臺灣博碩士論文加值系統

本論文之研究目的在於研製高機電能轉換效率之半導體與高分子型壓電材料，設計與製作可獵取機械能之撓性壓電發電系統。在室溫下，利用射頻磁控濺鍍法沉積具高C軸優選氧化鋅(ZnO)壓電薄膜；近場靜電紡絲直寫具高壓電β相結晶之聚偏氟乙烯(PVDF)壓電奈米纖維於聚乙烯對苯二甲酸酯(PET)與聚酰亞氨(PI)等撓性高分子基板上，且不需經過退火及高壓再極化的處理，即可產生高壓電特性之壓電薄膜與壓電奈米纖維。本論文依材料特性的差異分兩部份探討。

第一部份: 氧化鋅撓性壓電發電機於自電式與多頻寬獵能系統之應用。ZnO自電式獵能系統之應用，搭配撓性基板之特性使壓電薄膜經振動產生電勢差。並針對ZnO薄膜的壓電特性、機械能傳遞結構、整流儲能電路及系統可靠度等關鍵指標進行深入的探討。撓性壓電發電機之結構是選用PET為基板，由上(Cu箔)下電極(Al薄膜)層中間結合ZnO壓電薄膜的壓電複合薄板，並於基板表面直接建構紫外光固化膠之塊狀結構為質量塊。初期利用有限元素分析軟體ANSYS之耦合場模擬，配合模態與簡諧分析預測壓電複合薄板之最佳基板厚度、內部應力、操作頻率以及輸出電壓。為了避免複合薄板在振動過程中，內部的交互應變同時出現在壓電層而降低電壓輸出，因此配合精確的公式計算與模擬數據相互驗證找出最佳的撓性基板厚度。此外，壓電複合薄板理論、振動能轉電能機制與輸出功率也一併討論。製程上以射頻磁控濺鍍法於室溫下沉積高C軸優選之ZnO壓電薄於Al/PET基板，Al薄膜良好的導電特性與PET基板有良好的附著性，晶格常數亦吻合ZnO薄膜，因此該壓電複合結構應具有好的結構穩定性。高分子質量塊製作上，以近場靜電紡絲配合立體光刻技術直接成型經結構配置設計的光固化膠於PET基板上。電性測試結果顯示，單一撓性壓電發電機在共振頻率下，產生開路電壓4 V，接上2 MΩ之負載產生功率輸出 1.247 μW/cm2 。此自電式獵能系統在共振狀態與非共振狀態都能成功驅動一警示用發光二極體模組。多頻寬獵能系統則由四組不同結構設計之單一撓性壓電發電機所組成，結合整流儲能模組，操作頻寬可介於100 Hz-400 Hz。為了提高系統整體發電功率，結合了兩組單一撓性壓電發電機使之電路串聯，形成壓電增壓懸臂樑獵能系統。該系統操作於15 Hz，產生最大直流電壓3.18 V，接上3.5 MΩ之負載產生直流功率輸出2.89 μW/cm2。此外，軟性電子屬多層膜的複合結構，當結構承受外力作用時，界面間的變形、黏附機制及導電性質是非常值得探討的。藉由控制濺鍍參數沉積導電性良好的氧化銦錫(ITO)與Al薄膜於PET基板上，接續濺鍍ZnO壓電薄膜於導電薄膜上方，形成ZnO/ITO/PET及ZnO/Al/PET撓性壓電複合薄板。經週期性外力測試後，以奈米壓痕、刮痕系統(在此選用Berkovich和Conical探針)及四點探針，分別量測多層膜的機械性質、薄膜間的接合強度及導電性，並比較外力測試前後之差異。結果顯示，ZnO/Al/PET撓性壓電複合薄板具有優異的結構穩定性。

