Anthropogenetic environmental deterioration and climate change caused by energy production and consumption pose a significant threat to the future of humanity. Renewable, environmentally friendly, and cost-effective energy sources are becoming increasingly important for addressing future energy demands. Mechanical power is the most common type of external energy that can be converted into useful electric power. Because of its strong electromechanical coupling ability, the piezoelectric mechanism is a far more successful technique for converting mechanics energy to electrical energy when compared to electrostatic, electromagnetic, and triboelectric transduction systems. Currently, the scientific community has maintained a strong interest in piezoelectric micro-power generators because of their great potential for powering a sensor unit in the distributed network nodes. A national network usually has a large mass of sensor units distributed in each city, and a self-powered sensor network is eagerly required. This paper presents a comprehensive review of the development of piezoelectric micro-power generators. The fundamentals of piezoelectric energy conversion, including operational modes and working mechanisms, are introduced. Current research progress in piezoelectric materials including zinc oxide, ceramics, single crystals, organics, composite, bio-inspired and foam materials are reviewed. Piezoelectric energy harvesting at the nano- and microscales, and its applications in a variety of fields such as wind, liquid flow, body movement, implantable and sensing devices are discussed. Finally, the future development of multi-field coupled, hybrid piezoelectric micropower generators and their potential applications are discussed.

Kirigami‐ and oirigami‐inspired techniques have emerged as effective strategies for material structure design; however, the use of these techniques is usually limited to soft and deformable materials. Piezoelectric ceramics, which are typical functional ceramics, are widely used in electronic and energy devices; however, the processing options for piezoelectric ceramics are limited by their brittleness and feedstock viscosity. Here, a design strategy is proposed for the preparation of lead‐free piezoelectric ceramics inspired by kirigami/origami. This strategy involves direct writing printing and control over the external gravity during the calcination process for the preparation of curved and porous piezoelectric ceramics with specific shapes. The sintered BaTiO 3 ceramics with curved geometries produced using this strategy exhibit a high piezoelectric constant ( d 33 = 275 pC N −1 ), which is 45% higher than that of conventionally sintered sheet ceramics. The curved structure of the ceramics is well‐suited for use in the human body and it was determined that these curved ceramics can detect pulse signals. This strategy can be applied in the large‐scale and low‐cost production of other piezoelectric ceramics with various curved shapes and provides a new approach for the preparation of complex‐shaped ceramics.

It is desired to obtain a piezoceramic with a high piezoelectric coefficient and low dielectric loss simultaneously for energy harvester application. Herein, it is reported that the 0.025Pb(Mn 1/3 Nb 2/3 )O 3 ‐0.525Pb(Ni 1/3 Nb 2/3 )O 3 ‐0.135PbZrO 3 ‐0.315PbTiO 3 (PMNN‐PZT) ceramic exhibits superior piezoelectric charge parameter e 33 of 37.74 pC m −2 and low dielectric loss tanδ of 0.45%. Furthermore, a PMNN‐PZT ceramic‐based magneto‐mechano‐electric coupled energy harvester (MMEC‐EH) with a varying‐stiffness cantilever is designed and fabricated, which shows the strong self‐resonance effect in response to a random, transient impulse vibration or magnetic field stimulus. The investigations show that under a 0.6 s pulse vibration stimulus, the MMEC‐EH can tune itself into self‐resonance damping oscillation lasting for six seconds at its resonance frequency of 16 Hz, and the produced maximum power is 1.96 mW RMS . Under both weak magnetic field and force‐field dual‐stimulus ( H ac = 0.5 Oe and a = 0.05 g), the generated power density is about 60 mW RMS Oe −2 g −2 cm −3 , which is one to two orders of magnitude higher than those previously reported MMEC‐EHs. Finally, the MMEC‐EH is successfully demonstrated to power temperature/humidity sensors, indicating its potential for harvesting both weak vibration and magnetic field energy from environments for self‐powered sensor application.

