Energy Harvesting from Vibration Source Using Piezo- MEMS Cantilever. Kaushik Sarma

Transcription

1 Energy Harvesting from Vibration Source Using Piezo- MEMS Cantilever by Kaushik Sarma A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering in Mechatronics Examination Committee: Prof. Gabriel Louis Hornyak (Chairperson) Prof. Manukid Parnichkun Assoc. Prof. Erik L.J Bohez Dr. Tanujjal Bora Nationality: Previous Degree: Indian Bachelor of Engineering in Electrical & Electronics Engineering Gauhati University Assam, India Asian Institute of Technology School of Engineering and Technology Thailand May 2018

2 ACKNOWLEDGEMENTS First of all, I would like to express my heartiest gratitude to my advisor Prof. Gabriel Louis Hornyak for his continuous motivation, suggestion and also the resources that he had provided throughout my research period. I am deeply indebted to Dr. Adrien Dousse and Dr. Tanujjal Bora for giving their precious time and assistance during my thesis work. In addition, I am also thankful to Prof. Manukid Parnichkun and Assoc. Prof. Erik L.J. Bohez for their valuable suggestions. I want to take the opportunity to express my sincere gratitude to Dr. Noppadon Nuntawong from National Electronics and Computer Technology Centre (NECTEC) Thailand, for allowing me to use his laboratory facilities. And I would also like to acknowledge Asian Institute of Technology for granting me the AIT fellowship. I revere the patronage and moral support extended with love, by my beloved father Late Mr. Trailokya Nath Deva Sarma and mother Mrs. Premada Goswami without whose blessings and financial support, it wouldn t have been possible for me to complete this research. My joy knows no bounds in expressing my cordial gratitude to my friends and classmates from Center of Excellence in Nanotechnology, Asian Institute of Technology (AIT). ii

3 ABSTRACT In this thesis, we successfully modelled and simulated MEMS/NEMS Piezoelectric (ZnO) energy harvester which can work under low frequency under ambient condition. In addition to our research work a prototype of an Energy Harvester has been developed. Our simulated energy harvester model showed maximum output voltage of 0.32 V. Maximum output voltage delivered from our prototype is approx. 87 mv. This energy harvester can be operated under low frequency ambient condition moreover after implementing further modification can be used in Wireless Sensor Nodes. Keywords: MEMS, NEMS, FEM, COMSOL, magnetron sputtering, energy harvester iii

4 TABLE OF CONTENTS CHAPTER TITLE PAGE TITLE PAGE ACKNOWLEDGEMENTS ABSTRACT TABLE OF CONTENTS LIST OF FIGURES LIST OF TABLES LIST OF ABBREVIATIONS 1 INTRODUCTION Background Statement of the Problems Objectives of Research Scope and Limitations 4 2 LITERATURE REVIEW Piezoelectricity Energy Harvesters Theory of MEMS resonators Related Works 11 3 METHODOLOGY Overview Experimental procedure 14 4 RESULTS AND DISCUSSION Simulation results in MEMS cantilever beam 23 using COMSOL software 4.2 Experimental results from the Energy Harvester 25 prototype 5 CONCLUSIONS AND FUTURE RECOMMENDATIONS 29 REFERENCES 30 i ii iii iv v vi vii iv

5 LIST OF FIGURES FIGURE TITLE PAGE Figure 1(a) Architecture of a self-powered sensor node 1 Figure 1(b) MEMS cantilever 3 Figure 1(c) Electrode arrangement in microcantilever 3 Figure 2 Directions of forces affecting a piezoelectric element 2 Figure 2.2 Energy flow diagram of energy harvester 7 Figure Basic components of energy harvesting system 8 Figure Cantilever sensor modes of operation 9 Figure Block diagram of a resonant sensor 10 Figure 2.4(a) A series triple layer type cantilever. (b) A parallel triple 11 layer type cantilever (c) A uni-morph cantilever Figure 2.4(d) Piezoelectric cymbal circular shaped cantilever 11 Figure 2.4(e) Schematic of a PZT uni-morph cantilever 12 Figure 3.2.1(a) Flow chart of developing Piezo-bimorph MEMS cantilever 14 in COMSOL Figure.3.2.1(b) MEMS cantilever design in COMSOL 5.3 Multiphysics 15 Figure 3.2.1(c) Mesh model in COMSOL Figure 3.2.2(a) 3D model of the cantilever in SOLIDWORKS 18 Figure 3.2.2(b) Front view of the cantilever drawings in SOLIDWORKS 18 Figure 3.2.2(c) Tilted view of the cantilever drawing in SOLIDWORKS 19 Figure 3.2.2(d) Bottom view of the cantilever drawing in SOLIDWORKS 19 Figure 3.2.2(e) DC magnetron sputtering chamber 20 Figure 3.2.2(f) Final ZnO magnetron sputtered sample 21 Figure 3.2.3(a) Final setup of the energy harvester model 22 Figure 3.2.3(b) Block diagram of the final setup of energy harvester 22 Figure 4.1(a) 1 st mode resonant frequency Hz 23 Figure 4.1(b) 2 nd mode resonant frequency Hz 24 Figure 4.1(c) 3 rd mode resonant frequency Hz 24 Figure 4.1(d) Voltage vs frequency plot of the energy harvester 25 Figure 4.2(a) Output voltage plot from energy harvester using finger tip 26 vibration frequency source Figure 4.2(b) Output voltage plot from energy harvester using vortex 27 vibration as a frequency source Figure 4.2.1(a) Output voltage curve with respect to the position 28 of electrode Figure 4.2.2(b) Von Mises stress analysis of the proposed microcantilever 28 v

