DESIGN AND FABRICATION OF SELF-POWERED MICRO-HARVESTERS

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3 DESIGN AND FABRICATION OF SELF-POWERED MICRO-HARVESTERS

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5 DESIGN AND FABRICATION OF SELF-POWERED MICRO-HARVESTERS ROTATING AND VIBRATING MICRO-POWER SYSTEMS C. T. Pan National Sun Yat-Sen University, Taiwan Y. M. Hwang National Sun Yat-Sen University, Taiwan Liwei Lin University of California, Berkeley, USA Ying-Chung Chen National Sun Yat-Sen University, Taiwan

6 This edition first published John Wiley & Sons Singapore Pte. Ltd. Registered office John Wiley & Sons Singapore Pte. Ltd., 1 Fusionopolis Walk, #07-01 Solaris South Tower, Singapore For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as expressly permitted by law, without either the prior written permission of the Publisher, or authorization through payment of the appropriate photocopy fee to the Copyright Clearance Center. Requests for permission should be addressed to the Publisher, John Wiley & Sons Singapore Pte. Ltd., 1 Fusionopolis Walk, #07-01 Solaris South Tower, Singapore , tel: , fax: , enquiry@wiley.com. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The Publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the Publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. MATLAB is a trademark of The MathWorks, Inc. and is used with permission. The MathWorks does not warrant the accuracy of the text or exercises in this book. This book s use or discussion of MATLAB software or related products does not constitute endorsement or sponsorship by The MathWorks of a particular pedagogical approach or particular use of the MATLAB software. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. It is sold on the understanding that the publisher is not engaged in rendering professional services and neither the publisher nor the author shall be liable for damages arising herefrom. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Library of Congress Cataloging-in-Publication Data applied for. A catalogue record for this book is available from the British Library. ISBN: Typeset in 11/13pt Times by Laserwords Private Limited, Chennai, India

7 Contents About the Authors Preface Acknowledgments xi xiii xv 1 Introduction Background Energy Harvesters Piezoelectric ZnO Energy Harvester Vibrational Electromagnetic Generators Rotary Electromagnetic Generators NFES Piezoelectric PVDF Energy Harvester Overview 5 2 Design and Fabrication of Flexible Piezoelectric Generators Based on ZnO Thin Films Introduction Characterization and Theoretical Analysis of Flexible ZnO-Based Piezoelectric Harvesters Vibration Energy Conversion Model of Film-Based Flexible Piezoelectric Energy Harvester Piezoelectricity and Polarity Test of Piezoelectric ZnO Thin Film Optimal Thickness of PET Substrate Model Solution of Cantilever Plate Equation Vibration-Induced Electric Potential and Electric Power Static Analysis to Calculate the Optimal Thickness of the PET Substrate Model Analysis and Harmonic Analysis Results of Model Analysis and Harmonic Analysis 23

8 vi Contents 2.3 The Fabrication of Flexible Piezoelectric ZnO Harvesters on PET Substrates Bonding Process to Fabricate UV-Curable Resin Lump Structures on PET Substrates Near-Field Electro-Spinning with Stereolithography Technique to Directly Write 3D UV-Curable Resin Patterns on PET Substrates Sputtering of Al and ITO Conductive Thin Films on PET Substrates Deposition of Piezoelectric ZnO Thin Films by Using RF Magnetron Sputtering Testing a Single Energy Harvester under Resonant and Non-Resonant Conditions Application of ZnO/PET-Based Generator to Flash Signal LED Module Design and Performance of a Broad Bandwidth Energy Harvesting System Fabrication and Performance of Flexible ZnO/SUS304-Based Piezoelectric Generators Deposition of Piezoelectric ZnO Thin Films on Stainless Steel Substrates Single-Sided ZnO/SUS304-Based Flexible Piezoelectric Generator Double-Sided ZnO/SUS304-Based Flexible Piezoelectric Generator Characterization of ZnO/SUS304-Based Flexible Piezoelectric Generators Structural and Morphological Properties of Piezoelectric ZnO Thin Films on Stainless Steel Substrates Analysis of Adhesion of ZnO Thin Films on Stainless Steel Substrates Electrical Properties of Single-Sided ZnO/SUS304-Based Flexible Piezoelectric Generator Characterization of Double-Sided ZnO/SUS304-Based Flexible Piezoelectric Generator: Analysis and Modification of Back Surface of SUS Electrical Properties of Double-Sided ZnO/SUS304-Based Piezoelectric Generator Summary 66 References 67

