https://scholars.lib.ntu.edu.tw/handle/123456789/168186
標題: | 以古典及第一原理分子模擬探討莫來石基本材料性質及熱學性質與點缺陷的關連 Molecular-dynamics and first-principles study of point defects on static and thermal properties of mullite |
作者: | 陳仁彰 Chen, Jen-Chang |
關鍵字: | 莫來石;點缺陷;傳統分子動力學;第一原理計算;晶格常數;熱膨脹;擴散;mullite;point defect;classical molecular dynamics;first-principles simulation;lattice constants;thermal expansion;diffusion | 公開日期: | 2007 | 摘要: | 莫來石由於具有優異的熱機械性質與耐久性,因此廣泛地被應用於高溫結構陶瓷。從晶體結構觀點來探討莫來石的性質,發現其結構具有大量的氧空缺以及鋁原子會取代特定矽原子,到目前為止試驗結果的解釋也都圍繞著點缺陷的影響來探討莫來石的性質,然而有時不同性質的解釋,會提出不同的模型來分析,往往造成矛盾,因此本研究利用電腦模擬來研究單一點缺陷與缺陷組合對基本結構性質與熱學性質的影響。 由於莫來石的晶體結構相當複雜,我們首先提出一個合適的原子模型,進而使用傳統分子動力學來研究不同比例的點缺陷對晶格常數的影響以及在高溫下莫來石的晶格常數變化,藉以推算熱膨脹係數,另外也計算各原子受到高溫驅動下的自體擴散係數。為了釐清單一缺陷與特定缺陷組合對常溫與高溫下的晶格常數影響,我們使用第一原理全能量計算,另外我們也利用該計算方法來計算產生單一缺陷與特定缺陷組合所需的能量,以及透過施加外壓力於原子模型,然後利用狀態方程式擬合,藉以計算體模數係數來明瞭壓力對特定缺陷結構的影響。另外我們也使用了第一原理分子動力學來探討高溫下莫來石的結構變化,最後為了釐清氧擴散的機制,我們利用微彈性帶(Climb-image nudged elastic band)方法來計算兩種氧擴散路徑所需要的能量。 研究結果顯示莫來石的平均結構模型是非常適合用來電腦模擬,晶格常數受不同氧缺陷含量與鋁取代矽的交互影響造成b軸會隨著缺陷量增加而往下掉,而單一氧缺陷會造成晶格縮小,而鋁取代矽造成晶格膨脹,莫來石特有的缺陷組合(氧缺陷加上兩個鋁取代矽)會造成a軸增加,b軸減少。外加等向壓力於晶格上,無論何種缺陷結構,晶格都會縮小。高溫下莫來石的晶格常數是隨著溫度增加而增加,然而到攝氏1200度後,a軸與b軸斜率突然大增,而c軸變的比較小。在常溫下,AlO6的鍵長是影響莫來石的晶格變化最主要的因素,然而在高溫下,以莫來石而言最主要的是SiO4的鍵長變化,透過能量分析(Energy landscape)證實鍵長變化跟玻璃相轉換的形成有關。 氧空缺在O(C)位置以及鋁取代矽所需要的缺陷能量最低,因此這兩種缺陷最容易產生,而以最小平方法來估算原子的自體擴散係數似乎不適合用在莫來石上,計算的自體擴散係數與試驗值有很大的誤差。莫來石的氧擴散是氧O(C)位置移動,如果是完全的氧原子躍遷到已存在的結構缺陷,所需的能量非常大,約是15電子伏特,然而從試驗的活化能與微彈性帶計算的活化能顯示,氧原子並沒有完全跑到結構缺陷中,只是稍微往該缺陷方向動一下而已,而從第一原理分子動力學計算也觀察到這個現象。 本研究所建立的研究方法與流程適合應用在陶瓷材料的模擬與分析,而我們已經探討了莫來石的缺陷對晶體結構、晶格常數、熱膨脹、體模數、缺陷能、氧擴散的影響,接下來可以朝向以下幾點深入研究,包含研究熱狀態方程式、找出最適當的方法來計算擴散係數、提出更好的分子勢能模型、研究莫來石在高溫下的相變化與力學性質。 Mullite has extraordinary thermo-mechanical properties and durability; therefore, it is a strong candidate for high-temperature structural applications. From the structural viewpoints, such properties are strongly interwoven with the oxygen vacancies and Si atoms replaced by Al atoms. Based on experimental observations, several mechanisms have been proposed to explain mullite properties. However, some of them are controversial in which further insights are much needed. The objective of this work is to fulfill the needs through atomistic simulations. We applied classical molecular dynamics and first-principles simulations to study the effect of point defects on static and thermal properties of mullite. Due to the existence of partial atom occupancy of oxygen, aluminum, and silicon atoms in mullite, we first proposed suitable atomistic models of mullite for computer simulations. We then utilized classical molecular dynamics to calculate the lattice constants with different mullite compositions, the change of lattice constants with temperatures, and self-diffusivity of each species at high-temperature. In order to clarify the effects of the single point defect and defect-pair, the ab initio total-energy calculation was used to compute the lattice constants of different defective structures and defect formation energy for each single point defect and the defect pair. We also applied the third Birch-Murnaghan equation-of-state fitting to calculate bulk modulus of mullite with different external pressures. Furthermore, ab- initio molecular dynamics simulation was applied