6 LIST OF TABLES TABLE TITLE PAGE Table 2.1 Piezoelectric charge coefficients 5 Table Comparative analysis of piezoelectric material 13 Table 3.2.1(a) Dimensional properties of a MEMS cantilever in COMSOL 16 Table 3.2.1(b) Materials properties of MEMS cantilever in COMSOL 16 Table Geometric properties of the actual prototype 20 vi

7 LIST OF ABBREVIATIONS PZT - ZnO - AIN - PIE - DSP - FOM - Lead Zirconate Titanate (piezoelectric ceramic material) Zinc Oxide Aluminum Nitride Piezoelectric inverse-effect Digital Signal Processing Figure Of Merits FGTD - Floating Gate Transistor Diodes WSN - Wireless Sensor Node vii

8 CHAPTER 1 INTRODUCTION 1.1 Background Lately, utilization of remote sensors and embedded medicinal electronic frameworks has developed quickly. These contraptions are frequently controlled by standard batteries which have decently a shorter lifetime. The fundamental purpose for the organization of remote sensor nodess (WSN) is the substitution or revival of the battery. Also, with the approach of coordinated circuit innovation, batteries are generally a substantial element in WSN. Vibration-based MEMS control generators gives an amazing answer for productively supply power to remote sensors. Power output generated from these generators (energy harvester) ranges from mw to μw level. There are a few distinctive transduction components and techniques, for example, piezoelectric and methods such as piezoelectric, electromagnetic and electrostatic methods through which vibration harvester converts ambient kinetic energy to electric energy. Piezoelectric material holds high electromechanical coupling impact, no outer voltage sources and it is particularly good with MEMS/NEMS innovation. Wireless sensor nodes are widely used in pipelines as a piping health monitoring system. To make it understand my research in a simpler way an application such as Piping health monitoring system will be described in the below paragraph. Pipeline system plays a vital role in industries and human life. This system connects to one section of the industry to transport petroleum and natural gases domestically as well as internationally. Natural gas as well as petroleum accounts for large percentage of world s energy consumption and leakage of it may cause hazardous accident and also will contribute a huge loss to the economy. Pipes are the essential part of human life as it takes part in the distribution of non-industrial applications such as water, LPG. Safety and preventive actions are of great concern and so will be explained in the below paragraph. Fluid run along the pipelines produces vibrations due to the change in pressure, flow etc. Two types of vibrations are generally assigned for the vibrations in steady-state, they are either low frequency (< 500 Hz) or high frequency (> 500 Hz). they are either low recurrence (< 500 Hz) or high recurrence (> 400 Hz). Low recurrence vibration can result to side-wise movement of the pipe while high recurrence vibrations leads to severe damage in the pipe as it causes both flexural vibration in the pipe walls and lateral movement of pipe. Excessive noise and vibrations get produced when there is a continuous run of fluid having a particular flow rate and pressure rate. In order to avoid from fracture of pipeline and to maintain safety, it is necessary to have a sensing device which can detect early before the accident. array of the sensor is clamped on the effective place of the piping system framework. Sensor node is linked with the control room through a server so as to take preventive and corrective measures to avoid hazardous accident and also avoid wastage [14][15]. Figure 1(a): Architecture of a self-powered sensor node 1

