材料科學導論
六. 成核、結晶生長與擴散作用

 

1. 成核 (nucleation):
        
      定義— The formation of very small (often submicroscopic) particles, or nuclei, of the new phase,
                 which are capable of growing.
 
      A. 均質成核 (Homogeneous Nucleation)
 
              (driving force)— (Gv)
                  
                    GN = net free energy change
                    Gv = bulk free energy difference
                      r   = radius of a nucleus
                     = surface energy
              
              Critical radius of the stable nucleus (r*)
                  
 
      “        Free energy barrier to nucleation
                  
                        
              Effect of supercooling (undercooling)
                  
                  
                       HF = latent heat of fusion
                        TM = melting point
                       T = supercooling
 
      B. 非均質成核 (Heterogeneous Nucleation)
              大多數情形
              interfaces, grain boundaries, precipitates, and inclusions.
              critical nucleus is smaller, rate of formation is greater.
              unknown interactions between nucleus and other “impurities” difficult to analyze

 

2. 結晶生長 (crystal growth)
 
      固體中的顆粒長時間在高溫下會漸漸長大
      driving force reduction in interfacial area between grains.
 
      A. 固體/液體間平滑界面上的結晶生長
      B. 晶面上成核(Surface Nucleation)後「階」的生長
      C. 結晶的螺旋生長(Spiral Growth)
      D. 晶界上過冷(undercooling)對結晶生長速率的影響
      E. 燒結中的結晶生長 – 晶界移動
      F. 常用的人造晶體方法 (Synthetic Crystals Growing)
              (1)  Czochralski
              (2)  flame fusionc
              (3)  floating-zone
              (4)  hydrothermal


3. 擴散作用 (diffusion)
 
      定義— The movement of atoms or molecules in a material.
      [Kinetics—the totality of time-dependent mass transport and phase transformation phenomena in solids.]
 
      A. 擴散作用介紹
              擴散機制:空位擴散與插入物擴散(Vacancy diffusion and Interstitial diffusion)
              擴散作用的活化能 (activation energy, Q)
              擴散作用的種類 (types of diffusion)
              擴散作用的實例-- 銅-鎳擴散
              
      B. 穩定狀態下的擴散(steady-state diffusion, and D is a constant)
              Fick’s First Law
 
      C. 不穩定狀態下的擴散(non-steady-state diffusion, and D is a constant)
              Fick’s Second Law
              擴散來源的化學濃度可分為(1)固定和(2)變化
 
      D. 擴散係數(diffusion coefficient, D)隨溫度的變化
              
              Arrhenius plot
 
      E. 擴散接合 (diffusion bonding)
 
      F. 燒結作用 (sintering)
              球狀顆粒的燒結
              燒結作用的三種擴散機制—體、晶界、表面。
              燒結作用進行時的擴散情形
 
      G. 擴散作用有利的情況
              高溫 (at high T)
              擴散的原子小 (the diffusion atom is small [e.g., C in Fe])
              晶格鬆散 (the packing factor of the host structure is low [e.g., bcc vs. fcc])
              鍵結弱 (the bonds of the host structure are weak [e.g., low-melting materials])
              材料有缺陷 (there are defects in the material [e.g., vacancies or GB])
 

 

          問 題：為什麼在家裏冰箱做出來的冰塊是清澈透明的，而外面餐廳的冰塊常呈
                           乳白色？
          問 題：為什麼汽水在乾淨的玻璃杯中比較不易冒泡？

 

 

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