Dielectric Elastomers for Energy Harvesting

Dielectric elastomers are a type of electroactive polymers that can be conveniently used as sensors, actuators or energy harvesters and the latter is the focus of this review. The relatively high number of publications devoted to dielectric elastomers in recent years is a direct reflection of their diversity, applicability as well as nontrivial electrical and mechanical properties. This chapter provides a review of fundamental mechanical and electrical properties of dielectric elastomers and up-to-date information regarding new developments of this technology and it’s potential applications for energy harvesting from various vibration sources explored over the past decade

An electret-based thermoacoustic-electrostatic power generator

Sections PDFPDF Tools Share Summary This study reports a new concept for power generation from thermal energy, which integrates a thermoacoustic engine (TAE) with a contact‐free electret‐based electrostatic transducer. The TAE converts thermal energy into high‐intensity acoustic energy, while the electret‐based electrostatic transducer converts the generated acoustic energy into electricity. The experiments demonstrate the feasibility and potential of the proposed electret‐based thermoacoustic‐electrostatic power generator (TAEPG). The dynamic response of the electrostatic transducer and energy conversion inside the TAE are further investigated using a lumped element model and a frequency‐domain reduced‐order network model. Good agreement is achieved between experimental measurements and theoretical predictions. Furthermore, a parametric study is performed to study the effect of key parameters including the external heating power, air gap, and resistive load on the performance of the TAEPG. Results show that an open‐circuit voltage amplitude of 4.7 V is produced at a closed‐end pressure amplitude of 480 Pa in the experiment, and it is estimated that 25.2% of the acoustic power generated by the TAE has been extracted by the electret‐based electrostatic transducer. In this case, the maximum electric power output is measured to be 2.91 μW at a resistive load of around 2.2 MΩ. By increasing the external heating power, the TAEPG can produce a maximum voltage amplitude of 8 V. This work shows that the proposed concept has great potential for developing miniature heat‐driven power generators

