【作者】 张勇；

【导师】 唐元洪；

【作者基本信息】 湖南大学 ， 材料科学与工程， 2010， 博士

【摘要】 纳米碳纤维（简称CNFs）由于其纳米尺度的影响具有优异的物理和化学特性,可用作航天、航空、国防军工尖端技术领域的新材料和民用工业更新换代的新材料。因此,低成本、大批量纳米碳纤维的制备及应用成为世界各国重点研究的项目,期望能占领该技术领域的制高点。我国目前批量生产纳米碳纤维的技术不够成熟,开展纳米碳纤维生长工艺以及反应装置的研究对于纳米碳纤维工业化生产具有重要意义。本文以化学气相沉积（简称CVD）原理为基础,设计研制了双温卧式反应炉和立式沸腾炉;采用溶胶-凝胶和低温自蔓延燃烧技术探索研究了低成本、高效率NiO/MgO催化剂前驱体的制备方法,并以自制的双温卧式反应炉和立式沸腾炉为反应设备,以MgO负载Ni、纳米Ni和二茂铁的乙醇溶液为催化剂制备纳米碳纤维,重点研究了工艺参数对纳米碳纤维产率和结构的影响。首先,设计研制了双温卧式反应炉和立式沸腾炉。双温卧式反应炉采用两段式加热,可控制温度梯度并适合多种气氛工作,两段发热元件可以单独或联合操作对炉膛加热,并且能够准确获取并控制蒸发区和生长区的温度,从而能够达到生长不同形态的纳米材料的目的。立式沸腾炉反应器分别采用螺杆送料装置和电子蠕动泵装置将固态和液态催化剂连续导入立式炉中,解决现有的反应装置无法同时对不同形态催化剂连续投放的难题,以达到纳米碳纤维的批量生产。其次,低成本、高效率的催化剂是批量生产纳米碳纤维的关键步骤。本文以柠檬酸为还原剂,硝酸镁和硝酸镍为氧化剂,采用溶胶-凝胶和低温自蔓延燃烧制备出NiO/MgO催化剂前驱体。研究了柠檬酸与硝酸盐的配比对催化剂前驱体粒径和自蔓延燃烧的起燃温度的影响,并通过与直接焙烧所得催化剂进行比较,研究了温度对催化剂的烧结性能的影响,以及不同还原温度条件下NiO/MgO催化剂前驱体的催化活性规律;提出了柠檬酸络合低温自蔓延燃烧合成NiO/MgO催化剂前驱体的形成机理。研究结果表明:随柠檬酸与硝酸盐摩尔配比从1:1增加1:4时,催化剂前驱体的颗粒尺寸逐渐减小,自蔓延燃烧起燃温度从200℃升高到350℃。烧结性能的研究表明:直接焙烧所得的催化剂随焙烧温度的升高,催化剂前驱体容易团聚,直至熔结成大颗粒,失去催化活性,而低温自蔓延燃烧法制备催化剂前驱体具有合成温度低,能耗少,产物团聚少,催化效率高等特点,更适合于纳米碳纤维的批量制备。第三,以自制的双温卧式反应炉为实验设备,NiO/MgO为催化剂前驱体,研究了影响纳米碳纤维产率的主要因素。研究表明:柠檬酸与硝酸盐的摩尔配比、Ni的载荷量、反应时间、反应温度等对产率具有重要影响。通过对这些影响因素的分析可以找出最佳催化剂前驱体和纳米碳纤维的制备参数。NiO/MgO前驱体的配比为:Ni的载荷量为30⁴0wt%,硝酸镁和硝酸镍与柠檬酸的摩尔比为1:2。纳米碳纤维的生长工艺参数为:以NiO/MgO为催化剂前驱体,通入Ar+H2（10%）还原约30min,Ar+H2（10%）与CH₄流量比控制在3:1或4:1,纳米碳纤维的生长温度600℃左右,生长时间40⁶0min。这些研究参数为在立式沸腾炉中连续大量制备纳米碳纤维提供了很好的实验基础。第四,以自制立式沸腾炉为反应设备,以最佳摩尔配比的NiO/MgO催化剂前驱体、纳米Ni和二茂铁酒精溶液为催化剂,研究了立式炉中总气流量、原料气空速、风扇转速、催化剂种类等对纳米碳纤维产率的影响,探索了以NiO/MgO为催化剂前驱体进行大量制备纳米碳纤维的工艺,对比了各种催化剂制备的纳米碳纤维的宏观形貌和微观结构。研究表明以Ni/MgO为催化剂,原料气总流量控制在0.8¹.0 m3/h,原料气空速选择范围约为0.16⁰.2 m3/（g.h）之间,搅拌风扇转速在1000¹500rpm时纳米碳纤维的产率最高可达每克负载Ni催化剂可以生产14.5g,完全可以达到或超过单炉产量100g/h的生产能力。以纳米Ni作为催化剂时,沸腾炉中纳米镍粉处于悬浮状态,分散性好,不易团聚,但使用纳米Ni作为催化剂价格昂贵。使用二茂铁的乙醇溶液为催化剂制备纳米碳纤维存在明显的不足,铁颗粒的粒径不好控制、反应温度太高（900¹200℃）,能耗较大。第五,研究了石墨化处理工艺对纳米碳纤维微结构和性能的影响,并提出了纳米碳纤维的石墨化机理。HRTEM、Raman、XRD、NEXAFS以及TGA的分析研究表明通过2800℃的热处理,石墨化纳米碳纤维中非晶碳的含量大幅度降低,晶面间距减小,石墨化程度明显提高,抗氧化温度由617℃升高到857℃。本文研究表明,采用立式沸腾炉作为反应设备, NiO/MgO为催化剂前驱体生产纳米碳纤维,具有低成本、低能耗、节约人力、工艺简单、重复性好的特点,是较理想的工业化生产的方法。更多还原

【Abstract】 Carbon nanofibers (CNFs) have many unique physical and chemical properties due to the nanometer scale effect,which can be applied in aerospace, aviation, national defense, military and civil industry technology field. Therefore bulk production and application of CNFs at a low cost will become key research. At present, large scaled production of CNFs is not available for our country.In this paper, a horizontal double-temperature-zone reaction furnace and a vertical ebullition stove were designed to synthesize CNFs on large scale during the chemical vapor deposition (CVD) process. The cheap and efficient NiO/MgO catalyst precursor has been prepared by a sol-gel self-propagation process in low temperature. CNFs have been synthesized using the double-temperature horizontal reaction furnace and the vertical ebullition furnace as the reaction equipment, Ni / MgO, nano-Ni