9 Fig. 1(a) depicts the overall architecture of a sensor node which contains the individual sensor, DSP processors, radio transreciever, power management device and a battery. Sensor acquires information from physical quantities and then convert into an electric signal which is further processed by DSP processors and finally transmitted by radio transceiver to the central processing unit [3]. Major problem arises when there is a need for replacement of the battery due to its short life span which is infeasible. These types of sensors can be specially used in underground pipes, sensor nodes in concrete structures as frequent replacement of battery is not very easy [4]. Certain alternation and modification is a mere need for the enhancement of pipe monitoring system which can be achieved by using the energy harvester embedded with sensor node. Special care should be taken while mounting the energy harvester as shown in Fig 3 [1]. Piezoelectric material experiences a strain due to pressure ripples and by the phenomenon of piezoelectricity piezoelectric material produces a voltage difference due to alternation on the center of gravity of charges. Voltage multiplier is used to raise the voltage to the desired value and finally stored in a super capacitor. Supercapacitor is the final replacement of the battery. This paper is trailed by blueprint of a selfpowered MEMS ZnO energy harvesting framework for remote sensor hubs. The harvester will be possible to work or respond under low frequency and low acceleration vibrations as ambient vibration (e.g vibration of civil engineering structure and heavy machinery) is always stochastic, intermittent, and mainly distributes in low frequency (< 100 Hz). Fundamental concentrate will be on the plan strategies for how to improve the power yield and voltage of the energy harvester and voltage of the energy harvester and to make possible for working of the energy harvesters under low frequency vibrations. As most of the cantilever s natural frequency lies in a narrow frequency range and mismatch of cantilever s natural frequency (resonant frequency) and ambient vibration frequency. Design method which we would be implementing is the technique for smaller resonant frequency of the vibration EH. Use of Zinc Oxide as a piezoelectric film material all together to enhance the voltage yield as well the power output. My proposed harvester system will give self-powered supply answer for remote sensor hubs which will be a reference technique for battery less sensor node. The cantilever will be designed in order to vibrate (resonate) in specific low vibration frequency which can be made possible by using a large silicon proof mass attached at the tip of the cantilever. The bending moment of the cantilever will produce a strain distributed along it, therefore the strain will be translated into electrical energy by the phenomenon of piezoelectricity. The input vibration applies acceleration to the structure of the beam and and because of the viable mass, it transfigures the input acceleration into force. ZnO retains excellent piezoelectric properties which will be imprinted on the top surface of the cantilever. Zinc oxide film when get compressed or tensed will induce a shift of the center of gravity of the charge particle as well as accumulation due to effect caused by piezoelectricity. Relative displacement of the beam causes the deformation of the piezoelectric layer. Aluminum electrodes attached on the metal layer and the piezoelectric layer as shown in Fig.1(c), collects the produced charge. Architecture design of a MEMS/NEMS cantilever is shown in Fig. 1(b). 2

10 Figure 1(b): MEMS cantilever Figure 1(c): Electrode arrangement in microcantilever 1.2 Statement of the Problems Two main problems are found after an analysis conducted on previous research works on MEMS/NEMS vibration energy harvesters. firstly, in vast majority of the uses of MEMS vibration Energy collectors are under high frequency conditions, which put constraints on the extent of use for these devices as the frequencies are generally low. Secondly, MEMS/NEMS micro-generators delivers relatively low output. 3

11 1.3 Objectives of Research Design of an Energy harvester based on MEMS cantilever will be followed by : Simulation of MEMs cantilever via COMSOL FEA using vibration frequency of the piping system. Optimize the cantilever dimensional parameters and implement low frequency method (proof mass) for frequency fitting. Show electricity generation from the simulated ZnO piezo-cantilever. Development of macroscopic mechanical analog Build steel based cantilever device. Coating with ZnO piezo thin film on the device. Testing the device. Voltage dependence w.r.t to shift in electrode position 1.4 Scope and Limitations Scope: Limitation: It will be working under low frequency ambient conditions It will be able to generate more power output or voltage Low cost and environment friendly energy harvester The only prototype of the proposed energy harvester was implemented in our study. However further modification is need in terms of the hardware and the prototype which would require more time and resources. 4

12 CHAPTER 2 LITERATURE REVIEW 2.1 Piezoelectricity Piezoelectricity is otherwise called piezoelectric impact, is the capacity to produce power indicated as AC voltage when subjected to mechanical pressure or vibration or to vibrate when subjected to AC voltage. The crystal when experiences a vibration or any type of mechanical stress is converted to feeble AC signal [20]. Piezoelectric coefficients develop an interlinkage between electrical stimulus and mechanical retort or vice versa. It determines the change in volume when piezoelectric material is applied to electric field or delivers a indication of direction of polarization of piezoelectric material [16]. Figure 2. Directions of forces affecting a piezoelectric element Table 2.1: Piezoelectric charge coefficients 5

