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研究生：	林浩東 Lin, Hao-Tung
論文名稱：	氧化鋁-碳化鉻奈米複合材料的製備及其性質之研究 Preparation, Microstructure Development and Mechanical Properties of Alumina - Chromium Carbide Nano-composite
指導教授：	黃肇瑞 Huang, Jow-Lay
學位類別：	博士 Doctor
系所名稱：	工學院 - 材料科學及工程學系 Department of Materials Science and Engineering
論文出版年：	2006
畢業學年度：	94
語文別：	中文
論文頁數：	109
中文關鍵詞：	有機化學氣相沉積、碳化鉻、流體床、氧化鉻、氧化鋁
外文關鍵詞：	fluidized bed, chromium carbide, Al2O3, MOCVD, Cr2O3
相關次數：	點閱：96 下載：3
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利用有機化學氣相沉積法結合流體化床技術，以六羰鉻Cr(CO)6為先導物和氧化鋁顆粒為基材，製備奈米級的鉻化物顆粒均勻的分散在氧化鋁顆粒上。先導物在流體床內裂解後的主要組成為氧化鉻(Cr2O3)、介穩相碳化鉻(CrC1-x)和碳(C ) 。當流體床的腔體溫度為300 oC和400 oC時，先導物裂解後沉積顆粒分別具有非晶和結晶的Cr2O3 相。經過1150 oC，10-4torr真空下，2小時高溫碳化熱處理後，300 oC流體床粉末中的氧化鉻碳化成碳化鉻(Cr3C2)，而400 oC流體床粉末則碳化成碳化鉻-Cr3C2和Cr7C3的混合相。若在真空度為100 torr時進行熱處理，由於易形成一氧化碳(CO)而致導氧化鉻並無法碳化。當加入大量的碳時，在同樣的熱處理條件下，其碳化成Cr7C3。若在甲烷-氫氣的混合氣氛中，其可在800oC中形成Cr3C2。

以流體床粉末作為燒結的起始粉末，在高壓燒結過程中Cr2O3 和Al2O3可形成固溶體(Al2O3-Cr2O3)，另一方面Cr2O3 可以和碳反應形成碳化鉻(Cr3C2、Cr7C3)顆粒。因此本系統中同時存在固溶強化以及奈米第二相顆粒強化二種強化機制。在1000 oC持溫1小時而後升到1400 oC進行熱壓燒結，形成大量的固溶體，而使晶粒明顯成長。而若在1150 oC持溫1小時而後升到1400 oC進行熱壓燒結，形成大量的碳化鉻- Cr3C2強化顆粒，由於其有抑止晶粒成長的特性而使其氧化鋁晶粒最小。若直接升溫到1400 oC 時其形成的碳化鉻包含Cr3C2、Cr7C3兩相。

從電子顯微鏡觀察中發現碳化鉻-Cr3C2和氧化鋁間屬於不一致(non-coherence)介面，然而碳化鉻-Cr7C3和氧化鋁間則屬於半一致(semi-coherence)介面。此二介面皆為没有玻璃相存在，顯示介面間存在很強的鍵結，又因為其細化氧化鋁晶粒因而強化材料。另外，由於強化相碳化鉻和基材氧化鋁之間因為熱膨脹係數差異產生殘留應力，導致在氧化鋁晶粒內形成差排，這些差排將可能對裂縫的前進路徑產生影響，而助於提升其強度和韌性，另外當裂縫遇到奈米的碳化鉻時產生的裂縫偏折及裂縫分支的作用也可提升其韌性。

Nanoscaled Cr2O3 powders coated on alumina particles have been produced by means of metal organic chemical vapor deposition (MOCVD) in a fluidized bed, using the pyrolysis of Cr(CO)6 precursor. Amorphous and crystalline Cr2O3 particles were obtained when pyrolysis temperature of was raised to 300 oC and 400oC, respectively. In order to prepare the nanoscaled Cr3C2 powder from the Cr2O3, carbonizing behavior of the Cr2O3 particles was investigated.