9 Contents vii 3 Design and Fabrication of Vibration-Induced Electromagnetic Microgenerators Introduction Comparisons between MCTG and SMTG Magnetic Core-Type Generator (MCTG) Sided Magnet-Type Generator (SMTG) Analysis of Electromagnetic Vibration-Induced Microgenerators Design of Electromagnetic Vibration-Induced Microgenerators Analysis Mode of the Microvibration Structure Analysis Mode of Magnetic Field Evaluation of Various Parameters of Power Output Analytical Results and Discussion Analysis of Bending Stress within the Supporting Beam of the Spiral Microspring Finite Element Models for Magnetic Density Distribution Power Output Evaluation Fabrication of Microcoil for Microgenerator Microspring and Induction Coil Microspring and Magnet Tests and Experiments Measurement System Measurement Results and Discussion Comparison between Measured Results and Analytical Values Conclusions Analysis of Microgenerators and Vibration Mode and Simulation of the Magnetic Field Fabrication of LTCC Microsensor Measurement and Analysis Results Summary 113 References Design and Fabrication of Rotary Electromagnetic Microgenerator Introduction Piezoelectric, Thermoelectric, and Electrostatic Generators Vibrational Electromagnetic Generators Rotary Electromagnetic Generators Generator Processes Lithographie Galvanoformung Abformung Process Winding Processes LTCC 123

10 viii Contents Printed Circuit Board Processes Finite-Element Simulation and Analytical Solutions Case 1: Winding Generator Design Analytical Formulation Simulation Fabrication Process Results and Discussion (1) Results and Discussion (2) Case 2: LTCC Generator Simulation Analytical Theorem of Microgenerator Electromagnetism Simplification Analysis of Vector Magnetic Potential Analytical Solutions for Power Generation Fabrication LTCC Process Magnet Process Measurement Set-up Results and Discussion Design Analytical Solutions Fabrication 170 References Design and Fabrication of Electrospun PVDF Piezo-Energy Harvesters Introduction Fundamentals of Electrospinning Technology Introduction to Electrospinning Alignment and Assembly of Nanofibers Near-Field Electrospinning Introduction and Background Principles of Operation Process and Experiment Summary Continuous NFES Introduction and Background Principles of Operation Controllability and Continuity Process Characterization Summary 211

11 Contents ix 5.5 Direct-Write Piezoelectric Nanogenerator Introduction and Background Polyvinylidene Fluoride Theoretical Studies for Realization of Electrospun PVDF Nanofibers Electrospinning of PVDF Nanofibers Detailed Discussion of Process Parameters Experimental Realization of PVDF Nanogenerator Summary Materials, Structure, and Operation of Nanogenerator with Future Prospects Material and Structural Characteristics Operation of Nanogenerator Summary and Future Prospects Case Study: Large-Array Electrospun PVDF Nanogenerators on a Flexible Substrate Introduction and Background Working Principle Device Fabrication Experimental Results Summary Conclusion Near-Field Electrospinning Continuous Near-Field Electrospinning Direct-Write Piezoelectric PVDF Future Directions NFES Integrated Nanofiber Sensors NFES One-Dimensional Sub-Wavelength Waveguide NFES Biological Applications Direct-Write Piezoelectric PVDF Nanogenerators 258 References 258 Index 265

