以鎳/鎂及鈷/鎂催化劑成長奈米碳管與奈米碳纖維及其導電特性之研究

Growth of Carbon Nanotubes and Carbon Nanofibers Using Ni/Mg and Co/Mg Catalysts and Their Electrical Conductivities

吳定宇(Ting-Yu Wu)

指導教授 ： 曾信雄

大同大學/工程學院/材料工程學系所/博士(2010年)

摘要

本研究以共析法製備鎳/鎂及鈷/鎂催化劑前軀物，並於空氣下煅燒形成鎳/鎂及鈷/鎂催化劑，而後經化學氣相沉積法成長不同微結構之奈米碳纖維及奈米碳管。實驗主要從鎳/鎂及鈷/鎂催化劑之催化特性差異、改變化學氣相沉積法中的反應溫度與製程氣氛的種類等不同製程參數，來探討不同催化劑特性及製程參數與成長奈米碳纖維及奈米碳管間的關連性，並透過拉曼光譜分析及兩點加壓導電測試分析纖維及碳管微結構之差異及其對粉末導電度的影響。實驗結果顯示，鎳/鎂催化劑可於鎳含量5 at% ~ 25 at%且高反應溫度(800℃及900℃)下成長具有網狀分佈型態的單層奈米碳管；而鎳含量15 at% ~ 75 at%且低反應溫度(500℃~700℃)下則成長不同微結構之奈米碳纖維。HRTEM結果可知，奈米碳纖維具有三種不同微結構，分別為魚骨狀結構(550℃)、中心空孔結構(600℃~650℃)及竹節狀結構(700℃)，製程中的反應溫度高低及碳源氣氛中的氮氣含量多寡是影響纖維微結構變化之主要因素。在鈷/鎂催化劑方面，其結晶相組成受煅燒溫度影響而由不同比例的鈷/鎂岩鹽相及鈷/鎂尖晶石相組成，其中含有鈷/鎂尖晶石相之催化劑(鈷含量40 at% ~ 75 at%)雖可於500℃~700℃間成長奈米碳纖維，但產量較低且產物型態不佳；而具有鈷/鎂岩鹽相之催化劑(鈷含量5 at% ~ 25 at%)，則可於900℃成長具有網狀型態的奈米碳管。HRTEM結果顯示以鈷/鎂催化劑於500℃下生長之纖維微結構為魚骨狀結構，而於700℃下則為管狀結構。降低鈷/鎂催化劑的還原溫度雖可改變催化特性，但對產量增加並無助益。拉曼圖譜分析鎳/鎂催化劑成長之奈米碳管可知，800℃產物應為具有金屬特性及半導性的奈米碳管，而當成長溫度應為900℃時則為半導特性的奈米碳管；而對不同微結構之奈米碳纖維而言，拉曼光譜結果顯示I2D/IG強度比隨纖維微結構由魚骨狀轉變為竹節狀而增加，且2D特性峰具有強度增強且半高寬變窄的趨勢。在奈米碳管與奈米碳纖維之導電度的量測方面，多層奈米碳管受限緻密度較低而電導率並不佳(4.11 S/cm)；對原始不同微結構之奈米碳纖維而言，三者於壓力0.07MPa下皆具相似的體積密度及電導率(2.78~2.90 S/cm)，但於壓力1.44MPa時，產量較低之原始竹節狀結構纖維因產物中具有較多未反應催化劑，造成粉末緻密度較高而電導率(5.96 S/cm)；硝酸酸洗不僅大幅降低奈米碳纖維中的雜質含量，且在酸洗過濾時造成粉末緻密度之增加亦是酸洗纖維電導率增加的主因；對2400℃熱處理奈米碳纖維而言，高溫熱處理造成之纖維結晶大幅增加是此奈米纖維電導率增加的主因。

關鍵字

奈米碳管 ； 奈米碳纖維 ； 微結構 ； 導電性

並列摘要

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) with different microstructures were grown by chemical vapor deposition using Ni/Mg and Co/Mg catalysts derived from Ni/Mg and Co/Mg brucite-like compounds prepared using co-precipitation. The correlation between the catalytic characteristics in Ni/Mg and Co/Mg catalysts and the growth of CNTs and CNFs were investigated by a series of study of processing parameters, including different metal ratios in catalysts, reaction temperatures and reaction atmospheres. Some more detailed structural variations in CNTs and CNFs were characterized by Raman spectrum, and the electrical conductivities of CNFs and CNTs were measured under compression at room temperature by the two-probe method. For the Ni/Mg catalysts, single-walled CNTs could be obtained at higher temperature (800℃ and 900℃) when using the Ni/Mg catalysts with a Ni content in the range of 5 ~ 25 at%, while CNFs with different microstructures could be synthesized at lower temperatures (500℃~700℃) when using the Ni/Mg catalysts with a Ni content in the range of 15 ~ 75 at%. The HRTEM results revealed that three different microstructures, herringbone structure (500℃), internal conic cavity structure (650℃) and bamboo structure (700℃), were observed. It is noted that the microstructural change in CNFs was mainly affected by the reaction temperature and the nitrogen content in gas mixture. For the Co/Mg catalysts, two different crystal structures of rock-salt and spinel were found in Co/Mg catalysts, and the ratio of these two different crystal structures could be altered by changing the calcination temperature. CNFs with poor morphology and lower yield were grown at 500℃~700℃ when using Co/Mg catalysts with the spinel phase (Co content: 40 ~75 at%), and CNTs with a network morphology were synthesized at 900℃ when using Co/Mg catalysts with the rock-salt phase (Co content: 5~25 at%). HRTEM observations indicated that CNFs synthesized at 500℃ have a herringbone structure while a tubular structure (multi-walled CNTs) was observed at 700℃. It is found that the catalytic characteristics in Co/Mg catalysts could be modified by decreasing the reduction temperature, but the carbon yield of CNFs was still not improved. For the Raman study of CNTs synthesized using Ni/Mg catalysts, it is found that both of metallic and semiconducted CNTs were obtained at 800℃, but only semiconducted CNTs was observed at 900℃. For the CNFs of different microstructures, the intensity ratio of I2D/IG increase when the microstructure was transformed from a herringbone structure to a bamboo structure, and shaper peak and smaller FWHM were also found in their 2D peaks. For the electrical conductivities of MWNTs and CNFs with different microstructures, a lower electrical conductivity (4.11 S/cm) was measured for the MWNTs due to their lower packing density. Comparison of as-prepared CNFs with three different microstructures indicated that similar results in volume densities and electrical conductivities (2.78~2.90 S/cm) were measured at a compressive pressure of 0.07 MPa, while a higher electrical conductivity (5.96 S/cm) in CNFs with a bamboo structure was observed because of their higher packing density due to more residual catalysts. After a nitric acid treatment, the electrical conductivities of all CNFs were raised. In addition to the removal of unreacted catalysts, the reason for the improvement in electrical conductivity is mainly due to the increase of packing density during the filtration. For the 2400℃ heat-treated CNFs, the increase in electrical conductivity can be attributed to the significant improvement of crystallinity.

並列關鍵字

carbon nanotubes ； carbon nanofibers ； microstructure ； electroconductivity

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