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DESIGN ANDFABRICATIONOF SELF-POWEREDMICRO-HARVESTERS
DESIGN ANDFABRICATIONOF SELF-POWEREDMICRO-HARVESTERSROTATING AND VIBRATINGMICRO-POWER SYSTEMS
C. T. PanNational Sun Yat-Sen University, Taiwan
Y. M. Hwang
National Sun Yat-Sen University, Taiwan
Liwei LinUniversity of California, Berkeley, USA
Ying-Chung Chen
National Sun Yat-Sen University, Taiwan
This edition first published 2014
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1 2014
Contents
About the Authors xi
Preface xiii
Acknowledgments xv
1 Introduction 11.1 Background 1
1.2 Energy Harvesters 2
1.2.1 Piezoelectric ZnO Energy Harvester 3
1.2.2 Vibrational Electromagnetic Generators 3
1.2.3 Rotary Electromagnetic Generators 4
1.2.4 NFES Piezoelectric PVDF Energy Harvester 4
1.3 Overview 5
2 Design and Fabrication of Flexible Piezoelectric GeneratorsBased on ZnO Thin Films 7
2.1 Introduction 7
2.2 Characterization and Theoretical Analysis of Flexible ZnO-BasedPiezoelectric Harvesters 10
2.2.1 Vibration Energy Conversion Model of Film-Based FlexiblePiezoelectric Energy Harvester 10
2.2.2 Piezoelectricity and Polarity Test of Piezoelectric ZnO ThinFilm 12
2.2.3 Optimal Thickness of PET Substrate 15
2.2.4 Model Solution of Cantilever Plate Equation 15
2.2.5 Vibration-Induced Electric Potential and Electric Power 18
2.2.6 Static Analysis to Calculate the Optimal Thickness of thePET Substrate 19
2.2.7 Model Analysis and Harmonic Analysis 21
2.2.8 Results of Model Analysis and Harmonic Analysis 23
vi Contents
2.3 The Fabrication of Flexible Piezoelectric ZnO Harvesterson PET Substrates 27
2.3.1 Bonding Process to Fabricate UV-Curable Resin LumpStructures on PET Substrates 27
2.3.2 Near-Field Electro-Spinning with StereolithographyTechnique to Directly Write 3D UV-Curable Resin Patternson PET Substrates 29
2.3.3 Sputtering of Al and ITO Conductive Thin Films on PETSubstrates 29
2.3.4 Deposition of Piezoelectric ZnO Thin Films by Using RFMagnetron Sputtering 31
2.3.5 Testing a Single Energy Harvester under Resonant andNon-Resonant Conditions 34
2.3.6 Application of ZnO/PET-Based Generator to Flash SignalLED Module 39
2.3.7 Design and Performance of a Broad Bandwidth EnergyHarvesting System 40
2.4 Fabrication and Performance of Flexible ZnO/SUS304-BasedPiezoelectric Generators 48
2.4.1 Deposition of Piezoelectric ZnO Thin Films on StainlessSteel Substrates 48
2.4.2 Single-Sided ZnO/SUS304-Based Flexible PiezoelectricGenerator 50
2.4.3 Double-Sided ZnO/SUS304-Based Flexible PiezoelectricGenerator 51
2.4.4 Characterization of ZnO/SUS304-Based FlexiblePiezoelectric Generators 52
2.4.5 Structural and Morphological Properties of PiezoelectricZnO Thin Films on Stainless Steel Substrates 54
2.4.6 Analysis of Adhesion of ZnO Thin Films on Stainless SteelSubstrates 56
2.4.7 Electrical Properties of Single-Sided ZnO/SUS304-BasedFlexible Piezoelectric Generator 59
2.4.8 Characterization of Double-Sided ZnO/SUS304-BasedFlexible Piezoelectric Generator: Analysis and Modificationof Back Surface of SUS304 61
2.4.9 Electrical Properties of Double-Sided ZnO/SUS304-BasedPiezoelectric Generator 63
2.5 Summary 66
References 67
Contents vii
3 Design and Fabrication of Vibration-Induced ElectromagneticMicrogenerators 71
3.1 Introduction 71
3.2 Comparisons between MCTG and SMTG 74
3.2.1 Magnetic Core-Type Generator (MCTG) 74
3.2.2 Sided Magnet-Type Generator (SMTG) 76
3.3 Analysis of Electromagnetic Vibration-Induced Microgenerators 76
3.3.1 Design of Electromagnetic Vibration-InducedMicrogenerators 77
3.3.2 Analysis Mode of the Microvibration Structure 78
3.3.3 Analysis Mode of Magnetic Field 81
3.3.4 Evaluation of Various Parameters of Power Output 84
3.4 Analytical Results and Discussion 88
3.4.1 Analysis of Bending Stress within the Supporting Beamof the Spiral Microspring 90
3.4.2 Finite Element Models for Magnetic Density Distribution 93
3.4.3 Power Output Evaluation 97
3.5 Fabrication of Microcoil for Microgenerator 103
3.5.1 Microspring and Induction Coil 103
3.5.2 Microspring and Magnet 105
3.6 Tests and Experiments 106
3.6.1 Measurement System 106
3.6.2 Measurement Results and Discussion 107