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13 About the Authors Dr. C.T. Pan was born in Nauto, Taiwan, in He received master and doctoral engineering degrees in 1993 and 1998 respectively, from the Power Mechanical Engineering Department of National Tsing Hua University in Hsinchu, Taiwan. He was a researcher in the field of laser machining polymer at TU Berlin (IWF) in Germany from 1997 to 1998 and a researcher in the MEMS Division of MIRL/ITRI, Hsinchu, Taiwan from 1998 to He joined National Sun Yat-Sen University, Kaohsiung, Taiwan, as an Assistant Professor in 2003, then earned his associate professorship and full professorship in 2005 and 2008, respectively. He won the Outstanding Professor Award ( ) from National Sun Yat-Sen University. From June 2009 to June 2010, he was a visiting professor at the department of ME 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, Republic of China in He received his Bachelor s (1981) and Master s (1983) degrees in power mechanical engineering from National Tsing Hua University in Hsinchu, Taiwan. He earned his Doctor s degree (1990) in industrial mechanical engineering from Tokyo University in Japan. He has been a professor at the Department of Mechanical and Electro-Mechanical Engineering (MEME), National Sun Yat-Sen University (NSYSU), Kaohsiung, Taiwan, since He has served as the department chair ( ) of MEME. His research interests have been in the area of metal forming, machine design and mechanics. He won the Best Paper Award (1992) and Outstanding Engineering Professor Award (2007) from the Chinese Society of Mechanical

14 xii About the Authors Engineers in Taiwan. He earned the Fellow title from Japan Society for Technology of Plasticity (JSTP), Japan (2012) and Distinguished Professor of NSYSU (2012). Dr. Liwei Lin is a Professor at the Department of Mechanical Engineering at the University of California at Berkeley, and Co-Director of the Berkeley Sensor and Actuator Center. He received his B.S. degree (1986) in Power Mechanical Engineering from the National Tsing Hua University, Taiwan, and M.S. (1991) and Ph.D. (1993) degrees from UC Berkeley in Mechanical Engineering. After graduation, Professor Lin held the position of Senior Research Scientist at BEI Electronics Inc., Associate Professor at the National Taiwan University, Taiwan and Assistant Professor at the University of Michigan, Ann Arbor, USA before joining the faculty at UC Berkeley in His research interests and activities include MEMS, NEMS, Nanotechnology, design and manufacturing of microsensors and microactuators, development of micromachining processes by silicon surface/bulk micromachining, micro molding process, and mechanical issues in MEMS such as heat transfer, solid/fluid mechanics and dynamics. Professor Lin is the co-inventor of 16 US patents in MEMS and has co-authored more than 130 journal and 200 refereed conference papers. He has supervised 29 Ph.D. and 30 M.S. students. Dr. Ying-Chung Chen was born in Tainan, Taiwan, R.O.C., on 4 November He received the M.S. and Ph.D. degree in electrical engineering from National Cheng Kung University, Tainan, Taiwan, in 1981 and 1985 respectively. Since 1983, he has been at National Sun Yat-Sen University (NSYSU), Kaohsiung, Taiwan, where he is a professor of electrical engineering. Previously, he served as the department chair ( ) of EE, the Dean of the College of Engineering ( ) and won the Distinguished Professor Award and Outstanding Professor Award ( ) from National Sun Yat-Sen University. His current research 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 Society for Materials Science and a registered electrical engineer in Taiwan.

15 Preface Energy harvesting is known as power harvesting or energy scavenging to store and capture ambient energy which is natural, self-regenerating, or renewable. For example, ambient energy may include wind, solar, hydro, geothermal, and tide. Energy harvesting takes advantage of these sources to provide energy that is renewable and eco-friendly as compared with energy derived from fossil fuels. As an enabling technology from ambient vibrational energy sources, energy harvesting could find potential applications in low-power devices such as sensors, actuators, and electronics with ecological advantages in reducing chemical wastes from batteries and 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 only a limited number of introductory books or handbooks in the global community. Compared to other semiconductor disciplines, such as MEMS and LIGA, the gap in advanced knowledge of energy harvesting has yet to be filled. This book partially fills this gap by documenting the latest and most frequently cited research results of a few key energy harvesting processes. Scientists and researchers from various disciplines have contributed heavily to the related energy harvesting literature. Our hope with the current book is to provide reliable and practical techniques for analytical models of piezoelectric and electromagnetic energy harvesters and their relevant phenomena. This book presents a state-of-the-art understanding of diverse aspects of energy harvesting with a focus on broadband energy conversion, as well as new concepts in designs 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, broad bandwidth vibrational energy harvesting systems, and double-sided piezoelectric energy harvesters. The design and fabrication of vibration-induced electromagnetic generators is presented in Chapter 3. Chapter 4 focuses on the design, fabrication,

16 xiv Preface test, and application of in-plane rotary electromagnetic micro-generator. Chapter 5 describes the design and fabrication of PVDF electrospun piezo-energy harvesters with interdigital electrodes, details of the fabrication process involved in flexible piezoelectric 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 Lin September 2013

17 Acknowledgments Although four authors worked on this text, it would not have been written without support from various sources. We express our thanks to Z. H. Liu and Y. C. Candice Chen, undergraduate students who helped with edits and figures.

