Search

Optimal orthodontic forces need to be applied to induce appropriate biological and mechanical reactions in the periodontium, thus leading to proper tooth movements, while excessive orthodontic forces could harm the tooth roots and periodontal tissues during treatment. However, estimating orthodontic forces currently relies on clinical experience, lacking objective data and analysis; thus monitoring the in-vivo orthodontic force is essential to reducing side effects and improving orthodontic treatment. Unfortunately, in-vivo orthodontic force sensor systems are currently not available, and in-vitro ones are mostly bulky and costly. Here, we report a battery-free and wireless orthodontic force sensing system (BWS) designed for in-vivo, contactless, and real-time measurement of the orthodontic force. We developed a 915 MHz far-field RF energy harvesting system to power the intraoral BWS, and achieved an in vivo wireless measurement distance of 220 mm. This system allows in-vivo orthodontic force monitoring at anytime and anywhere, so it can be applied to improve clinical treatments and orthodontic research. Besides, in vivo experiments demonstrate that the system can harvest a voltage of 2 V even at a 350 mm distance, which highlights the device's potential to serve as a versatile platform for monitoring various oral environmental parameters through different sensors. [Display omitted] • The intraoral battery-freesystem enables real-time orthodontic force monitoring up to a distance of 220 mm. • Real-time data transfer and visualization allow precise control and remote monitoring of orthodontic force. • The developed system can be potentially expanded for real-time sensing in the oral environment. [ABSTRACT FROM AUTHOR]

Thin cylindrical shells are commonly employed to connect adjacent disks in large rotating machinery. The vibration characteristics of coupled cylindrical shell and annular plate components have a significant influence on the rotor dynamics. This paper provides a general approach for the vibration analysis of a rotating cylindrical shell coupled with an annular plate. The Sanders shell theory and Mindlin plate theory are employed to calculate the strain energy of the shell and plate, respectively. The Coriolis and centrifugal effects due to the rotation are taken into account. The artificial spring technique is adopted to simulate the coupling and boundary conditions. By taking Chebyshev polynomials as the admissible functions, the Rayleigh–Ritz method is employed to derive the motion equations for the rotating shell-plate combination. The approach proposed is validated by comparing with the available results in literature and finite element analysis results. The traveling wave motion of the coupled shell and plate structure is investigated. The effects of the geometric parameters and the boundary and coupling conditions on the vibration behavior of the coupled structure are evaluated. The present approach can not only evaluate free vibrations of coupled shell and plate structures, but also be extended to vibration analysis of disk–drum rotors involving cylindrical shells. [ABSTRACT FROM AUTHOR]

We proposed a nonlinear multi-mode tri-leg compliant orthoplanar spring piezoelectric vibration-based energy harvester (COPS-PVEH), which provided the improved performance using a pure geometric nonlinearity, in terms of a wide bandwidth, and a high-voltage output in forward and reverse sweeps. This paper presents an application of magnetic force interaction to further enhance the performance of the COPS-PVEH. A repulsive magnetic force is added to the spring to change the harvester system’s stiffness. A lumped parameter model is derived and analytically solved for the designed harvester with a magnetic coupling. The experiments are carried out by adding a moving magnet at the center of the spring and a pair of fixed magnets at the top and bottom sides of the harvester. Both experimental data and simulation results show the benefits brought through the dynamics of a magnetic oscillator. In an experiment by adding magnetic force, a slight increase is observed in the operating bandwidth and a 10% increase in the magnitude of voltage output is achieved at the same excitation level. The magnetic interaction on the COPS-PVEH with an addition of multiple masses has also achieved an increase in voltage output and bandwidth. In addition, the gap between the three vibration modes has been reduced. [ABSTRACT FROM AUTHOR]

Piezoelectric energy harvesting technology is regarded as a remedy to the short-life battery issue in low-power electronic devices, and has been extensively studied in the past two decades. Although a variety of new structures and mechanisms have been proposed for piezoelectric energy harvesters (PEHs), there is not much progress made on the piezoelectric materials that play a determinant role in PEHs performance. Most energy harvesters are still constructed with lead zirconate titanate (PZT). Recently, the new-generation piezoelectric materials PMN-PT and PZN-PT single crystals are proposed and start to be used in transducers. Their electromechanical coupling factor ( k 33 ) is over 90%, and the piezoelectric coefficient ( d 33 ) can reach three times of that of PZT. This study characterizes the PMN-PT and PZN-PT single crystals used in energy harvesters, compared with the polycrystalline PZT, in terms of charging capacitors. Systematic discussions are presented on the voltage, power and energy responses, as well as the reverse coupling effect on the mechanical response and the self-discharge effect of capacitors. Furthermore, a self-powered synchronized switch harvesting on inductor (SSHI) circuit is constructed and tested with the new single-crystal based energy harvesters. Experiments indicate that PMN-PT and PZN-PT can significantly boost the charging capability of energy harvesters, and that performance can be further enhanced by the SSHI circuit. [ABSTRACT FROM AUTHOR]

