以原子尺度模擬探討莫來石之氧空缺跳躍對熱膨脹係數之影響

Abstract

莫來石(Mullite)在陶瓷材料中一直都是非常熱門的研究主題之一，其擁有優異的熱機械性質與耐久性的特性，主要應用在高溫環境。其結構中具有大量的氧空缺以及鋁原子會取代特定矽原子，與矽線石(Sillimanite)非常的相似，目前實驗也都圍繞著結構中缺陷所造成的影響來探討莫來石的性質，在實驗上，到目前為止還無法完全解釋高溫相變化(phase transformation)的情形，因此本研究利用分子動力模擬(Molecular Dynamics Simulation)來研究從低溫到攝氏1200度的加溫過程中，對莫來石的熱膨脹係數的影響，進而分析莫來石中氧原子跳躍(Hopping)的機制。 論文中使用Matsui勢能(Matsui, 1994 and 1996)(Matsui’s potential)，模擬不同配置空缺方法的莫來石模型，計算結果發現晶格常數的趨式與實驗相近，並且發現莫來石在高溫時熱膨脹係數有不規則現象的發現，而不會出現在矽線石上，除此之外還發現莫來石中高溫時氧與氧空缺會發生動態交換情況，進行分析了解其路徑。

Mullite is the major phase of many conventional silicate-based ceramics (e.g., porcelains and aluminosilicate refractory) and of various advanced ceramics, coatings, fibers and ceramic matrix composites. The high creep resistance and thermal/chemical stability of mullite have made it one of the best candidates for structural and high-temperature applications. Recently, a few anomalies associated with mechanical and thermal properties of mullite have been identified. In this thesis, the anomaly related to thermal expansion was studied using molecular dynamics simulation. The oxygen vacancy hopping mechanisms and their correlation with the anomaly were investigated. The Matsui interatomic potential was adapted herein. We calculated lattice constants of sillimanite and mullite at difference temperatures. The calculated results agreed well with the experiment measurements. Thermal expansion anomaly at high temperature was found in mullite but not in sillimanite. Oxygen vacancy densities were plotted. The dynamic site-exchange processes between O(C) and vacancy sites were found in mullite. These microscopic processes resulted in thermal expansion anomaly.