本研究利用分子動力學模擬以Buckingham勢能進行不同粒徑3.36nm和4.49nm之ThO2奈米陶瓷顆粒的燒結研究。本研究創新地設計三維燒結模型，將同一尺寸顆粒在三維空間中以面心方式堆積建立三維燒結模型，進行不同燒結溫度1800K至2200K的燒結模擬，可以直接計算出各種控制變因對於孔隙率變化的影響。由於實際上燒結樣品中每個晶粒之間可能有各種不同排列方式，而不同排列方式可能存在各種初始接觸面形式，可能影響燒結結果，我們將探討不同初始接觸面組合對燒結緻密程度之影響。但由於模擬受限於計算時間等因素，無法完全達到實際上之條件，因此本研究僅以有限的例子進行計算。首先為了驗證有限的初始接觸面種類模型是否有辦法近似實際燒結的情形，我們針對以不同初始接觸面組合模型進行三次燒結模擬測試，結果顯示粒徑3.36nm小顆粒模型在溫度1800K時，顆粒之間部分相互吸引也有些相互排斥，使顆粒無法均勻分布使燒結結果重複性低，但當溫度1900K以上時，無論是粒徑3.36nm或4.49nm的顆粒，所有初始接觸面組合模型皆有很高的一致性。證明在有限的模型模擬結果，當溫度夠高時無論晶粒初始排列方式為何，若顆粒分布均勻燒結結果都能較高的一致性，可符合實際上燒結實驗時晶粒方向任意排列之情況。
接著本研究分別探討在燒結初期時燒結溫度、顆粒尺寸等條件對ThO2燒結之影響，我們亦觀察到燒結初期有顆粒旋轉的現象，可能是受到顆粒偶極-偶極相互作用力所致。關於燒結溫度對緻密化行為的影響，在實際上燒結緻密度和溫度正相關，模擬也可以做相似的研究，在本研究中以孔隙率描述緻密化行為，結果顯示無論粒徑大小，當燒結溫度1900K以上時燒結結果一致性佳，且皆有較低之孔隙率。此燒結溫度實驗值2200K相較已證明這是最低的燒結溫度。接下來討論顆粒尺寸和分布均勻度之影響結果發現未施加外壓時，若顆粒均勻混和則溫度1900K以上小顆粒平均孔隙率可達約9%-10%，大顆粒則大約在11%-14%。我們也針對小顆粒在1800K時顆粒不均勻導致高孔隙率的模型進行施加外壓500MPa的模擬測試，結果顯示給予外壓後顆粒可均勻分布使最後孔隙率降低。
本研究中的燒結緻密機制是可分成顆粒旋轉機制、接觸面微調機制和表面擴散機制三種，在燒結過程中的第一階段為燒結開始至大約0.04ns，顆粒會旋轉調整晶向並逐漸接近直到接觸，旋轉趨動力推測為顆粒間的偶極相互作用力。到了第二階段(t=0.04~0.4ns)有些顆粒仍有些微旋轉以進行接觸面的調整，第二階段的主要機制為接觸面的微調作用，目的為讓接觸面達到最低能量之特定某些接觸面。第三階段是從0.4ns直到2.5ns，此時僅剩下表面氧原子仍繼續擴散。故第三階段的主要燒結機制為表面氧的擴散作用。根據本研究之結果，當顆粒排列均勻的前提下，無論顆粒排列方式只要燒結溫度在1900K以上，最終孔隙率可達9-10%，相較於Kutty, et al. [1]的單相ThO2燃料芯塊(pellet)的燒結實驗，ThO2在溫度1650oC (1923K)常壓下燒結的孔隙率13%。我們已知當顆粒粒徑越小時燒結緻密度越高，由於其實驗中顆粒粒徑為微米尺度，而我們的模型中顆粒為奈米尺度，因此會有較低的孔隙率。

In recent times, thorium-based materials have regained interest as a nuclear fuel due to their unique characteristics such as abundant resource, long fuel cycles, high burn up, and improved waste form characteristics, etc. However, the higher sintering temperature (>2200K) has hampered the commercialization of thorium dioxide fuel. To overcome the issue, molecular dynamics (MD) simulations is used to study the microstructure evolution of ThO2 nanoparticles during sintering at T=1800K-2200K below melting point. We used two sizes of ThO2 nanoparticles 3.36nm and 4.49nm in different crystal orientations, and the innovative three-dimensional fcc model was adopted to mimic the real sintering process. The effect of sintering temperature, porosity, the angle of particle reorientation, mean square displacement of Th and O in different regimes of ThO2 particle and grain growth were investigated.
By the results, we found that the system densified very well as the particle size is small and as T≧1900K. Here we classify three governing sintering mechanisms: particle reorientation, contact plane adjustment and surface diffusion in different stages. The simulation results would provide useful information on the synthesis of ThO2 nuclear fuel pellet.

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