Humans have evolved to walk highly efficiently to conserve energy, making it challenging to develop assistive and energy-harvesting devices for walking. Herein, we report a heel pad-based assistance device for walking that not only optimizes the energetic economy of walking and prevents plantar fasciitis but also harvests energy from heel impact. Our footwear-embedded device improves the walking economy by offering shock absorption and walking assistance, while simultaneously providing energy-harvesting functions. We demonstrate that the use of our device reduces the activation of the gastrocnemius and soleus muscles during the foot strike by 5.8 ± 1.0 % and 4.1 ± 0.6 %, respectively. The collisional energy conserved from the impact at the touchdown is transformed into 3.8 ± 0.3 watts of electrical power (mean ± SEM). Compared with walking in normal shoes, the energy savings with the device imply that walking endurance could be increased by as much as 10 % without extra effort from the wearer. Our findings demonstrate the potential of the heel pad-based device that enhances the energy economy of walking and human bipedal locomotion. [Display omitted] • Highly integrated footwear for shock absorbing, gait assisting, and energy harvesting. • A power density of 27.5 mW/cm3 (peak power: 3.8 Watts) is achieved for a single step. • Walking with the device does not significantly interfere with the normal gait pattern. • The footwear device shows stable durability and robustness in daily locomotion. [ABSTRACT FROM AUTHOR]

Piezoelectric energy harvesting is envisioned as an ideal complement to batteries for long-life operation of wireless or remote electronic devices. Most piezoelectric energy harvesters (PEHs) utilize piezoelectric plates or discs as the energy transducing elements, and they cannot generate enough power for most potential applications. In this study, we explore a new way of using arc-shaped piezoelectric patches as the core transducing elements in energy harvesters to improve their performance. An analytical model is developed via the Castigliano’s theorem to elucidate the low stiffness characteristic of the arc-shaped structure. A finite element analysis is conducted, and the results indicate that high and evenly-distributed stress can be easily induced in the arc-shaped PEHs. Prototypes are fabricated with arc-shaped piezoelectric elements and flat piezoelectric plates, respectively, and tested under the same conditions. The experimental data demonstrate that the arc-shaped PEHs are capable of generating 2.55–4.25 times as much power as the equivalent flat-plate PEHs. This study lays the foundation to enhance energy harvester performance by utilizing arc-shaped piezoelectric structures. [ABSTRACT FROM AUTHOR]

Vibration energy harvesting has been a research subject of growing interest over the past few years, and is envisaged as a remedy to the unsatisfactory battery issue in low-power electronic devices. In this study, we propose a new magnetostrictive transducer to harvest energy from finger tapping. Galfenol is selected as the transducing material due to its high piezomagnetic coefficient and excellent machinability. To effectively harness energy in low-frequency conditions, we develop a frequency up-conversion mechanism that succeeds in converting vibration below 10 Hz from finger tapping up to the system's resonance of a few hundred Hz. Furthermore, multiple impacts are induced in each working cycle deliberately to boost the efficiency. A comprehensive model is built and solved to analyze the mechanical-magnetic-electrical coupling system. Based on the model, we elucidate the design criteria for high-performance magnetostrictive transducers. A prototype is fabricated with a Galfenol beam of 0.5 × 5 × 25 mm 3 and, under finger tapping, it generates 5.3 mW power and instantaneously lights up 10 commercial LEDs and a numeric LCD. [ABSTRACT FROM AUTHOR]

Bistable nonlinear energy harvesters are effective structures for scavenging broadband energy from frequency-varying vibration sources. The bistable characteristic of such structures is typically induced by introducing magnetic fields, prestress, and nonuniform geometries or fixtures. However, the complicated structures or single deformation modes of these structures limit their practical application. Inspired by the structure of soybean pods, a bionic metallic nanomaterial-based bistable piezoelectric energy harvester is developed in this study. The nanoplate substrate with a local bistable region mimics the configuration of a soybean pod. To the best of the authors' knowledge, the local bistable configuration represents a novel energy harvesting strategy. The local bistable nanoplate is fabricated using a mature surface mechanical attrition treatment technique that generates a gradient nanostructure to enhance the mechanical properties of the bistable structure. The energy harvesting performance and nonlinear dynamic characteristics of the energy harvester are evaluated through frequency-sweep and fixed-frequency vibration tests and numerical simulations with a new two-step finite element (FE) model. The harvester characteristics in three vibration modes (single-well vibration (SV), intermittent cross-well vibration (ICV), and continuous cross-well vibration (CCV)) are discussed. The experimental and numerical results demonstrate that the voltage output and working bandwidth of the proposed harvester increase by five times in the ICV and CCV modes when the excitation acceleration increases from 0.5 g to 3.0 g. The bionic nanomaterial-based bistable piezoelectric energy harvester can be potentially used in various applications such as vehicle suspension systems, tires, and vehicle-bridge systems. [Display omitted] • Soybean pod-mimic nanoplate-based broadband bistable piezoelectric energy harvester for vibration energy harvesting. • Nanoplate with a local bistable region processed by a facile method of surface mechanical attrition treatment (SMAT). • Novel two-step FE model for analyzing local bistable/multistable problems. • Three vibration modes including single-well vibration, continuous cross-well vibration, and intermittent cross-well vibration. • 500% improvement in working bandwidth and voltage output generated by nonlinear cross-well vibrations of the bionic harvester. [ABSTRACT FROM AUTHOR]

Vibration energy harvesting has a great potential to achieve self-powered operations for wireless sensors, wearable devices and medical electronics, and thus has attracted much attention in academia and industry. The majority of research into this subject has focused on the piezoelectric effect of synthetic materials, especially the perovskite PZT ceramics. Recently the new-generation piezoelectric materials PMN-PT and PZN-PT single crystals have gained significant interest because of their outstanding piezoelectric properties. They can be used to replace the widely-adopted PZT ceramics for improving energy harvesters’ performance substantially. However, there is little research on comparing PMN-PT and PZN-PT energy harvesters against PZT harvesters. In this paper, we present a systematic comparison between vibration energy harvesters using the PMN-PT, PZN-PT single crystals and those using the PZT ceramics. Key properties of the three materials are summarized and compared. The performance of the PMN-PT and PZN-PT energy harvesters is characterized under different conditions (beam length, resistance, frequency, excitation strength, and backward coupling effect), and is quantitatively compared with the PZT counterpart. Furthermore, the effect of the synchronized switch harvesting on inductor (SSHI) circuit on the three harvesters is discussed. The experimental results indicate that energy harvesters using the PMN-PT and PZN-PT single crystals can significantly outperform those using the PZT ceramics. This study provides a strong base for future research on high-performance energy harvesters using the new PMN-PT and PZN-PT single crystals. [ABSTRACT FROM AUTHOR]