3.6.3 Comparison between Measured Results and Analytical Values 110
3.7 Conclusions 112
3.7.1 Analysis of Microgenerators and Vibration Mode andSimulation of the Magnetic Field 112
3.7.2 Fabrication of LTCC Microsensor 112
3.7.3 Measurement and Analysis Results 113
3.8 Summary 113
References 114
4 Design and Fabrication of Rotary ElectromagneticMicrogenerator 117
4.1 Introduction 117
4.1.1 Piezoelectric, Thermoelectric, and Electrostatic Generators 119
4.1.2 Vibrational Electromagnetic Generators 119
4.1.3 Rotary Electromagnetic Generators 120
4.1.4 Generator Processes 121
4.1.5 Lithographie Galvanoformung Abformung Process 122
4.1.6 Winding Processes 123
4.1.7 LTCC 123
viii Contents
4.1.8 Printed Circuit Board Processes 124
4.1.9 Finite-Element Simulation and Analytical Solutions 126
4.2 Case 1: Winding Generator 126
4.2.1 Design 127
4.2.2 Analytical Formulation 132
4.2.3 Simulation 134
4.2.4 Fabrication Process 138
4.2.5 Results and Discussion (1) 139
4.2.6 Results and Discussion (2) 142
4.3 Case 2: LTCC Generator 146
4.3.1 Simulation 147
4.3.2 Analytical Theorem of Microgenerator Electromagnetism 148
4.3.3 Simplification 152
4.3.4 Analysis of Vector Magnetic Potential 153
4.3.5 Analytical Solutions for Power Generation 154
4.4 Fabrication 157
4.4.1 LTCC Process 157
4.4.2 Magnet Process 159
4.4.3 Measurement Set-up 160
4.5 Results and Discussion 162
4.5.1 Design 162
4.5.2 Analytical Solutions 168
4.5.3 Fabrication 170
References 178
5 Design and Fabrication of Electrospun PVDF Piezo-EnergyHarvesters 183
5.1 Introduction 183
5.2 Fundamentals of Electrospinning Technology 187
5.2.1 Introduction to Electrospinning 187
5.2.2 Alignment and Assembly of Nanofibers 190
5.3 Near-Field Electrospinning 191
5.3.1 Introduction and Background 191
5.3.2 Principles of Operation 194
5.3.3 Process and Experiment 196
5.3.4 Summary 202
5.4 Continuous NFES 202
5.4.1 Introduction and Background 202
5.4.2 Principles of Operation 202
5.4.3 Controllability and Continuity 205
5.4.4 Process Characterization 208
5.4.5 Summary 211
Contents ix
5.5 Direct-Write Piezoelectric Nanogenerator 2115.5.1 Introduction and Background 2115.5.2 Polyvinylidene Fluoride 2125.5.3 Theoretical Studies for Realization of Electrospun PVDF
Nanofibers 2135.5.4 Electrospinning of PVDF Nanofibers 2165.5.5 Detailed Discussion of Process Parameters 2195.5.6 Experimental Realization of PVDF Nanogenerator 2235.5.7 Summary 241
5.6 Materials, Structure, and Operation of Nanogenerator with FutureProspects 2415.6.1 Material and Structural Characteristics 2415.6.2 Operation of Nanogenerator 2435.6.3 Summary and Future Prospects 248
5.7 Case Study: Large-Array Electrospun PVDF Nanogenerators on aFlexible Substrate 2485.7.1 Introduction and Background 2485.7.2 Working Principle 2495.7.3 Device Fabrication 2495.7.4 Experimental Results 2515.7.5 Summary 252
5.8 Conclusion 2535.8.1 Near-Field Electrospinning 2535.8.2 Continuous Near-Field Electrospinning 2545.8.3 Direct-Write Piezoelectric PVDF 254
5.9 Future Directions 2555.9.1 NFES Integrated Nanofiber Sensors 2555.9.2 NFES One-Dimensional Sub-Wavelength Waveguide 2565.9.3 NFES Biological Applications 2575.9.4 Direct-Write Piezoelectric PVDF Nanogenerators 258References 258
Index 265
About the Authors
Dr. C.T. Pan was born in Nauto, Taiwan, in 1969. Hereceived master and doctoral engineering degrees in 1993and 1998 respectively, from the Power Mechanical Engi-neering Department of National Tsing Hua University inHsinchu, Taiwan. He was a researcher in the field of lasermachining polymer at TU Berlin (IWF) in Germany from1997 to 1998 and a researcher in the MEMS Divisionof MIRL/ITRI, Hsinchu, Taiwan from 1998 to 2003. Hejoined National Sun Yat-Sen University, Kaohsiung, Tai-wan, as an Assistant Professor in 2003, then earned his
associate professorship and full professorship in 2005 and 2008, respectively. He wonthe Outstanding Professor Award (2009–2013) from National Sun Yat-Sen Univer-sity. From June 2009 to June 2010, he was a visiting professor at the department ofME in UC Berkeley. His current research interests focus on MEMS, nanofabrication,micro-scale energy, and LIGA process.
