可撓曲光電元件薄膜撓曲疲勞性質研究;Flexural Fatigue Properties of Flexible Thin Films for Use in Optoelectronic Devices

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题名:	可撓曲光電元件薄膜撓曲疲勞性質研究;Flexural Fatigue Properties of Flexible Thin Films for Use in Optoelectronic Devices
作者:	覃鑫富;Tran, Dinh-Phuc
贡献者:	機械工程學系
关键词:	可撓曲光電元件薄膜撓曲疲勞性質研究;Fatigue;Flexible Thin Films
日期:	2018-07-27
上传时间:	2018-08-31 15:02:46 (UTC+8)
出版者:	國立中央大學
摘要:	本研究目的在探討可撓曲光電元件之薄膜撓曲疲勞性質。在第一部分，探討聚對苯二甲酸(PET)基板上沉積氧化銦錫(ITO)的薄膜，其導電性受反覆撓曲變形作用及退火處理之影響，ITO/PET薄膜於室溫中承受不同的循環動態及靜態撓曲負載，並同步量測其導電性質的變化。實驗結果顯示，隨著變形程度的提高，其達到一定電阻改變量的疲勞壽命會隨之下降。不同循環負載模式伴隨靜態撓曲的組合會大大影響ITO/PET薄膜之疲勞性質，循環撓曲結合靜態張力撓曲比起其結合靜態壓力撓曲、中性持時或純循環撓曲會造成更大的損傷以及降低疲勞壽命。當承受較小撓曲疲勞負載時，ITO/PET薄膜之導電耐久性將會隨退火溫度的升高而增加，然而，退火溫度對承受大撓曲變形之ITO/PET薄膜的疲勞壽命影響有限。本研究亦建構一個三維有限元素分析模型，用以分析介面脫層及層狀挫曲破損機制與ITO/PET薄膜導電性改變之間的關聯性。數值模擬分析結果顯示，承受張力負載之ITO/PET薄膜的挫曲量明顯高於其承受壓應力負載時之挫曲量，提供了介面脫層及層狀挫曲破損機制造成ITO/PET薄膜導電性改變的證據。另外，當ITO厚度及既存缺陷尺寸變大時，ITO/PET薄膜承受張應力之挫曲量將會明顯地提升。ITO/PET薄膜受拉伸負載的方向與橢圓形裂紋長軸之間的夾為0°時，挫曲量將會小於其他負載角度。當橢圓裂紋的長短軸比增加時，ITO/PET薄膜的挫曲量亦會增加。在第二部分，主要探討純循環撓曲以及循環結合靜態撓曲作用對封裝薄膜之阻絕能力的影響，封裝薄膜在承受循環撓曲作用後，透過鈣測試法量測其水氣滲透率。實驗結果顯示，在承受一定次數之循環撓曲後，封裝薄膜會產生微裂縫，這些微裂縫會提升封裝薄膜之水氣滲透率。在同一撓曲半徑下，撓曲作用次數愈多，會對封裝薄膜造成更多傷害進而造成更大程度的溼氣侵入。在較少循環撓曲作用次數時(≤ 104週期)，愈小的撓曲半徑將會對封裝薄膜帶來較大的傷害。然而，在承受105次循環撓曲作用後，撓曲負載對封裝薄膜帶來的傷害和曲率半徑的大小無關。本研究亦建構一個簡化的三維有限元素數值分析模型，用來分析溼氣在封裝薄膜內部的擴散情形。數值模擬分析結果顯示，在有裂紋以及介面脫層存在的情況下，水氣滲透率明顯上升。實驗結果之水氣滲透率與模擬結果之水氣滲透率有良好的一致性，證明撓曲作用產生的損傷與裂紋及介面脫層有關，說明此有限元素分析模型可有效預測撓曲損傷封裝薄膜之水氣滲透率。在結合循環與靜態撓曲負載的作用下，較短的靜態撓曲作用時間對封裝薄膜的損傷影響有限，循環撓曲對封裝薄膜之傷害較為明顯。然而，當撓曲半徑較小時，靜態撓曲作用時間拉長之後，靜態撓曲亦會對封裝薄膜造成明顯的損傷。再者，於循環撓曲結合靜態撓曲作用下，小曲率半徑對封裝薄膜所造成之傷害大於大曲率半徑對封裝薄膜所造成之傷害。 ;The aim of this work is to investigate the long-term durability of highly flexible thin film components for use in flexible optoelectronic devices. In the first phase, effects of cyclic deformation and annealing treatment on conductive durability of indium tin oxide (ITO) thin film deposited on polyethylene terephthalate (PET) substrate are investigated. In-situ electrical and mechanical tests of ITO/PET sheet under various combinations of cyclic and static loadings are conducted at room temperature. Experimental results show that the number of cycles to failure is significantly decreased with an increase in displacement amplitude, given a specific extent of electrical resistance change of ITO/PET sheet. A static holding period of 1000 s in various loading modes plays a role in influencing the failure of ITO/PET sheet. Cyclic bending combined with a static tensile holding generally generates more damage and a smaller number of cycles to failure than does that combined with a compressive holding, neutral holding, or no holding. Under a small fatigue loading, the conductive durability of ITO/PET sheet is increased with an increase in annealing temperature. However, there is little effect of annealing temperature on ITO/PET fatigue life under a larger displacement amplitude of fatigue loading. Using a surface-based cohesive modeling technique, a simplified three-dimensional (3D) finite element analysis (FEA) micromodel subjected to tensile and compressive loadings is numerically analyzed to clarify the failure mechanism of interfacial and buckling-like delamination which governs the change in electrical conductivity of ITO/PET sheet. Modeling results indicate that buckle height of the ITO/PET micromodel subjected to tensile loading is significantly greater than that of compressive loading, providing more evidence of the aforementioned effect of loading mode. In addition, buckle height of the ITO/PET micromodel subjected to tensile loading is significantly increased with an increase in ITO thickness and size of preexisting crack. Buckle height of the ITO/PET micromodel subjected to an angle of 0° between tensile loading and major axis of an elliptical crack is much smaller than that of other loading directions. Buckle height is also increased with an increase in aspect ratio of elliptical crack. In the second phase, effects of pure cyclic and combined cyclic-static bending on the encapsulation properties of a barrier thin film are investigated. Water vapor transmission rate (WVTR) of the given barrier film is measured after variously cyclic bending conditions using a calcium (Ca) corrosion test technique. Experimental results show that microcracks in the barrier thin film are found after applying a certain number of bending cycles. They are responsible for an increase in WVTR. Given a bending radius, a greater number of bending cycles leads to a larger amount of damage, and consequently a greater extent of moisture ingress. A smaller bending radius produces a greater amount of damage than a larger one, in a short period of loading time (≤ 104 cycles). However, after 105 cycles of cyclic bending, the amount of damage reaches a saturated level regardless of bending radius, as all the WVTR values become comparable. A simplified 3D FEA model is established in microscale to analyze the moisture diffusion mechanism. Numerical results show that, with the presence of cracking and delamination, the WVTR value increases significantly. Good agreement between the simulation and experimental measurements on WVTR confirms that the failure mechanism involves cracking and delamination under cyclic bending. The 3D FEA modeling developed could offer a method to predict the change of WVTR in correlation with cracking and delamination in the barrier thin film. For a short holding period of static bending in the combined cyclic-static case, there is no obvious effect of static bending on barrier performance. Cyclic bending takes a main role in damaging the given barrier film. However, given a longer holding period of static bending, the contribution of static bending to deterioration in the encapsulation performance of barrier thin film is clear for a smaller bending radius. Moreover, a cyclic bending combined with static holding under a smaller bending radius generates a larger amount of damage than that of a larger bending radius.
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