It was found that the amorphous Cr2O3 transformed into Cr3C2 at 1150 oC in graphite furnace in vacuum (10-4 torr), while the crystalline Cr2O3 transformed into a mixture of Cr7C3 and Cr3C2. Cr2O3 can’t carbonize at 1150oC in vacuum level of 100 torr. However, Cr2O3 carbonizes into Cr7C3 when it is mixed with sufficient graphite before the heat treatment. Cr2O3 also carbonizes into Cr3C2 if it was heated in a mixed gas of CH4 and H2 at 800 oC.

The composition of decomposed precursor of Cr(CO)6 includes Cr2O3, CrC1-x, and C. Cr2O3 react with Al2O3 to form a solid solution and also it react with carbon to transform into chromium carbide. Solid solution (Al2O3 - Cr2O3) and nanosized Cr-carbide (Cr3C2 and Cr7C3) particles dispersing uniformly on the Al2O3 matrix were formed after the hot-pressing. The microstructures show that the interface between the nanosized particle and matrix is non-glassy, the grain size in nanocomposite is smaller than that in monolithic Al2O3. The fracture mode of nanocomposite is transgranular and there are network dislocations near the Cr-carbide particles. The strength and toughness of nanocomposite are better than pure alumina. The possible strengthening mechanisms resulting from the solid solution and reinforced particles are discussed in this study.

中文摘要................................I
Abstract................................III
誌謝....................................V
總目錄..................................VII
表目錄..................................X
圖目錄................................. XI
第一章	緒論...........................1
1.1 前言 ...............................1
1.2 研究動機與重點......................2
第二章	理論基礎.......................4
2.1 奈米陶瓷複合材料(Ceramic Nanocomposites).4
2.2 化學氣相沈積法及流體床技術...............9
2.2.1 MOCVD製程  ............................9
2.2.2 MOCVD製程中六羰鉻使用之探討............11
2.2.3 流體床技術.............................12
2.3 第二相對陶瓷基複合材料之影響.............13
2.3.1陶瓷基複合材料強化機制..................13
2.3.2陶瓷基複合材料韌化機制..................16
2.4碳化鉻與氧化鉻............................20
第三章  實驗步驟.............................24
3.1 實驗設計.................................24
3.2 實驗設備及合成奈米複合粉體之製程.........26
3.2.1 MOCVD與流體床之實驗設計................26
3.2.2 實驗材料...............................28
3.2.3奈米複合粉體之合成......................28
3.3 流體床粉末之熱處理.......................31
3.4 複合材料之熱壓燒結製程...................31
3.5 複合粉體之性質分析.......................31
3.5.1 碳硫分析儀測碳、Ｘ射線螢光分析儀測沉積粒子的量.........................................31
3.5.2比表面積特性分析及熱差/熱重分析.........33
3.5.3 複合粉體之相分析.......................33
3.5.4 複合粉體之XPS成分分析..................33
3.5.5 複合粉體之表面型態之觀察...............34

3.6 燒結體的物理性質測定.....................34
3.6.1密度的測定..............................34
3.6.2 機械性質之量測.........................35
3.6.2.1彎曲強度的測定........................35
3.6.2.2表面維氏硬度測試......................36
3.6.2.3破壞韌性..............................36
3.7 燒結體之微結構觀察.......................37
3.7.1 場發射掃瞄式電子顯微鏡（FE-SEM）試樣之製作及觀察.......................................37
3.7.2 電子微探儀（EPMA）試樣之製作及觀察.....37
3.7.3穿透式電子顯微鏡（TEM）試樣之製作及觀察...........................................38
第四章  結果與討論...........................39
4.1先導物六羰鉻Cr(CO)6的裂解.................39
4.2 奈米氧化鉻的碳化.........................51
4.2.1流體床粉末中碳含量對於氧化鉻碳化的影響..51
4.2.2真空度對於氧化鉻碳化的影響..............63
4.2.3甲烷對於氧化鉻碳化的影響................69
4.3 燒結體中的第二相碳化鉻及固溶相氧化鋁-氧化鉻的形成..........................................74
4.3.1微結構的觀察.............................74
4.3.2機械性質.................................82
4.3.2.1密度...................................82
4.3.2.2破壞強度和韌性.........................86
第五章 結論...................................96
參考文獻.....................................100
研究成果目錄.................................107
作者簡歷.....................................109