Dr. Y.M. Hwang was born in Chanhwa, Taiwan, Repub-lic of China in 1958. He received his Bachelor’s (1981)and Master’s (1983) degrees in power mechanical engi-neering from National Tsing Hua University in Hsinchu,Taiwan. He earned his Doctor’s degree (1990) in industrialmechanical engineering from Tokyo University in Japan.He has been a professor at the Department of Mechanicaland Electro-Mechanical Engineering (MEME), NationalSun Yat-Sen University (NSYSU), Kaohsiung, Taiwan,since 1996. He has served as the department chair
(2002–2005) ofMEME. His research interests have been in the area of metal forming,machine design and mechanics. He won the Best Paper Award (1992) and Outstand-ing Engineering Professor Award (2007) from the Chinese Society of Mechanical
xii About the Authors
Engineers in Taiwan. He earned the Fellow title from Japan Society for Technologyof Plasticity (JSTP), Japan (2012) and Distinguished Professor of NSYSU (2012).
Dr. Liwei Lin is a Professor at the Department of Mechani-cal Engineering at the University of California at Berkeley,and Co-Director of the Berkeley Sensor and Actuator Cen-ter. He received his B.S. degree (1986) in Power Mechan-ical Engineering from the National Tsing Hua University,Taiwan, and M.S. (1991) and Ph.D. (1993) degrees fromUC Berkeley in Mechanical Engineering. After graduation,Professor Lin held the position of Senior Research Scientistat BEI Electronics Inc., Associate Professor at the NationalTaiwan University, Taiwan and Assistant Professor at the
University of Michigan, Ann Arbor, USA before joining the faculty at UC Berkeleyin 1999. His research interests and activities include MEMS, NEMS, Nanotechnol-ogy, design and manufacturing of microsensors and microactuators, development ofmicromachining processes by silicon surface/bulk micromachining, micro moldingprocess, and mechanical issues in MEMS such as heat transfer, solid/fluid mechan-ics and dynamics. Professor Lin is the co-inventor of 16 US patents in MEMS andhas co-authored more than 130 journal and 200 refereed conference papers. He hassupervised 29 Ph.D. and 30M.S. students.
Dr. Ying-Chung Chen was born in Tainan, Taiwan,R.O.C., on 4 November 1956. He received the M.S.and Ph.D. degree in electrical engineering from NationalCheng Kung University, Tainan, Taiwan, in 1981 and 1985respectively. Since 1983, he has been at National SunYat-Sen University (NSYSU), Kaohsiung, Taiwan, wherehe is a professor of electrical engineering. Previously, heserved as the department chair (2006–2009) of EE, theDean of the College of Engineering (2011–2014) andwon the Distinguished Professor Award and Outstanding
Professor Award (2009–2013) from National Sun Yat-Sen University. His currentresearch interests are in the areas of electronic devices, surface acoustic wave devices,thin-film technology, and electronic ceramics. He is a member of the Chinese Societyfor Materials Science and a registered electrical engineer in Taiwan.
