聲波振動同心噴流火焰合成奈米碳結構

Flame Synthesis of Carbon Nanostructures in Acoustically Modulated Coaxial Jet Flames

鄭重役(Chung-Yi Cheng)

指導教授 ： 侯順雄

崑山科技大學/工程學院/機械工程研究所/碩士(2011年)

摘要

奈米碳結構的合成須具備碳源、熱源和金屬觸媒三大要件。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，因此頗具有發展的潛力。本研究之目的在於利用聲波振動同心噴流火焰來進行燃燒合成奈米碳結構的分析。
研究中，固定內環出口流速為20 cm/s、外環出口流速20 cm/s，並且對於內環施以聲波振動，改變外環氧氣/氮氣混合濃度、聲波振盪頻率，探討火焰型態、溫度場對火焰燃燒合成奈米結構的影響。首先進行火焰型態觀察和量測溫度，接著使用鎳網格，將其置於距離燃燒器中心軸出口上方Z = 5和10 mm 位置，當作沉積基板來合成奈米碳結構，沉積取樣時間為2分鐘。最後，再以掃描式電子顯微鏡(FE-SEM)和高解析場發射掃描穿透式電子顯微鏡(HR-TEM)觀察分析不同實驗條件下所生成奈米碳結構的微結構和形態。結果發現在頻率區間10~30Hz為自然擺盪頻率及60~70Hz與120~140Hz為系統自然頻率耦合，火焰形成雙層結構，至於其他頻率下其雙層火焰結構則不明顯。而雙層火焰中心亮黃色火焰柱顯示中心軸處為高濃度裂解前驅物之富碳環境，在合適溫度和觸媒作用下，營造出有利於奈米碳結構燃燒合成的環境。當氧氣濃度為21%、30%、35%及40%時，未施加聲波振動時，Z = 5和10 mm兩處之沉積物經SEM分析均顯示奈米碳結構物之生成極為稀少。當f = 10∼30Hz，Z = 5 mm(氣體溫度範圍為260∼560 ℃)、以及f = 10∼20Hz，Z = 10 mm(氣體溫度範圍為390∼605 ℃)，沉積取樣分析都發現有大量葡萄狀堆疊之奈米碳球生成。此外，當振動頻率60∼70Hz時，Z = 5 mm(氣體溫度範圍為510∼805 ℃)、及Z = 10 mm(氣體溫度範圍為610∼750 ℃)，亦可發現大量奈米碳球生成。而頻率120∼140Hz時，Z = 5 mm(氣體溫度範圍為425∼790 ℃)、及Z = 10 mm(氣體溫度範圍為500∼720 ℃)，亦可發現大量奈米碳球生成。在高氧氣濃度下其奈米碳結構的生成範圍及數量都優於在低氧濃度。此外，在較高的沉積取樣高度，奈米碳結構之生成範圍比低沉積取樣位置時來得廣。此乃因低沉積取樣位置相對於高沉積取樣位置，其可合成奈米碳結構的碳原子濃度較低及滯留時間較短，故較高沉積取樣位置所合成奈米碳結構之結果會比低沉積取樣位置時來得佳。

關鍵字

聲波振動 ； 火焰合成 ； 氧氣濃度 ； 奈米碳結構 ； 奈米碳管 ； 奈米碳球

並列摘要

Ethylene/oxygen-enriched jet diffusion flames modulated by acoustic excitation in an atmospheric environment were used to synthesize carbon nano-onions (CNOs) on a catalytic nickel substrate. The formation of CNOs was significantly enhanced by acoustic excitation at frequencies near either the natural flickering frequency or the acoustically resonant frequency. The rate of yield of CNOs was high at 10 ~ 30 Hz (near the natural flickering frequency) for a sampling position Z = 5 mm above the burner exit where the gas temperature was about 260 ~ 560℃, and at 10 and 20 Hz for Z = 10 mm with the gas temperature ranging from 390 ~ 605℃. Additionally, for both Z = 5 and 10 mm, a quantity of CNOs could be obtained at 60 ~ 70 Hz, near the acoustically resonant frequency, where the gas temperature was between 510 ~ 805℃ and 610 ~ 750℃. Furthermore, for both Z = 5 and 10 mm, a quantity of CNOs could be obtained at 120 ~ 132 Hz, near the acoustically resonant frequency, where the gas temperature was between 425 ~ 790℃ and 500 ~ 720℃. Almost no CNOs were produced for the other frequencies due to low temperature or lack of carbon sources. The enhanced synthesis of CNOs was caused by strong mixing of the fuel and oxidizer due to the acoustic excitation at the resonant frequencies. Moreover, at high oxygen concentrations a large amount of CNOs could be synthesized at low oxygen concentrations.

並列關鍵字

Flame synthesis ； Acoustic modulation ； Oxygen concentration ； Carbon Nanotubes ； Carbon Nano-onions

參考文獻

[83] 成功大學微奈米科技研究中心。

[1] Iijima, S., “Helical Microtubules of Graphitic Carbon,” Nature, Vol. 354, pp. 56-58, 1991.

[4] Ebbesen, T. W. and Ajayan, P. M., “Large-Scale Synthesis of Carbon Nanotubes,” Nature, Vol. 358, pp. 220-222, 1992.

[5] Dillon, A. C., Parilla, P. A., Alleman, J. L., Perkins J. D. and Heben, M. J., “Controlling Single-Wall Nanotube Diameters with Variation in Laser Pulse Power,” Chem. Phys. Lett., Vol. 316, pp. 13-18, 2000.

[6] Pan, Z. W., Xie, S. S., Chang, B. H., Sun, L. F., Zhou, W. Y. and Wang, G., “Direct Growth of Aligned Open Carbon Banotubes by Chemical Vapor Deposition,” Chem. Phys. Lett., Vol. 299, pp. 97-102, 1999.