以平衡分子動力學模擬探討碲化鉍雙晶界結構對熱傳導性質之影響

An Investigation of the Thermal boundary resistance associated with the Twin Boundary in Bismuth Telluride in use of the Equilibrium Molecular Dynamics Simulation

黃重穎(Chung-Ying Hwang)

指導教授 ： 黃美嬌

國立臺灣大學/工學院/機械工程學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201701471

摘要

本論文採用平衡分子動力學數值工具來研究碲化鉍單晶系統及具雙晶界之系統的聲子色散關係、聲子態密度及晶格熱傳導係數。原子間的交互作用力考慮了二體勢能函數、三體勢能函數及庫倫勢能函數。
在單晶系統的模擬中，於聲子色散關係的部份，考慮三體勢能會使聲頻支斜率增加，並使得低頻區聲子態密度分佈範圍增加，庫倫勢能則不會造成明顯影響。晶格熱傳導係數乃透過Green-Kubo relation求得，並以擬合函數得相關時間常數。研究發現熱傳導係數隨溫度上升而下降，且有明顯的非等向性。有考慮三體勢能時所得的熱傳導係數較大，額外添加庫倫勢能則略微降低熱傳能力。
在雙晶界系統的模擬中，發現具Te1原子層雙晶界之系統其界面能量改變不明顯，而具Te2及Bi原子層雙晶界之系統其界面能量略為上升，但對於聲子色散關係及聲子態密度的影響不大，僅多出一些橫波光頻支。與單晶系統相比，雙晶界會造成熱傳導係數在in plane及cross plane方向上約有20%~40%的下降，表示雙晶界會增加聲子的散射機會。其中，以具Te2及Bi原子層雙晶界之系統有較Te1雙晶界系統大的下降幅度；隨著雙晶界週期變小，熱傳導係數有降低的趨勢。

關鍵字

平衡分子動力學 ； 聲子色散關係 ； 聲子態密度 ； 晶格熱傳導係數 ； 碲化鉍雙晶界

並列摘要

This thesis employs the equilibrium molecular dynamics (EMD) simulation method to calculate the phonon dispersion relations, the phonon density of states (DOS) and the lattice thermal conductivity of bismuth telluride which has either a single crystal structure or is embedded with twin boundaries. We consider the two-body potential, the three-body potential and the coulomb potential to describe the interactions between atoms. Effects of the latter two are studied.
In the simulations of Bi2Te3 single crystal, the group velocities of acoustic phonons are larger and the low-frequency region of the DOS spectrum becomes broader when the three-body potential is taken into consideration. However, the Coulomb potential has no significant impact on the vibration modes. We obtain the lattice thermal conductivity in use of the Green-Kubo relation; a low-frequency filtering process, and an exponential fitting process with two time scales are utilized. The calculation results show that the thermal conductivity is anisotropic and decreases with increasing temperature. The thermal conductivities become larger when the three-body potential is included and drop a little by the Coulomb force.
When embedded with Te1 twin boundaries, the interfacial energy of the system does not change obviously, when embedded with Te2 or Bi twin boundaries, the interfacial energy of the system increases slightly, but the phonon dispersion relation and DOS are barely affected except the presence of a few new optical branches. Compared to the simulations of the single crystal, both the in-plane and cross-plane thermal conductivities decrease about 20%-40% by the twin boundaries, especially, the Te2 and the Bi twin boundaries. Furthermore, the thermal conductivities decrease as the period of twin boundaries decreases.

並列關鍵字

Equilibrium molecular dynamics ； phonon dispersion relation ； phonon density of states ； lattice thermal conductivity ； twin boundary of bismuth telluride

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