Harnessing ambient vibration energy using piezoelectric materials is a promising alternative solution to batteries to inexhaustibly power small-scale mobile devices. This paper presents an innovative energy harvester, named high-efficiency compressive-mode piezoelectric energy harvester (HC-PEH). It consists of a pair of elastic beams and a flex-compressive center where the piezoelectric element bears a dynamic compressive load. The compressive mode is preferred than the commonly-used bending mode and tensile mode due to the superior compressive strength of piezoceramics. To cater to the characteristic of piezoceramics: large ultimate stress, but extremely small ultimate strain, a multi-stage force amplification mechanism is developed in the design. The HC-PEH shows superior capability of high power output and favorable nonlinear phenomena at low frequency range. Under an acceleration of 0.5 g (g = 9.8 m/s 2 ), a maximum power of 19 mW is produced at 21 Hz, which is about one order of magnitude higher than the power output of energy harvesters previously reported. The capability of high power output of the HC-PEH demonstrates the practicability of using energy harvesters to power most of wireless sensors. [ABSTRACT FROM AUTHOR]

Previous droplet-based electricity generator has made breakthroughs in efficient water energy harvesting through taking advantage of the contact electrification, however, the kinetic energy of water remains to be tapped into. In this work, we develop a leaf-mimic rain energy harvester (REH) that allows for the collection of electrostatic and kinetic energy simultaneously based on the synergy of the liquid-solid contact electrification and the piezoelectric effect. Impinged by a water droplet, the REH generates a boosted transferred charge value (101 nC) and high output power density (82.66 W m−2). We also show that such performances enable the construction of self-charged wireless sensor systems with continuous operational capability. The REH is also low-cost and facile for fabrication, paving a new way towards high-efficiency power generation from raindrops as well as other water sources. [Display omitted] • A mechanism of harvesting energy from both the liquid-solid contact and kinetic motion inspired by the leaf-rain interaction. • A new explicit theoretical model that links key parameters on the liquid-solid contact electrification. • Continuous operation of a wireless sensor system powered by the proposed droplet energy harvesters. [ABSTRACT FROM AUTHOR]

Wrist-worn smart devices such as smartwatches and wristbands are rapidly penetrating our daily life for health, security and fashion purposes. However, the battery-powered wearables have a limited lifetime and need to be frequently recharged, which degrades user experience and further poses environmental problems. Inspired by the structure of the woodpecker's head, this paper reports a bionic two-layer band piezoelectric energy harvester for self-powered smartwatches and wristbands. An inner band and an outer band of the harvester mimic two branches of the hyoid bone of the woodpecker. The energy harvesting unit employs a piezoelectric array structure. A finite element analysis is conducted to identify the optimal positions of piezoelectric patches in the array and an improved rectifier circuit is developed to manage the multiple AC inputs. We design two operation modes, i.e., movement mode and impact mode, to accommodate energy harvesters in different human activities. A maximum power output of 15.41 mW is achieved in experiments when tapping the prototype, corresponding to an equivalent power density of 1049.01 μW/cm3, around five times higher than previous wrist-worn harvesters. The design demonstrates its capability of continuously powering a watch and a screen during a normal walking process. The concept of shifting from the watch body to watch bands provides the design of energy harvesters a larger space and higher user-friendliness, which, fused with the piezoelectric array method, shows the potential to merge the gap of the power generation of energy harvesters and the power consumption of wrist wearables. [Display omitted] • Woodpecker-inspired two-layer band design for wrist-worn wearables. • A piezoelectric array structure as an energy harvesting unit. • Two working modes including movement mode and impact mode. • The maximum power output of 12.25 mW in the movement motion and 15.41 mW in the impact mode. • Able to continuously power a watch or a screen during normal walking. [ABSTRACT FROM AUTHOR]

As a typical ferroelectric material, the PbZr 1−x Ti x O 3 (PZT) thin film with a homogeneous component has drawn extensive attention to the application in self-powered flexible electronics. The design of compositionally graded PZT thin films (i.e. artificially control the Zr/Ti ratio in different compositional layers in one film) has shown its immense potentials due to some novel phenomena induced by strain gradient, such as the shifted hysteresis loop, generation of the built-in electric field, enhancement in the motion of domain wall. However, the effect of the compositionally graded heterostructure on the piezoelectric response and its performance on the device level has seldom been investigated. To fill this knowledge gap, we prepare PZT thin films with the compositionally graded structure and based on that, fabricate flexible piezoelectric nanogenerators (PENGs). Furthermore, we explore the 2D mica as the substrate of PENGs and avoid the commonly used costly and troublesome laser-lifting or etching transfer procedure. The PENG prototypes integrated with 1 µm thick PZT films are flexible, translucent, and show high electric responses under different mechanical stimuli. Experiments indicate that up-graded PZT films, of which Zr composition varies from 20% near the bottom to 80% at the top, outperforms the down-graded counterpart. The up-graded PENG achieves a maximum voltage response of 2 V and a current density of 10.0 μA/cm2 when bent by 90 °. This work demonstrates that the strain-tuned compositionally graded structure is an effective approach to improve the PENG performance. [Display omitted] • A new method to control the piezoelectric properties via the strain engineering. • The first performance-enhanced compositionally graded PZT thin film at the device level. • The up-graded PZT thin films exhibit a performance comparable to 52/48 multilayer films. [ABSTRACT FROM AUTHOR]

