Research On Bulk Production Technology And Device For Carbon Nanofibers

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.