D33 Mode Research Articles

Diagnosis and assessment of cyclic freeze–thaw damage in tunnel lining concrete using piezoelectric-based electromechanical impedance technique

Cold regional tunnel linings extensively suffer from severe cyclic freeze–thaw damage, which adversely influences their stability, durability and in-service safety. Consequently, the diagnosis and assessment of cyclic freeze–thaw damage in concrete linings have garnered worldwide attention. This study presents a pioneering effort in using the PZT-based electromechanical impedance (EMI) technique to monitor the evolving damage within lining concrete concurrently exposed to bending loads and freeze–thaw cycles. First of all, ultrasonic measurements and four-point bending tests were performed on cyclically freeze-thawed concrete specimens under two different bending loads. Both the degrading flexural strength and the diminishing ultrasonic wave velocity were measured. Particularly, the damage evolution was characterized by the flexural strength degradation ratio, which exponentially increased by 44.81% and 64.28% under two bending scenarios, respectively. Moreover, conductance signatures driven by two vibration modes (d31 and d33 modes) of embedded piezoelectric transducers were recorded, and both localized and overall variations in the signals were analyzed. It is concluded that both the resonant frequency shift and the statistic metric RMSD within the d31-dominated frequency range could accurately indicate the cyclic freeze–thaw damage of lining concrete. Their linear correlation coefficients with the damage variable reached 0.957 and 0.935, respectively. This research establishes a quantitative framework for monitoring the progressive damage of tunnel lining subjected to freeze–thaw cycling using the EMI technique and lays the groundwork for a broader range of future applications in structure damage diagnosis and assessment.

Enhanced piezoelectric performance of PVDF/MWCNTs energy harvester through a 3D-printed multimodal auxetic structure for smart security systems

Piezoelectric energy harvesters (PEHs) have attracted considerable attention over recent decades for their ability to convert mechanical energy into electrical energy. However, the current landscape is characterized by a lack of structural variety, which constrains the potential for notable performance improvements and the expansion of applications of these devices. To address these challenges, three distinct metamaterial structures are investigated through numerical simulations in this study. An auxetic, reentrant shape with carefully optimized dimensions was found as the best choice for the development of an efficient Polyvinylidene fluoride/multi-walled carbon nanotubes (PVDF/MWCNTs) PEH with optimized composition. This structure exhibits a remarkable capability to activate three different piezoelectric modes. Given it is polarization in the z-direction, the structure activates both d31 and d32 modes upon stretching in the x-direction, benefitting from the structure's negative Poisson's ratio and generating an open circuit voltage of 14.8 V. In contrast, compression in the z-direction activates the d33 mode, resulting in an open-circuit voltage of 28.2 V. This design allows the harvester to charge a 1 μF capacitor to 22 V within 260 s. The structure's feasibility is further evidenced by its fabrication using Fused Deposition Modeling (FDM) 3D printing technology. To further extend its practicality and real-world application, the PEH can be seamlessly integrated into large-scale commercial, industrial, and residential flooring, offering a novel approach to security systems. In this innovative approach, the PVDF/MWCNTs PEH converts the compressive force of footsteps into electrical signals. These signals serve as triggers for an integrated alarm system. Essentially, the electrical signal functions as a switch: its transmission acts like a closed switch that triggers the activation of the alarm system. Conversely, the absence of the signal is an open switch, indicating that no external force is acting on the tiles and a safe condition is maintained. Remarkably, the PEH can produce a voltage of 19.8 V when activated by the footsteps of a 60 kg person, demonstrating its practical utility.© 2017 Elsevier Inc. All rights reserved.