13 Equations that governs the piezoelectric phenomena are as follows: To study the accurracy of cantilever beam Euler Bernoulli model is been adopted. No assumption has been taken into account that the piezo-plate twists on its neutral axis. In this way higher order beam hypothesis i.e Euler Bernoulli's conventions are utilized for analysis. Therefore higher order beam theory i.e Euler Bernoulli s conventions are used for analysis [20] [21]. 2.2 Energy Harvesters Energy harvesters are the energy storing device from unused or wasted energy resources. Generally, there are three mechanisms can be used for energy harvesting from the vibration, for example, electrostatic, electromagnetic and piezoelectric techniques. Piezoelectric materials are the best appropriate ones for harvesting energy from encompassing vibration sources, in light of the fact that they can perform transformation of mechanical strain to an electric charge with no external power and have the least complex structure [34], [39]. Piezoelectric sensors are also known as piezoelectric resonators. It has the potent to compensate the energy deficit as well as to produce feasible power sources from our environment. Energy harvesting plays a promising role as it facilitates alternative scope to scarcity of fossil fuel. Nowadays, smart grids acquire a great influence in energy sectors and for the enhancement of energy efficiency in the smart grid harvesters has its major contribution. Piezoelectric collector converts direct energy from vibrations and mechanical disfigurements to helpful electrical energy. The phenomenon behind this energy conversion is the direct piezoelectric phenomena and as a result piezoelectric detection technique is used. In this technique energy generated from external forces and mechanical vibrations [16] [17]. Energy harvesters can both work as a sensor and also in energy storage too. Quality of a piezoelectric material can be determined from the figures of merit (FOM) of a material. Great piezoelectric materials indicates high piezoelectric charge consistent (d), voltage steady (g), electromechanical coupling factor (k), and mechanical quality factor (Qm), which are the FOM of the piezoelectric material necessary for energy harvesting. Every Parameters are as follows [18]: 6

14 Figure 2.2: Energy flow diagram of an energy harvester The Building Blocks of an Energy Harvesting System The procedure of energy harvesting takes diverse structures based on the source, amount, and kind of energy being changed over to electrical energy. In its simplest form, the energy collector system requires a wellspring of energy, for example, heat, light, or vibration, and the accompanying three key parts. [40] 7

15 Figure Basic components of energy harvesting system [40] Transducer/harvester: Typical transducers incorporate photovoltaic for light, thermoelectric for heat, inductive for magnatic, RF for radio recurrence, and piezoelectric for vibrations/kinetic energy. Power conversion circuit: converts AC to DC, voltage regulation, voltage step up Energy storage: Such as a battery or super capacitor. Power management: This condition the electrical energy into an appropriate form for its application. Typical conditioners incorporate regulators and complex control circuits that can deal with the power management, based on energy demand and the accessible power. 2.3 Theory of MEMS resonators Microcantilever Beam Resonator Microcantilever is a MEMS based device which can also function as a physical, chemical, or biological sensors as well as energy harvesters by sensing the variation in the bending of cantilever or vibrational frequency. Cantilever is a long mechanical projecting beam which one end is fixed and the other end is free to move. These cantilevers achieve high sensitivity in various sensing applications. Pressure sensor, chemical sensor and mass sensor are some few examples uses static as well as dynamic mode so as to measure variation or change in physical environment Modes of operation Working of microcantilever sensor are based on two commonly used approaches i.e dynamic mode or static mode. Shift in the resonance frequency is the prime concern when the sensor is operating in dynamic mode. This change in resonance frequency forwarded the scope for measurement of the mass absorbed on the surface of the mass sensor which is purely based on microcantilever. Normally cantilever is excited near to its resonance frequency. 8

16 Deviation in resonance frequency occurs when additional mass is bonded to the surface of the cantilever. As the adsorbed mass increases frequency of the cantilever also get lowered. Equation 1 depicts the relation between frequency change and the adsorbed mass Δm [8][9]. Where fo = natural frequency and mo= initial mass of the cantilever Figure 2.3.2: Cantilever sensor modes of operation Cantilever sensor methods of operation: (A) dynamic mode distinguishing mass changes on the cantilever by changes in resonance frequency; (B) bimetallic mode distinguishing temperature changes by a static bending because of various thermal expansion of the metal layer and silicon cantilever; (C) static mode where asymmetrical molecule retention cause elastic pressure, brings about bowing the cantilever upwards; (D) static mode where asymmetrical molecule absorption cause compressive stress, results in bending the cantilever downwards [10] In static mode by observing the bending moment of the cantilever can sense the variation in the external measurands. Surface stress, pressure or temperature can be the main cause behind the deflection of the cantilever. In most of the biosensors sensing is conducted with the involvement of static mode, dynamic mode is not considered as an efficient approach as because of some demerits observed when the biosensor is operated in fluid environment. Quality factor gets drastically reduced due to the viscous damping of the fluid [11]. Bending moment of the cantilever is entirely dependent on the type of stress applied. Tensile stress causes upward movement and the downward movement due to compressive stress. Equation 2 shows Stoney s formula governs the relationship between the beam deflection, surface stresses and beam dimensions [10]. Where = change in surface stress amongst top and base surface of the cantilever, z is the cantilever deflection, E is Young's modulus, is Poisson proportion and L, t are the length and the thickness of the beam separately. 9