The performance of vibration-based energy harvesters deteriorates when used in rotational environments with an offset distance. To address this issue, we propose a method of using magnets to improve the performance of the high-efficiency compressive-mode piezoelectric energy harvester (HC-PEH) with the offset configuration in rotational environments. The performance of the magnetically coupled rotational HC-PEH system with offset distances is experimentally studied. The results show the proposed method is feasible and significantly improves the performance (peak-peak voltage) by 254.2%. To attain comprehensive insight into the electromechanical behaviors of the system, we develop a theoretical model with consideration of nonlinear stiffness, nonlinear damping, and nonlinear piezoelectricity. The governing equations of the system are obtained by using Hamilton's principle and their approximate analytical solution is derived via the harmonic balance method. The simulation results closely render the experimental data and are used to characterize the system. Based on the developed theoretical model, a parametric study is performed and the results indicate that the best performance improvement can be obtained via tuning the parameters related to the magnetic force, the centrifugal force, and the initial deformation of the elastic beams. This work helps overcome a major obstacle for the application of rotational energy harvesting. • Verification on adverse effects of the offset distance in rotational environments. • A brand-new solution by using magnetic repulsion and its experimental verification. • The peak-peak voltage of the energy harvester shows a maximum increase by 254.2%. • Model of the magnetically coupled rotational HC-PEH system with offset distances. • Discussions on the dynamical characteristics of the system. [ABSTRACT FROM AUTHOR]

In this article, we report a wearable millimeter-size energy generator that yields instantaneous power of 2.1 W level from kinetic energy of human running motions. The generator, with a hybridization of piezoelectric and electromagnetic transductions, integrates mainly two major novel techniques: impact induced frequency up conversion effect for the piezoelectric generation and abrupt magnetic flux density change for the electromagnetic generation. The generator proves to be able to effectively harness energy from human motions, producing a power density over one order of magnitude higher than that of state-of-the-art work. This result is further validated by the charging performance, i.e. high charging rate and voltage into mF-level capacitors. Moreover, this wearable generator demonstrates the capability to charge a Li-on battery in dozens of minutes. This work can be of much significance for the development of self-charging battery powered wearable electronic devices. ga1 • Novel hybridization of high-performance piezo stack and electromagnetic generators. • Adopting abrupt magnetic flux density change in the electromagnetic generator. • Matched impedance of piezo generator lowered to 100 Ω by frequency conversion effect. • Total instantaneous power reaching 2.1 W of a millimeter-size configuration. • Achieving fast charging towards a 180mAh Li-on battery based on running motions. [ABSTRACT FROM AUTHOR]

Sensors that provide critical information about jet engine performance are widely installed on static components but are rarely found on rotors because of their inaccessibility and extremely high rotation speeds. We present a new monitoring method, integrating energy harvesting technology with wireless sensors to achieve real-time self-powered engine monitoring. Energy harvesters, used to generate power from ambient vibration, are sustainable alternatives to batteries for achieving self-sustained long-term operation of electronic devices. By utilising structural nonlinearity, force amplification mechanism, and the piezoelectric effect, we show a 22.52-g energy harvester capable of high power output (78.87 mW), broad working bandwidth (22.5 Hz), and strong reliability (2100 RPM). Our approach breaks limitations from wired connections that are weighty and vulnerable to failures. We theoretically and experimentally analyse the nonlinear responses and demonstrate the harvester by constantly lighting 112 LEDs and a self-powered wireless sensor system in a jet engine. This work paves a new way for developing future monitoring systems for advanced jet engines and other rotating machinery applications. The self-powered wireless sensor system consists of an energy harvester as the power supply and a wireless sensor node as the communication device. When the jet engine operates, the energy harvester that installs on the rotational component generates power for the wireless sensor node and then the real-time monitoring information of the engine components will be sent wirelessly to a local area network (LAN) in the aircraft for working condition analysis and failure prediction. The energy harvester of the monitoring system exhibits great compatibility in the rotational environment of jet engines and excellent power generation capability (max. to 78.87 mW) and working bandwidth (22.5 Hz in 0–2100 RPM). A combination of such characteristics guarantees the realization of the self-powered and wireless operation of sensor systems in jet engines. Image 1 • A brand-new sensor architecture for real-time and self-powered jet engine monitoring. • A maximum output power 78.87 mW and efficiency 36.43%, capable of powering 112 LEDs simultaneously. • A demonstration of a self-powered wireless sensor system in jet engine components for the first time. • A comprehensive study on the compressive-mode piezoelectric energy harvester used in the rotational environment. • A comparative study the compressive-mode and bending-mode PEHs under vibration and rotational excitations. [ABSTRACT FROM AUTHOR]