and ferrocene ethanol solution as catalyst. The effects of technological parameters on the yield and structure of the CNFs have been investigated in detail.Firstly, the horizontal double-temperature zone reaction furnace and the vertical ebullition stove were designed. The horizontal double-temperature zone furnace consists of a two stage heating system to control the temperature which is suitable for various gas atmospheres. The furnace can be heated by the two-stage heating element separately or jointly in order to precisely control temperatures of the evaporation and the growth zone, which can synthesize different nanoscale materials with different morphologies. The vertical ebullition stove has two kinds of feeding modes for the bluk production of CNFs. The screw feeding setup and electronic wriggling pump setup can continuously import the solid catalyst and the precursors of liquid catalysts to the vertical reactor, respectively. Thus the large scale production of CNFs is finally achieved by controlling the equipment parameters.Secondly, catalysts with low cost and high efficiency are the key step for the mass production of CNFs. NiO/MgO catalyst precursors have been prepared using the citrate as reduction agent, Mg(NO3)2 and Ni(NO3)2 as oxidizing agents by sol-gel and low temperature self-propagating combustion methods. The effects of the ratio of citrate and salt nitrate on the particle size of the catalyst precursors and combustion temperature of the self-propagating combustion have been analyzed. The effect of the temperature on the sintering ability of the catalysts, and the catalyst activity regularity of the NiO/MgO catalyst precursors at different reduction temperature have been investigated by the comparison of the catalyst obtained by the direct sintering. At the same time, the formation mechanism of the Ni/MgO catalyst precursors synthesized by citrate salt self-propagating combustion process has also been proposed. The particle size of the catalyst precursors gradually decreases and combustion temperature increases from 200℃to 350℃with the decrease of the mole ratio of the nitrate slat with citrate acid from 1:1 to 1:4. The catalyst precursor by the direct sintering process aggregates easily to become large particles and lose the catalyst activity with the increase of the sintering temperature. However, the method for the synthesis of catalyst precursors by the low temperature self-propagating method takes great advantages of low synthesis temperature, low power, small aggregation and high catalyst efficiency which is suitable for the mass preparation of CNFs.Thirdly, the parameters on the yield of the CNFs have been investigated using the horizontal double-temperature zone reaction furnace as the reaction equipment and NiO/MgO as catalyst precursors. The results show that the ratio of salt nitrate and citrate acid, the loading