17 Commonly used excitation principles and detection principles Resonant sensor consists of mechanical resonant part as well as the excitation source for which is responsible for the vibration of the mechanical part. Basic components of this sensors are shown in below Fig In order to achieve the desired output from the sensor a feedback control circuitary is connected back to the excitation source to ensure the resonator maintains the desired resonance mode in spite of deviation or shift in resonant frequency. Change in the measurand is always responsible for the frequency shift [12]. Figure 2.3.3: Block diagram of a resonant sensor The six most commonly used excitation techniques and also their respective excitation method are as follows: i) Electrostatic excitation and capacitive detection ii) Resistive heating excitation and piezo resistive excitation iii) Dielectric excitation and capacitive detection iv) Piezoelectric excitation and detection v) Magnetic excitation and detection vi) Optical heating and detection As the work in this paper is based on the piezoelectric excitation and detection but the main focus is to the detection principle so detail explanation is given in the methodology part. iv) Piezoelectric excitation and detection: PZT materials, AIN or ZnO have excellent piezoelectric properties and have its wide application in the phenomenon of piezoelectricity. These materials are used in the excitation and detection of beam vibrations. Piezoelectric materials consist of two unique properties which contradicts among them. Properties are: a) direct PIE, b) inverse PIE effect. Inverse piezoelectric property has its application in the excitation technique where an piezoelectric material experiences an electric potential undergoes a deformation in the lattice structure of the crystal as a result excitation of resonators occurs. Fabrication of the beam is done in such a way that the top surface of it is embedded with a pair of electrodes on which the AC voltage is applied. And in the detection principle direct piezoelectricity is used. Vibrations in the beam causes physical alternation of the piezoelectric layer consequences generation of electricity. By measuring the intensity of the electric field, magnitude of vibration is measured [13]. 10

18 2.4 Related Works Many researchers work have been reported for the improvement of piezoelectric energy harvesters. Johnson et al. [26] demonstrated that by using the uni-morph cantilever beam configuration as shown in Fig.2.4 highest electrical power output can be generated under lower excitation frequencies and load resistances. Uni-morph structure cantilever comes with one piezoelectric layer whereas the bimorph structure with two piezoelectric layers. These types of cantilever are low cost compared to bimorph. Figure 2.4: (a) A series triple layer type cantilever. (b) A parallel triple layer type cantilever. (c) A uni-morph cantilever Study conducted by Anderson and Sexton [27] finally proved that cantilever geometrical structure also plays an important role for the upgradation of harvester efficiency. Roundy et al. [28], found that the strain distribution throughout the structure is more in trapezoidal shaped cantilever also found that the same volume of a trapezoidal can deliver more than twice the energy than a rectangular cantilever beam. Similarly, Baker et al. [29], experimentally proved that triangular trapezoidal beam can generate 30% more energy compared to the same volume of a rectangular beam. Kim et al. [30] developed a circular shaped structure called cymbal consists of two dome-shaped metal bonded to piezoelectric circular plate. Fig. 2.4(d) depicts the piezoelectric cymbical shaped cantilever. Figure 2.4(d): Piezoelectric cymbal circular shaped cantilever 11