In this paper, we originally report an approach for cantilever piezoelectric energy harvesters (PEHs) for frequency tuning and performance improvement. The proposed scheme implements embedment of revolute joint(s) into PEH structure. We investigated two ways of embedment: the prior one is to replace the clamped edge partially with a joint and the other is to incorporate joint(s) into the substrate plane. Introducing joint mechanism reduces stiffness of the harvester, leading to more intense vibration and higher voltage of the piezoelectric element. We conducted FEA to validate hypothetical stiffness reduction by deriving resonant frequency considering parameters of joint width, length and number. Furthermore, experimental studies were conducted to compare open-circuit voltages in the frequency domain, power generation and capacitor-charging capabilities. The resonant frequencies from experiments display a good agreement with those from our simulation estimations. The embedded joint(s) effectively lowers resonant frequency by ∼43.1%. The output voltage of jointed cases increases ∼71.2 V, 2.83 times as high as that of the counterpart case. The mean power of the jointed cases reaches 0.49 mW, 7.3 times as high as that of the counterpart case. Furthermore, experimental charging results indicate remarkable improvement in charging capabilities regarding much higher charging rates and higher saturated voltage. • Original study of favorable effects of joints on piezoelectric energy harvesters. • Embedded joints can effectively tune the resonant frequency of the harvester. • A 630% output power improvement due to the joint embedment. • Much higher charging rates and voltages in charging performance of capacitors. [ABSTRACT FROM AUTHOR]

• Multiple nonlinear energy harvesters are developed based on compliant orthoplanar springs and piezoelectric materials. • Experiments with both harmonic and random excitations are performed. The proposed COPS harvesters could show an extended operational bandwidth in both conditions. • The study quantitatively demonstrates the effectiveness of interaction of multiple nonlinear vibration modes on harvester performance enhancement, and which effect can be tuned by the addition of multiple masses and spring legs. • The quad-leg design shows a broad bandwidth of 35 Hz while maintaining a voltage response of 4 V. This paper compares the performance of three nonlinear compliant orthoplanar springs, namely, bi-leg, quad-leg and pent-leg designs to study the vibration mode interaction effect for widening the operational bandwidth of piezoelectric vibration energy harvester. All the designs have three or more vibration modes below 150 Hz with the addition of multiple masses. Finite element analysis results of spring designs and vibration mode shapes are presented and compared with experiments. The prototypes were manufactured and experimentally tested to analyze their dynamic voltage-frequency responses and operational bandwidths under sinusoidal and band-limited excitations. The experimental results demonstrate that the operational bandwidth is significantly broadened by the geometric nonlinearity of the structure. The addition of multiple masses brings the vibration modes of the system closer, and thus further helps to extend the operational bandwidth. In addition, we introduce multiple piezoelectric plates into harvesters to enable energy harvesting in all vibration modes. The quad-leg design shows the largest frequency bandwidth of 35 Hz, maintaining a peak-to-peak voltage of 4 V under the forward frequency-sweep excitations. Under the reverse frequency-sweep excitations, the voltage response and the operational bandwidth are comparable to those of the forward-sweep ones as a result of continuous overlapping of linear and nonlinear voltage peaks. Random excitation experiments demonstrate the effectiveness of the proposed mutli-leg structure in harvesting energy from wideband environmental vibrations. [ABSTRACT FROM AUTHOR]

Highlights • Hybridization of piezoelectric and triboelectric energy generators. • Bifurcation induced superharmonic resonances for low-frequency energy harvesting. • High power outputs with force amplification effect by a truss mechanism and impact. • Capability of harnessing energy from a broad frequency range. • Frequency up conversion approach for higher output power. Abstract In this paper, we report a hybrid generator to harness energy from low-frequency ambient vibrations. The generator is designed with a piezoelectric energy harvester (PEH) patch, a triboelectric nanogenerator (TENG) patch, a spring-mass system and an amplitude limiter. The spring-mass system receives energy from excitations and applies forces to the piezoelectric element and the triboelectric layers. The unique amplitude limiter is deliberately introduced into the system, not only achieving the desired frequency up-conversion effect, but also boosting the voltage responses significantly. The limiter also causes hardening nonlinearity and dynamic bifurcation triggering superharmonic resonance so that the generator resonates at a frequency of about 3 Hz. Furthermore, the proposed PEH adopts the strong compressive operation mode, and employs a truss mechanism to effectively amplify the impact forces. In experiments, open-circuit voltages are 58.4 V from PEH and 60 V from TENG under an excitation of 1.0 g at resonance. The hybridized generator is capable of achieving a maximum power of 19.6 mW from the two sources with matched impedances. The working bandwidths of the PEH and the TENG reach up to 5.39 Hz and 7.25 Hz, respectively, out of our targeted frequency domain [2.5 Hz, 10 Hz] due to the multiple resonances and nonlinearity. In the applications to charge capacitors, the high saturation voltage and relatively short charging time validate the effectiveness of our power management technique. Furthermore, the generator proves to be able to effectively scavenge energy from human body motions and charge a capacitor of 4.7 μF to 7.6 V in around 50 s, which indicates a great potential of practical applications in wearable devices. [ABSTRACT FROM AUTHOR]

[Display omitted] Bulk piezoceramics are widely used and functional materials, but they are hard to be shaped to adapt to complex 3D surfaces. Most existing ceramic shaping techniques, like 3D printing, come at the cost of physical properties such as piezoelectricity and thermal stability. Here to fabricate 3D piezoceramic sheets, we report a mold-assisted sintering (MAS) method that can replicate 3D surfaces with nonzero Gaussian curvatures while maintaining the intrinsic properties. Assisted with supporting molds, the MAS exploits elastic-to-viscous transition of piezoceramic powder compacts during sintering, bypassing the boundaries of deformation and surface development in an elastic continuum. We successfully replicate shapes to sintered lead zirconate titanate (PZT) ceramics from designed molds with saddle, sine and spherical surfaces, respectively. The PZT ceramic replicas show high piezoelectricity and excellent conformability towards nonplanar surfaces (maximum deviation 90 μm). We further demonstrate the advantage of the conformal curved piezoceramics by using them for wirelessly transferring power over a pipeline. The MAS method is low-cost, simple, scalable and applicable to various 3D energy transducers. [ABSTRACT FROM AUTHOR]