Micromachined piezoelectric sensor with radial polarization for enhancing underwater acoustic measurement

This work presents an underwater acoustic sensor featuring a radial polarized piezoelectric diaphragm, which aims to work in d33 mode to enhance its performance. For the sake of accomplishing the in-plane polarization, interdigital electrodes based on semi-circular ring patterns are designed on both sides and aligned along the thickness direction of the diaphragm. Effects of electrode parameters on the polarization orientation are numerically analyzed by the conversion relationship between different coordinates. Using the Kirchhoff plate theory and Rayleigh-Ritz method, a mathematical model is developed to investigate the dynamic properties of the sensor. The results indicate that the output voltages increase with the increase of the electrode inner radius, width, and spacing but decrease with the increase of the diaphragm thickness and radius. Sensor prototypes are fabricated by the microelectromechanical system (MEMS) technology in a clean room and characterized by impedance spectrums. An underwater testing system is established to conduct the experimental verification, whose results are in good accord with those of mathematical modeling. The sensor output voltage has fine linearity with the underwater acoustic pressure and achieves a flat frequency response. Moreover, the sensor has a sensitivity of −172.7 dB (Ref. 1 V/μPa) superior to the reported devices in the same categories. This work provides significant guidance for modeling radial field piezoelectric diaphragms and designing more efficient piezoelectric microsensors, which have a fine application prospect in underwater acoustic measurement.

Electro-mechanical transfer matrix modeling of piezoelectric actuators and application for elliptical flexure amplifiers

Mechanically amplified piezoelectric actuators (APAs) can be found in many scenarios of precision engineering and instrumental devices. However, the electro-mechanical design of curvilinear APAs is difficult due to the existence of curved flexure beams and multi-domain electro-elasto dynamics. In this paper, the electro-mechanical behavior of elliptical APAs is studied by exploiting a novel electro-mechanical transfer matrix method. The motivation is to facilitate both the static and dynamic analyses of such a complex APA with curvilinear flexure beams by considering the multi-domain dynamics of piezoelectric stacks and compliant mechanisms from an electro-mechanical viewpoint. To this end, an analytical electro-elasto transfer matrix of piezoelectric stacks operating at the d33 mode is derived in the form of Taylor's series based on Timoshenko beam theory. The dynamic response spectrum of displacement and impedance for elliptical APAs are insightfully captured by such an electro-mechanical model. Different topologies of elliptical APAs are also compared and the optimal configuration is suggested. At last, a proof-of-concept prototype is fabricated and tested with a special focus on experimental evaluation of the electro-mechanical coupling model.

Ferroelectret Polypropylene Foam-Based Piezoelectric Energy Harvester for Different Seismic Mass Conditions

Energy harvesting technologies and material science has made it possible to tap into the abundant amount of surrounding vibrational energy to efficiently convert it into useable energy providing power to portable electronics and IoT devices. Recent investigations show that the piezoelectric effect is created in cellular polymers called ferroelectrets. These cellular-compliant polymers with polarized pores have a piezoelectric response to generate electrical energy when subjected to mechanical strain or surrounding vibration. It is found that there is a significant difference between ferroelectret polarized cellular polypropylene foam and traditional piezoelectric polymers such as polyvinylidene fluoride (PVDF). The former has approximately ten times higher piezoelectric coefficient than the latter. This means that with an acceleration of 9.81 m/s2 force on this material, ferroelectrets generate up to 39 (µW/g/mm3) power output. Designing a polypropylene-based piezoelectric energy harvester based on the d33 mode of vibration can be challenging due to several factors, as it requires balancing multiple factors such as mechanical stability, piezoelectric response, circuit topology, electrode size, spacing, placement relative to the piezoelectric material, and so on. This paper proposes the preliminary experimental investigation of ferroelectret cellular polypropylene foam in harvesting performance. Suggestions of different approaches for the structural design of energy harvesters are provided. The vibration-dependent response and generated output are examined concerning pulse or sinusoidal input excitation. The voltage generated for both excitations is compared and suggestions are provided regarding the suitable kind of excitation for the chosen ferroelectret material. Finally, conclusions and prospects for ferroelectret materials used in energy-harvesting applications are given.