Preface
Energy harvesting is known as power harvesting or energy scavenging to storeand capture ambient energy which is natural, self-regenerating, or renewable.For example, ambient energy may include wind, solar, hydro, geothermal, andtide. Energy harvesting takes advantage of these sources to provide energy that isrenewable and eco-friendly as compared with energy derived from fossil fuels. Asan enabling technology from ambient vibrational energy sources, energy harvestingcould find potential applications in low-power devices such as sensors, actuators, andelectronics with ecological advantages in reducing chemical wastes from batteriesand is maintenance-free.This book covers recent advances in energy harvesting using different transduction
mechanisms, including mechanics, semiconductor process, and electrical circuitry.The dissemination of this technology is important for the industry but there are onlya limited number of introductory books or handbooks in the global community.Compared to other semiconductor disciplines, such as MEMS and LIGA, the gap inadvanced knowledge of energy harvesting has yet to be filled. This book partially fillsthis gap by documenting the latest and most frequently cited research results of a fewkey energy harvesting processes. Scientists and researchers from various disciplineshave contributed heavily to the related energy harvesting literature. Our hope withthe current book is to provide reliable and practical techniques for analytical modelsof piezoelectric and electromagnetic energy harvesters and their relevant phenomena.This book presents a state-of-the-art understanding of diverse aspects of energyharvesting with a focus on broadband energy conversion, as well as new concepts indesigns and fabrication processes.The book is arranged in five chapters to describe the research and development
of energy harvesters. Chapter 1 introduces the background of energy harvesting,as well as the development of electromagnetic and piezoelectric energy harvesters.Chapter 2 describes the development of the single ZnO energy harvesters, broadbandwidth vibrational energy harvesting systems, and double-sided piezoelectricenergy harvesters. The design and fabrication of vibration-induced electromagneticgenerators is presented in Chapter 3. Chapter 4 focuses on the design, fabrication,
xiv Preface
test, and application of in-plane rotary electromagnetic micro-generator. Chapter 5describes the design and fabrication of PVDF electrospun piezo-energy harvesterswith interdigital electrodes, details of the fabrication process involved in flexiblepiezoelectric composites and the demonstration of energy harvesters from NFES(Near-Field Electrospinning) PVDF fibers.
C. T. Pan, Ying-Chung Chen, Y. M. Hwang, and Liwei LinSeptember 2013
Acknowledgments
Although four authors worked on this text, it would not have been written withoutsupport from various sources. We express our thanks to Z. H. Liu and Y. C. CandiceChen, undergraduate students who helped with edits and figures.
1Introduction
1.1 Background
Various approaches in developing clean and green energy systems have been exten-sively explored in the past few years. Much research has focused on properties suchas small size, light weight, high density in power, economically efficient, and envi-ronmentally friendly. Possible sources of ambient energy include light energy, windpower, kinetic energy, and thermal energy.In many applications, researchers have been investigating the use of microactuators
and microsensors in MEMS (microelectromechanical systems) where an independentpower source is needed. A possible solution is to design the power supply at the samescale as actuators, sensors, and electronics. The conventional solution is to use bat-teries, but batteries can be undesirable for many reasons: they tend to be quite bulky,contain a finite amount of energy, have a limited life, and contain chemicals that couldcause a hazard. Vibration energy harvesters and energy scavengers recover mechan-ical energy from their surrounding environment and convert it into usable electricityas sustainable self-sufficient power sources to drive micro-to milli-watt-scale poweredinstruments independently.Mechanical kinetic energy is ubiquitous in real environments. The conversion of
ambient mechanical vibration to electrical energy is considered one of the likelymethods of powering a wireless sensor, without hazardous byproducts related to
Design and Fabrication of Self-Powered Micro-Harvesters: Rotating and Vibrating Micro-Power Systems, First Edition.C. T. Pan, Y. M. Hwang, Liwei Lin and Ying-Chung Chen.© 2014 John Wiley & Sons Singapore Pte Ltd. Published 2014 by John Wiley & Sons Singapore Pte Ltd.
2 Design and Fabrication of Self-Powered Micro-Harvesters
power generation. The power source does not need to be replaced and fuel does notneed to be replenished from time to time like batteries.There are two types of harvesting systems of mechanical kinetic energy under
investigation in this book, as follows.
1. Electromagnetic microgenerator: based on Faraday’s law to harvest energy.2. Piezoelectric energy harvester: using piezoelectric material to convert strain
energy into electricity.
Microsystems have to be self-powered, so efficient energy scavenging is crucial.The self-powered microsystems are designed to avoid the replacement of energy cellsand miniature sensing devices. Vibration-based energy harvesting is a process of cap-turing ambient kinetic energy and converting it into usable electricity. The growingdemand of cell phone devices such as miniature wireless sensor networks and therecent advent of extremely low-power controlled circuit and MEMS devices makesuch renewable power sources very attractive.In addition, the energy harvesting process must be compatible with environmen-
tal vibrations such as running machines and human body movement. However, thewide range of environmental vibration frequencies means the harvester operates atlow efficiency when deployed in a stochastic surrounding vibration.