19 Figure 2.4(e): Schematic of a PZT uni-morph cantilever Piezoelectric material selection of this research will be followed by ZnO, because of its high piezoelectric coefficient and high electric potential. One of the main property of the Vibration type energy harvester, its good sensitivity i.e must have the capacity to react to the low frequency and low accelerration vibrations that more often exist in the ambiance. In the meantime, the energy collector ought to have the capacity to produce maximum energy as conceivable to supply the subsequent burdens. As of late, with the application of MEMS/NEMS technology, energy harvester as well as piezoelectric energy harvesters were developed. Renaud et al. proposed a manufactured MEMS-based PZT cantilever smaller scale generator with a coordinated verification mass that can produce a normal energy of 40 μw at 1.8 khz vibration recurrence [31]. Jeon et al. built up a d33 mode thin film PZT control creating gadget with interdigitated terminals that can produce a normal energy of 1.0 μw from 108 m/s2 vibration increasing speeds at its full recurrence of 13.9 khz [32]. In any case, the above resonnant frequencies retained from the two energy harvesters are high. Fang et al. created a MEMS-based PZT cantilever controlled generator with a segregated Ni seismic mass that can produce 2.16 μw from 10 m/s2 vibration increasing velocities at its full recurrence of 609 Hz. The nickel metal mass on the tip of the cantilever is utilized to dwindle the structure's resonant frequency for the application under low-recurrence vibration, yet it can't to be smaller scale machined by MEMS innovation [33]. Like the past work, Liu et al. utilized the past cantilever structure to build a power generator array to enhance control yield and frequency adaptability. In spite of the fact that they showed that power thickness was high, the tip mass was not likewise coordinated with the cantilever which will be an extra trouble during manufacturing [34]. Muralt et al. composed and manufactured a micro scale power generator of thin film PZT covered cantilever with seismic mass and interdigitated electrodes which could produce a voltage of 1.6 V and energy of 1.4 μw when energized under 20 m/s2 vibration speeding up at 870 Hz full recurrence [35]. Throughout the years, there were dependably been some adjustment in the MEMS structure or use of new piezoelectric materials to vitality collectors keeping in mind the end goal to accomplish low full recurrence and considerably more yield control. One of the critical interfacing gadget required for these gatherers is the power molding circuit for the change of high inward yield impedance about the piezoelectric energy collector is once in a while been examined in these references [36-37]. Subsequent to considering the surveys on the past work led by the specialists, no less than three conclusions can be reached and encapsulated takes are as follows: Firstly, in the majority of the utilizations of MEMS vibration vitality gatherers are under high frequenccy conditions, which put impediments on the extent of utilization for these devices on the grounds that the frequencies are moderately low under accessible encompassing vibration sources. Furthermore, MEMS/NEMS miniaturized scale generators conveys moderately 12

20 low yield. Thirdly, there is a much need of an interfacing power change circuit for these energy harvesters keeping in mind the end goal is to retain the maximum power from the EH, voltage control and regulation is taken into account so as to compromise the demands of load, for example, remote senser nodes [38] Comparative analysis of piezoelectric material, (L=2500 micrometer, W=500 micrometer, T=6 micrometer) Table 2.4.1: Comparative analysis of piezoelectric material Researchers [22] conducted comparative analysis on various piezoelectric material through response of cantilever. Results after the analysis are depicted in table 2. Taking reference from the above table it is convenient and helpful for me to decide best suitable material for harvesting energy. Materials having higher electric potential can generate more electric power. Although Lead Zirconium Titanate PZT has a higher voltage potential [23]-[24] than ZnO, taking environmental factors into consideration Zinc oxide a lead free material is selected. As presence of lead causes harmful effects on the environment. Here are some of the unique properties of ZnO which made it exceptional from other piezoelectric materials are listed below: (a) It possesses high piezoelectric coupling coefficient compared to AIN (b) Dielectric constant found lower in comparison to PZT. (c) Low deposition Temperature and well know standard sputtering deposition technique. (d) It can get well bonded to silicon substrate. (e) Polling or post deposition annealing not required as it is a non-ferroelectric material. (f) Environmental friendly as it prevents severe environmental pollution compared to PZT. 13

21 CHAPTER Overview METHODOLOGY Our methodology was followed by the modelling and simulation of bimorph piezoelectric based MEMS/NEMS cantilever in which maximization of electrical output will be taken care of and moreover it can be operated under low ambient frequency. Also, development of prototype has been further considered in this work. 3.2 Experimental procedure Modelling and Simulation of the bimorph MEMS/NEMS cantilever The proposed bimorph cantilever beam is designed using COMSOL Multiphysics 5.3 software as shown in Fig 1. Steps involved in the flow chart will be well explained throughout the modelling and simulation steps on the upcoming sections. Figure 3.2.1(a): Flow chart of developing Piezo-bimorph MEMS cantilever in COMSOL 14

22 Modelling of vibration based piezoelectric energy harvester is performed using COMSOL 5.3. The resonance frequency used in designing the model ranges between 570 Hz to 585 Hz. Basically, our proposed cantilever beam is a bimorph configuration in which two layer of piezoelectric material ZnO is imprinted on both top and bottom surface of it and the base layer is made of steel. One end of the cantilever is fixed whereas the free end is connected to the tip mass. So, this proposed model has been tested under the above-mentioned frequencies using COMSOL Multiphysics (Structural mechanics module). Selecting Physics and study type Finite element modelling of our MEMS/NEMS energy harvester was conducted in COMSOL 5.3 using structural mechanics module associated with piezoelectric devices application mode which has been selected from PHYSICS tree and also selecting Eigen frequency from STUDIES Geometric modelling and material defining Three dimensional (3-D) micro cantilever is successfully built using COMSOL Multiphysics 5.3 drawing tools. The 3D model of the energy harvester consists of a steel rectangular beam, two piezoelectric (ZnO) rectangular film layer and a seismic steel mass. Orientation of the cantilever beam is along the global x-axis. Figure and Table encapsulates both geometric range as well as material range of the cantilever. Figure 3.2.1(b): MEMS cantilever design in COMSOL 5.3 Multiphysics 15