Electroactive Materials for Applications in the Field of Wearable Technologies

The objective of this PhD thesis is to present the most performing EAP-based materials, technologies and devices developed by our lab (Ch.4, 5 and 6) also in collaboration with other research groups (Ch.1 and 2) for sensing, actuating and energy harvesting, with reference to their already demonstrated or potential applicability to electronic textiles and wearable technologies in general. Over the last decade great strides have been made in the field of wearable technology: thanks to new discoveries in materials science and miniaturized electronics, tissues and "smart" devices for monitoring vital parameters, rehabilitation and tele-assistance were born. However, a complete and self-powered system, able to exchange information with the external environment, to generate power using the usual movements of the human body (walking, work, sport) and to drive wearable devices, is not yet available on the market and it would find a considerable number of applications (monitoring physiological parameters for athletes and special forces officers in emergency situations, etc.). After a first survey of the state of the art concerning the so-called "smart materials” and technologies currently available for " wearable " activities, the work has developed on three major directives consisting in: energy generation and storage, sensing and actuation. Energy generation and storage. An experimental study, conducted mainly during the first year of PhD, has identified possible candidate materials (piezoelectric PVDF, electret PP) for the energy harvesting and subsequent generation of power from movement and gestures by exploiting the piezoelectric properties of selected materials. These materials have been either found on the market or processed in laboratory. In collaboration with the University of Pavia, a circuit for the storage of electric charges generated was made. Both the commercial materials and those obtained in laboratory were electromechanically tested and the generation of electric charges has been used to develop a demonstrator generator-LED embedded in a shoe. Sensing. During the first and second year, different sensor configurations of "dry" piezoelectric PVDF sensors were tested for the monitoring of vital parameters (heart and breathing rate). Such sensors, prepared in collaboration with the University of Lodz (TUL, Poland), our partners in the PROETEX European project (6th FP 2006-2009), were woven into fabrics to be easily integrated into clothing, and their response was studied. Signal intensities comparable to those of common 3M medical electrodes have been observed. A further development of these materials should be turn to reduce noise, while a computational study might deal with the signal filtering and elimination of motion artifacts. Along with the study of piezoelectric sensors mentioned above, during the third PhD year the production and characterization of dielectric elastomers for sensing applications (artificial skin) was developed too, in collaboration with the Genoa DIST (Dipartimento di Informatica, Sistemistica e Telematica). Such elastomers, characterised by high dielectric constants and restrained compressive elastic moduli, were develop in order to act as dielectric medium in piezocapacitive sensing devices. The obtained materials will be used as artificial skin in robotic systems. Actuating. In parallel with the two lines described above, the activity was concentrated, throughout the period of PhD, on the development of new dielectric elastomer actuators, to be used as high dielectric constant, low elastic modulus and, especially, low electric driving fields devices so that they can be used once inserted inside the clothing (simplified prototype actuators able to change the porosity / texture of different textiles were developed during the first year of activity for the FLEXIFUNBAR European project (6th FP 2005-2008)). The "blend" approach has been privileged over the "composite" approach, previously studied in the master thesis, and has led to promising results both from the applicative point of view, with an increase in the electromechanical performance, and on a fundamental level, for the implications emerging from the interaction between different phases in the study of dielectric response of partially heterogeneous systems. Electromechanical encouraging results were then obtained during the second year of activity with the development of silicone/polyurethane (SI/PU) blends prepared by appropriate volume fractions. Further improvements have also been achieved during the third year of doctoral studies, when it was introduced in the same mixtures a third component, the conjugated polymer poly-(3-hexylthiophene-2,5-dyil) (P3HT), already used by our group for its high polarizability in order to increase the dielectric constant of silicon actuators. The obtained samples, dielectrically, mechanically and electromechanically tested, showed that the conjugated polymer leads to a further significant increase in the electromechanical response of the blend only when added at levels of 1 wt%. This polymer shows, in fact, a certain influence on the microscopic distribution of the SI and PU "phases" in the blend. This effect is maximized for the 1 wt% concentration at which the presence of interfaces is maximized and thus a larger surface polarization, combined with the characteristic high polarizability of P3HT, leads to dielectric constant and strain further implementations. Similar increases in performance, compared to pure components, were also found in mixtures prepared using other polyurethanes and silicones adopting, when necessary, appropriate steps to modify the kinetics of reaction (addition of solvents). The results obtained with this "blending" approach are supported by the Intephase Theory (IT), recently introduced to complete the well known Effective Medium Theory (EMT) which, although applicable to a variety of particle composite structures, is not suitable to describe the behaviour of systems where the presence of an interphase between filler and matrix is significant. The EFT demonstrates that border regions, showing dielectric characteristics different from those of the starting components, can strongly influence the system performance. Through theoretical and experimental evidence, in fact, it is known that, while the inner parts of the matrix polymer chains are able to adopt a configuration that minimizes spontaneous conformational energy, at the interface they are linked or otherwise conditioned in their movements, giving rise to a region where the electrical properties (in some cases also thermal and mechanical) are different from those of both the pure material composing the mixture. During the third year, the production and characterization of elastomeric foams with dielectric properties suitable for sensing (artificial skin) and actuating applications were also developed. The electromechanical performance of these polyurethane-based foams, after appropriate polarization under very high electric fields (Corona poling), were compared with those of two commercial products, which were also subjected to corona poling. Studies have been conducted also on the life of the induced polarization produced by poling in the foam and on the influence of electric field exposure time on the final response of the material. The slightly positive results obtained in terms of increased dielectric constants and strains have opened a new line of activity that represents an innovation in the field of dielectric elastomers, that is the preparation of elastomeric foams with electret properties

A novel nonlinear dielectric elastomer generator for vibration energy harvesting

The recent development of small scale electronics working within an integrated wireless sensor network has led to the massive potential monitoring of human and structure health. Such devices however are often limited by their battery life, thus there is a great need for energy harvesting to increase the lifespan of these devices. This thesis presents a novel vibration energy harvester based upon dielectric elastomers. A number of numerical models and device setups are investigated, where the device was subjected to a wide variety of harmonic excitation conditions as well as random vibrations. The numerical model was developed through experimentation to determine the nonlinear material properties of a commonly used material, V HBTM 4910. This allowed the device to be compared favourably against other energy harvesters of similar volume. The proposed device is capable of producing a maximum energy density of 9.15J/kg at an excitation frequency of 35Hz. Although observations made regarding the influence of the material nonlinearity have predicted that with a slight increase in the material nonlinearity, the device could significantly increase its energy density to 4359J/kg which would occur at the extremely useful frequency of 3Hz. The creation of this numerical model to simulate an energy harvester also allowed the direct comparison between a promising new electrical scheme and a well developed conventional scheme. Specific investigations were carried out on the device size and orientation, which highlighted an extremely effective setup which can harvest energy from a wide range of excitation conditions and orientations