1.2 Energy Harvesters
Microtransducers have been of the focus of much attention since the rapiddevelopment of MEMS technology based on micro-wireless sensors and actuatorswith different applications in the communication, military, and biomedical industries.Research in the field of kinetic energy harvesters using piezoelectric materials andmagnetic modules that harvest environmentally random energy, such as irregularvibrations, light airflow, and human activity, have attracted considerable attention.This type of mechanical energy available in our environment has a wide spectrum offrequencies and time-dependent amplitudes, however. The design of high-sensitivityenergy harvesters is therefore crucial, including electromechanical conversion ofmaterials, efficient energy transfer of mechanical structures, and controlled circuit toobtain good performance.In this book, electromagnetic transduction mechanisms include vibrational and
rotary power generators. Microgenerators to harvest vibration-induced and rotary-induced energy such as human motion are an attractive study topic. According toFaraday’s law, a permanent magnet moving relative to a coil can induce an EMF(electromotive force). Conventional microcoil fabrication processes include LIGA(Lithographie, Galvanoformung, Abformung), LIGA-like, filament winding, printedcircuit board etching, and LTCC (low-temperature co-fired ceramic). LIGA andLIGA-like processes consist of expensive methods such as thick photoresistantexposure, thin film, and a high-aspect-ratio electroplating process. The latest research
Introduction 3
in the field of LTCC technology applications for the field of microgenerators areincluded in this book.Much research has focused on electromechanical coupling effects of various
ambient energy sources using PZT (lead zirconate titanate), ZnO (zinc oxide), PVDF(polyvinylidene fluoride), and AlN (aluminum nitride) -based piezoelectric thin films.The use of piezoelectric PZT devices has been restricted because of environmentalrelated issues. Using ZnO and PVDF piezoelectric materials is attractive in terms oftheir low cost, high resistance to fatigue, and environmentally friendly applications.Significantly, the deposition process of sputtering ZnO thin films with high c-axispreferred orientation and electrospun PVDF fibers with high piezoelectric 𝛽-phasecrystallization are controlled at room temperature; they therefore do not requirepost-annealed and electrical repoling processes to obtain an excellent piezoelectricity.The sputtering ZnO thin films and well-aligned electrospun piezoelectric PVDFfibers can be directly deposited on flexible substrates at room temperature, such asPET (polyethylene terephthalate) and PI (polyimide). Utilizing ZnO and PVDF asthe main elements of piezoelectric materials has the following additional advantages:
1. they have high piezoelectric coupling coefficients,2. they do not cause environmental pollution; and3. the deposition process is controlled at room temperature, which is suitable for all
flexible substrates.
1.2.1 Piezoelectric ZnO Energy Harvester
In the fabrication process, the piezoelectric ZnO thin film is deposited between theelectrodes. ZnO thin film with excellent distribution of crystal grains with a highdiffraction peak (002) was deposited using radio frequency (RF) magnetron sputter-ing. To fabricate the selectively deposited UV-curable resin lump structures as a proofmass, electrospinning techniques were used to construct various patterns on the backof the PET-based composite plate to control the resonant frequencies. PiezoelectricZnO thin-film harvesters can therefore harvest energy in small working frequencies.This ZnO piezoelectric energy harvester consists of a piezoelectric laminated can-tilever and a mass, and harvests vibrational energy. Through the resonant inertialoscillation of the cantilever, the power of a piezoelectric generator is generated bythe inherent piezoelectric effect. The application of stress on a piezoelectric materialgenerates a corresponding electric charge. The mechanical energy of a piezoelectricenergy harvester is converted into electrical energy. A piezoelectric energy harvestermust operate at the resonance frequency to harvest higher power; the friction anddamping reduce its output. These obstacles are addressed in Chapter 2.
1.2.2 Vibrational Electromagnetic Generators
Microelectromagnetic generators demonstrate the advantages of power output andintegrated circuit (IC) package processing, thereby saving energy and improving
4 Design and Fabrication of Self-Powered Micro-Harvesters
generator efficiency. An electromagnetic generator converts mechanical energy into
electrical energy. According to Faraday’s law, a permanent magnet moving relative to
a coil creates an EMF. Electromagnetic transduction mechanisms include vibrational
and rotational power generators. When a permanent magnet installed on a mass
of a vibrational electromagnetic generator moves relative to a coil, it induces an
EMF. The magnetic microgenerator includes an LTCC microinductor, a magnetic
core element, a magnet module, an oscillation mass, and an attached adjustable
resistor. The magnet module is composed of two NdFeB magnets of thickness 2mm
attached on the upper and lower surfaces of the LTCCmicroinductor. NdFeBmagnets
are commercially available and contribute towards cost-reduced mass production.
A cylindrical hole is located at its center to accommodate the microspring oscillation.
The magnetic elements also provide the supporting frame for mechanical oscillation.
The power of the vibrational generator is limited by natural resonance frequencies
and the operation of the vibrational source.