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19 1 Introduction 1.1 Background Various approaches in developing clean and green energy systems have been extensively explored in the past few years. Much research has focused on properties such as small size, light weight, high density in power, economically efficient, and environmentally friendly. Possible sources of ambient energy include light energy, wind power, kinetic energy, and thermal energy. In many applications, researchers have been investigating the use of microactuators and microsensors in MEMS (microelectromechanical systems) where an independent power source is needed. A possible solution is to design the power supply at the same scale as actuators, sensors, and electronics. The conventional solution is to use batteries, 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 could cause a hazard. Vibration energy harvesters and energy scavengers recover mechanical energy from their surrounding environment and convert it into usable electricity as sustainable self-sufficient power sources to drive micro-to milli-watt-scale powered instruments independently. Mechanical kinetic energy is ubiquitous in real environments. The conversion of ambient mechanical vibration to electrical energy is considered one of the likely methods 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 John Wiley & Sons Singapore Pte Ltd. Published 2014 by John Wiley & Sons Singapore Pte Ltd.

20 2 Design and Fabrication of Self-Powered Micro-Harvesters power generation. The power source does not need to be replaced and fuel does not need 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 cells and miniature sensing devices. Vibration-based energy harvesting is a process of capturing ambient kinetic energy and converting it into usable electricity. The growing demand of cell phone devices such as miniature wireless sensor networks and the recent advent of extremely low-power controlled circuit and MEMS devices make such renewable power sources very attractive. In addition, the energy harvesting process must be compatible with environmental vibrations such as running machines and human body movement. However, the wide range of environmental vibration frequencies means the harvester operates at low efficiency when deployed in a stochastic surrounding vibration. 1.2 Energy Harvesters Microtransducers have been of the focus of much attention since the rapid development of MEMS technology based on micro-wireless sensors and actuators with different applications in the communication, military, and biomedical industries. Research in the field of kinetic energy harvesters using piezoelectric materials and magnetic modules that harvest environmentally random energy, such as irregular vibrations, light airflow, and human activity, have attracted considerable attention. This type of mechanical energy available in our environment has a wide spectrum of frequencies and time-dependent amplitudes, however. The design of high-sensitivity energy harvesters is therefore crucial, including electromechanical conversion of materials, efficient energy transfer of mechanical structures, and controlled circuit to obtain good performance. In this book, electromagnetic transduction mechanisms include vibrational and rotary power generators. Microgenerators to harvest vibration-induced and rotaryinduced energy such as human motion are an attractive study topic. According to Faraday 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, printed circuit board etching, and LTCC (low-temperature co-fired ceramic). LIGA and LIGA-like processes consist of expensive methods such as thick photoresistant exposure, thin film, and a high-aspect-ratio electroplating process. The latest research