Wearable bending wireless sensing with autonomous wake-up by piezoelectric and triboelectric hybrid nanogenerator

The Internet of Things calls for the demand for wearable self-powered sensors for human motion monitoring and interaction. However, the redundant data and large amount of power consumption limit the application in sustainable wireless sensing. In this study, a flexible hybrid nanogenerator with bridge structure design for self-powered wearable sensor is proposed. When harvesting energy from the finger bent motion, it combines contact separation mode for triboelectric nanogenerator (TENG) with bending d31 mode for piezoelectric generator (PEG). To highlight the significance, we propose an autonomous wake-up wireless sensing method by hybrid generator in wearable bending. Different from recording and transmitting data all the time, the TENG signal as a trigger to record the PEG voltage amplitude as angle sensing data for wireless transmitting. Concise and accurate sensing data with autonomous wake-up is expected to reduce the computational burden and power consumption for wireless sensing and provide more possibilities for wearable wireless monitoring and human-computer interaction. The wireless manipulator interaction experiment based on this hybrid nanogenerator as wearable bending sensor verifies the feasibility of this scheme, which has a great application prospect in wearable self-powered sensors field such as virtual reality and robot control.

Design and analysis of a d15 mode piezoelectric energy generator using friction-induced vibration

Research works have been conducted on transverse and longitudinal mode piezoelectric energy generation to collect energy from ambient vibrations. However, the inconsistency with the frequency of the energy source and low output power density remain problems for high energy output. In this work, we propose a shear mode piezoelectric energy generator, which utilizes the friction-induced vibration (FIV) and high shear mode piezoelectric coefficient to improve the energy output. A piezoelectric coupled FIV mathematical model is developed to accurately calculate the dynamic vibration response and voltage output. The dynamic voltage response is validated by experiment, and it proves the possibility of continuous friction-induced high-frequency vibration. The energy generation process is evaluated by transient charging simulation of a storage capacitor through an iteration process, which was experimentally validated in the literature. Parameter studies have been conducted to investigate the influences of the piezoelectric patch dimensional parameters, vibration system parameters, friction model parameters, methods of electrical connections, and different piezoelectric materials on the energy generation performance to provide guidance for better design. Under ideal experiment conditions with proper parameters, a volume of m3 PZT4 piezoelectric material indicates root mean square (RMS) charging power density of Wm−3 and Wm−3 with electrically in parallel and electrically in series, respectively. While using the same amount of material and structural setup, the single crystal PMN-PT piezoelectric material shows RMS charging power density of Wm−3 and Wm−3 with electrically in parallel and electrically in series, correspondingly. These promising results demonstrate that close to W-level RMS charging power output may be realized by structure optimization of energy generator design and incorporating multiple generators together for operation. Possible incorporation into vehicle braking systems can be considered to utilize the wasted friction energy, and it may offer an energy supply for low-power wireless devices.

Enhanced performance of bionic ciliary piezoelectric microsensor for hydrodynamic perception

This work presents a bionic ciliary piezoelectric microsensor in d33 mode for enhancing performance of self-powered hydrodynamic perception. The sensor is composed of a cylindrical pillar emulating the fish cilium and a piezoelectric sensing diaphragm imitating the hair cell. Interdigital electrodes are designed on both sides of the sensing diaphragm to realize the radial polarization, whose pattern consists of a few semi-circular rings connected to each other. Based on the receptance method and Kirchhoff plate theory, a mathematical model is established to evaluate the resonance frequency of the sensor. The output characteristics of the sensor are numerically obtained by coupling the drag force acquired by computational fluid dynamics into the finite element model. The effects of key structural parameters on the sensor response are further investigated for the purpose of optimizing the device design. Prototypes are fabricated by the microelectromechanical systems technology and stereolithography, characterized using the impedance spectrum and tested employing a dipole stimulus. The experimental values of the sensor response to hydrodynamic flow velocities are in fine agreement with the numerical simulation. The results indicate a high sensitivity of 2.483 V/(m/s) and a relatively flat frequency response of the sensor. This work provides important guidance for designing the more efficient ciliary microsensor that can conduct flow field perception for underwater autonomous vehicles.