1.2.3 Rotary Electromagnetic Generators
The recycling of mechanical energy has been the focus of many previous stud-
ies. Rotary electromagnetic generators show potential for the development of
self-sufficient energy sources. Machines using planar rotary permanent magnets
have been developed for various applications due to their ability to eliminate field
excitation losses. The frequencies of rotary electromagnetic generators do not have
specific restrictions, making them environmentally friendly. Furthermore, as long
as there is movement between the coil and the magnet, energy can be continuously
created, ensuring performance in machines of all sizes. The microgenerator com-
prises a multilayer planar LTCC silver (Ag) or copper (Cu) microcoil and multipole
hard magnets of neodymium/iron/boron (Nd/Fe/B). Finite-element simulations
and analytical solutions are performed to predict the induced voltage. Various
configurations of planar microcoils were investigated: sector-shaped, circular, and
square microcoils. Recently, hub dynamos have become popular for powering bicycle
lights. Rotary electromagnetic generators are suitable for use in bicycle dynamos.
The principle of a rotary electromagnetic generator, which can be easily installed due
to its rotary motion, is similar to that of a vibrational electromagnetic generator, and
is not limited to use at the resonance frequency.
1.2.4 NFES Piezoelectric PVDF Energy Harvester
Other research efforts have focused on energy harvest and actuation properties of
the modified NFES (near-field electrospinning) piezoelectric PVDF fibers. A direct-
write electrospinning technique by means of NFES was developed to produce the
controllable nanofiber deposition. Compared to the conventional electrospinning
process, it shows that decreasing electrical field in continuous NFES results in
Introduction 5
smaller line-width fiber deposition. Piezoelectric fibers have the characteristics ofelectromechanical energy conversion and offer the advantages of nanoscale size.Piezoelectric fibers can harvest relatively low-frequency kinetic energy, such asbody movement and muscle stretching. Researchers have published many studies onpiezoelectric fibers, wires, and rods. A direct-write electrospinning techniques usingNFES was developed to achieve controllable fiber deposition for various materials.Unlike the conventional electrospinning process, NFES only needs a small electricfield to produce continuous fibers with fine diameters. The fabrication processof PVDF fibers in this study is compatible with a flexible substrate and does notinvolve any complex processes. PVDF is a potential piezoelectric polymer becauseof its high flexibility, biocompatibility, and low cost. These features make PVDFattractive for energy conversion applications involving microelectromechanicaldevices, electromechanical actuators, and energy harvesters.
1.3 Overview
The book is arranged in five chapters to describe the research and development ofenergy harvesters. Chapter 1 describes the background of energy harvesting, the devel-opment of electromagnetic generators and piezoelectric materials for flexible energyharvesters, and the use of electrospun piezoelectric nanofibers.Chapter 2 describes the design and fabrication of flexible piezoelectric generators
based on ZnO thin films. The development of a single ZnO broad-bandwidthvibrational energy harvesting system and double-sided piezoelectric energy harvesterare described, including experiments, measurements, and interfacial adhesion ofPET-based substrate.In Chapter 3 the design and fabrication of vibration-induced electromagnetic micro-
generators are discussed. The manufacturing technology of LTCC applied to theconstruction of multilayer silver microinduction coils and spiral ceramic microspringin microgeneration is also introduced. Compared to silicon-based processes, ceramicand silver are not easily broken or fractured after sintering. Microgeneration hashigh-current output because of the low resistance of the multilayer structures.Chapter 4 focuses on the design, fabrication, testing, and application of an in-plane
rotary electromagnetic microgenerator to obtain a high-power output. Finite-elementsimulations have been performed to observe electromagnetic information. The studyalso establishes analytical solutions for the microgenerator to predict the induced volt-age for three different configurations of planar microcoils.Chapter 5 describes the design and fabrication of a PVDF electrospun piezo energy
harvester with interdigital electrode and the fabrication processes of flexible piezo-electric composites and NFES PVDF fibers. Furthermore, adding modified CNTs(carbon nanotubes) to reinforce PVDF nanofibers can enhance the crystallinity of the𝛽 phase and increase its ability to store charges. NFES can potentially be reduced to thenanometer scale and form any shape for various sensing and actuation applications.
2Design and Fabrication of FlexiblePiezoelectric Generators Basedon ZnO Thin Films
2.1 Introduction
The study of energy has recently been gaining more attention. Various clean, green,and renewable energy systems have been extensively developed and explored tohelp address the problem of energy shortage. Alternative energy systems have beenactively researched worldwide while regarding considerations such as compact size,light weight, high density in power, economy, safety, and environmental concerns.The numerous renewable sources of green energy include hydroelectric [1], wind [2],oceanic [3], solar [4–6], and vibration [7, 8]. However, environmental constraintsgenerally limit recycling power applications. For instance, hydroelectric power plantsare always built with turbines, such as the Pelton turbine or the chain turbine.
Solar power is the most frequently used energy source. However, applying solarpower is restricted by day-and-night limitations [9]. The development of green energy
Design and Fabrication of Self-Powered Micro-Harvesters: Rotating and Vibrating Micro-Power Systems, First Edition.C. T. Pan, Y. M. Hwang, Liwei Lin and Ying-Chung Chen.© 2014 John Wiley & Sons Singapore Pte Ltd. Published 2014 by John Wiley & Sons Singapore Pte Ltd.
