以第一原理模擬探討莫來石在高溫下氧跳動之動態變換機制與氧擴散路徑

First Principle Study on Oxygen Hopping, Dynamical Site-Exchange and Diffusion Path in Mullite

郭品岑(Pin-Tsen Kuo)

指導教授 ： 陳俊杉

國立臺灣大學/工學院/土木工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.00152

摘要

莫來石是許多先進陶瓷、塗料、纖維和陶瓷複合材料的主要成分。它具有比較低的熱膨脹係數、低熱傳導、耐高溫等特性，使得其在高溫下依然可以保持穩定的特性質，因此，莫來石被視為一個重要的高溫陶瓷材料。
由於莫來石在高溫下的物理性質會有特異性，產生的相關機制一直是莫來石研究的議題之一。一般認為，此特異性和莫來石中氧離子的擴散(跳動)現象相關。本研究中，我們利用密度泛函理論為主要的模擬方法和微動彈性帶的計算來探討莫來石在高溫下產生特異性的原因。我們設計了兩條擴散路徑來模擬莫來石在不同溫度下氧離子跳動(擴散)到氧空缺的現象。其一是氧離子沿著c軸做跳動，使得空缺在c軸方向遷移。另一個路徑則是空缺在a-b平面上作遷移，這兩個路徑互相垂直。
從跳動的模擬結果，我們可以觀察到溫度大約在1200K時氧離子才會開始有明顯的跳動現象。這個結果和德國學者Schneider提出的莫來石動態變換機制互相呼應，同時，也說明了氧離子在高溫下的跳動現象和莫來石在物理性質的特異性上有著關聯性。在比較兩個不同路徑的擴散結果和實驗值後，我們認為本研究中所設計a-b平面的擴散路徑是合理且接近事實的。而我們也提出了一套機制解釋了莫來石中氧離子的擴散現象，同時釐清了結構中的空缺在此所扮演的腳色。

關鍵字

莫來石 ； 模擬 ； 密度泛函理論 ； 跳動 ； 擴散

並列摘要

Mullite is the major phase of conventional and advanced silicate-based ceramics, coatings, fibers and ceramic matrix composites. It has low thermal expansion, low thermal conductivity, good chemical stability and high creep resistance in high temperature. These features have made mullite materials one of the best candidates for structural and high-temperature applications.
Mullite is known to exhibit an anomalous heat capacity and thermal expansion and isotropic character of diffusion. In this study, the first-principles density functional theory (DFT) simulation and nudge elastic band calculations were applied to elucidate the mechanisms responsible for these counter-intuitive phenomena at high temperature. We design two migration paths. The first one is oxygen hopping to vacancy along the c-axis. The other one is vacancy diffuses on the a-b plane. The two paths are perpendicular to each other.
　　　From the calculations, we found that oxygen starts hopping intensely after temperature exceeds 1200K. This findings support the “dynamical site-exchange” mechanism proposed previously and is deemed as the main reason for anomaly of heat capacity in the temperature after 1200 degrees (K). Comparing the result of diffusivity of two different paths to experimental data, we asserted that the result of vacancy diffusion on the a-b plane is the plausible path. The proposed in-plane diffusion path supports the isotropic character of diffusion coefficient measurements and clarifies the role of structural vacancies for diffusion in mullite.

並列關鍵字

Mullite ； Simulation ； Density functional theory(DFT) ； Diffusion ； Hopping

參考文獻

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