六方氮化硼的力學以及熱學性質研究

Abstract

六方氮化硼(Hexagonal boron-nitride, h-BN)為一種結構類似於石墨烯(Graphene)的二維材料，由於在實驗以及計算上發現h-BN具有本質能隙(intrinsic bandgap)的重要電學特性，使其繼石墨烯之後成為極具潛力之電子元件材料。目前利用機械或化學剝離法、化學氣象沉積法所生長出來的h-BN經常伴隨缺陷的產生，因此我們將調查各種缺陷對於h-BN的力學與熱學性值影響。 本文所利用的方法為分子動力學模擬(Molecular dynamics)計算方法，使用在熱學性質上較為準確的力場函數(potential)，幫助我們提高計算的可靠性。接著我們以此為基礎計算了完美h-BN晶格的熱傳以及力學特性，並研究了受到缺陷時在這些缺陷上的變化。 首先在奈米帶的情形下，我們探討了手性角(chiral angles)對奈米帶的熱傳以及力學影響，並得到在兩個最常見角度；鋸齒形(Zigzag)以及手扶椅形(Armchair)會有最高的熱傳能力，而在中間角度10度~13度時熱傳能力最低，而力學性質方面各角度的奈米帶楊氏係數值差異不大，但皆小於完美h-BN狀態的數值。而在孔洞缺陷的部分，我們首先從能量觀點出發，得到在刪除同樣數目的原子下，由多個原子所構成的孔洞缺陷能量，會比單原子孔洞的能量低，同時在熱傳能力的計算上我們得到在同樣的刪除原子數目上，大洞對熱傳影響小於小洞的影響。 最後我們將討論晶界缺陷的影響，由第一定理計算得到晶界中的差排存4|8環的排列形式，利用分子動力學模擬生成同樣結構，並以原子級應力計算觀察差排中的4環以及8環分別受到壓縮以及拉伸應力。而差排的存在對於力學的影響上，我們發現差排的存在會使斷裂強度下降，但隨著差排密度的提高，斷裂強度反而有所提升。

Hexagonal boron-nitride (h-BN) nanosheets are promising materials for the next generation electronic devices. In this work, we systematically investigated the dependencies of h-BN thermal conductivities on nanoribbon edge chiral angles, vacancy concentration by carrying out a series of non-equilibrium molecular dynamics (NEMD) simulations. Our simulation results indicate the thermal conductivities of BN nanoribbons have similar edge chiral angle dependencies with graphene nanoribbons, and longer BN nanoribbons yield higher thermal conductivities. Furthermore, the present study also reveals that thermal conductivity of BN nanosheets undergoes significant drops due to phonon scattering induced by vacancies. We also found that large vacancies are energetically more favorable than small vacancies, implying the aggregation of small vacancies into vacancy clusters, thereby minimizing thermal conductivity drops of BN nanosheets. We also construct the dislocations and grain boundaries based on the geometries calculated by first principle method. In these results, we have learned that 4|8dislocation pairs can exists more energetically favorable than 5|7 dislocation pair due its unpolar property. With this statement, we started our study by constructing more grain boundaries spread in a wide misorientation angles. Our results have showed an contradiction to dislocation theorem which dislocations would reduce the mechanical strength several orders from pristine structure. But it is actually performing that mechanical strength can’t decrease over an order of the strength, and even increased when dislocation density become higher.