Stress Online Monitoring Method Based on Multifrequency SH0 Guided Wave Information Fusion

This article proposes a stress monitoring technique based on multifrequency SH0 guided wave information fusion. Its nondispersive acoustoelastic characteristic facilitates joint analysis in the whole frequency band. Moreover, the typical representation of broadband multifrequency signal is equivalent to realizing multiple measurements simultaneously, effectively improving the measurement efficiency and reliability. First, a generalized multifrequency comprehensive measurement model is established, which combines with specific data analysis and postprocessing criteria to construct a new stress characterization method. Then, the acoustoelastic effect of SH0 guided wave is investigated. Next, the wideband shear piezoelectric transducer based on the out-of-plane d15 mode is designed. The frequency-domain finite-element method (FDFEM) is applied to optimize the SH0 guided wave excitation and reception interval. Finally, the laboratory experiments of 6061-T6 aluminum plates with a thickness of 1 mm are conducted to verify the proposed strategy. The fitting result of multifrequency component fusion is better than that of any single-frequency algorithm. The actual measurements from the same batch are carried out through the calibration results. The maximum error does not exceed 2.5 MPa and the maximum relative standard deviation (RSD) is 7% in the range of 15–85 MPa, demonstrating good measurement accuracy and consistency.

Piezoelectric characteristics of PVA/DL-alanine polycrystals in d33 mode

In this study, polyvinyl alcohol (PVA)-mixed DL-alanine (PVA/DL-alanine) polycrystals are fabricated, and their piezoelectric characteristics in the d33 mode are investigated. The d33 piezoelectric coefficients of the PVA/DL-alanine polycrystals are found to increase with an increase in the weight ratio of DL-alanine, and the PVA/DL-alanine polycrystal composed of PVA and DL-alanine in a weight ratio of 1:3 exhibits a d33 of ∼5pC/N. The piezoelectric characteristics of the PVA/DL-alanine polycrystals are discussed in terms of the crystal structure by employing scanning electron microscopy and X-ray diffraction analyses. To confirm the piezoelectric performance of the polycrystals, the piezoelectric voltages of a piezoelectric device composed of a single layer of ZnO thin film and heterostructured devices consisting of a layer of PVA/DL-alanine polycrystal and a ZnO thin film layer are measured and compared. This study presents PVA/DL-alanine polycrystals as a potential piezoelectric material for bio-friendly piezoelectric-device applications.

Analysis and experiment of auxetic centrifugal softening impact energy harvesting from ultra-low-frequency rotational excitations

The ubiquitous ultra-low-frequency rotational motions have become a kind of potential source for energy harvesting. Despite this, achieving high energy output at ultra-low rotational frequencies via simple energy harvesting structures appears to be a significant challenge. Therefore, this paper proposes an auxetic centrifugal softening impact energy harvester (ASIEH) to break through this bottleneck. The ASIEH consists of a centrifugal softening driving beam that impacts two rigid auxetic piezoelectric beams to generate electric energy during rotation. The modeling and simulation of the ASIEH and the plain centrifugal softening impact energy harvester (PSIEH) indicate that the high stress distribution and the simultaneous operation of d31 and d32 modes of the auxetic energy harvester can improve the energy output. Under the centrifugal effect, the driving beam experiences the vibration, impact and trapping modes subsequently, among which the impact mode produces the highest energy output. Experiments are conducted to validate the model and show that the peak power of the ASIEH (0.673 mW) can be increased by 200.45% compared with that of the PSIEH (0.224 mW) at 3.5 Hz. Parametric studies indicate that no matter how the impact distance and rotational radius are varied, the peak power and bandwidth of the ASIEH are higher than those of the PSIEH. Owing to the combination of both the centrifugal effect and the auxetic structure, the proposed ASIEH exhibits the great potential in ultra-low-frequency rotational energy harvesting with both high power density (41.23 μW/g) and high output power.