8 Design and Fabrication of Self-Powered Micro-Harvesters
should therefore address the disadvantages of solar power. In many applications, forexample, solar power has been applied in microactuators and microsensors in micro-electromechanical systems (MEMS) that require an independent and embedded powersource without an outside connection. A possible solution is to design the power sup-ply to the same scale as actuators, sensors, and electronics [10]. The conventionalsolution involves using batteries however, which can be undesirable for many rea-sons: they tend to be relatively bulky, contain a finite amount of energy, have a limitedlife, and contain hazardous chemicals [11].
In many applications, microsystems must be self-powered; efficient energy scav-enging is therefore crucial. Mechanical vibration is ubiquitous in real environments.Converting ambient mechanical vibration to electrical energy is considered one of thelikely methods for powering wireless sensors without hazardous byproducts relatedto power generation. In addition, the power source of wireless sensors does not needto be replaced and the fuel does not require periodical replenishment as for batteries[12]. Piezoelectric materials have electrical-mechanical coupling effects, and are theleading candidates for converting mechanical energy into electricity [13]. White et al.[14] developed an inertial piezoelectric generator that uses 7-μm-thick film of leadzirconate titanate (PZT) with a 100-μm-thick steel substrate. The device produces anoutput of 2.1 μW from vibrations in the environment. Roundy et al. [15] improved thegeometry of the scavenger’s piezoelectric bimorph. Using the same volume of PZTand a trapezoidal geometry can supply more than twice the energy of rectangulargeometry. Fang et al. [16] used a generator structure of a nickel-metal composite can-tilever. At a resonant frequency of approximately 608 Hz, 608 mV of AC voltage valuewith a 2.16 μW power level was obtained. Minazara et al. [17] used a mechanicallyexcited unimorph piezoelectric membrane transducer. A power output of 0.65 mWwas generated at the resonance frequency (1.71 kHz) across a 5.6 kΩ optimal resistor.
Several harvesting systems of resonant mechanical vibration energy are currentlyunder investigation. Three considerations potentially affect transducer technologyselection:
• Electromagnetic: a coil attached to a mass that vibrates through a magnetic fieldto induce voltage according to Faraday’s law [18–23].
• Piezoelectric: using piezoelectric material to convert strain energy into electricity[9–18, 24–26].
• Electrostatic: inducing capacitor voltage through the movement of a mass that hasits permanent charges electrically arranged [15, 27].
Dutoit et al. [27] compared these three generators according to energy density. Ofthese three mechanisms, the electrostatic transducer has the lowest energy storagedensity and conversion efficiency. Further, electromagnetic generators usually require
Design and Fabrication of Flexible Piezoelectric Generators Based on ZnO Thin Films 9
a large operating space, complex instrumentation, and continual management offacilities, all of which are expensive. Many researchers have therefore examinedmicropiezoelectric power generators and their fabrication from mostly piezoelectricmaterials (Pb,Zr)TiO3 (PZT), aluminum nitride (AlN), and zinc oxide (ZnO).Table 2.1 lists the general characteristics and properties of PZT, AlN, and ZnOfilms [28].
The production of PZT piezoelectric material causes considerable environmentalpollution. This study adopted ZnO as a piezoelectric thin film for three main reasons:(1) it has relatively high piezoelectric coefficients compared to piezoelectric AlN [24];(2) producing ZnO thin film does not cause substantial environmental pollution com-pared to piezoelectric PZT thin film; and (3) it is a non-ferroelectric material thatrequires no poling or post-deposition annealing. These factors make ZnO piezoelec-tric material useful for elucidating the application of piezoelectric energy harvester inenvironments.
This chapter is organized in four sections to describe the research and developmentalefforts. Section 2.1 briefly introduces the background of green and renewable energysystems, the development of generator technologies, and the materials of piezoelectricthin films for piezoelectric generators.
Section 2.2 focuses on the theoretical analysis and simulations of flexible piezo-electric generators based on ZnO thin films on polyethylene terephthalate (PET)substrates. A piezoelectric cantilever plate was designed and simulated usingcommercial software ANSYS FEA (finite element analysis) to determine the optimalthickness of the PET substrate, internal stress distribution, operation frequency, andelectric potential.
Section 2.3 describes the relationship between the model solution of piezoelectriccantilever plate equation, vibration-induced electric potential, and electric power.Section 2.4 discusses the development of high-performance piezoelectric generatorsusing single-sided and double-sided ZnO thin films on a flexible stainless steelsubstrate (SUS304). Relevant fabrication processes, experiments, measurements, andpiezoelectric responses are addressed.