表 目 錄

Table2.1 Properties of the powders used in this study.........................................21                                                                
Table 3.1 Characteristics of Al2O3 powders....30
Table 4.1 The content of Al and Cr of 300oC fluidized powder..............................49
Table 4.2 The specific surface area and carbon content of specimens..........................58
Table 4.3 Compositions of mixture 1 and mixture 2 used in the TG/DTA experiments...............59
Table 4.4 Sintering conditions of samples......75
Table 4.5 Literature reported mechanical properties of pure Al2O3.......................87
















圖 目 錄
Fig. 2.1 The classification of ceramic nanocomposites...............................5
Fig. 2.2 Schematic diagram of internal stresses around the particles and crack propagations in the case of Al2O3-SiC .......................7
Fig. 2.3 Schematic diagram of internal stresses around the particles and crack propagations in the case of Al2O3-Cr3C2......................8
Fig. 2.4 The five consecutive processes of metal-organic chemical vapor deposition...........10
Fig. 2.5 Dislocations around the nanoparticle after sintering.............................15
Fig. 2.6 Schematic of the fracture-resistance curves of the alumina-silicon carbide nanocomposite and the monolithic alumina polycrystal..................................18
Fig. 2.7 The formation of secondary crack front between two reinforced particles.............19
Fig. 2.8 Cr-C phase diagram..................22
Fig. 2.9 Cr2O3-Al2O3 phase diagram...........23
Fig. 3.1 The experimental flowchart..........25
Fig. 3.2 The schematic diagram of MOCVD and fluidized bed reactor........................27
Fig. 3.3 The schematic diagram of precursor Cr(CO)6 showing its vaporization, decomposition and deposition...............................29
Fig. 3.4 The temperature and pressure profiles for hot-pressed alumina composites...........32
Fig. 4.1 DSC (differential scanning calorimetry ) curves of the precursor Cr(CO)6..40
Fig. 4.2 TEM micrograph of nano-particles prepared in fluidized at 300oC.................41
Fig. 4.3 X-ray photoelectron spectra of the Cr 2p regions of decomposed Cr(CO)6  prepared at 300oC in fluidized bed.........................43
Fig. 4.4 X-ray photoelectron spectra of the C1s regions of decomposed Cr(CO)6 prepared at 300 oC in fluidized bed..............................44
Fig. 4.5 TEM diffraction patterns of the decomposed Cr(CO)6 (a) prepared at 300oC in fluidized bed, showing amorphous phase; (b), and (c) prepared at 400 oC in fluidized bed. (b) Cr2O3 phase with hexagonal structure, and (c) CrC1-x phase with NaCl (B1) structure, respectively..................................45
Fig. 4.6 XRD pattern of particles decomposed Cr(CO)6 prepared in fluidized bed at (a) 300 oC and (b) 400 oC................................47
Fig. 4.7 (a) TEM micrograph of nanometer-sized particles dispersed on alumina particle which was prepared at 300 oC in fluidized bed for 2 h. (b) EDS spectrum of the coating particle A in (a)...........................................48
Fig. 4.8 XRD patterns of the 300 oC fluidized powder thermally treated in a vacuum in a graphite furnace at a variety of temperatures (a) 700oC, (b) 800 oC, (c)900 oC, (d)1000 oC, and (e) 1150 oC for 2 hours..................52
Fig. 4.9 TEM diffraction patterns of 300oC fluidized powder thermally treated in graphite furnace in vacuum at 1150oC for 2 hours.(a) bright field, (b) dark field and (c) diffraction pattern of particle marked by arrow............53
