臺灣博碩士論文加值系統

本研究藉由以往觀察θ→α-Al2O3相轉換過程的機制，依據等溫試驗的活化能分析原理，探討該相轉換過程所包含的（1）θ-晶粒成長、（2）θ-臨界晶粒轉換成α-核晶、（3）α-核晶聚合成長達基礎晶粒dp-Al2O3等各階段的活化能。實驗以三種凝聚程度較低，平均晶徑不同的θ-Al2O3粉末（S、M和L），採用三種不同的溫度持溫熱處理進行等溫試驗。所得樣品，則個別分析其θ-Al2O3的平均晶徑、α-Al2O3及dp-Al2O3的生成量以與時間建立關係。藉Grain growth與JMA速率式，觀察θ-晶粒成長與相轉換活化能，相轉換過程的α-核晶生成與聚合成長活化能，由Exponential rate law及連續一階反應理論分析，另由此等結果分析影響個別階段活化能所存在的影響因素。
分析主要影響相轉換活化能的因素，應為θ-晶粒的大小與各晶粒間的空間分佈（距離）。二者影響θ-晶粒的成長過程及後續的相轉換各階段動作會不一致。
反之如一θ-粉體系統中所有的θ-晶粒均可同時發生θ至α相轉換，也即系統中之θ-晶粒皆可同時或已達相轉換臨界晶徑，則α-核晶可同時出現，α-核晶聚合成長至基礎晶徑也可同時發生，則可獲致粒徑均一的α-Al2O3晶粒，而此條件也使相轉換活化能降至最低。
分析各階段的活化能可發現：θ-晶粒成長的活化能，隨起始θ-粉末的平均晶徑之變大而升高。原因可能有二，一為粗θ-晶粒受熱驅動擴散較細θ-晶粒不易，另一則是粗θ-晶粒粉末中，部分先達成相轉換臨界晶徑的θ-晶粒將先相轉換，這些早出現的α-Al2O3晶粒有阻礙θ-晶粒的成長現象，因而粗θ-晶粒系統的θ-晶粒其成長活化能變高。推測此影響因素在粉末愈細時，漸不明顯。當θ-晶粒為11 nm時，其成長活化能約為81 kJ/mol。
由等溫及非等溫的試驗結果發現，α-核晶的生成活化能隨起始θ-粉末系統的平均晶徑之趨近相轉換臨界晶徑而降低。此原因係粗的起始θ-粉末有較多接近相轉換臨界晶徑的θ-晶粒，可省去θ-晶粒成長階段所需的能耗即發生相轉換。在由一顆達臨界晶徑的θ-晶粒轉換為一顆α-核晶的活化能約為85 kJ/mol。而由α-核晶聚合成長至基礎晶粒的活化能，如發生於θ-晶粒皆為相轉換臨界晶徑之條件下為215 kJ/mol。
探究真實θ-粉體系統在相轉換過程的孕核與聚合成長活化能，發現各階段都會受其前一階段已產生結果的影響；θ-晶粒成長為臨界晶徑的結果影響α-核晶生成的活化能；同樣α-核晶間的距離影響聚合的活化能。另外θ-臨界晶粒（dcθ）的達成速率（即α-核晶的生成速率），也可能為聚合成長的另一影響因素，將使活化能改變。觀察α-核晶生成及α-核晶聚合成長至基礎晶徑的個別段的活化能大小，若無阻礙反應的因素存在，當有足量的α-核晶生成時，聚合成長應可旋即發生。因此，亦說明θ→α-Al2O3相轉換的控制步驟，可能需視原始粉末的θ-晶粒的大小與其空間分佈關係而定。
本研究推測在θ→α-Al2O3相轉換出現的三個階段，其活化能數值分別為：θ-晶粒成長至臨界晶徑的活化能：81∼114 kJ/mol、θ-臨界晶粒轉換成α-核晶的活化能：85∼130 kJ/mol、α-核晶聚合成長至基礎晶徑的活化能：215 kJ/mol（但各階段的實際值可能較上述推測值更低）。整體的相轉換活化能應為380∼460 kJ/mol。
細的θ-Al2O3粉體提供系統接近類均質孕核反應，但θ-晶粒成長需消耗較多的能量。而粗的θ-Al2O3粉體系統，相轉換能量低，卻受制於反應時間無法集中發生。因此，控制反應過程於時間及空間的一致性，為製備均一晶徑的奈米級（~50 nm）α-Al2O3粉末的重要關鍵。