In this paper, we originally present a gravity induced self-excited vibration generator based on a cantilever piezoelectric resonator. The configuration is composed of a sliding rod, a sleeve, a transduction unit. To estimate output voltage and investigate dynamic responses of the generator, an equivalent model based on mechanical structure was established and numerically solved. The effects of the cantilever length, tip mass and linear coupling factor on the open circuit voltage were discussed. In addition, we also conducted series of experiments to validate the theoretical responses. In a 10s cycle (namely 0.1Hz excitation frequency) scenario, the transduction unit responds with structural resonant frequency of 8.621Hz. The results of the experiments show that the open circuit voltage of the generator increases with increment of the cantilever beam length and the tip mass, which agrees well with simulation estimations. We also conducted impedance matching experiments to examine power response in the load spectrum, of which the results indicate that the prototype peak power reaches maximum 1.897 mW. We also discussed the change of gravity potential energy of the prototype and compared it with electric energy. This work provides a novel approach for piezoelectric energy harvesting from extremely low frequency excitations. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper originally investigates the influence of frequency-up conversion effect on piezoelectric stack generators for high-performance energy harvesting. A compressive-mode piezoelectric generator is proposed comprised of a piezostack and a spring-mass system for electromechanical transduction and mechanical excitation, respectively. The frequency-up conversion effect is induced by amplitude truncations of the mass under external excitation. A control group, i.e. without truncation, is set for performance comparisons. Experimental results indicate that, with the frequency-up conversion, instantaneous power and average power are enhanced by over 1000 and 177 times, respectively. Each voltage response cycle consists of two stages: high-frequency high-amplitude oscillation and the rest of low-frequency low-amplitude harmonic vibration. The frequency of the former stage determines impedance matching. Due to the frequency converted up by 69 times, and intrinsic high capacitance of the stack, the matched resistance is significantly reduced from over 5 kΩ to 73.10 Ω, resulting much higher power response than that without conversion. Additionally, increase of spring stiffness causes the decrease of voltage responses and less effectiveness of frequency conversion effect. In this work we conclusively report that an instantaneous peak power output of 0.32 W can be generated from a millimeter-size piezoelectric generator, which can foster development of high-viability self-powered applications. [Display omitted] • Original study on frequency-up conversion effect on the piezostack-based generators. • Achieving over two orders of magnitude enhancement of instantaneous power. • Matched impedance is lowered from 5.1 kΩ to 73Ω. • Average power enhanced by over 200 times after frequency conversion. • Study on the restoration spring stiffness affecting frequency converting behaviors. [ABSTRACT FROM AUTHOR]

• A novel hybrid scheme of piezoelectric and electromagnetic energy harvesting. • The harvester can yield a peak power of 0.33 W with a compact size. • The piezoelectric transduction operates in compressive mode. • The magnet array in electromagnetic transduction unit is arranged alternatively. • Applications of building self-powered sensors and electronics with the harvester. In this paper, we present a 0.33 W piezoelectric and electromagnetic hybrid energy harvester to tackle the problem of efficiently harnessing energy from low-frequency vibrations. Two types of transduction cores-one piezoelectric element and two sets of magnets and coils, are embedded into a compact design. Several techniques are employed to enhance the performance of the harvester. A pair of truss mechanisms are added into the system to amplify the strain of the piezoelectric element. A stopper is used to induce impact to take advantage of the frequency up conversion effect as well as nonlinearity. We used an array made of multiple small cubic magnets and alternative magnet arrangement rather than one magnet block, for abrupt magnetic flux density changes. Experimental results based on a fabricated prototype validate the proposed techniques that work collectively and enable the harvester to yield a maximum peak power more than 0.33 W at resonance under an excitation of 0.70 g. We also examined the capability of the harvester for self-powered applications. The harvester displays excellent charging performance to millifarad or farad-scale capacitors. An LED array consisting of 99 diodes was lit up in real time. More importantly, experimental results indicate that not just can the harvester simultaneously power a temperature and humidity sensor, and a calculator, but also when the excitation stops, the remaining charges stored can power them for ~20.38 min and ~8.33 min, respectively. This study can be of great significance for high-performance energy harvesting and further development of self-powered sensing and battery-free electronic systems. [ABSTRACT FROM AUTHOR]

The pyroelectric effect provides an efficient route to convert low-grade waste heat into electricity. (Bi 0.5 Na 0.5)TiO 3 -based ceramic is one of the most attractive lead-free pyroelectric candidates due to its high polarization. However, the low breakdown strength (BDS) restricts its ability to achieve a high polarization, hindering the application for thermal energy harvesting based on the Olsen cycle. In this work, by incorporation of AlN into BNT-BZT ceramics to form the BNT-BZT: AlN composites, the BDS is enhanced from 160 kV cm−1 to 260 kV cm−1, leading to a high energy density of 1.54 J cm−3 which is ∼5 times that of the pristine BNT-BZT. The reason for the high energy harvesting performance is due to the addition of AlN that can depress the conductivity of the matrix, resulting in the high BDS that permits high electric fields to be applied on the samples to induce high polarization that changes pronouncedly with temperature. [ABSTRACT FROM AUTHOR]