Electromechanical coupling behavior of dielectric elastomer transducers

Dielectric elastomer transducers with large deformation, high energy output, light weight and low cost have been drawing great interest from both the research and industry communities, and shown potential for versatile applications in biomimetics, dynamics, robotics and energy harvesting. However, in addition to multiple failure modes such as electrical breakdown, electromechanical instability, loss-of-tension and fatigue, the performance of dielectric elastomer transducers are also strongly influenced by the hyperelastic and viscoelastic properties of the material. Also, the interplay among these material properties and the failure modes is rather difficult to predict. Therefore, in order to provide guidelines for the optimal design of dielectric elastomer transducers, it is essential to first develop accurate and reliable models, and efficient numerical methods to investigate their performance. First, this thesis purposes a boundary-constraint method to eliminate the electromechanical instability of dielectric elastomer actuators under voltage-control loading condition and improve their actuation deformation. Second, based on the finite-deformation viscoelasticity model, the natural frequency tuning process of viscoelastic dielectric elastomer resonators is examined in this work. It is found that the tuned natural frequency is highly affected by the material viscoelasticity. Also, it is concluded that the electrical loading rate only influences the tunable frequency range and the safe operation voltage of the resonator, but not the tuned natural frequency when the applied voltage is within the safe range. Third, with the finite-deformation viscoelasticity model, the energy conversion efficiency of dielectric elastomer generators under equi-biaxial loading is also investigated in this work. Simulation results show that increasing the maximum stretch ratio and the rate of deformation, and choosing a proper bias voltage can lead to an improvement of the energy conversion efficiency. Furthermore, the fatigue life of dielectric elastomer devices under cyclic loading is explored in this work for the first time. Simulation results have demonstrated that the energy conversion efficiency of dielectric elastomer generators is compromised by their fatigue life. To tackle the critical challenges for the development and design of dielectric elastomers transducers, this research develops theoretical models and numerical methods that are able to capture the nonlinear electromechanical coupling, the material properties, the typical failure modes and different operating conditions of dielectric elastomer transducers. With more accurate and reliable modeling methods, this work is expected to provide a comprehensive understanding on the fundamentals and technologies of dielectric elastomer transducers and trigger more innovative and optimal design of such devices

Ferroelectrets: from material science to energy harvesting and sensor applications