21 Introduction 3 in the field of LTCC technology applications for the field of microgenerators are included 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 environmental related issues. Using ZnO and PVDF piezoelectric materials is attractive in terms of their low cost, high resistance to fatigue, and environmentally friendly applications. Significantly, the deposition process of sputtering ZnO thin films with high c-axis preferred orientation and electrospun PVDF fibers with high piezoelectric β-phase crystallization are controlled at room temperature; they therefore do not require post-annealed and electrical repoling processes to obtain an excellent piezoelectricity. The sputtering ZnO thin films and well-aligned electrospun piezoelectric PVDF fibers can be directly deposited on flexible substrates at room temperature, such as PET (polyethylene terephthalate) and PI (polyimide). Utilizing ZnO and PVDF as the main elements of piezoelectric materials has the following additional advantages: 1. they have high piezoelectric coupling coefficients, 2. they do not cause environmental pollution; and 3. the deposition process is controlled at room temperature, which is suitable for all flexible substrates Piezoelectric ZnO Energy Harvester In the fabrication process, the piezoelectric ZnO thin film is deposited between the electrodes. ZnO thin film with excellent distribution of crystal grains with a high diffraction peak (002) was deposited using radio frequency (RF) magnetron sputtering. To fabricate the selectively deposited UV-curable resin lump structures as a proof mass, electrospinning techniques were used to construct various patterns on the back of the PET-based composite plate to control the resonant frequencies. Piezoelectric ZnO thin-film harvesters can therefore harvest energy in small working frequencies. This ZnO piezoelectric energy harvester consists of a piezoelectric laminated cantilever and a mass, and harvests vibrational energy. Through the resonant inertial oscillation of the cantilever, the power of a piezoelectric generator is generated by the inherent piezoelectric effect. The application of stress on a piezoelectric material generates a corresponding electric charge. The mechanical energy of a piezoelectric energy harvester is converted into electrical energy. A piezoelectric energy harvester must operate at the resonance frequency to harvest higher power; the friction and damping reduce its output. These obstacles are addressed in Chapter Vibrational Electromagnetic Generators Microelectromagnetic generators demonstrate the advantages of power output and integrated circuit (IC) package processing, thereby saving energy and improving

22 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 2 mm attached on the upper and lower surfaces of the LTCC microinductor. NdFeB magnets 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 Rotary Electromagnetic Generators The recycling of mechanical energy has been the focus of many previous studies. 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 comprises 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 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 directwrite 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

23 Introduction 5 smaller line-width fiber deposition. Piezoelectric fibers have the characteristics of electromechanical energy conversion and offer the advantages of nanoscale size. Piezoelectric fibers can harvest relatively low-frequency kinetic energy, such as body movement and muscle stretching. Researchers have published many studies on piezoelectric fibers, wires, and rods. A direct-write electrospinning techniques using NFES was developed to achieve controllable fiber deposition for various materials. Unlike the conventional electrospinning process, NFES only needs a small electric field to produce continuous fibers with fine diameters. The fabrication process of PVDF fibers in this study is compatible with a flexible substrate and does not involve any complex processes. PVDF is a potential piezoelectric polymer because of its high flexibility, biocompatibility, and low cost. These features make PVDF attractive for energy conversion applications involving microelectromechanical devices, electromechanical actuators, and energy harvesters. 1.3 Overview The book is arranged in five chapters to describe the research and development of energy harvesters. Chapter 1 describes the background of energy harvesting, the development of electromagnetic generators and piezoelectric materials for flexible energy harvesters, 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-bandwidth vibrational energy harvesting system and double-sided piezoelectric energy harvester are described, including experiments, measurements, and interfacial adhesion of PET-based substrate. In Chapter 3 the design and fabrication of vibration-induced electromagnetic microgenerators are discussed. The manufacturing technology of LTCC applied to the construction of multilayer silver microinduction coils and spiral ceramic microspring in microgeneration is also introduced. Compared to silicon-based processes, ceramic and silver are not easily broken or fractured after sintering. Microgeneration has high-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-element simulations have been performed to observe electromagnetic information. The study also establishes analytical solutions for the microgenerator to predict the induced voltage 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 piezoelectric composites and NFES PVDF fibers. Furthermore, adding modified CNTs (carbon nanotubes) to reinforce PVDF nanofibers can enhance the crystallinity of the β phase and increase its ability to store charges. NFES can potentially be reduced to the nanometer scale and form any shape for various sensing and actuation applications.

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25 2 Design and Fabrication of Flexible Piezoelectric Generators Based on 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 to help address the problem of energy shortage. Alternative energy systems have been actively 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 constraints generally limit recycling power applications. For instance, hydroelectric power plants are always built with turbines, such as the Pelton turbine or the chain turbine. Solar power is the most frequently used energy source. However, applying solar power 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 John Wiley & Sons Singapore Pte Ltd. Published 2014 by John Wiley & Sons Singapore Pte Ltd.