amounts of nickel in catalyst precursor, reaction time and reaction temperature have important roles in the yield of the CNFs. The best synthesis parameters for the catalyst precursors and CNFs have been obtained. The loading amount of Ni is 30~40wt%, the molar ratio of magnesium nitrate and nickel nitrate with citrate acid is 1: 2. The parameters for the growth of the CNFs are as follows: NiO/MgO is the catalyst precursor, Ar+H2(10%)is provided as the reducing agent for about 30mins, the flow ratio of Ar+H2(10%)with CH4 is 3:1 or 4:1, and the growth temperature of the CNFs is about 600℃for about 40-60min. A very good experimental foundation for continuous mass production of CNFs by use of vertical ebullition stove has been provided by these results.Fourthly, the roles of the total gas feed ratio, gas airspeed of the starting materials, rotational speed of the fan, different kinds of catalysts on the yield ratio of the CNFs have been explored using the vertical ebullition furnace as the reaction equipment, Ni / MgO, nanoscale Ni and ferrocene ethanol solution as the catalysts. The morphologies and microstructures of the CNFs synthesized by different catalysts have been compared. The yield of the CNFs is the highest with up to 14.5 g CNFs using 1 g Ni catalyst which can exceed the production ability of 100 g/h when controlling the total gas feed ratio of the starting materials as 0.8~1.0 m3/h, gas airspeed of the starting materials as 0.16~0.2 m3/(g.h) and the rotational speed of the fan as 1000~1500 rpm. The efficiency of the catalysts can be improved increasing the yield of the CNFs using the nanoscale Ni as the catalyst owing to the suspension state of nanoscale Ni in the vertical ebullition furnace with good dispersion. However, the nanosale Ni is expensive. Using the ferrocene ethanol solution as the catalysts precursor takes the disadvantage of the broad Fe particle size distribution. In addition, the growth temperature of the CNFs is high (900-1200℃) with high power consumption.Fifthly, the microstructures of carbon nanofibers affected by graphitization process have been analyzed by high resolution transmission electron microscopy (HRTEM), Raman spectra, X-ray diffraction (XRD), near-edge-X-ray absorption fine structure spectroscopy (NEXAFS) and thermogravimetric analysis (TGA). The results show that the content of the amorphous carbon and the spacing between graphite sheets decrease obviously, the oxidation-resistance temperature of the CNFs increases from 617℃to 857℃after heat treatment at 2800℃. The graphitization mechanism including four stages was also proposed.In the paper, a simple, economization, low cost, low energy consumption and good reproducibility method is demonstrated to the large production of CNFs in the vertical ebullition stove with NiO/MgO as catalyst precursors. It is a promising method for the mass production of CNFs.更多还原