摘要
在壓電觸媒的研究領域中，水熱法合成的二硫化鉬(MoS2)奈米花具有少數層片狀的結構特性，此方法有別於一般商業的粉狀MoS2使材料壓電特性顯著提升。然而，奈米花結構除了有效提升壓電特性之外，若施加外力在花瓣上，使得花瓣上下擾動產生非對稱中心應變，則會產生撓曲電效應，此外撓曲電的尺度效應說明當尺度越小，則撓曲電效應愈強，在奈米尺度下甚至可比擬壓電效應。本研究透過有限元素模擬軟體COMSOL Multiphysics 5.4 (以下簡稱COMSOL) 將奈米花結構簡化為二維片狀懸臂梁結構模擬，透過壓電模組模擬壓電特性；另外以壓電模組維原型進行模組方程式改寫為撓曲電效應模組，進行撓曲電特性模擬，將結果與壓電特性進行比較。除此之外，在撓曲電領域之中，係數的研究一直是個難題。相較於壓電係數18個分量，撓曲電係數總共有54個分量，除了透過點群對稱減少需考慮的係數之外，本研究中利用COMSOL力學模組分析各種不同機械力下的應變梯度貢獻百分比進行更進一步的係數縮減，為計算撓曲電係數提供了一個更為便利的研究方法。


關鍵字：撓曲電、二硫化鉬、奈米花、懸臂樑、二維材料、COMSOL
 

Abstract
In a research area of piezo-catalyst, molybdenum disulfide (MoS2) synthesized through hydrothermal method with nanoflowers structure demonstrated superior piezoelectric properties compared to commercial powder. Besides, nanoflowers structure also possess superb flexoelectricity at nanoscale with when bending to the direction of layer In this work, finite element method (FEM) in COMSOL Multiphysics 5.4 was utilized to view nanoflowers structure as a cantilever to simulate piezoelectricity and flexoelectricity, particularly in nanoscale condition: nano-cantilever. In addition, flexoelectricity model which was derived from piezoelectricity analyze the nano-cantilever. In the field of flexoelectricity, percentage of strain gradient is difficult in calculation in experiments. However, it can be analyzed when a model was given a mechanical force in COMSOL. There are 54 numbers of components in flexoelectric coefficients. The numbers of components can be reducing by point group symmetry and percentage of gradient of strain. It lead to measuring flexoelectricity in an easy way that combine first principle, percentage of strain gradient and experiment.


Keywords: MoS2, nanoflowers, cantilever, piezoelectricity, flexoelectricity, COMSOL

目錄 ----------------------------------------------------V
圖目錄 ---------------------------------------------------VI
表目錄 ----------------------------------------------------X
第一章 緒論 --------------------------------------------1
1.1 前言 --------------------------------------------1
1.2 研究動機 --------------------------------------------2
第二章 文獻回顧 --------------------------------------------4
2.1 水解產氫 --------------------------------------------4
2.1.1 光觸媒 --------------------------------------------4
2.2 壓電觸媒 (piezo-catalyst) --------------------8
2.2.1 壓電效應的起源與原理 ----------------------------8
2.2.2 壓電催化之過程 -----------------------------------15
2.2.3 ZnO和BaTiO3壓電觸媒產氫 ---------------------------17
2.3 二維壓電材料 -----------------------------------20
2.3.1 二維過渡金屬硫屬化合物 (transition metal dichalcogenide ; TMD or TMDC) 壓電觸媒 ---------------------------21
2.3.2 TMDC之壓電觸媒研究 ---------------------------24
2.4 撓曲電效應 (Flexoelectricity effect；又稱彎曲電效應、正撓曲電效應) ---------------------------------------------------28
2.4.1 撓曲電效應之原理 -----------------------------------28
2.4.2 逆撓曲電效應 (converse flexoelectric effect) 與正撓曲電效應之轉換關係 ---------------------------------------------------29
2.4.3 撓曲電效應之對稱性 ---------------------------32
2.4.4 撓曲電效應之起源與近年來相關之研究 -------------------36
第三章 模擬分析與結果討論 -----------------------------------42
3.1 研究背景 -------------------------------------------42
3.2 COMSOL原理之介紹 -----------------------------------42
3.2.1 有限元素法 (Finite Element Method; FEM) 基本概念 ---42
3.2.2 COMSOL Multiphysics 5.4基本介紹 -------------------43
3.2.3 COMSOL 中壓電模組之介紹 ---------------------------45
3.2.4 COMSOL撓曲電模組之介紹 ---------------------------47
3.3 COMSOL應變梯度之分析 ---------------------------48
3.3.1 MoS2奈米磚應變梯度之分析 ---------------------------49
3.3.2 MoS2單層懸臂樑彎曲應變梯度之分析 -------------------59
3.4 MoS2撓曲電之非對稱中心缺陷效應 -------------------62
3.5 MoS2撓曲電之尺度效應 ---------------------------72
3.6 MoS2懸臂樑壓電效應與撓曲電效應 -------------------73
3.6.1 MoS2 層數堆疊效應 ---------------------------74
3.6.2 懸臂樑的撓曲電效應 ---------------------------78
3.6.2.1 懸臂樑彎曲與應變梯度的關係 -------------------------78
3.6.2.2 懸臂樑彎曲之撓曲電效應 -----------------------------82
3.6.3 懸臂樑之撓曲電效應與壓電效應比較 -------------------85
第四章 結論 -------------------------------------------87
第五章 未來展望 -------------------------------------------88
第六章 參考文獻 -------------------------------------------89

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