to study the structural change of mullite at high temperature. Finally, we used the climb-image nudged elastic band method to evaluate the activation energies of two migration paths for oxygen atoms. The simulation results show that the average structure model of mullite is suitable for the computer simulations. The increase of oxygen vacancies and Si atoms replaced by Al atoms in mullite causes the decrease of lattice constant b. The single oxygen vacancy reduces the lattice constants, and the single Si atom replaced by Al atom expands the lattices. The special defect pair (one oxygen vacancy with two Si atoms replaced by Al atoms) for mullite causes the increase of lattice constant a but decrease of lattice constant b. External pressure on all defective structures shortens all lattice constants. The lattice constants of mullite expand while temperature is increasing from 0℃ to 1200℃. At 1200℃-1400℃, lattice constants a and b increase rapidly, but lattice constant c decreases slightly. The bond lengths in AlO6 units affect lattice constants at room temperature, but at high temperature the bond lengths of SiO4 mainly influence lattice constants of mullite. We assert that such phenomenological change is related to the glass transition. This assertion is further reinforced by analysis of energy landscape. Oxygen vacancy at O(C) site and the Si atom replaced by Al atom have the lowest formation energy; therefore, these might explain why the two defects occur normally in mullite. The calculated self-diffusivity values for each species using the mean-square-displacement method do not match the experimental ones. This indicates that the mean-square-displacement method might not appropriate to study the diffusion of mullite. From nudged elastic band analysis, oxygen diffusion in mullite is the migration of oxygen atom at O(C) site. The activation energy for the oxygen to be fully hopping into the existed vacancy is around 15 eV. The value is quite high and thus such migration path is not likely happen during the diffusion. According to the experimental values and the nudged elastic band calculation, the oxygen at O(C) site moves slightly toward the existed structural vacancy only. We also confirmed such small movement in the ab-initio molecular dynamics simulation. We have created a research process for conducting the atomistic simulation of ceramics. The anomalies of lattice constants and thermal expansion and oxygen diffusion have been revealed in this study. Moreover, the effects of point defects within the structure and equation of state of mullite with different pressure have also been studied. Despite significant progress has been made in this study on mullite simulation, much work remains to be done in the future. For example, it is stall need to study the thermal equation-of-state, to find a proper methodology for calculating self-diffusivity, to propose a better potential model for mullite and sillimanite, to calculate mechanical properties at high temperature, and to analyze the structural transformation at high temperature. |
URI: | http://ntur.lib.ntu.edu.tw//handle/246246/50266 | 其他識別: | en-US |
顯示於: | 土木工程學系 |
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