第二部份 : 中空滾筒近場靜電紡絲製作預應變壓電聚偏氟乙烯奈米纖維陣列應於撓性能量之轉換。一維奈米發電機應用於環境能量之擷取已被廣泛的重視，然而現行藉由微成型技術所製作出的壓電奈米線可能無法輕易的控制線材的線徑及長度。此研究創新提出高壓電性之中空滾筒近場靜電紡絲技術，研製具永久壓電特性PVDF奈米纖維，透過高電場之原位極化與強的機械拉伸效應，可大量直寫出有序排列之具高壓電β相結晶的PVDF壓電奈米纖維。而直寫於滾筒上的預應變壓電奈米纖維，兼具低成本、線材幾何結構高控制性等製程特性可應用於奈米發電機、感測元件以及致動元件。首先，1號奈米發電機，是將PVDF奈米纖維陣列製作於一組平行式電極，並以PET基板進行封裝以維持結構穩定性。在0.05%的應變與5 Hz 振動頻率下，最大輸出電壓與電流分別為-50 mV與-10nA。2號奈米發電機是將奈米纖維陣列製作於指叉式電極上方，並以PI膜進行封裝，經高壓電場再極化程序，形成大量的奈米發電機單元使之電路並聯，以提升電流的數量級。在0.14%的應變與6 Hz 振動頻率下，最大輸出電壓與電流分別為20.2 mV與39 nA。在15 Hz 衝擊頻率下，最大輸出電壓與電流分別為24.4 mV與130 nA。3號奈米發電機則是測試單根壓電奈米纖維的發電功率。0.05-0.1%的應變與5 Hz 振動頻率下，最大輸出電壓與電流分別為-45 mV與 -3.9 nA。接上6.8 MΩ之負載產生最大功率輸出18.45 pW/cm2。此外，利用有限元素分析軟體ANSYS之耦合場分析，模擬PVDF壓電奈米纖維於電場下的致動行為並與實驗值進行比對驗證。為了探討奈米纖維的致動行為是否單純來自壓電效應，亦比較有極化跟無極化之奈米纖維在電場下的致動行為。

Vibration energy harvesters, or energy scavengers, recover mechanical energy from their surrounding environment and convert it into useable electricity as sustainable self-sufficient power sources to drive micro-to milli-Watt scale power electronics in small, autonomous, wireless devices and sensors. Using semiconducting, organic piezoelectric nanomaterials are attractive in low-cost, high resistance to fatigue, and environmentally friendly applications. Significantly, the deposition processes of sputtering ZnO (zinc oxide) thin films with high c-axis preferred orientation and electrospun PVDF (polyvinylidene fluoride) nanofibers with high piezoelectric β-phase crystallisation are controlled at room temperature. Thus they don’t have the necessity of post-annealed and electrical repoling process to obtain an excellent piezoelectricity, and are suitable for all flexible substrates such as PET (polyethylene terephthalate) and PI (polyimide). These works are divided into two parts.

Part 1: Flexible piezoelectric harvesters based on ZnO thin films for self-powering and broad bandwidth applications. A new design of Al (aluminum)/PET-based flexible energy harvester was proposed. It consists of flexible Al/PET conductive substrate, piezoelectric ZnO thin film, selectively deposited UV (ultraviolet)-curable resin lump structures and Cu (copper) foil electrode. The design and simulation of a piezoelectric cantilever plate was described by using commercial software ANSYS FEA (finite element analysis) to determine the optimum thickness of PET substrate, internal stress distribution, operation frequency and electric potential. With the optimum thickness predicted by developed accurate analytical formula analysis, the one-way mechanical strain that is efficient to enhance the induced electric potential can be controlled within the piezoelectric ZnO layer. In addition, the relationship among the model solution of piezoelectric cantilever plate equation, vibration induced electric potential and electric power was realized. ZnO thin film of high (002) c-axis preferred orientation with an excellent piezoelectricity was deposited on the Al/PET by RF (radio-frequency) magnetron sputtering in room temperature. Al was sputtered on the PET substrate as the bottom electrode because of its low sheet resistance, superior adhesion with PET, and lattice constants matching with ZnO thin film. The selectively deposited UV-curable resin lump structures as proof mass were directly constructed on flexible piezoelectric plate using electrospinning with a stereolithography technique. One individual harvester achieves a maximum OCV (open-circuit voltage) up to 4V with power density of 1.247 μW/cm2. This self-powered storage system can drive the warning signal of the LED (light emitting diode) module in both resonant and non-resonant conditions. We also succeeded in accomplishing a broad bandwidth harvesting system with operating frequency range within 100 Hz to 400 Hz to enhance powering efficiency. This system comprises four units of individual ZnO piezoelectric harvester in the form of a cantilever structure connected in parallel, and rectifying circuit with storage module. In addition, a modified design of a flexible piezoelectric energy-harvesting system with a serial bimorph of ZnO piezoelectric thin film was presented to enhance significantly higher power generation. This high-output system was examined at 15 Hz. The maximum DC (direct current) voltage output voltage with loading was 3.18 V, and the maximum DC power remained at 2.89 μW/cm2.
Furthermore, in order to examine the deformation between interfaces and the adhesion mechanism of multi-layer flexible electronics composites (e.g., ITO (indium tin oxide)/PET, Al/PET, ZnO/ITO/PET, and ZnO/Al/PET), nanoscratching and nano-indention testing (nanoindenter XP system) were conducted to analyze the adhesion before and after the vibration test. The plastic deformation between the ductile Al film and PET substrate is observed using SEM (scanning electron microscopy). Delamination between the ZnO and Al/PET substrate was not observed. This indicates that Al film provides excellent adhesion between the ZnO thin film and PET substrate.

