鋯基合金因具備良好的抗腐蝕能力以及低熱中子接收截面，再加上適當的機械性質，而被廣泛應用於核能產業多年。然而在日本福島核電廠事故後，全世界對於核能安全議題更加的重視，燃料護套材料在嚴苛環境下的腐蝕行為以及強化抗腐蝕性質研究因此積極的發展。本文研究探討鋯-4合金在850℃下分別於純水氣、空氣，以及水氣與空氣混合氣氛中的氧化機制。同時，透過非平衡磁控濺鍍，鍍著高溫抗氧化能力極佳的氮化鉻薄膜作為擴散阻障層，以舒緩鋯-4燃料護套材料在高溫環境中的腐蝕行為。
研究結果顯示，鋯-4燃料護套材料於水氣與空氣環境下的氧化行為類似，在初期快速氧化後伴隨著由氧化性氣氛擴散主宰的週期性氧化行為，這樣的氧化模式與氧化層的破裂有密切的關聯。同時，由於空氣中氮氣參與反應的影響，造成多孔洞的氧化結構，進而導致氧化速率隨著空氣分壓的增加隨之上升。另一方面，氮化鉻薄膜在氧化速率極快的空氣環境下表現出優良的抗氧化能力，然而，隨著水氣比例的上升，大量的氫氣累積在薄膜與基板介面進而破裂，喪失其作為阻障層的保護能力。因此，在使用鍍膜護套的情況下，由於空氣與水氣兩種氧化氛圍的競爭，導致空氣與水氣各半的情況下可得到最慢的氧化速率。

Zirconium-based alloys (zircaloys) have been widely used as the cladding materials in nuclear industries owing to their desirable properties including high corrosion resistance, low thermal-neutron capture cross-section and appropriate mechanical properties. However, after Fukushima accident, several severe conditions have been taken into account, because Zr-base alloys would lose their corrosion resistance at high temperature. Moreover, those alloys react with the steam and produce zirconia and hydrogen, which may lead to the leakage of fission products and hydrogen explosion caused by the degradation of fuel cladding and the formation of hydrogen. Therefore, studies of oxidation mechanism and improvement of high temperature corrosion resistance of zircaloys have been aggressively conducted in decades. In this study, oxidation kinetics and mechanism of Zr-4 under different steam/air ratios would be investigated. Furthermore, CrN thin film was deposited on Zr-4 by unbalanced magnetron sputtering system (UBMS) to enhance corrosion resistance of Zr-4 at high temperature.
The results showed that the oxidation kinetic is strongly related to crack propagation inward the metal core, which causes the cyclic linear and parabolic rate kinetics. This mechanism occurs at each atmosphere. Additionally, the transformation of zirconium from nitride to oxide induced the formation of cracks and pores, and leaded to acceleration of oxidation. Therefore, the oxidation kinetic is in proportion to partial pressure of air. For coated samples, the hydrogen produced by the reduction of steam would accumulate beneath the thin film and cause the fracture at the early oxidation stage, so the competition of two kinds of oxidants, oxygen and steam, leading to the slowest time for reaching equivalent cladding reacted of 17%.

摘要…………………………………………………………………………………….Ⅰ
Abstract………………………………………………………………………………….Ⅱ
致謝…………………………………………………………………………………….Ⅲ
Content IV
Table VII
Figure VII
Equation VIII
Chapter 1 Introduction 18
1.1 Research Background 18
1.2 Purposes of Study 19
Chapter 2 Literature Review 21
2.1 Introduction of Nuclear Accidents 21
2.2 Cladding Materials 31
2.2.1. Selection of Cladding Materials 31
2.2.2. The Evolution of Cladding Materials 34
2.3 Chromium Nitride Thin Film 37
2.3.1. Selection of CrN 37
2.3.2. Introduction of UBMS 41
2.4 High Temperature Oxidation Mechanism 44
Chapter 3 Experimental 54
3.1 Specimens 54
3.2 Coating Process 54
3.3 High Temperature Oxidation Tests 56
3.4 Characterization Methods 59
3.4.1. Oxidation Test 59
3.4.2. Scanning Electron Microscopy 59
3.4.3. Dual-Beam Focused Ion Beam System 60
3.4.4. Atomic Force Microscope 61
3.4.5. Nano-Indenter 61
3.4.6. X-ray Photoelectron Spectroscopy 62
3.4.7. X-ray Diffraction and Grazing Incident X-ray Diffraction 63
Chapter 4 Results 65
4.1 Properties of Substrates and Thin Film 65
4.1.1. Substrates 65
4.1.2. Thin Films 68
4.2 High Temperature Tests in Pure Steam 71
4.2.1. Mass Change 71
4.2.2. Bare Samples 72
4.2.3. CrN Coated Samples 76
4.3 High Temperature Oxidation in Mixture environment 81
4.3.1. High Temperature Oxidation in 75% Steam/25% Air 81
4.3.2. High Temperature Oxidation in 50%Steam/50%Air 91
4.3.3. High Temperature Oxidation in 25%Steam/75%Air 101
4.4 High Temperature Oxidation in Dry Air 111
Chapter 5 Discussion 120
5.1 High Temperature Oxidation Mechanism of Zr-4 120
5.2 Crack Propagation Mechanism in Zr-4 129
5.2.1. Lateral Cracks 129
5.2.2. Percolation Cracks 134
5.3 Oxidation Kinetics 141
5.3.1. Oxidation Kinetics of Bare Zr-4 141
5.3.2. Oxidation Kinetics of Coated Sample 147
5.3.3. Evaluation of Coating Method 155
5.3.4. Oxidation Mechanism under Air and Steam 157
Chapter 6 Conclusions 161

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