known that the lattice structure for a crystal can be manipulated through mechanical strain, temperature, an electric field, a magnetic field, and light. In the past, the photostriction commonly occurs at the surface and the bulk photostriction is very small in most semiconductors. Here, the 532 nm laser can excite the excess electron-hole pairs in the surface layer and consequently these carriers diffuse in the millimeter-thick MAPbBr3-xIx crystal and introduce a giant bulk photostriction of 0.17, 0.28, and 0.35% for the 0.5 mm-thick MAPbBr3-xIx single crystals at x = 0, 1, and 2, respectively. Furthermore, the displacement of each crystal linearly increases from hundreds of picometers to several micrometers when the light intensity increases from about 0.2 to 536 mW/cm2. Since both the maximum strain and the displacement accuracy are as good as those of PZT ceramics used in piezoelectric actuators, these crystals can be used in light-driven actuators for precise positioning.

Structural Tailing and Pyroelectric Energy Harvesting of P(VDF-TrFE) and P(VDF-TrFE-CTFE) Ferroelectric Polymer Blends.

The copolymer P(VDF-TrFE) is a normal ferroelectric because the bulky TrFE monomer improves its crystalline chain structure, while the terpolymer P(VDF-TrFE-CTFE) is a relaxor ferroelectric because the third monomer CTFE makes it amorphous. Herein, in order to induce a crystalline beta phase in the terpolymer, we blended a small amount of crystalline P(VDF-TrFE) into P(VDF-TrFE-CTFE) and investigated the effect of blending on the pyroelectric energy harvesting (PyEH) properties. The polarization-electric field hysteresis loops at different temperatures and energy densities were investigated. The PyEH energy density (N D) is compared with the electrical energy density (U E). The U E and N D at the ferroelectric-paraelectric transition temperature for the χ = 0.1 blend are reported as 3.18 and 5.04 J/cm3, respectively, which are higher than the other polymer blends. Interestingly, the N D of the χ = 0.9 blend is found to be 3.44 J/cm3 when operated at lower and higher temperatures, that is, at T L = 25 °C and T H = 40 °C, respectively, which is the highest possible energy density at the lowest possible transition temperature for the polymer blends.

Piezo-catalysis for nondestructive tooth whitening.

The increasing demand for a whiter smile has resulted in an increased popularity for tooth whitening procedures. The most classic hydrogen peroxide-based whitening agents are effective, but can lead to enamel demineralization, gingival irritation, or cytotoxicity. Furthermore, these techniques are excessively time-consuming. Here, we report a nondestructive, harmless and convenient tooth whitening strategy based on a piezo-catalysis effect realized by replacement of abrasives traditionally used in toothpaste with piezoelectric particles. Degradation of organic dyes via piezo-catalysis of BaTiO3 (BTO) nanoparticles was performed under ultrasonic vibration to simulate daily tooth brushing. Teeth stained with black tea, blueberry juice, wine or a combination thereof can be notably whitened by the poled BTO turbid liquid after vibration for 3 h. A similar treatment using unpoled or cubic BTO show negligible tooth whitening effect. Furthermore, the BTO nanoparticle-based piezo-catalysis tooth whitening procedure exhibits remarkably less damage to both enamel and biological cells.

Transparent, Flexible, Fatigue-Free, Optical-Read, and Nonvolatile Ferroelectric Memories.

Perovskite oxide films are widely used in various commercial industries. However, they are usually prepared at high temperature and in oxygen ambience, detrimental to most transparent and flexible substrates and bottom conductive electrodes such as indium tin oxide (ITO). It remains challenging to integrate perovskite oxides into transparent and flexible electronics. Here, the 1.2 wt % Ag-doped ITO (Ag-ITO) grown on a mica substrate is employed as the bottom electrode, which can withstand high temperature and repeated bending, and then we achieve the transparent, flexible, fatigue-free, and optical-read ferroelectric nonvolatile memories based on the mica/Ag-ITO/Bi3.25La0.75Ti3O12/ITO structures. The as-prepared memories show ∼80% transmittance for visible lights and fatigue-free performance after more than 108 writing/erasing cycles. These performances are stable after repeated bending down to 3 mm in a curvature radius. More importantly, the "1/0" state of the memory can be read out by the photovoltaic current rather than destructive polarization switching, an emergent functionality for many applications. This work substantially promotes the applications of perovskite oxide films in transparent and flexible electronics, including wearable devices.

Harvesting the Vibration Energy of BiFeO3 Nanosheets for Hydrogen Evolution.

In this study, mechanical vibration is used for hydrogen generation and decomposition of dye molecules, with the help of BiFeO3 (BFO) square nanosheets. A high hydrogen production rate of ≈124.1 µmol g-1 is achieved under mechanical vibration (100 W) for 1 h at the resonant frequency of the BFO nanosheets. The decomposition ratio of Rhodamine B dye reaches up to ≈94.1 % after mechanical vibration of the BFO catalyst for 50 min. The vibration-induced catalysis of the BFO square nanosheets may be attributed to the piezocatalytic properties of BFO and the high specific surface area of the nanosheets. The uncompensated piezoelectric charges on the surfaces of BFO nanosheets induced by mechanical vibration result in a built-in electric field across the nanosheets. Unlike a photocatalyst for water splitting, which requires a proper band edge position for hydrogen evolution, such a requirement is not needed in piezocatalytic water splitting, where the band tilting under the induced piezoelectric field will make the conduction band of BFO more negative than the H2 /H2 O redox potential (0 V) for hydrogen generation.