Additive manufacturing of arbitrary shapes and unique architecture provides remarkable flexibility and simplicity for the preparation of highly complex conformal electronics. This drives up demand for electronic materials with excellent process ability and functionality from one-step molding of microminiature to large-scale devices. Herein, a novel method is introduced for fabricating high-performance barium titanate (BaTiO3)-polydimethylsiloxane composites based on three-dimensional (3D)-printing-ordered structure of a metamaterial skeleton. When subjected to external mechanical stress, the metamaterial structure facilitates effective stress transfer, resulting in a significantly improved voltage output. In comparison to traditional low-dimensional ceramic polymer composites, metamaterial-structured BaTiO3 composites exhibit excellent electromechanical energy conversion properties, thereby enabling tactile imitation applications and mechanical energy harvesting. This study proposes a novel strategy for biological signal identification and implantable self-powered electronic applications.

Smart perovskite-based stimulus-responsive fluorescent materials have attracted considerable attention for confidential information protection, are mostly encapsulated in inorganic mesoporous materials, while little in easily processed polymers. Polymers, despite providing ultra-high resistance to humidity, heat and solvents, inevitably restrict the intrusion of external stimulation sources. In this study, based on perovskite nanocrystals/polymer composites, a weak solvent engineering strategy is rationally designed within the inaugural application in the encryption and decryption of confidential information. N,N-dimethylformamide and water are blended as weak solvents, which can swell and penetrate the polymer after spraying. This triggers the fluorescence quenching effect of the polar solvent on the encapsulated perovskite quantum dots and realizes information encryption. Recrystallization of perovskite can be easily achieved by high-temperature treatment to remove weak solvents, and hence regain its fluorescence properties. By introducing and removing the weak solvent, the perovskite-based composite can be quenched and restored, which enables reversible switching of the fluorescence signal. Surprisingly, the switch is highly effective in encrypting and decrypting various types of information. The optical encryption strategy via weak solvent modification not only provides a new path toward information security transmission but also, with its design friendliness, will contribute to the development and design of myriad innovations for data protection.

Besides the excellent sensitivity, linearity is also highly required for a miniaturization and low power consumption of the flexible sensor system. Herein, multi‐hierarchical microstructures to improve the linearity of capacitive pressure sensor based on poly(vinylidene fluoride)/reduced graphene oxide (PVDF/rGO) multilayer films is designed and constructed. The differentiated hierarchical microstructure in each layer comes from irregular protrusions with various roughness. Excellent sensor performances, such as higher sensitivity (0.18 kPa−1) in the linear range from 0.1 kPa to 11 kPa, lower limit of detection (100 Pa), rapid response time (56.3 ms) compared with some current reports, are obtained in the sensor based on three laminated films with the successive roughness (remarked as 150/400/800 Cw films). Moreover, the device exhibits outstanding stability over 3000 cycles at a pressure of 10 kPa, and successfully works on the falling leaf of 0.16 g and the weight of 100 g as well as high heel pressure. These outstanding properties are attributed to the compressibility of the hierarchical structure and efficient stress distribution between stacked multilayers, which is verified by using the finite‐element analysis (FEA). The work provides a simple and low‐cost route for fabricating high‐performance capacitive sensors with diverse potential applications in wearable electronics.

Traditionally, magnetization performance of bulk ferromagnetic materials is evaluated mainly based on Faraday's law of electromagnetic induction, magneto-optical effect, Superconducting Quantum Interference Devices (SQUID, only for small specimen), etc. Here, we report that the magnetoelectric (ME) coupling effect from a composited Terfenol-D alloy/PMN-PT single crystal under a constant-amplitude AC magnetic field excitation (H-ac = 0.25 Oe at 1 kHz) can be used to reveal magnetization behavior of an approaching bulk ferromagnetic material. Investigations show that the magnetostriction lambda(33,m), piezomagnetic coefficient d(33,m), ME voltage (V-ME) response spectrum of the ME composite have a strong dependence on the external ferromagnetic material. It is further found that the required DC magnetic field for domain switching in Terfenol-D alloy could be notably decreased from hundreds Oe to only 10 Oe once a high-mu permalloy is approached. This work shows that the ME composite has the potential to quickly, qualitatively evaluate the magnetization ( M ) or permeability (mu) behavior of an approaching bulk and heavy ferromagnetic material or alloy via ME effect. (c) 2021 Elsevier Ltd. All rights reserved.