Finger-inspired rigid-soft hybrid tactile sensor with superior sensitivity at high frequency

Among kinds of flexible tactile sensors, piezoelectric tactile sensor has the advantage of fast response for dynamic force detection. However, it suffers from low sensitivity at high-frequency dynamic stimuli. Here, inspired by finger structure—rigid skeleton embedded in muscle, we report a piezoelectric tactile sensor using a rigid-soft hybrid force-transmission-layer in combination with a soft bottom substrate, which not only greatly enhances the force transmission, but also triggers a significantly magnified effect in d31 working mode of the piezoelectric sensory layer, instead of conventional d33 mode. Experiments show that this sensor exhibits a super-high sensitivity of 346.5 pC N−1 (@ 30 Hz), wide bandwidth of 5–600 Hz and a linear force detection range of 0.009–4.3 N, which is ~17 times the theoretical sensitivity of d33 mode. Furthermore, the sensor is able to detect multiple force directions with high reliability, and shows great potential in robotic dynamic tactile sensing.

Damage detection in thick plate structures based on ultrasonic SH wave

Thick plate structures are widely used in large deep-sea scientific detection equipment, such as deep-sea human-occupied vehicles, but they are prone to local damage caused by fatigue due to the complex working environment load. To prevent structural failure due to damage accumulation, it is necessary to carry out structural health monitoring for thick plate structures. In this work, two half-ring thickness poled d15 mode omnidirectional shear horizontal (SH) wave piezoelectric transducers are used to form an exciter/receiver network, and the SH2 mode wave packet is selected as the damage location wave packet. Then, the reconstruction algorithm for the probabilistic inspection of damage and an algorithm based on the time of guided wave flight are applied for damage detection in a 30 mm-thick aluminium plate. Experimental results show that structural damage detection based on SH waves can effectively identify and locate thick plate structural damage and evaluate the damage depth.

Electromechanical Performance of Biocompatible Piezoelectric Thin-Films

The present study analyzed a computational model to evaluate the electromechanical properties of the AlN, BaTiO3, ZnO, PVDF, and KNN-NTK thin-films. With the rise in sustainable energy options for health monitoring devices and smart wearable sensors, developers need a scale to compare the popular biocompatible piezoelectric materials. Cantilever-based energy harvesting technologies are seldom used in sophisticated and efficient biosensors. Such approaches only study transverse sensor loading and are confined to fewer excitation models than real-world applications. The present research analyses transverse vibratory and axial-loading responses to help design such sensors. A thin-film strip (50 × 20 × 0.1 mm) of each sample was examined under volumetric body load stimulation and time-based axial displacement in both the d31 and d33 piezoelectric energy generation modes. By collecting evidence from the literature of the material performance, properties, and performing a validated finite element study to evaluate these performances, the study compared them with lead-based non-biocompatible materials such as PZT and PMN-PT under comparable boundary conditions. Based on the present study, biocompatible materials are swiftly catching up to their predecessors. However, there is still a significant voltage and power output performance disparity that may be difficult to close based on the method of excitation (i.e., transverse, axial, or shear. According to this study, BaTiO3 and PVDF are recommended for cantilever-based energy harvester setups and axially-loaded configurations.

Design and analysis of a d33 mode piezoelectric energy generator for vehicle braking system

A novel piezoelectric energy generator embedded in vehicle brake pads and excited by magnetic repulsion is developed. The generator is made of a cylindrical magnet and a piezoelectric stack. The generators are mounted on the backing plate of the brake pad through the perforated friction layer. Slotted brake rotor with embedded magnets is equipped to ensure the braking performance of the vehicle. During the braking process with slotted brake rotor, dynamic magnetic repulsion will be generated when the overlapping area of the embedded magnets in the brake pad and brake rotor is changing. The magnetic repulsion is generated when two magnets are close to each other, and the force is proportionally changing with the overlapping area of the two magnets. As a result of repulsion between the magnets, the piezoelectric stack will experience compressive forces, creating an electrical charge for generating energy. To illustrate the voltage generation, a mathematical model with experimental verification is established to calculate the electric charge and output voltage considering the charge dissipation. The energy harvesting process is evaluated by simulating the transient charging of the storage capacitor through the diode bridge, which was experimentally validated in literature. The influences of the dimensional and material properties of the piezoelectric stack, the vehicle speed, the magnetic repulsion, the diameter of the magnetic actuator, the capacitance of the storage capacitor and the distance between rotor center to the actuator on the root mean square (RMS) of the charging power are discussed. A total RMS power of 0.0710 W can be achieved with thirty-six generators embedded in both the inner and the outer brake pads within one brake caliper using APC850 (PZT4) material, and a total RMS power of 1.1226 W can be achieved using PMN-PT-B (PT = 0.3–0.33) material at 120 km h−1 speed of the vehicle. This novel generator will be useful for efficient and practical energy harvesting applications during vehicle braking process.