Table 2.1 General characteristics and properties of piezoelectric
materials [28]
Mass density
(kg m−3)
Electromechanical coupling
coefficient kt2 (%)
Dielectric
constant 𝜀r
PZT 1550 20.25 309.8
ZnO 5680 7.5 8.85
AlN 3270 6 9.14
10 Design and Fabrication of Self-Powered Micro-Harvesters
2.2 Characterization and Theoretical Analysis of FlexibleZnO-Based Piezoelectric Harvesters
2.2.1 Vibration Energy Conversion Model of Film-Based FlexiblePiezoelectric Energy Harvester
Figure 2.1a shows the composite cantilever plate used as a harvester with a rectifyingcircuit (B) and a capacitor (C) as the energy storage module. Ambient vibration ampli-tude on the anchor side induced vertical deflection at the front end of the cantileverplate. The piezoelectric layer on the composite structure deformed when the cantileverplate oscillated with mass. The bending results in a mechanical strain distributedalong the cantilever plate, which is then converted to alternating voltage through thetransverse-mode (d31) piezoelectric effect (the generated strain is perpendicular tothe electric field, which forms the d31 mode of the piezoelectric element), and over-comes the forward bias of the rectifying diodes. Subsequently, the full-wave rectifiedpotential can quickly charge the storage module.
Figure 2.1b shows a schematic model of vibration energy conversion. The mechan-ical behavior of a vibrating piezoelectric harvester can be modeled using a seismicmass m bonded on a spring k corresponding to the stiffness and a damping factor bm,which in turns corresponds to the mechanical losses of the structure. A displacementy(t) is used to vibrate the rigid house, resulting in differential movement x(t) betweenthe mass and the house; the relative displacement of the mass is therefore Uz(t).
This section reports the analysis of a new self-powered flexible Cu/ZnO/Al/PET-based piezoelectric energy harvester with a storage module. To verify the optimalthickness of the PET substrate evaluated by performing finite element model (FEM)analysis, an accurate analytical formula was developed. The optimal thickness ofthe substrate was calculated. Thus, the tensile and compressive strains do not occur
x(t)
Anchorz
x
Ambient vibration
B
Strain
+C R_
~V
out~
Direction of aligned dipoles
+-
Flexible composite cantilever(PET/Al/ZnO/Cu)
+-
+-
+-
+-
+-
+-
+-
+-
ZnO thin film
(a)
m
k
(b)
m, k, b m
Rigid house
Uz(t)
y(t)
bm Uz
Figure 2.1 Generic vibration energy conversion model: (a) schematic model of flexible
piezoelectric cantilever plate according to d31 conversion mode and (b) first-order model of a
resonant system [28]
Design and Fabrication of Flexible Piezoelectric Generators Based on ZnO Thin Films 11
on the piezoelectric ZnO layer simultaneously. Static analysis, modal analysis, andharmonic response analysis were performed to determine a suitable thickness ofthe PET substrate, structural modal parameters, frequency response functions, andelectric potential.
In the fabrication process, the piezoelectric ZnO thin film was deposited betweenthe Al and Cu electrodes. The Al thin film used as the bottom electrode was depositedon the PET substrate by using a sputtering deposition method because of its superioradhesion to the PET substrate and lattice constant matching with ZnO. The ZnO thinfilm with superior compactness and more even distribution of crystal grains with ahigh (002) diffraction peak at 2𝜃 = 34.44∘ and full-width at half-maximum (FWHM)of 0.422∘ was deposited using RF magnetron sputtering. To fabricate the selectivelydeposited ultraviolet- (UV)-curable resin lump structures as a proof mass, electrospin-ning and stereolithography techniques were used to construct various patterns on theback of the PET-based piezoelectric composite plate, which can adjust the resonantfrequencies of the composite cantilever plate to achieve larger deflection. Piezoelec-tric ZnO thin-film harvesters can therefore harvest energy in small work spaces. Inaddition, they demonstrate long-term stability, have a low operating frequency withlow-level excitation and a high deflection response compared to traditional PZT-basedbulk ceramic vibrational structures, and can be used at low cost in clean energyapplications. The schematic layout of the self-powered storage system and the designparameters of the flexible piezoelectric composite plate are shown in Figure 2.2.
In this study, a broad bandwidth harvesting system with a wide bandwidth of100–400 Hz was designed and fabricated. Four individual, flexible PET-basedZnO piezoelectric harvesters were assembled in parallel with a rectifying circuit
SwitchResistance
Flash LED module
z
x
y
Flexible power harvester
Rectifying and
wave filter module
Clamping apparatus
Electric wires
Storage module
Lump structure
Figure 2.2 Schematic illustration of self-powered storage system and flexible composite
structure [28]