Flexible pressure sensors with high sensitivity over a broad linear range and fast response have extension applications in wearable electronics. Herein, we prepared a tunable capacitive pressure sensors based on arbitrary micro‐protuberances geometry films via low‐cost soft lithography, employing the polyvinylidene fluoride/reduced graphene oxide (PVDF/rGO) composite with a high dielectric constant of 172 and low loss tangent of 0.48. The hierarchical microstructures’ different sizes endow the sensors adjustable pressure response, achieving the regulation of sensitivity and linear detection range. The superior microstructured film‐based sensor exhibits a high sensitivity (1.19 kPa−1) over 20 times that of the bulk film, wide linearity (1.3 kPa), a rapid response (43 ms), a low limit of detection (10.6 Pa), and remarkable durability over 5000 compression/release cycles. In addition, based on the excellent sensing performance above‐mentioned, the device is successfully employed in the scenario of monitoring pressure interaction and human activities by mounting on different parts of the human body or object. This work provides a means to optimize the sensor's sensitivity and linearity by hierarchical structure and contributes to its wearable electronic applications.

Additive manufacturing technology has promoted the development of piezoelectric devices, from the one-step moulding of micro-miniature devices to large-scale devices.

Piezoelectric ceramic materials such as Pb(Zr,Ti)O-3 (PZT) based ceramics have excellent electromechanical energy conversion ability, however, their rigid and undeformed characters are not suitable for flexible electronics application. Here, we report the design of flexible (0-3) connection ceramic-polymer composite made of poly-dimethylsiloxane (PDMS) elastomeric matrix doped with Ag-coated PNN-PZT (0.55Pb(Ni1/3Nb2/3)O-3-0.135PbZrO(3)-0.315PbTiO(3)) ceramic heterojunction particles, and the 3D printing method for fabricating complex three-dimensional grid architectures. It was found that the 3D-printing, non-stereolithographic grid-composite exhibits a greater flexible character after doping ceramic particles and also excellent electromechanical coupling with a piezoelectric voltage coefficient g(33) as high as 400 x 10(-3) m V N-1, which is one order of magnitude higher than that of PZT based ceramics. Under drop hammer (similar to 20 N) impact, it can instantly drive over 20 commercial red-LEDs lighting directly without using a charge storage capacitor. This work shows that 3D-printed flexible ceramic-polymer composite has potential to replace brittle piezoceramics for electromechanical energy conversion and touching force sensor applications, such as soft robotics, artificial muscles and biology signal identification.

A 3D-printed multilayer copolymer rugby ball-structured energy harvester is prepared, and a high peak output power of 16.4 mW cm−2 is obtained.

Designing topological and geometrical structures with extended unnatural parameters (negative, near-zero, ultrahigh, or tunable) and counterintuitive properties is a big challenge in the field of metamaterials, especially for relatively unexplored materials with multiphysics coupling effects. For natural piezoelectric ceramics, only five nonzero elements in the piezoelectric matrix exist, which has impeded the design and application of piezoelectric devices for decades. Here, we introduce a methodology, inspired by quasi-symmetry breaking, realizing artificial anisotropy by metamaterial design to excite all the nonzero elements in contrast to zero values in natural materials. By elaborately programming topological structures and geometrical dimensions of the unit elements, we demonstrate, theoretically and experimentally, that tunable nonzero or ultrahigh values of overall effective piezoelectric coefficients can be obtained. While this work focuses on generating piezoelectric parameters of ceramics, the design principle should be inspirational to create unnatural apparent properties of other multiphysics coupling metamaterials.