Fig. 4.10 TEM diffraction patterns of the 400oC fluidized powder treated at 1150 oC in graphite furnace under vacuum for 2hr, showing (a) Cr3C2 phase with orthorhombic structure. (b) Cr7C3 phase with hexagonal structure, respectively...55
Fig. 4.11 XRD patterns of the (a) 300 oC, and (b) 400 oC as-deposited powders after thermal treatment in graphite furnace at 1150 oC for 2 h in vacuum......................................56
Fig. 4.12 TG/DTA diagrams of the mixture 1.....60
Fig. 4.13 TG/DTA diagrams of the mixture 2.....61
Fig. 4.14 XRD patterns of powder heated in alumina tube in a vacuum (of level of 100 torr) at 1150 oC for 2 hours for (a) the 300oC fluidized powders, (b) fluidized powders mixed with 4.33 mole graphite.......................64
Fig.4.15 Influence of the CO pressure on the temperature of the Cr2O3-Cr3C2-C equilibrium...................................65
Fig. 4.16 The schematic diagram shows the formation of Cr3C2 and Cr7C3..................67
Fig.4.17 Isothermal section of the system of Cr-C-O at 1200oC.................................68
Fig. 4.18 XRD pattern of 300 oC fluidized powder thermal treated in CH4 - H2 mixed gas at 800oC for 5 hours...................................70
Fig. 4.19 TEM morphology of fluidized powder heated in the mixed CH4 – H2 atmosphere at 800 oC............................................71
Fig. 4.20 XRD patterns of hot pressed samples: (a) ALO, (b) Sample1, (c) Sample2, and (d) Sample3.......................................76
Fig. 4.21 Optical Micrographs of (a) Sample1:Al2O3-Cr2O3, (b) Sample2: Al2O3-4.5 vol%Cr3C2.........................................77
Fig. 4.22 HAADF STEM micrographs of samples, (a) Sample1: Al2O3-Cr2O3, (b) Sample2: Al2O3-4.5 vol%Cr3C2 and (c) Sample3: Al2O3-1.5 vol%Cr3C2+Cr7C3...................................79
Fig. 4.23 (a) SEM micrograph of Sample1, (b) Cr mapping of (a), showing center area of the exaggerative grain with less Cr than grain boundaries....................................81
Fig. 4.24 TEM diffraction patterns of sample hot pressed at 1400oC in a graphite furnace for 1 hour, (a) Cr3C2 phase with orthorhombic structure, and (b) Cr7C3 phase with hexagonal structure.....................................83
Fig. 4.25 Lattice images of the interfaces (a) between Cr3C2 and Al2O3, and (b) between Cr7C3 and Al2O3....................................84
Fig. 4.26 Bulk densities of composites hot-pressed in different sintering conditions in vacuum (5*10-4 torr)..........................85
Fig. 4.27 (a) Hardness, (b) four-point bending strength and (c) fracture toughness of hot-pressed samples................................88
Fig. 4.28 SEM micrographs of the fracture surfaces of (a) pure alumina and (b) Sample1: Al2O3-Cr2O3....................................89
Fig.4.29 (a) SEM micrograph of Sample 2, and (b) Cr mapping of (a), showing solid solution and Cr-carbide in Sample2.............................91
Fig. 4.30 Scanning electron micrographs of (a) monolithic Al2O3, (b) nanocomposite (Sample3)..92
Fig. 4.31 SEM micrographs of the interaction of nano-sized Cr-carbide with crack formed by Vickers indentaion on the composite (Sample3). The direction of crack propagation from top to buttom. (a) crack deflection (b) crack branching.....................................94
Fig. 4.32 TEM image of Sample3 shows dislocations near the nanosized Cr3C2 particles noted by dark arrows..........................95







                                    

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