26 8 Design and Fabrication of Self-Powered Micro-Harvesters should therefore address the disadvantages of solar power. In many applications, for example, solar power has been applied in microactuators and microsensors in microelectromechanical systems (MEMS) that require an independent and embedded power source without an outside connection. A possible solution is to design the power supply to the same scale as actuators, sensors, and electronics [10]. The conventional solution involves using batteries however, which can be undesirable for many reasons: they tend to be relatively bulky, contain a finite amount of energy, have a limited life, and contain hazardous chemicals [11]. In many applications, microsystems must be self-powered; efficient energy scavenging is therefore crucial. Mechanical vibration is ubiquitous in real environments. Converting ambient mechanical vibration to electrical energy is considered one of the likely methods for powering wireless sensors without hazardous byproducts related to power generation. In addition, the power source of wireless sensors does not need to be replaced and the fuel does not require periodical replenishment as for batteries [12]. Piezoelectric materials have electrical-mechanical coupling effects, and are the leading candidates for converting mechanical energy into electricity [13]. White et al. [14] developed an inertial piezoelectric generator that uses 7-μm-thick film of lead zirconate titanate (PZT) with a 100-μm-thick steel substrate. The device produces an output of 2.1μW from vibrationsin the environment.roundy et al. [15] improved the geometry of the scavenger s piezoelectric bimorph. Using the same volume of PZT and a trapezoidal geometry can supply more than twice the energy of rectangular geometry. Fang et al. [16] used a generator structure of a nickel-metal composite cantilever. At a resonant frequency of approximately 608 Hz, 608 mv of AC voltage value with a 2.16 μw power level was obtained. Minazara et al. [17] used a mechanically excited unimorph piezoelectric membrane transducer. A power output of 0.65 mw was generated at the resonance frequency (1.71 khz) across a 5.6 kω optimal resistor. Several harvesting systems of resonant mechanical vibration energy are currently under investigation. Three considerations potentially affect transducer technology selection: Electromagnetic: a coil attached to a mass that vibrates through a magnetic field to 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 has its permanent charges electrically arranged [15, 27]. Dutoit et al. [27] compared these three generators according to energy density. Of these three mechanisms, the electrostatic transducer has the lowest energy storage density and conversion efficiency. Further, electromagnetic generators usually require

27 Design and Fabrication of Flexible Piezoelectric Generators Based on ZnO Thin Films 9 a large operating space, complex instrumentation, and continual management of facilities, all of which are expensive. Many researchers have therefore examined micropiezoelectric power generators and their fabrication from mostly piezoelectric materials (Pb,Zr)TiO 3 (PZT), aluminum nitride (AlN), and zinc oxide (ZnO). Table 2.1 lists the general characteristics and properties of PZT, AlN, and ZnO films [28]. The production of PZT piezoelectric material causes considerable environmental pollution. 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 compared to piezoelectric PZT thin film; and (3) it is a non-ferroelectric material that requires no poling or post-deposition annealing. These factors make ZnO piezoelectric material useful for elucidating the application of piezoelectric energy harvester in environments. This chapter is organized in four sections to describe the research and developmental efforts. Section 2.1 briefly introduces the background of green and renewable energy systems, the development of generator technologies, and the materials of piezoelectric thin films for piezoelectric generators. Section 2.2 focuses on the theoretical analysis and simulations of flexible piezoelectric generators based on ZnO thin films on polyethylene terephthalate (PET) substrates. A piezoelectric cantilever plate was designed and simulated using commercial software ANSYS FEA (finite element analysis) to determine the optimal thickness of the PET substrate, internal stress distribution, operation frequency, and electric potential. Section 2.3 describes the relationship between the model solution of piezoelectric cantilever plate equation, vibration-induced electric potential, and electric power. Section 2.4 discusses the development of high-performance piezoelectric generators using single-sided and double-sided ZnO thin films on a flexible stainless steel substrate (SUS304). Relevant fabrication processes, experiments, measurements, and piezoelectric responses are addressed. Table 2.1 General characteristics and properties of piezoelectric materials [28] Mass density (kg m 3 ) Electromechanical coupling coefficient k t 2 (%) Dielectric constant ε r PZT ZnO AlN