Part 2: Pre-strained piezoelectric PVDF nanofiber array fabricated by near-field electrospining on cylindrical process for flexible energy conversion. In various methodologies of energy harvesting from ambient sources, one-dimensional nanoharvesters have been gaining more attention recently. However, these nanofibers fabricated by micro-forming technologies may not easily control their structural diameter and length. This study originally presented the HCNFES (hollow cylindrical near-field electrospining) process to fabricate permanent piezoelectricity of PVDF piezoelectric nanofibers. Under high in-situ electric poling and strong mechanical stretching effect during HCNFES process, large PVDF nanofiber array with high piezoelectric β-phase crystallisation was demonstrated. These pre-strained piezoelectric PVDF nanofibers fabricated by HCNFES with high process flexibility at low cost, availability in ultra-long lengths, various thicknesses and shapes can be applied at power scavenge, sensing and actuation. Firstly, PVDF nanofibers lay on a PET substrate, silver paste was applied at both ends of fibers to fix their two ends tightly on a Cu foil electrode pair. The entire structure was packaged inside a thin flexible polymer to maintain its physical stability. Repeatedly stretching and releasing the nanoharvester (NH 1) with a strain of 0.05% at 5 Hz vibration created a maximum peak voltage and current of -50 mV and -10 nA in forward connection, respectively. Secondly, a total of 44 parallel nanofibers have been fabricated and transferred onto an IDT (interdigital) electrode with 64 electrode pairs as a nanohavester (NH 2) to amplify current outputs under repeated mechanical vibration and impact tests. Under a repeated maximum strain of 0.14% at 6 Hz vibration, a peak current of 39 nA and peak voltage of 20.2 mV have been measured. Impact testing at 15 Hz, peak current of 130 nA has been collected with a voltage of 24.4 mV. Finally, the single PVDF fiber as nanoharvester (NH 3) with a strain of 0.05-0.1% at 5 Hz vibration created a maximum peak voltage and current of -45 mV and -3.9 nA, respectively. The maximum power remained at 18.45 pW/cm2 with a load resistor of 6.8 MΩ.
Based on the mechanism of converes piezoelectric effect, ANSYS software with coupled field analysis was used to realize piezoelectric actuation behavior of the PVDF fibers. From the observation of actuation property, a fixed-fixed single nanofiber was tested under different DC voltage supply. Comparing the polarized fiber with non-polarized fibers, the measurement of the center displacements as a function of electric field was conducted and characterized.