The purpose of this thesis is to develop innovative ferroelectrets that can be used in energy harvesting devices as well as mechanical sensors. In the first stage, the focus lies on the application of ferroelectrets as energy harvesters. The inability to control the environment where the energy harvesters will be applied, requires the use of materials that can be utilized in harsh environment such as high temperature or humidity. Therefore, new ferroelectrets based on polymers with excellent electret properties, such as fluoroethylene propylene (FEP) are developed. Two types of ferroelectrets are considered, one optimized for the longitidunal piezoelectric effect and the other one optimized for the transverse piezoelectric effect in these materials. Hereby, new void structures are achieved through thermally fusing such films so that parallel tunnels (parallel-tunnel ferroelectrets) are formed between them, or by fusing round-section FEP tubes together so that they form a band or membrane. The FEP tube configuration is optimized based on a finite element model showing that implementing a single tube structure (25 mm × 1.5 mm) as the energy harvester exhibits the largest output power. By building the energy harvester and modeling it analytically, it is demonstrated that the generated power is highly dependent on parameters such as wall thickness, load resistance, and seismic mass. Utilizing a seismic mass of 80 g at resonance frequencies around 80 Hz and an input acceleration of 1 g (9.81 m s−2), output powers up to 300 μW are reached for a transducer with 25 μm thick walls. The parallel-tunnel ferroelectrets (40 mm × 10 mm) are characterized and used in an energy harvester device based on the transverse piezoelectric effect. The energy harvesting device is an air-spaced cantilever arrangement produced by additive manufacturing technique (3D-printing). The device is tested by exposing it to sinusoidal vibrations with an acceleration a, generated by a shaker. By placing the ferroelectret at a defined distance from the neutral axis of the cantilever beam and using a proper pre-stress of the ferroelectret, an output power exceeding 1000 μW at the resonance frequency of approximately 35 Hz is reached. This demonstrates a significant improvement of air-spaced vibrational energy harvesting with ferroelectrets and greatly exceeds previous performance data for ferroelectret energy harvester of maximal 230 μW. In the second stage of the dissertation, the focus is shifted to develop ferroelectrets for chosen applications such as force myography, ultrasonic transducer and smart insole. Hereby, new arrangements and manufacturing methods are investigated to build the ferroelectret sensors. Furthermore, and following the recent requirements of eco-friendlier sensors, ferroelectrets based on polylactic acid (PLA) are investigated. PLA is a biodegradable and bioabsorbable material derived from renewable plant sources, such as corn or potato starch, tapioca roots, and sugar canes. This work relays a promising new technique in the fabrication of ferroelectrets. The novel structure is achieved through sandwiching a 3D-printed grid of periodically spaced thermoplastic polyurethane (TPU) spacers and air channels between two 12.5 μm-thick FEP films. Due to the ultra-soft TPU sections, very high quasistatic (22.000 pC N−1) and dynamic (7500 pC N−1) d33-coefficients are achieved. The isothermal stability of the d33-coefficients showed a strong dependence on poling temperature. Furthermore, the thermally stimulated discharge currents revealed well-known instability of positive charge carriers in FEP, thereby offering the possibility of stabilization by high-temperature poling. A similar approach is taken by replacing the environmentally harmful FEP by PLA. Large piezoelectric d33-coefficients of up to 2850 pC N−1 are recorded directly after charging and stabilized at about 1500 pC N−1 after approximately 50 days under ambient environmental conditions. These ferroelectrets when used for force myography to detect the slightest muscle movement when moving a finger, resulted in signal shapes and magnitudes that can be clearly distinguished from each other using simple machine learning algorithms known as Support Vector Machine (SVM) with a classification accuracy of 89.5%. Following the new manufacturing route using 3D-printing, an insole is printed using pure polypropylene filament and consists of eight independent sensors, each with a piezoelectric d33 coefficient of approximately 2000 pC N−1. The active part of the insole is protected using a 3D-printed PLA cover that features eight defined embossments on the bottom part, which focus the force on the sensors and act as overload protection against excessive stress. In addition to determining the gait pattern, an accelerometer is implemented to measure kinematic parameters and validate the sensor output signals. The combination of the high sensitivity of the sensors and the kinematic movement of the foot, opens new perspectives regarding diagnosis possibilities through gait analysis. By 3D-printing a PLA backplate and using it in combination with a bulk PLA film, a new possibility to build ultrasonic transducers is presented. The ultrasonic transducer consists of three main components all made from PLA: the film presenting the vibrating plate, the printed backplate with well-defined groves, and the printed holder. The PLA film and the printed backplate build together the ferroelectret with artificial air voids. The printed holder clamps the film on the backplate and fixes the ferroelectret together. The resulting sound pressure is measured with a calibrated microphone (Type 4138, Bruel & Kjaer) at a distance of 30 cm. The biodegradable ultrasonic transducer exhibits a large bandwidth of approximately 45 kHz and fractional bandwidth of 70%. The resulting sound pressure at the resonance frequency can be increased from 98 dB up to 106 dB for driving voltages from 30 to 70 V. respectively. The obtained theoretical and experimental results are an excellent base for further optimizing ferroelectrets to be accepted in the field of energy harvesting and mechanical sensors, where flexibility and high sensitivity are mandatory for the applications