The activation energy of θ- to α-Al2O3 phase transformation of well-dispersed θ-Al2O3 powder systems from derived boehmite was investigated in this study. The phase transformation are subdivided into three stages: (1) growth of θ-crystallite, (2) one θ-crystal transforms to oneα-crystal and (3) the growth of α-crystal. Calculation of activation energy than based on isothermal models of Grain growth, JMA and Exponential rate equation, and the results are 81∼114, 85∼130 and 215 kJ/mol, for (1), (2) and (3) stages, respectively. The total activation energy of the phase transformation is estimated amounting to 380∼460 kJ/mol.
The activation energy for θ-crystallite growth increases as the θ-crystallite’s size coarsening. This is because it is difficult for the coarse θ-crystallite to diffusion during the thermal treatment compared with that of the smaller size θ-crystallite. Additionally, the α-Al2O3 particles transformed earlier from the coarser θ-crystallites would retard the θ-crystallite growth. It is found, starting θ-crystallites size with 11 nm would show lowest activation energy of θ-crystallite being ∼81 kJ/mol.
The activation energy of α-nucleus formation was decreased as the θ-crystallite close to θ-critical size. It is because the existence of large amounts of coarser θ-crystallite, may save the energy consumption that requires for the crystallite growth of the θ-powder system.
The activation energies of α-nucleus formation and the dp-crystallite formation from α-nuclei may highly related to the agglomeration state of the starting θ-crystallites, because the inter-crystallite distance affects the growth of θ-crystallite as well as the coalescence of dp-crystallite from the previously formed α-nuclei. When the number of α-nuclei is sufficient then a dp-crystallite will occur immediately if the reaction can take place freely.
Although, it is more possible for θ-Al2O3 powder systems with finer crystallite sizes to obtain, a quasi-homogeneous reaction during θ- to α-phase transformation, it needs more energy consumption for θ-crystallite growth. Contrarily θ-powder systems with coarser sizes will show low activation energy of phase transformation, but the reaction can heterogeneous. Thus, how to obtain the reaction of θ→α-Al2O3 phase transformation through homogenous reaction at a low activation energy of reaction is the key point to fabricate nano-sized α-Al2O3 powders.

摘要Ⅰ
AbstractⅢ
致謝Ⅴ
表目錄Ⅷ
圖目錄Ⅸ
附錄ⅩⅢ

第一章 緒論1
1.1 前言1
1.2 研究目的2

第二章 理論基礎與前人研究4
2.1 θ→α-Al2O3相轉換4
2.1.1 Ostwald ripening的晶粒粗化現象4
2.1.2 成核成長理論6
2.1.3 臨界晶徑與基礎晶徑6
2.2 非均質與類均質的相轉換7
2.2.1 相轉換的示差熱行為7
2.2.2 相轉換的維差特性10
2.3 相轉換動力學12
2.3.1 晶粒成長反應速率式12
2.3.2 相轉換反應速率式13
2.3.3 θ→α-Al2O3相轉換活化能15

第三章 研究方法及步驟19
3.1 實驗原料19
3.2 實驗步驟20
3.3 特性分析20
3.3.1 粉末結晶相分析20
3.3.2 熱差分析20
3.3.3 晶徑及粒徑分析20
3.3.4 相轉換定量分析23
3.3.5 顯微影像及結構分析23
3.3.6 臨界晶徑及基礎晶徑數含量比例計算23

第四章 結果與討論25
4.1 粉末的結晶相分析25
4.2 酒精研磨的解凝效應25
4.3 θ-Al2O3的晶粒成長機制及活化能30
4.3.1 θ-Al2O3晶粒的粗化模式30
4.3.2 Grain growth速率式觀察θ-Al2O3成長活化能34
4.4 θ→α-Al2O3的相轉換活化能38
4.4.1 JMA速率式觀察相轉換活化能43
4.4.2 Exponential rate law觀察α-核晶生成及聚合成長活化能47
4.5 綜合討論58
第五章 結論60
參考文獻62
附錄67

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