摘要 I
Abstract V
List of Figures XIII
List of Tables XXIII
List of Abbreviatons XXV
Chapter 1: Introduction
1.1 Energy harvesting from ambient environment 1
1.2 Harvesting mechanical energy 2
1.3 Various piezoelectric transducer structures 3
1.3.1 Piezoelectric film-based harvesters for self-powered and broad bandwidth applications 3
1.3.2 Mechanical properties of multi-layer flexible composite interfaces 5
1.4 Vibration energy conversion model of film-based flexible piezoelectric energy harvester 6
1.5 Piezoelectric fibers possess the characteristics of electromechanical energy conversion 8
1.6 Two-dimensional piezoelectric materials for energy harvesters 9
1.6.1 Piezoelectric thin films 9
1.6.2 Reactive magnetron sputtering technique for ZnO thin films deposition 11
1.7 One-dimensional piezoelectric materials for energy harvesters 15
1.7.1 PVDF nanofiber 15
1.7.1.1 Conventional far-field electrospinning (FFES) 16
1.7.1.2 Near-field electrospinning (NFES) 17
1.7.2 ZnO nanofiber 19
1.7.3 PZT nanofiber 20
1.8 Chapter overviews 21
Chapter 2: Piezoelectricity of ZnO thin films and electrospun PVDF nanofibers
2.1 Characterization and theoretical analysis of flexible ZnO piezoelectric harvesters 23
2.1.1 Piezoelectricity and polarity test of piezoelectric ZnO thin film 25
2.1.2 The optimum thickness of PET substrate 28
2.1.3 Model solution of cantilever plate equation 29
2.1.4 Vibration-induced electric potential and electric power 33
2.2 Well-aligned PVDF nanofiber 34
2.2.1 Near-field electrospinning process and control capability 34
2.2.2 Hollow cylindrical near-field electrospining (HCNFES) 37
2.2.3 Mechanical strain rate induced electric potential 40
Chapter 3: Analysis of flexible ZnO piezoelectric harvesters with FEM
3.1 Static analysis to calculate the optimum thickness of the PET substrate 44
3.2 Model analysis and harmonic analysis 46
3.2.1 Setting boundary conditions 46
3.2.2 Model analysis and harmonic analysis results 50
Chapter 4: Detailed discussions on fabrication of flexible piezoelectric ZnO harvesters and NFES PVDF fibers
4.1 Bonding process to fabricate UV-curable resin lump structures on PET substrates 54
4.1.1 NFES with stereolithography technique to directly write 3D UV-curable resin patterns on PET substrates 56
4.1.2 Sputtering Al and ITO conductive thin films on PET substrates 58
4.1.3 Deposition piezoelectric ZnO thin films by RF magnetron sputtering 59
4.1.4 Scanning electron microscopy and X-ray diffraction analysis 59
4.1.5 Photoluminescence (PL) characteristics 62
4.1.6 Various flexible ZnO harvesters and thermal deformation effect 62
4.2 Interfacial characteristics of PET-based piezoelectric multi-layer films 63
4.2.1 Flexible multi-layer plate vibration testing 63
4.2.2 Nanoindentation and nanoscratching tests 64
4.3 The optimal parameters of NFES PVDF fibers 65
4.3.1 PVDF solution preparation 65
4.3.1.1 Internal diameter of needles and the diameter of PVDF fibers 67
4.3.1.2 Electric field and the diameter of PVDF fibers 68
4.3.1.3 X-Y stage speed and the diameter of PVDF fibers 69
4.3.1.4 Solution concentration and the diameter of PVDF fibers 70
4.3.1.5 MWCNT concentration and the diameter of PVDF fibers 71
4.3.2 Material and structural characterizations of PVDF fibers 72
4.3.2.1 SEM observations 72
4.3.2.2 XRD analysis 75
Chapter 5: Results and discussion
5.1 Testing one single energy harvester both resonant and non-resonant conditions 78
5.1.1 Experimental set-up and power generating testing 78
5.1.2 Electricity charging and energy harvesting system applications 83
5.2 Testing broad bandwidth vibrational energy harvesting system 85
5.3 Improving power output for flexible piezoelectric energy harvesters 94
5.3.1 Piezoelectric energy harvester based on serial bimorph of ZnO thin films 94
5.3.1.1 Design concept and fabrication process 95
5.3.1.2 Testing energy harvesting system under low frequency vibration 97
5.4 Analysis of adhesion of multi-layer flexible piezoelectric composites 100
5.4.1 Measuring the conductive property of conductive coatings 100
5.4.2 Measuring the mechanical properties of PET-based coatings 101
5.4.3 Interfacial adhesion of PET-based coatings 102
5.4.4 Nanoscratching test using conical probe 108
5.4.5 Lifetime testing results of harvesters 109
5.5 Large PVDF nanofiber array for flexible energy conversion 111
5.5.1 Analysis of induced electric potential of PVDF nanofiber array with IDT electrode 111
5.5.2 Fabrication of flexible nanohavesters 115
5.5.2.1 PVDF nanofiber array patterned on IDT electrode 116
5.5.2.2 Realigning dipoles in PVDF nanofiber using high-voltage field 118
5.5.3 Experimental set up and electric potential testing 118
5.5.3.1 Nanohavesters under low frequency vibration 118
5.5.3.2 Nanohavesters under impact 121
5.5.4 Power generation of PVDF single-fiber nanoharvester (NH 3) 124
5.6 Piezoelectric actuation of direct-write electrospun PVDF fibers 127
5.6.1 Model solution of single PVDF fiber 127
5.6.2 Analysis of actuation behavior of PVDF fiber with FEM 132
5.6.3 Observation of piezoelectric actuation based on PVDF fiber 134
Chapter 6: Conclusions
6.1 Flexible piezoelectric harvesters based on (002) c-axis-preferred orientation ZnO thin film 138
6.2 Piezoelectricity of large PVDF nanofiber arrays fabricated by the modified near-field electrospining process 139
References 141

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