Wave Propagation in Viscoelastic Dielectric Elastomer Media

Dielectric elastomers (DEs) are capable of producing large deformation under electric stimuli, which makes them desirable materials for a variety of applications including biomimetics, dynamics, robotics, energy harvesting, and waveguide devices. In general, DEs possess intrinsic hyperelasticity and viscosity. Such material properties may significantly affect the dynamic performance of DE-based devices. The delicate interplay among electromechanical coupling, large deformation, material viscosity and dynamics makes modeling of the performance of DE-based devices more challenging. Therefore, in order to provide guidelines for the optimal design of DE waveguide devices, it is essential to develop appropriate and reliable models, and efficient numerical methods to examine their performance first. In this thesis, by integrating the state-of-art finite-deformation viscoelasticity theory into the framework of small-amplitude wave propagation superposed on a finitely deformed medium, the Rayleigh-Lamb wave propagation in a viscoelastic DE medium is investigated. Simulation results have demonstrated the effects of material viscosity, status of relaxation, external electric load, and mechanical pre-stretch on the dispersion behavior of the wave. For both pure elastic and viscoelastic DE media, waves with certain frequencies could be filtered by actively tuning electric loads. Moreover, some interesting findings conclude that the material viscoelasticity may cause some significant changes in the wave dispersion behavior. Therefore, incorporating the material viscosity in modeling DE waveguide is expected to provide more accurate prediction on their performance. This thesis will help to better understand the fundamentals of wave propagation in DE media and trigger more innovative and optimal design for DE waveguide applications

Advanced Materials and Technologies in Nanogenerators

This reprint discusses the various applications, new materials, and evolution in the field of nanogenerators. This lays the foundation for the popularization of their broad applications in energy science, environmental protection, wearable electronics, self-powered sensors, medical science, robotics, and artificial intelligence

Miniaturized Power Electronic Interfaces for Ultra-compact Electromechanical Systems

Advanced and ultra-compact electromechanical (EM) systems, such as kinetic energy harvesting and microrobotic systems are deemed as enabling solutions to provide efficient energy conversion. One of the most critical challenges in such systems is to develop tiny power electronic interfaces (PEIs) capable of addressing power conditioning between EM devices and energy storage units. This dissertation presents technologies and topological solutions toward fabricating miniaturized PEIs to efficiently regulate erratic power/voltage for kinetic energy harvesting and drive high-voltage actuators for microrobotic systems. High-frequency resonant-switching topologies are introduced as power stages of PEIs that allow small footprint of the circuit without suffering from switching losses. Two types of bridgeless resonant ac-dc converters are first introduced and developed to efficiently convert arbitrary input voltages into a regulated dc output voltage. The proposed topologies provide direct ac-dc power conversion with less number of components, in comparison to other resonant topologies. A 5-mm×6-mm, 100-mg, 2-MHz and 650-mW prototype is fabricated for validation of capability of converting very-low ac voltages into a relatively higher voltage. A resonant gate drive circuit is designed and utilized to further reduce gating losses under high-frequency switching and light-load condition. The closed-loop efficiency reaches higher than 70% across wide range of input voltages and output powers. In a multi-channel energy harvesting system, a multi-input bridgeless resonant ac-dc converter is developed to achieve ac-dc conversion, step up voltage and match optimal impedance. Alternating voltage of each energy harvesting channel is stepped up through the switching LC network and then rectified by a freewheeling diode. The optimal electrical impedance can be adjusted through resonance impedance matching and pulse-frequency-modulation (PFM) control. In addition, a six-input standalone prototype is fabricated to address power conditioning for a six-channel wind panel. Furthermore, the concepts of miniaturization are incorporated in the context of microrobots. In a mobile microrobotic system, conventional bulky power supplies and electronics used to drive electroactive polymer (EAP) actuators are not practical as on-board energy sources for microrobots. A bidirectional single-stage resonant dc-dc step-up converter is introduced and developed to efficiently drive high-voltage EAP actuators. The converter utilizes resonant capacitors and a coupled-inductor as a soft-switched LC network to step up low input voltages. The circuit is capable of generating explicit high-voltage actuation signals, with capability of recovering unused energy from EAP actuators. A 4-mm × 8-mm, 100-mg and 600-mW prototype has been designed and fabricated to drive an in-plane gap-closing electrostatic inchworm motor. Experimental validations have been carried out to verify the circuit’s ability to step up voltage from 2 V to 100 V and generate two 1-kHz, 100-V driving voltages at 2-nF capacitive loads