23 Table 3.2.1(a): Dimensional properties of a MEMS cantilever in COMSOL Steel beam Piezo film layer Proof mass length = 0.02 m 0.02 m m breadth = m m m height = m m m Table 3.2.1(b): Material properties of MEMS cantilever in COMSOL Materials Property Value Unit ZnO Density 5680 Kg/m 3 Poisson s ratio 38.2 GPa Stainless Steel Young s 200 e9 Pa modulus Density 7850 Kg/m 3 Boundary conditions It is always very necessary to provide boundary conditions to the profound model so as to obtain the desired results. Boundary conditions provided to this microcantilever model is a proof mass of 1g provided the other end of the cantilever beam to be fixed on one end. Meshing The application of meshing defines the correlation between the 3-D structure and reference structure, involves solving of mesh smoothing equations inside the COMSOL to define the coordinate transformations of the beam. Free tetrahedral meshing is used for meshing the model. Fig 3.2.1(e) modelling of cantilever using mesh analysis is given below. 16

24 Figure 3.2.1(c): Mesh model in COMSOL Development of Prototype of MEMS/NEMS Unimorph cantilever Prior to the development of our prototype of the proposed model is done in CAD software (SOLIDWORKS). Development of the final working model is processed through two approaches. 1 st approach includes development of the steel based cantilever and 2 nd approach involves the deposition of one piezo layer film on the top surface of the steel based cantilever through magnetron sputtering. Hence the final prototype has been achieved. 1 st Approach: Modelling of steel based cantilever in SOLIDWORKS 3-D steel based cantilever model consists of 4-part elements are: beam, proof mass, anchor and base layer. These elements are drawn individually using part drawing in SOLIDWORKS software. Regarding dimensions of the actual model drawings in SOLIDWORKS are determined by scaling up the COMSOL simulated model by a factor of 3. Further in order to achieve the entire three-dimensional model of the cantilever all the distinct part drawings are assembled with the help of assembly drawing inbuilt in the CAD software. Hence the entire model of the cantilever is successfully drawn in SOLIDWORKS and this drawing is further processed for 3D printing for the development of the realistic physical model. Fig: depicts 3D model of the cantilever in SOLIDWORKS. Also for better understanding of the CAD drawings Fig 3.2.2(a), (b), (c) provides a clear view of the dimensional properties. Note: Deposition of ZnO film in microcantilever is not considered in 1 st approach. 17

25 Figure 3.2.2(a): 3D model of the cantilever in SOLIDWORKS Figure 3.2.2(b): Front view of the cantilever drawing in SOLIDWORKS 18

26 Figure 3.2.2(c): Tilted view of the cantilever drawing in SOLIDWORKS Figure 3.2.2(d): Bottom view of the cantilever drawing in SOLIDWORKS 19

27 Table 3.2.2: Geometric properties of the actual prototype Length Breadth Height Beam 60 mm 9 mm 15 mm Proof mass 12 mm 9 mm 15 mm Base 50 mm 4.5 mm 15 mm 2 nd Approach: deposition of piezoelectric (ZnO) thin film layer on the steel based cantilever The obtained steel based cantilever model from 3D printing is firstly disassembled into small parts from which the steel cantilever beam (sample) is taken for Zinc oxide thin film (piezo material) deposition using magnetron sputtering. Hence 500 nm ZnO film has been successfully deposited on the cantilever steel beam using magnetron sputtering. Fig 3.2.2(d) depicts the instrumentation of magnetron sputtering chamber and Fig 3.2.2(e) shows the final ZnO magnetron sputtered sample. Figure 3.2.2(e): DC magnetron sputtering chamber 20

28 Figure 3.2.2(f): Final ZnO magnetron sputtered sample Final configuration of the proposed energy harvester model After obtaining all the ZnO sputtered specimens as well as the steel based cantilever all the elements are assembled finally. Aluminum foil and copper wire were used as electrodes which were connected to the piezo layer deposited beam and the steel body respectively in order to obtain the output voltage. Aluminum foil (electrode) connected to the piezo beam surface with the help of silver conductive paste. Devices used in the complete setup were Vortex as a vibrational source, GDM 396 instek multimeter, RS32 cable. Electrodes are connected to the multimeter and the multimeter is interfaced to the PC by RS32 cable. GDM 396 instek interfacing software was installed so as to successfully achieve the entire interfacing of the multimeter reading to the PC. Multimeter readings can be seen and recorded clearly with wave graphs. The signal graph obtained after the test of the whole experimental setup are recorded in the PC as excel file. Fig depicts the final setup the energy harvester model. 21

