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研究生:鍾德華
研究生(外文):De-Hua Chung
論文名稱:旋轉流擴散火焰燃燒合成奈米碳管
論文名稱(外文):Combustion Synthesis of Carbon Nanotubes via Diffusion Flames in a Rotating Opposed-Jet Flow
指導教授:林大惠林大惠引用關係
指導教授(外文):Ta-Hui Lin
學位類別:碩士
校院名稱:國立成功大學
系所名稱:機械工程學系碩博士班
學門:工程學門
學類:機械工程學類
論文種類:學術論文
論文出版年:2006
畢業學年度:94
語文別:中文
論文頁數:89
中文關鍵詞:硝酸鎳旋轉流燃燒合成奈米碳管拉伸率對沖流滯留時間
外文關鍵詞:Stretch RateRotating flowResidence TimeCarbon NanotubesOpposed-Jet FlowCombustion Synthesis
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  奈米碳管的合成需具備碳源、熱源和金屬觸媒顆粒三大要件。在各種合成的方法中,燃燒合成法具有成本低、穩定性高且能大量產出的絕佳優勢,因此發展潛力十足。本研究之目的係利用旋轉對沖流擴散火焰來進行燃燒合成奈米碳管的分析。對沖流擴散火焰可提供較大面積和較佳的沉積物收集方式,而旋轉流的新方法可增進不飽和自由基於高溫流場的滯留時間,有助於奈米碳管的合成,使燃燒合成法更具經濟效益。研究中將量測在不同旋轉角速度下的熄滅極限、火焰位置與碳煙層(soot zone)的生成範圍,並以SEM和TEM分析奈米碳管生成的成長機制及其型態和結構,並尋求最佳的火焰操作條件。
  研究中首先分析甲烷和乙烯火焰在不同旋轉角速度下的熄滅極限、火焰位置與碳煙區的生成範圍。結果顯示,隨著旋轉角速度逐漸增加,火焰熄滅時的燃料濃度值有先逐漸減少,至最低點後,又逐漸增加的趨勢,亦即燃燒強度呈現先增強後減弱的現象。火焰位置則隨著上下兩邊燃料與氧化物噴流之擴散能力和流體動力的影響而移動。觀察碳煙區的生成邊界則發現,提高氧氣或燃料濃度、或者增加燃燒器之旋轉角速度皆對碳煙區的生成有所助益;但是增加出口速度則對碳煙生成有負面的影響。此外,上燃燒器為燃料/氮氣混合氣,下燃燒器為氧氣/氮氣混合氣時,碳煙區生成在火焰面外側,而上燃燒器為氧氣/氮氣混合氣,下燃燒器為燃料/氮氣混合氣時,碳煙區生成在火焰面內側,因此前者較容易觀察,且較易生成碳煙。
  接著利用上燃燒器通入乙烯/氮氣混合氣,下燃燒器通入空氣所形成之乙烯火焰,以鎳網格當做金屬基板,置入流場中2分鐘沉積取樣,探討不同燃料濃度、旋轉角速度和取樣位置對奈米碳管生成的影響。結果顯示,在固定的燃燒器出口流速且旋轉角速度為零的情況下,當出口速度V= 15 cm/s時,隨著燃料濃度的增加,低於碳煙生成的臨界燃料濃度前,基板網格的邊緣上之奈米碳管有逐漸增長的趨勢;超過臨界燃料濃度後,網格邊緣上之奈米碳管則有管壁增厚的現象。在出口流速15 cm/s時,當燃料濃度低於其碳煙生成之臨界燃料濃度可由TEM觀察發現奈米碳管的生成,但流速為20 cm/s則無法發現。此乃因出口速度20 cm/s的實驗中使用之燃料濃度較高,若在火焰面位置之軸向高度取樣,基板易沉積過多的碳顆粒,使得網格邊緣無法觀察到奈米碳管。此外,遠離對稱軸的徑向位置拉伸率較大,不飽和自由基的滯留時間較短,這兩項因素使得軸向高度遠離火焰面1 mm且徑向位置遠離對稱軸16.8 mm處可發現奈米碳管。而探討旋轉角速度對奈米碳管生成的影響可以發現,隨著轉速增加停滯分界面附近的拉伸率減少,不飽和自由基的滯留時間增加,碳顆粒層有增厚的趨勢,反而不利於高燃料濃度下的奈米碳管生成。最後, 以硝酸鎳為催化劑,發現硝酸鎳在可生成碳管的條件下,對於碳管的長度與密度有正面的效應;在某些無法生成碳管的條件,可幫助其生長,對於碳管的生成有極佳的助益。
  This study is aimed at investigating the conditions for carbon nanotube formation and growth via a new method of rotating counter-flow diffusion flames. In the experiments, the flame appearance, flame structure, flame extinction and soot layer were firstly observed using image processing techniques. Thereafter, the effect of rotation on synthesis of carbon nanotube were explored. Flow rotation can extend the residence time of reactive radicals in the combustion environment and in turn cause the radicals to form carbon nanotubes more easily. We employed the bare nickel grids as the catalytic metal substrate to collect deposit materials. The thickness of the nickel grid was 0.2 mm with a diameter of approximately 3 mm. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and SEM-EDS were utilized to characterize the morphology and microstructure of condensed carbon deposits and their primary formation mechanism in the combustion environment.