Parametric analysis of hybrid tribo-piezoelectric energy harvester

Energy harvesters are widely used in various fields for environmental energy harvesting purposes. Due to the less power output of a single energy harvester, researchers are working on hybridizing energy harvesters. This paper presents a hybrid model of d33 mode piezoelectric energy harvester (PEH) and vertical contact mode triboelectric energy harvester (TEH). A Polyvinylidene fluoride (PVDF) composite film is used for the PEH unit, and a Polydimethylsiloxane (PDMS) composite film is used for the TEH unit. The effect of various design parameters like material, structural and experimental parameters are numerically investigated for a periodic motion of hybrid tribo- piezoelectric energy harvester (HTPEH). A genetic algorithm has been used to maximize the harvested energy. This study will help design the prototype of HTPEH according to applications. The HTPEH can generate a significant amount of energy which can be helpful in the applications like designing self-powering electronic devices, robotics, sensors, and harvesting the vibration energy from human, wind, or ocean motion.

Piezoelectric MEMS Acoustic Transducer with Electrically-Tunable Resonant Frequency.

The paper presents a technique to obtain an electrically-tunable matching between the series and parallel resonant frequencies of a piezoelectric MEMS acoustic transducer to increase the effectiveness of acoustic emission/detection in voltage-mode driving and sensing. The piezoelectric MEMS transducer has been fabricated using the PiezoMUMPs technology, and it operates in a plate flexural mode exploiting a 6 mm × 6 mm doped silicon diaphragm with an aluminum nitride (AlN) piezoelectric layer deposited on top. The piezoelectric layer can be actuated by means of electrodes placed at the edges of the diaphragm above the AlN film. By applying an adjustable bias voltage Vb between two properly-connected electrodes and the doped silicon, the d31 mode in the AlN film has been exploited to electrically induce a planar static compressive or tensile stress in the diaphragm, depending on the sign of Vb, thus shifting its resonant frequency. The working principle has been first validated through an eigenfrequency analysis with an electrically induced prestress by means of 3D finite element modelling in COMSOL Multiphysics®. The first flexural mode of the unstressed diaphragm results at around 5.1 kHz. Then, the piezoelectric MEMS transducer has been experimentally tested in both receiver and transmitter modes. Experimental results have shown that the resonance can be electrically tuned in the range Vb = ±8 V with estimated tuning sensitivities of 8.7 ± 0.5 Hz/V and 7.8 ± 0.9 Hz/V in transmitter and receiver modes, respectively. A matching of the series and parallel resonant frequencies has been experimentally demonstrated in voltage-mode driving and sensing by applying Vb = 0 in transmission and Vb = −1.9 V in receiving, respectively, thereby obtaining the optimal acoustic emission and detection effectiveness at the same operating frequency.

Flexible, Piezoelectric Aluminum-Doped Zinc Oxide Energy Harvesters with Printed Electrodes for Wearable Applications

Aims: Recent advancements in sensing technology and wireless communications have accelerated the development of the Internet of Things (IoT) which promote the usage of wearable sensors. An emerging trend is to develop self-sustainable wearable devices, thus eliminating the necessity of the user to carry bulky batteries. In this work, the development of a flexible piezoelectric energy harvester that is capable of harvesting energy from low frequency vibrations is presented. The target application of this energy harvester is for usage in smart shoes. Objectives: The objectives of this research is to design, fabricate and test an energy harvester on PET substrate using Aluminum Zinc Oxide as its piezoelectric layer. Methods: The energy harvester was designed as a cantilever structure using PET/AZO/Ag layers in d33 mode which can generate large output voltages with small displacements. The electrodes were designed as an interdigitated structure in which two significant design parameters were chosen, namely the effect of gap between electrodes, g and number of interdigital electrodes (IDE) pairs, N to the output voltage and resonant frequency. Results: The sputtered AZO on PET showed c-axis orientation at 002 peak with 2 values of 34.45° which indicates piezoelectric behaviour. The silver IDE pairs were screen-printed on the AZO thin film. Functionality of the device as an energy harvester was demonstrated by testing it using a shaker. The energy harvester was capable of generating 0.867 Vrms output voltage when actuated at 49.6 Hz vibrations. Conclusion: This indicates that the AZO thin films with printed silver electrodes can be used as flexible, d33 energy harvesters.