29 Figure 3.2.3(a): Final setup of the energy harvester model \ Figure 3.2.3(b): Block diagram of the final setup of energy harvester 22

30 CHAPTER 4 RESULTS AND DISCUSSION 4.1 Simulation results in MEMS cantilever beam using COMSOL software Simulation and modelling of vibrational based piezoelectric energy harvester is conducted in COMSOL 5.3. It has been observed different beam displacement corresponding to resonance frequency from which the three eigen frequency modes are determined. Eigen frequencies (resonant frequency) obtained are Hz, Hz, Hz. As the proposed energy harvester model is based on low frequency operation so the least resonant frequency (575.5 Hz) is considered as its fundamental resonant frequency. Fig 4.1(a)(b)(c) depicts the three resonant modes of the microcantilever. Figure 4.1(a): 1 st mode resonant frequency Hz 23

31 Figure 4.1(b): 2 nd mode resonant frequency Hz Figure 4.1(c): 3 rd mode resonant frequency Hz 24

32 Boundary conditions provided to this microcantilever model is a proof mass of 1g acceleration provided the other end of the cantilever beam to be fixed on one end. Fig 4.1(d) below shows the result plot of maximum voltage harvested from the proposed model against resonance frequency. Figure 4.1(d): Voltage vs frequency plot of the energy harvester By referring to the above plot it is been observed that resonance frequency is in between the frequency range of 570 Hz to 585 Hz. Hence the maximum output voltage obtained is 0.32V against resonant frequency approximately Hz. The reason behind obtaining peak voltage from the energy harvester because at resonance frequency the microcantilever vibrates at its maximum frequency due to which the piezoelectric (ZnO) film layer subjected to large deflection and according t0 the phenomenon of piezoelectricity maximum amount of voltage has been achieved. 4.2 Experimental results from the Energy Harvester Prototype The prototype has successfully delivered output voltage in the ranges of mv level provided tip load of g acceleration and a vibrational source. There were two vibrational sources provided using vortex and fingertip respectively. 25

33 The curve obtained by the frequency provided with the help of fingertip is given below Figure 4.2(a): Output voltage plot from energy harvester using fingertip vibration frequency source In the above plot four voltage peaks were observed among which the second peak represents the highest peak voltage (72.5 mv) which occurs due to the maximum deflection on the ZnO thin film layer (piezoelectric layer). 26

34 The curve obtained by the frequency provided with the help of vortex vibration is shown below: Figure 4.2(b): Output voltage plot from energy harvester using vortex vibration as a frequency source From the above plot we can observe many peak voltage curves. The mechanism of the peaks occurring is the same as the graph obtain in which fingertip were used as a frequency source Effect of the change of position of the electrodes There was always a change in output voltage level with the change of position of the electrode on the cantilever beam. Fig 4.2.1(a) below depicts the output voltage curve with respect to the position of electrode. From the below curve its been observed that voltage suddenly fell to zero with the change in the electrode position from fixed end to the tip end. 27

35 Figure 4.2.1(a): Output voltage curve with respect to the position of electrode The main reason behind the sudden decrease of the peak voltage level can be explained by Von mises stress analysis of the cantilever beam which has been shown in Fig 4.2.1(b). With the help of this analysis we can study the degree of stress level on each and every element of the cantilever. Stress level is higher towards fix end side compared to other end of the cantilever, more stress is subjected to more displacement on the ZnO (piezo layer) which results in more output voltage based on the phenomenon of piezoelectricity. Figure 4.2.2(b): Von Mises stress analysis of the proposed microcantilever 28

36 CHAPTER 5 CONCLUSIONS AND FUTURE RECOMMENDATIONS Modelling and simulation of the proposed energy harvester is carried out successfully with an output voltage of 0.32 V. Moreover, a prototype of energy harvester has been developed in order to demonstrate the concept behind energy harvesting from low vibrational source using piezoelectric material. Modelling and simulation of energy harvester using COMSOL finite element analysis Simulation of MEMs cantilever via COMSOL FEA using vibration frequency of the piping system. Frequency fitting achieved and finalization of MEMS cantilever dimensional parameter. Electrical generation is shown from the simulated ZnO piezo-cantilever. Development of macroscopic mechanical analog Successfully built a steel based cantilever device. Successfully coated ZnO piezo thin film on the cantilever device. Electrical output achieved. Shift in electrode position can have a large impact on the output voltage. Future Works Optimization of the energy harvester parameters such as to improve the output voltage. Requirement of a smart interfacing circuit for maximizing the voltage level as well overall power management between source and the load. 29

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