- 學位論文
Crystallization dynamics in quenched 2D Yukawa liquids
Crystallization dynamics in quenched 2D Yukawa liquids
摘要
二微焠火湯川液體已透過分子動力模擬來建構並觀察，我們發現在結晶化的過程中，粒子會合作的結構重整，同時，晶粒也會不斷成長並與其他晶粒結合。我們更進一步的重要發現為：晶粒的結合是必須透過晶界前端的晶粒的破裂，將原本的晶粒破裂成更小塊且同時轉動的晶粒，待小塊晶粒完成小角度的旋轉後，晶軸重新被調整癒合成大塊晶粒，晶粒與晶粒間即透過這種動力方式不斷結合，晶界也透過stick-slip的方式向前推進。晶粒的破裂是從結構上的脆弱點也就是dislocation的點開始順著其Burgers vector的方向破裂，晶粒破裂的同時也會引發dislocation的移動。當兩個擁有120° Burgers vector 夾角的dislocations結合時，往往也會伴隨著三個小塊晶粒的同時旋轉，待兩個dislocations結合後，系統中的拓樸缺陷數隨之減少，晶界也同時往前推進。這更解釋了晶界的曲率與晶粒結合的關係，因為曲率大的晶粒才有可能延續並確保dislocation的結合進而使得晶粒結合，反之，一平滑的晶界就意味著當中的dislocations其Burgers vectors是互相平行的，這樣彼此平行的Burgers vector是無法使dislocation結合而讓拓樸缺陷數減少的，在這樣的狀態下會引發晶粒結合的停滯。
並列摘要
The cooperative structural rearrangement in the crystallization of a quenched 2D Yukawa liquid is investigated through molecular dynamics simulation. It is found that, in the grain coalescence, cracking the region in front of the grain boundary into smaller sub-grains co-rotating with small angle, followed by healing, is the key for aligning the lattice mis-orientation and inducing grain boundary stick-slip motion. Cracking is initiated from the weakly interlocked dislocation along its Burgers vector, which in turn causes dislocation motion along the crack. The recombination of two dislocations with 120$o$ Burgers vector angle difference through exciting three co-rotation sub-grains is the dominant process for defect reduction associated with grain boundary migration. A grain boundary with large curvature and roughness can support cascaded dislocation recombination process and induces high mobility of the grain boundary. Along the smooth grain boundary, the parallel Burgers vectors of the string of dislocations forbid defect reduction and induce coalescence stagnation.