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As society advances, the shift from passive medical care to health management and preventive medical care has become an important issue, with the realization of wearable monitors becoming desirable. In light of the COVID‐19 pandemic, the number of patients who are in urgent need of the monitoring of biological information is increasing. This review focuses on piezoelectric materials and composites that convert kinetic energy into electrical energy to realize self‐powered wearable monitoring sensors, outlining the recent research activity on sensors for use in healthcare monitoring. First, a general description of the principles of piezoelectric monitoring sensors is given. Next, the development status of piezoelectric materials and composites aimed at the application of detecting tiny motions of the human body is introduced, and then the research trends on the detection of larger human body movements are highlighted. Finally, after presenting the performance of current piezoelectric sensors and future research guidelines for developing multifunctional systems in the post COVID‐19 era, the achievements are summarized. Overall, this review will provide guidance to researchers who are seeking to design and develop highly sensitive self‐powered piezoelectric sensors that monitor human motion and physiological signals. [ABSTRACT FROM AUTHOR]

The electromechanical and thermoelectric responses of piezoelectric composites under mechanical and thermal loading are numerically and experimentally examined. Piezoelectric composites are fabricated using epoxy and BaTiO3 particles, and three types of single‐layer composites and two types of functionally graded piezocomposites (single‐ and six‐layer composites) are prepared. The resonant frequencies and induced voltages of the composites are measured to determine the influences of mechanical loading and temperature change. Finite element analysis is used to calculate the resonant frequency, stress, and induced voltage due to mechanical loading and temperature change, and the results are compared with the experimental results. Single‐ and six‐layer piezocomposite models are employed in the analysis. By adopting a phenomenological model that considers the temperature dependence of material properties, the effect of the temperature dependence of a physical property on the internal stress and voltage is investigated. It is found that the variations in the temperature‐dependent material constants largely influence the induced voltages of the piezocomposites. In conclusion, functionally graded piezocomposites can be used to control the internal stress and obtain induced voltage from different power sources. It may be useful in designing energy‐harvesting devices for use in vibration‐rich environments and/or environments with severe temperature variations. [ABSTRACT FROM AUTHOR]

Various types of sensors such as piezoelectric ceramics and strain gauges are widely used to monitor the damage of automobile components, buildings, and so on. Among these sensors, piezoelectric sensors have a high‐energy conversion efficiency, that is a high sensitivity. Therefore, we fabricated a mullite fiber‐reinforced resin sheet with piezoelectric particles (Piezo‐MFRS) and evaluated their sensor performance. Piezo‐MFRS was fabricated by the hand layup method—the polyamide resin was mixed with piezoelectric particles and infused into the mullite fiber fabric. After that, the piezo‐MFRS was polarized by a corona discharge. We investigated the relationship between the applied impact load or vibration load to the piezo‐MFRS and the generated voltage. The piezo‐MFRS induced an output voltage of 20 mV under a compressive stress of 4 MPa, and an output voltage amplitude of 2 mV due to bending vibration of amplitude 0.5 mm. Therefore, it seems that the piezo‐MFRS is suitable for use as a load sensor. Furthermore, we confirmed that the tensile properties of piezo‐MFRS were not reduced by the addition of piezoelectric particles. Consequently, it is likely that piezo‐MFRS is a promising material for a rugged piezoelectric sensor and opens the door to monitoring large structures. [ABSTRACT FROM AUTHOR]

In the coming era of the internet of things (IoT), wireless sensor networks that monitor, detect, and gather data will play a crucial role in advancements in public safety, human healthcare, industrial automation, and energy management. Batteries are currently the power source of choice for operating wireless network devices due to their ease of installation; however, they require periodic replacement due to capacity limitations. Within the scope of the IoT, battery maintenance of the trillion sensor nodes that may be implemented will be practically infeasible from environmental, resource, and labor cost perspectives. In considering individual self‐powered sensor nodes, the idea of harvesting energy from ambient vibrations, heat, and electromagnetic waves has recently triggered noticeable research interest in the academic community. This paper gives an overview of energy harvesting materials and systems. Three main categories are presented: piezoelectric ceramics/polymers, magnetostrictive alloys, and magnetoelectric (ME) multiferroic composites. State‐of‐the‐art harvesting materials and structures are presented with a focus on characterization, fabrication, modeling and simulation, and durability and reliability. Some perspectives and challenges for the future development of energy harvesting materials are also highlighted. [ABSTRACT FROM AUTHOR]