28 10 Design and Fabrication of Self-Powered Micro-Harvesters 2.2 Characterization and Theoretical Analysis of Flexible ZnO-Based Piezoelectric Harvesters Vibration Energy Conversion Model of Film-Based Flexible Piezoelectric Energy Harvester Figure 2.1a shows the composite cantilever plate used as a harvester with a rectifying circuit (B) and a capacitor (C) as the energy storage module. Ambient vibration amplitude on the anchor side induced vertical deflection at the front end of the cantilever plate. The piezoelectric layer on the composite structure deformed when the cantilever plate oscillated with mass. The bending results in a mechanical strain distributed along the cantilever plate, which is then converted to alternating voltage through the transverse-mode (d 31 ) piezoelectric effect (the generated strain is perpendicular to the electric field, which forms the d 31 mode of the piezoelectric element), and overcomes the forward bias of the rectifying diodes. Subsequently, the full-wave rectified potential can quickly charge the storage module. Figure 2.1b shows a schematic model of vibration energy conversion. The mechanical behavior of a vibrating piezoelectric harvester can be modeled using a seismic mass m bonded on a spring k corresponding to the stiffness and a damping factor b m, which in turns corresponds to the mechanical losses of the structure. A displacement y(t) is used to vibrate the rigid house, resulting in differential movement x(t) between the mass and the house; the relative displacement of the mass is therefore U z (t). This section reports the analysis of a new self-powered flexible Cu/ZnO/Al/PETbased piezoelectric energy harvester with a storage module. To verify the optimal thickness of the PET substrate evaluated by performing finite element model (FEM) analysis, an accurate analytical formula was developed. The optimal thickness of the substrate was calculated. Thus, the tensile and compressive strains do not occur Rigid house k m U z (t) b m x(t) Ambient vibration z x Anchor m, k, b m Flexible composite cantilever (PET/Al/ZnO/Cu) U z Strain y(t) B C + _ R ~V out ~ ZnO thin film Direction of aligned dipoles (a) (b) Figure 2.1 Generic vibration energy conversion model: (a) schematic model of flexible piezoelectric cantilever plate according to d 31 conversion mode and (b) first-order model of a resonant system [28]

29 Design and Fabrication of Flexible Piezoelectric Generators Based on ZnO Thin Films 11 on the piezoelectric ZnO layer simultaneously. Static analysis, modal analysis, and harmonic response analysis were performed to determine a suitable thickness of the PET substrate, structural modal parameters, frequency response functions, and electric potential. In the fabrication process, the piezoelectric ZnO thin film was deposited between the Al and Cu electrodes. The Al thin film used as the bottom electrode was deposited on the PET substrate by using a sputtering deposition method because of its superior adhesion to the PET substrate and lattice constant matching with ZnO. The ZnO thin film with superior compactness and more even distribution of crystal grains with a high (002) diffraction peak at 2θ = and full-width at half-maximum (FWHM) of was deposited using RF magnetron sputtering. To fabricate the selectively deposited ultraviolet- (UV)-curable resin lump structures as a proof mass, electrospinning and stereolithography techniques were used to construct various patterns on the back of the PET-based piezoelectric composite plate, which can adjust the resonant frequencies of the composite cantilever plate to achieve larger deflection. Piezoelectric ZnO thin-film harvesters can therefore harvest energy in small work spaces. In addition, they demonstrate long-term stability, have a low operating frequency with low-level excitation and a high deflection response compared to traditional PZT-based bulk ceramic vibrational structures, and can be used at low cost in clean energy applications. The schematic layout of the self-powered storage system and the design parameters of the flexible piezoelectric composite plate are shown in Figure 2.2. In this study, a broad bandwidth harvesting system with a wide bandwidth of Hz was designed and fabricated. Four individual, flexible PET-based ZnO piezoelectric harvesters were assembled in parallel with a rectifying circuit Switch Resistance Flash LED module Flexible power harvester Lump structure Storage module Rectifying and wave filter module Electric wires z y x Clamping apparatus Figure 2.2 Schematic illustration of self-powered storage system and flexible composite structure [28]