Abstract Colloidal quantum dots (CQDs) have shown their advantages in gas-sensing applications due to their extremely small particle size and facile solution based processes. In this study, a high sensitivity of surface acoustic wave (SAW) NO 2 sensor was demonstrated using SnS CQDs as the sensing layer. The delay line based SAW device with a resonant frequency of 200 MHz were fabricated on ST-cut quartz substrate. The SnS CQDs with average sizes of 5.0 nm were synthesized and deposited onto SAW sensors using a spin-coating method. The fabricated SAW sensor was capable of detecting a low concentration of NO 2 gas at room temperature with a good efficiency and selectivity e.g., with a 1.8 kHz decrease of center frequency of the SAW delay line when exposed to 10 ppm NO 2 at room temperature. Highlights • Small-size and high-crystallinity SnS CQDs were synthesized via hot injection. • The SnS CQDs were successfully integrated onto the SAW delay lines by spin-coating. • The SnS CQD-coated SAW sensor exhibited high response to NO 2 at room temperature. [ABSTRACT FROM AUTHOR]

Highlights: • Improved harvester power output via abrupt magnetic-flux-density changes. • Fabricated magnet arrays of alternating north and south poles to abrupt flux changes. • Constructed alternative and Halbach arrays with triangular and cubic magnets. • Achieved a max RMS power output of 35.5 mW under an excitation of 9.8 m/s2. Abstract Electromagnetic energy harvesters have been studied intensively in the past decade as non-stop power solutions for low-power wireless electronic devices. We here exploit a new way, i.e., forming an abrupt change of magnetic flux density, to improve harvester performance. A new transducer is proposed, which is primarily composed of a set of magnet array, a pair of springs and a coil array. Four cases are investigated: cubic magnets in Halbach and alternative configurations; triangle magnets in Halbach and alternative configurations. We examined the magnetic flux density distribution via the finite element method (FEM) and ensured the nonexistence of phase difference in two contiguous coils. We fabricated a prototype and conducted comprehensive tests, of frequency sweep with and without external resistances, of matching impedance and of obtaining maximum RMS power output. The FEM analysis indicates that the harvester in the cubic alternative case has the largest changing rate of the magnetic flux density (MFD) in terms of the magnitude and the distance range of the change. Experiments verify the FEM simulation and the prototype with the cubic alternative magnet array shows the highest voltage response of 20 V and a maximum RMS power output of 35.5 mW under a harmonic excitation of 9.8 m/s2, which is much higher than those in the other cases. In term of the volume and mass power density, the alternative arrangement of cubic magnets displays the most desirable outputs at 0.4955 mW/cm3 and 0.28 mW/g, respectively, which are three and two times as high as those of the second best case - the triangle Halbach case. This detailed study reveals the considerable benefits brought by the magnet arrays of alternating polarity and configuration, and paves a new way to improve the performance of electromagnetic energy harvesters. [ABSTRACT FROM AUTHOR]

• The RMS voltage response of the energy harvester is increased by up to 1225%;. • We present a new quasi-zero stiffness mechanism for energy harvesters;. • We derive a stable solution to the adverse effect of the offset distance;. • The nonlinear dynamics of the proposed systems are analyzed comprehensively. The self-powered sensing technology based on gravity-rotation-excited vibration energy harvesting (GRE-VEH) enables information acquisition directly from rotatory machines online. However, the centrifugal effect due to the distance between the mass centroid of the energy harvester and the rotating axis (i.e., offset distance) seriously limits its broad engineering applications. To address this issue, we propose methods of using a tensile force provided by pre-deformed springs to counter the constant centrifugal force around the target rotation speed for improving the performance of the energy harvester affected by offset configurations. The GRE-VEH system with a linear spring is first studied by experiments and simulations, where a distributed-parameter model is developed and experimentally validated. The results well validate the proposed mechanism, where the performance of the harvester prototype significantly increases (by up to 1225%). A parametric study is also performed theoretically. The results indicate that the best performance improvement can be obtained by tuning the parameters related to the tensile length, the stiffness, and the damping of the spring. To enable the proposed mechanism in large centrifugal force cases, a pair of negative springs is introduced to the system for constructing a quasi-zero stiffness (QZS) structure together with the linear spring. The new GRE-VEH system is theoretically studied by an updated model, where the system's main characteristics are revealed. The simulation results well indicate the feasibility of the method using the QZS mechanism. This work paves a new way of solving the offset distance problem for the application of the GRE-VEH and also provides new insight into the application of the QZS mechanism. [Display omitted] [ABSTRACT FROM AUTHOR]

• T-phage-inspired multilegged design for microrobot. • A rotationally asymmetric structure. • Multiple working modes: forward motion, backward motion, steering, climbing, and swimming. • The maximum velocity of 120 mm/s under a driving voltage of 50 V p-p and the maximum carrying weight of 10.6 gs, 6 times more than the weight of the microrobot. • Miniaturization capability and ability to work in various environments, i.e., a narrow tube and water. T-phages use contractile tails to recognize host bacterial and move on the surface of bacterial. This virus demonstrates remarkable locomotive capabilities in recognition and infection progress. The field of microrobotics focuses on achieving these functions in mobile robotic systems of centimeter size. However, owing to the structure size and weight, it has been a challenge to satisfy high robustness, fast motion, adequate carrying capacity, low driving voltage, and versatile environmental adaptability in one robot. In addition, functional microrobots require multimodal locomotive strategies that reconcile the constraints imposed by different applications. Inspired by the unique structure of T-phage in the micro-world, we develop a T-phage Mimic MicroRobot (TMMR), which is composed of a triangular prism core with piezoelectric elements and legs attached on bases, mimicking the core and the tail fibers of the T-phage, respectively. The triangular prism structure avoids rollover while moving, indicating the robustness of the design. TMMR of 2.5 × 2.5 × 2.6 mm3 weighs 1.74 gs and can be easily further miniaturized. It is capable of moving forward, backward, and steering at a low voltage of 25 V. Working mechanisms for each type of motion are analyzed using the finite element method. Results show that TMMR reaches 120 mm/s, 4.8 body lengths per second (BL/s), and carry an object weighing 10.6 gs, 6 times more than its weight. By sealing the body and adding passive flaps, TMMR achieves a "swimming" motion with a velocity of 16 mm/s. TMMR shows potential for versatile applications in a narrow and constrained environment. [Display omitted] [ABSTRACT FROM AUTHOR]