Electromechanical simulation and analysis of perovskite piezoelectric ‎unimorph cantilever nanogenerator with interdigitated electrodes in ‎vibration energy harvesting application

This paper addresses a unimorph cantilevered piezoelectric nanogenerator having high ‎output power through vibrational energy harvesting that is simulated using finite element ‎method (FEM). The simulations are done for three types of perovskite piezoelectric ‎materials including PZT, PMN-PT and MAPbI3. The interdigitated electrodes were ‎exploited‏ ‏to obtain longitudinal vibration mode using d33 mode of piezoelectric layer during ‎the bending of nanogenerator structure. The presented structure consists of a piezoelectric ‎nanolayer with gold interdigitated electrodes on it, which is placed on a flexible PET ‎polymeric substrate. To encapsulate the piezoelectric layer, an SU-8 epoxy is placed over ‎the surface. The poling process is also simulated by applying high voltage through IDs to ‎piezoelectric layer. Generally, the electric potential distribution of the piezoelectric layer ‎must be performed by applying mechanical loadings. Then the output voltage, ‎power for free vibrations and base excitation (0.25-2g) of the nanogenerator at resonance ‎frequency are investigated. The resonance frequency of the PZT, PMN-PT, and MAPbI3 were ‎calculated to be 549 Hz, 560.5 Hz and 631 Hz, respectively. We found that PZT ‎piezoelectric materials yields maximum output voltage and electrical power values of 91.69 ‎V and 350 mW which shows better performance in vibrational energy harvesting ‎application. In comparison, the results of the simulation implied a good agreement with ‎other experimental studies. The unimorph piezoelectric energy harvester system generates ‎high voltage and output power in response to sub-kilohertz ambient vibration‎.‎‎

Enhanced Piezoelectric Performance of Various Electrospun PVDF Nanofibers and Related Self-Powered Device Applications.

The piezoelectric polymer poly(vinylidene fluoride) (PVDF) has been widely employed for energy harvesting or sensors. Its piezoelectricity originates from the unique crystal structure and the oriented electric dipoles. Generally, electrospinning (ES) is able to improve the crystallinity and piezoelectricity of PVDF. In this investigation, three types of ES, including far-field ES with random (R-PVDF-FFES) or aligned distribution (A-PVDF-FFES) and near-field ES (PVDF-NFES), are applied to fabricate a one-step polarized PVDF membrane. To compare the piezoelectricity of separated PVDF-NFES fibers, the array of PVDF-NFES fibers is encapsulated in polylactic acid (PLA). The result shows that the piezoelectricity of PVDF is dramatically enhanced by NFES and FFES. In d31 mode, the current density of PVDF-NFES-PLA is 75.63 pA cm-2 with a 2 Hz shear force, which is four times larger than A-PVDF-FFES (17.62 pA cm-2) or seven times larger than R-PVDF-FFES (10.63 pA cm-2). Due to its outstanding property, the PVDF-NFES-PLA membrane is designed to be applied as a self-powered device. The unimorph cantilevers are prepared to harvest the vibration energy and their output power density reaches 95.3 μW m-2 at a resonance frequency of 31 Hz. Meanwhile, the flexible bending sensor and pressure sensor are fabricated with a PVDF-NFES-PLA membrane, which shows the stable response for pressure with sensitivity values of 864.68 and 22.6 mV kPa-1, respectively.