This article studies the dielectric and piezoelectric behavior of unpoled and poled barium titanate (BaTiO3) polycrystals with oxygen vacancies. A phase field model is employed for BaTiO3 polycrystals, coupled with the time-dependent Ginzburg–Landau theory and the oxygen vacancies diffusion, to demonstrate the interaction between oxygen vacancies and domain evolutions. To generate grain structures, the phase field model for grain growth is also used. The hysteresis loop and butterfly curve are predicted at room and high temperatures. The permittivity, and longitudinal and transverse piezoelectric constants of the BaTiO3 polycrystals are then examined for various grain sizes and oxygen vacancy densities. [ABSTRACT FROM AUTHOR]

This paper investigates theoretically and experimentally the cryogenic fracture behavior of cracked piezoelectric ceramics under electric fields. Fracture tests were performed in three-point bending with the single-edge precracked-beam specimens at room temperature and liquid nitrogen temperature (77 K), and the fracture loads under electric fields were obtained. Plane strain finite element analysis was also carried out using temperature-dependent material properties of the piezoelectric ceramics, and the dependence of the energy release rate on the electric field and temperature was discussed. In addition, possible mechanisms for cryogenic fracture were examined by scanning electron microscopy. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines the nonlinear electromechanical response of 1-3 piezoelectric/polymer composites. The piezocomposites contain square or circular piezoelectric rods in an epoxy matrix. Experiments were conducted to measure the displacement versus electric field curves, using the device specimen of the 1-3 piezocomposites. Three dimensional finite element analysis was also carried out to study the electromechanical fields in the 1-3 piezocomposites by introducing a model for polarization switching. Comparison was then made between simulation and experiment. [Copyright &y& Elsevier]

This article discusses the delayed fracture and localized polarization switching near a crack tip in three-point bending piezoceramics under electromechanical loading. We have used finite element analysis to study delayed fracture experiments with single-edge precracked-beam method. A nonlinear finite element analysis was employed to calculate the fracture mechanics parameters such as energy release rate for the permeable, impermeable, and open crack face boundary conditions. Recent proposed energetically consistent boundary conditions were also considered. The effects of applied electric field and localized polarization switching on the fracture mechanics parameters were then examined. [ABSTRACT FROM AUTHOR]

Three-dimensional electroelastic formulation is developed to study the response of functionally graded rectangular piezoelectric structures/plates. The electroelastric properties are assumed to vary linearly and nonlinearly in the thickness direction, and exact expressions for all relevant electroelastic variables are obtained through a state vector methodology. Numerical examples are presented and discussed for simply-supported bi-layer actuators with functionally graded piezoelectric layers under surface and/or internal electric potential loads. The results presented in this paper are important in the design and characterization of functionally graded piezoelectric materials in applications as microelectromechanical systems. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the nonlinear bending behavior of piezoelectric laminated actuators as a function of the amplitude and frequency of large electric fields both analytically and experimentally. Laminated beam theory is expanded to allow for the cantilever piezoelectric/metal/piezoelectric actuator, and the effects of alternating current (ac) electric fields on the tip deflection are analyzed. A simple phenomenological model of a vibrating domain wall in electric fields is used, and the macroscopic actuator response is predicted. A nonlinear three-dimensional finite element analysis is also employed. Experimental results are presented to validate the predictions using bimorph-type bending actuators made with thin soft lead zirconate titanate (PZT) layers and a thin metal sheet. Calculations show reasonable agreement with the experimental data, and support a role for domain wall motion in the nonlinear material response of the soft PZT under ac electric fields. [Copyright &y& Elsevier]