• A comprehensive electrode coverage design principle is developed. • Equivalent stiffness and damping are introduced to evaluate coupling coefficients. • Strongly coupled harvesters are compatible with a wide range of electrode coverage. Cantilevers are widely used for piezoelectric vibration energy harvesters (PEHs), thanks to the low resonance frequency and large mechanical strain under weak excitations. Electrode coverage optimization is essential for maximizing output power. In this study, bimorph cantilevers with segmented electrodes are developed to investigate the correlation between effective electrode coverage (EEC) and the harvesting behavior of piezoelectric cantilevers. A nonlinear electromechanical model is established using Hamilton's principle, numerically solved with the method of harmonic balance, and further validated by experimental measurements. Equivalent linear stiffness and equivalent mechanical damping ratio are firstly introduced for evaluating the equivalent effective electromechanical coupling coefficients and identifying strong and weak coupling harvesters. Both harvesters fabricated with and without proof mass behave mainly as strongly coupled systems and can obtain the maximum possible power at a very low EEC. Variation of EEC in higher range acquires almost constant maximum power on dramatically changing resistance. A comprehensive EEC design principle for acquiring the maximum output power is obtained for cantilevered PEHs. An optimal EEC obtaining the maximum effective electromechanical coupling coefficient is needed for acquiring the unique maximum power pick for weakly coupled harvesters, whereas a wide range of EEC can be used to maximize output power for harvesters those mainly behave as strongly coupled systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • Propose an auxetic nonlinear piezoelectric energy harvester. • Utilize a pure mechanical structure to enhance both efficiency and bandwidth. • Perform finite element analysis to analyse the performance of the harvester. • Experimentally validate the proposed design and model and conduct parametric studies. In this paper, we design and experimentally validate an auxetic nonlinear piezoelectric energy harvester. This harvester adopts a clamp-clamp beam with auxetic structures, which can improve the efficiency and bandwidth of the energy harvesting based on a pure mechanical structure without outer magnets or stoppers. Finite element analysis is performed to analyse the effects of the auxetic structures on the efficiency and nonlinearity of the energy harvester. The lumped parameter model is utilized to predict the performance of the energy harvester, which well matches the experimental results. In the experimental validation, at 0.1 g base acceleration, the power output of the two types of auxetic energy harvesters is 173% and 94% higher than the conventional nonlinear energy harvester. Besides, the bandwidths of the two types of auxetic energy harvesters are broadened by 1556% and 2142% compared with the linear systems. [ABSTRACT FROM AUTHOR]

Researchers have recently focused on improving the efficiency of piezoelectric energy harvesters by redesigning their substrate beams. Two approaches have been explored, including changing the shape of the cross-section area, i.e., width variation, and changing the thickness of cantilever beams. The width changing approach suffers from the deduction of output power from the piezoelectric elements of the trimmed-off regions, while performance deterioration does not occur in the thickness-variable approach. The benefit of thickness-variable beams on energy harvesting has been analyzed theoretically, but its detailed experimental study is still absent. In this study, we for the first time fabricate a thickness-variable harvester based on the Garolite FR-4 epoxy laminate, and validate the superiority of the variable thickness beam on the harvester performance enhancement. A theoretical model is developed based on the Euler-Bernoulli beam theory and numerically solved via the Rayleigh-Ritz method. We compare the thickness-variable composite beam configuration with a conventional uniform-thickness counterpart and unveil the beneficial effect of evenly-distributed strain on performance enhancement. Experiments indicate that the thickness-variable method increases on the energy harvester efficiency by 78%. [ABSTRACT FROM AUTHOR]

Ocean, as a natural system containing a tremendous amount of energy, can be used for either the large-scale power grid network or small-scale distributed off-grid electronic devices via the energy harvesting technology. As one low-cost and effective way to capture flow energy, the vortex induced vibration (VIV) energy harvesting is attracting more and more attention. However, the direction of water flow in a natural water environment is changeable while most existing VIV harvesters are limited by their directional sensitivity. These harvesters only respond to flow excitations from one fixed direction and become insufficient once the flow direction varies. In this paper, we take the lead to address the unidirectional sensitivity issue and propose a novel direction-adaptive energy harvester. We establish theoretical models to analyze the Kármán vortex street, the torque excitation, and the vortex-induced pressure oscillations. Prototypes are fabricated and tested to characterize the direction-adaptive capability of the proposed design under different flow conditions. The experiments demonstrate that the energy harvesting angle span is extended by the guide wing from 40° to 360° under a wide flow velocity range. The guide-wing method endows harvesters with an all-around multidirectional sensitivity, and thus will accelerate energy harvesters' applications in oceans. • Introduce the directional self-adaptability to VIV energy harvesters. • Develop the first all-around 360° direction-adaptive flow energy harvester. • Design and analyze a unique guide wing theoretically and experimentally. • Can be adopted by other harvesters to improve their environmental adaptability. [ABSTRACT FROM AUTHOR]