High-efficient synergy of piezocatalysis and photocatalysis in bismuth oxychloride nanomaterial for dye decomposition.

In this work, it is found that the hydrothermally-synthesized bismuth oxychloride can behave both the piezocatalysis and photocatalysis for the Rhodamine B dye decomposition. ∼99% decomposition efficiency is achieved after both vibrating and lighting the Rhodamine B dye solution for ∼96 min with the addition of bismuth oxychloride catalyst, while the ∼72% and ∼26% decomposition efficiencies are obtained for only photocatalysis or only piezocatalysis respectively. In bi-catalysis, the mechanical strain produced due to vibration will directly provide an electric field that will increase the separation between the photo-induced electron-hole pairs, yielding to the enhanced decomposition performance of bi-catalysis. There is no significant change in the bi-catalytic performance of bismuth oxychloride nanomaterial observed after being recycled four times. Bismuth oxychloride catalyst is potential for the bi-catalytic decomposition treatment of wastewater through harvesting both the environmental vibration energy and light energy.

Reversible Structural Transformation between Polar Polymorphs of Li2GeTeO6.

Li2GeTeO6 prepared at ambient pressure adopts the corundum derivative ordered ilmenite structure (rhombohedral R3). When heated at 1073 K and 3-5 GPa, the as-made Li2GeTeO6 can convert into a LiSbO3-derived Li2TiTeO6-type phase (orthorhombic Pnn2), which is the third LiSbO3-derived double A2BB'O6 phase in addition to Li2TiTeO6 and Li2SnTeO6. This Pnn2 Li2GeTeO6 phase spontaneously reverts to the R3 phase if annealed up to 1023 K at ambient pressure. Although the crystal structural analyses and second harmonic generation measurements clearly demonstrate the polar nature of both the R3 and Pnn2 phases, P( E) and dielectric measurements do not show any convincing ferroelectric response. Given the large estimated spontaneous polarization (17 and 80 µC/cm2), the absence of ferroelectric behavior could be attributed to the random domain distribution and leakage due to Li-ion migration.

Self-Organized Ferroelectric Domains Controlled by a Constant Bias from the Atomic Force Microscopy Tip.

The ferroelectric polarization switching along an external electric field is most important for the applications of ferroelectric memories and piezoelectric sensors and actuators; however, the depolarization commonly occurs randomly and cannot be controlled exactly until now. Here, a tip bias introduces the polarization switching and a ∼10 µm-scale domain in a triglycine sulfate crystal, and then the polarization backswitching as a special depolarization introduces a series of ordered granular domains along a line being parallel to the c axis and through the tip which divides the original domain to two similar parts. Such backswitching is controlled by the surface charge change as a result of the interplay among polarization charges, mobile H+ ions at the surface, and the strong crystal anisotropy. The self-organized ferroelectric domains offer us a new freedom to design novel ferroelectric or piezoelectric devices in future.

Flexible, Fatigue-Free, and Large-Scale Bi3.25La0.75Ti3O12 Ferroelectric Memories.

Flexible, fatigue-free, large-scale, and nonvolatile memory is an emerging technological goal in a variety of fields, including electronic skins, wearable devices, and other flexible electronics. Perovskite oxide films deposited on rigid substrates (e.g., Si and SrTiO3) at 500-700 °C and >1.0 Pa oxygen ambience have been widely used in electronic industries. However, their applications in flexible electronics are challenging, if not impossible. Here, the Bi3.25La0.75Ti3O12 ferroelectric films with SrRuO3 or Pt electrodes were prepared on the two-dimensional mica substrates, and then the flexible Pt/SrRuO3/Bi3.25La0.75Ti3O12/Pt memories have been achieved through reducing the mica to ∼10 µm thickness. These memories show the saturated polarization of Ps ∼ 20 µC/cm2, and either the <1% bending strain or a normal light illumination hardly overcomes the potential barrier among different polarizations which originate from the noncentral symmetry of the atomic structure. As a result, they can undergo 109 write/erase cycles and/or 10000 times bending with 1.4 mm in radius without any fatigue or damage. Furthermore, they can withstand the operation at 20-200 °C or under light illumination. In short, these flexible oxide memories provide comprehensive performance for industrial applications.

Flexible, Semitransparent, and Inorganic Resistive Memory based on BaTi0.95 Co0.05 O3 Film.

Perovskite ceramics and single crystals are commonly hard and brittle due to their small maximum elastic strain. Here, large-scale BaTi0.95 Co0.05 O3 (BTCO) film with a SrRuO3 (SRO) buffered layer on a 10 µm thick mica substrate is flexible with a small bending radius of 1.4 mm and semitransparent for visible light at wavelengths of 500-800 nm. Mica/SRO/BTCO/Au cells show bipolar resistive switching and the high/low resistance ratio is up to 50. The resistive-switching properties show no obvious changes after the 2.2 mm radius memory being written/erased for 360 000 cycles nor after the memory being bent to 3 mm radius for 10 000 times. Most importantly, the memory works properly at 25-180 °C or after being annealed at 500 °C. The flexible and transparent oxide resistive memory has good prospects for application in smart wearable devices and flexible display screens.