It is well known that the curing residual stress induced during a fabrication process has a great influence on the performance of piezoelectric composite devices. The purpose of this work was to evaluate the residual thermal stress of lead zirconate titanate piezoelectric fiber aluminum (Al) matrix (piezoelectric fiber/Al) composites generated during fabrication numerically and experimentally and to understand the effect of the residual thermal stress on the electromechanical response. The three-dimensional finite element method was employed, and the residual stress generated during the solidification process of the Al matrix was calculated. The output voltage was also calculated in the analysis when putting stresses on the composite materials in the length direction of the piezoelectric fiber. It was shown that the cooling from higher temperatures increases the electromechanical conversion capability. Furthermore, we also performed the simulation, and we recorded the output voltage under concentrated load to investigate its application as a load position detection sensor, and we also discussed the influence of the position by changing the modeling with a different fiber position in the Al. The residual stress of hot press molded piezoelectric fiber/Al composite was then measured, and the comparison was made with the calculated values. The simulation results revealed that our model predictions reproduced and explained the experimental observations of curing residual stress. After this study, similar models of composite materials can be analyzed by this simulation, and the result can be used to design piezoelectric composite materials. [ABSTRACT FROM AUTHOR]

Wearable energy harvesting devices attract attention as the devices provide electrical power without inhibiting user mobility and independence. While the piezoelectric materials integrated shoes have been considered as wearable energy harvesting devices for a long time, they can lose their energy harvesting performance after being used several times due to their brittleness. In this study, we focused on Fe–Co magnetostrictive materials and fabricated Fe–Co magnetostrictive fiber integrated shoes. We revealed that Fe–Co magnetostrictive fiber integrated shoes are capable of generating 1.2 µJ from 1000 steps of usual walking by the Villari (inverse magnetostrictive) effect. It seems that the output energy is dependent on user habit on ambulation, not on their weight. From both a mechanical and functional point of view, Fe–Co magnetostrictive fiber integrated shoes demonstrated stable energy harvesting performance after being used many times. It is likely that Fe–Co magnetostrictive fiber integrated shoes are available as sustainable and wearable energy harvesting devices. [ABSTRACT FROM AUTHOR]

Energy harvesting technology is becoming increasingly important with the appearance of the Internet of things. In this study, a magnetostrictive clad steel plate for harvesting vibration energy was proposed. It comprises a cold-rolled FeCo alloy and cold-rolled steel joined together by thermal diffusion bonding. The performances of the magnetostrictive FeCo clad steel plate and conventional FeCo plate cantilevers were compared under bending vibration; the results indicated that the clad steel plate construct exhibits high voltage and power output compared to a single-plate construct. Finite element analysis of the cantilevers under bending provided insights into the magnetic features of a clad steel plate, which is crucial for its high performance. For comparison, the experimental results of a commercial piezoelectric bimorph cantilever were also reported. In addition, the cold-rolled FeCo and Ni alloys were joined by thermal diffusion bonding, which exhibited outstanding energy harvesting performance. The larger the plate volume, the more the energy generated. The results of this study indicated not only a promising application for the magnetostrictive FeCo clad steel plate as an efficient energy harvester, related to small vibrations, but also the notable feasibility for the formation of integrated units to support high-power trains, automobiles, and electric vehicles. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates experimentally and numerically the delamination behavior in woven glass fiber reinforced polymer (GFRP) composite laminates with surface-bonded piezoelectric ceramic actuators under Mode I loading at cryogenic temperatures. The piezoelectric actuators were used to control the delamination behavior in a composite double cantilever beam (DCB) specimen, and tests were performed at liquid nitrogen temperature (77K). The finite element analysis of the DCB specimen with piezoelectric actuators was also carried out using temperature-dependent material properties of both the woven GFRP laminates and the piezoelectric ceramics, and the numerical predictions were compared with the experimental data. The dependence of the Mode I delamination behavior on the applied electric fields and the temperature was examined based on the experimental and numerical results, and the feasibility of piezoelectric control of cryogenic delamination behavior in composite laminates was demonstrated. [Copyright &y& Elsevier]