  It was found that for the methane or ethylene flames, the fuel concentration at extinction firstly decreases and then increases with increasing rotating angular velocity under a fixed injection velocity. With increasing the oxygen concentration, fuel concentration or the rotating angular velocity, the soot zone was reduced. However, increasing the injection velocities of jet flows, the soot formation was suppressed. Soot zone was found to be formed more easily when the fuel/nitrogen and oxygen/nitrogen mixtures were, respectively introduced into the upper and lower burners.
  Under a fixed injection velocity (V) without rigid-body rotation, increasing fuel concentration (which was lower than the critical fuel concentration where soot formation initiated), the carbon nanotubes synthesized on the edge of a substrate (TEM grid) were quite long. However, increasing the value of fuel concentration higher than the critical fuel concentration of soot formation, the wall of carbon nanotubes was gradually thickened. When the injection velocity was 15 cm/s, carbon nanotubes were observed in the TEM images at the relatively low fuel concentration (which was lower than the critical fuel concentration at the onset of soot formation). Whereas, at V= 20 cm/s, carbon nanotubes were not found at the relatively low concentration. At V= 20 cm/s and the fuel concentration higher than the critical fuel concentration at the onset of soot formation, if the sampler was positioned at a distance farther away from flame surface and the axisymmetric axis, the concentration of radical pool was reduced (which reduced the formation of soot particle but increases the synthesis of carbon nanotube) and the stretch rate was increased (which shortened the residence time of reactive radicals). These two factors caused the occurrence of carbon nanotubes at the position of 1 mm above the flame surface plane and at a distance 16.8 mm apart from the axisymmetric axis. Finally, increasing the rotating angular velocity lead to the decrease of stretch rate and the extension of residence time and thus at high fuel concentration the soot layer was increased but the growth of carbon nanotubes was suppressed.
總目錄
總目錄 Ⅰ
表目錄 Ⅲ
圖目錄 Ⅳ
符號說明 Ⅶ
一、前言 1
1-1 碳管的性質與應用 1
1-1-1 碳管的結構與分類 2
1-1-2 碳管的性質 3
1-1-3 碳管的應用方向 3
1-2 碳管的合成與製備方式 6
1-2-1 觸媒和催化劑的影響 6
1-2-2 常見的合成方法 7
1-3 燃燒合成奈米碳管 9
1-3-1 富燃料預混火焰(rich premixed flames)合成法 10
1-3-2 擴散火焰(diffusion flames)合成法 11
1-3-3 其他火焰機構 14
1-3-4 煙灰(soot)與奈米碳管 14
1-4 研究目的 15
二、實驗設備與方法 18
2-1 實驗設備及量測儀器 18
2-1-1 燃燒器系統 18
2-1-2 氣體供應系統 20
2-1-3 金屬觸媒基板沉積物取樣系統 20
2-1-4 影像處理系統 21
2-1-5 奈米碳管健測設備 21
2-2 實驗進行方法與步驟 22
2-2-1 實驗參數設定 23
2-2-2 火焰形態觀測和火焰熄滅分析 2-3 燃燒器系統 24
2-2-3 火焰位置和碳煙層(soot layer)觀測 2-4 安全及控制系統 25
2-2-4 沉積物取樣及奈米設備分析 25
三、結果與討論 26
3-1 火焰特性分析 26
3-1-1 甲烷火焰特性分析 26
3-1-2 乙烯火焰特性分析 31
3-2 奈米碳管之分析 33
3-2-1 燃料濃度對奈米碳管生成的影響 34
3-2-2 出口速度對奈米碳管生成的影響 37
3-2-3 沉積位置對奈米碳管生成的影響 38
3-2-4 旋轉角速度對奈米碳管生成的影響 40
3-2-5 硝酸鎳對奈米碳管生成的影響 42
四、結論 44
五、參考文獻 46
六、圖表 52
五、參考文獻
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