| Carbon nanotubes(CNTs)are one-dimensional carbon materials made by winding one or more layers of graphite sheets.Because of its special structure,CNTs have good mechanical properties,electrical conductivity,thermal conductivity and chemical properties,etc.,and have a wide application prospect in electronic devices,absorbing materials,catalyst loading and other fields.Carbon nanotube preparation methods,such as those based on arc discharge,laser evaporation and chemical vapor deposition,have complicated processes,low yield and high cost,which make CNTs expensive and hinder their commercial application.The synthesis of high efficiency catalysts to improve the nucleation and growth kinetics of CNTs and the development of new high efficiency preparation methods is an important research direction of low cost carbon nanotube preparation technology,which is of great significance to promote the application of carbon nanotubes.In this paper,Ni-Mo-Mg catalyst was synthesized by template method,and the influence of the ratio of raw materials on the composition and structure of the catalyst was studied.Then,carbon nanotubes were prepared by blending PP with Ni-Mo-Mg catalyst and sintering with PP as carbon source.The effects of catalyst composition and sintering conditions on the yield,structure and properties of CNTs were studied.Finally,the polyvinyl alcohol(PVA)/CNTs composite nanofiber membrane was prepared by electrospinning method,and the effects of the addition of carbon nanotubes on the structure and properties of PVA were studied.Specific research contents and results are as follows:(1)Preparation of Ni-Mo-Mg catalyst by template method.Three kinds of Ni/Mo/Mg catalysts with different ratios of Ni/Mo/Mg were synthesized by impregnation with different ratios of nickel nitrate,magnesium nitrate and ammonium molybdate using viscose fiber cloth as template.The effects of Ni/Mo/Mg ratio on the particle size and composition of the catalysts were studied.The results show that the crystal size of the catalyst is uniform and does not grow into large crystals because the crystal growth is restricted in the void of the amorphous zone of the cellulose.When the ratio of Ni/Mo/Mg is 7/0.21/1,the average particle size of the catalyst is 7.74 nm,and the composition of the catalyst is mainly face centered cubic Mg O-supported Ni O.With the increase of Mo content(the ratio of Ni/Mo/Mg was 7/0.7/1),the composition and structure of the catalyst did not change significantly.The average particle size of the catalyst decreased to 6.83 nm,and Mo content continued to increase(the ratio of Ni/Mo/Mg was 7/1.4/1).Monoclines Mg Mo4 appeared in the catalyst components.The particle size distribution becomes wider and the average particle size increases.On this basis,under the catalytic action of these three different ratios of Ni-Mo-Mg catalysts,carbon nanotubes were prepared with PP as carbon source.The catalytic performance of PP was studied.It was found that the ratio of Ni/Mo/Mg was 7/0.7/1,and the catalytic efficiency of PP to carbon was the highest.(2)Study on the correlation between sintering process and yield and structure of carbon nanotubes.Carbon nanotubes were prepared by melting and blending polypropylene and Ni/Mo/Mg with a ratio of 7/0.7/1,and then by one-step sintering in a tubular furnace.The effects of sintering temperature and the amount of catalyst on the yield and structure of CNTs were studied.The results showed that:When the amount of fixed catalyst was 5 wt%,the yield and graphitization degree of CNTs increased at first and then decreased with the sintering temperature from 750?to 950?.When the sintering temperature was 850?,the yield of CNTs could reach 12.71%,the ratio of I_G/I_D in Raman spectra could reach 1.33,and the diameter was about 20 nm.The number of wall layers is about 24.At the fixed sintering temperature of 850?,the yield and graphitization degree of carbon nanotubes increased first and then decreased with the addition of catalyst from 1 wt%to 10wt%.When the addition of catalyst was 7 wt%,the yield of carbon nanotubes reached 19.15%,and the ratio of I_G/I_D in Raman spectra was 1.00.The diameter of the tube is about 20 nm,and the number of wall layers is about 20.(3)Study on synergistic effect of Fe/Ni-Mo-Mg composite catalyst.On the basis of conclusion(2),the synergistic effect of Fe/Ni-Mo-Mg composite catalyst was studied by using three kinds of carbonyl iron,Ni-Mo-Mg and Fe/Ni-Mo-Mg catalysts to further improve the yield.The results showed that:When the total catalyst content was the same,the yield of carbon nanotubes prepared by using carbonyl iron as catalyst was very low but the graphitization degree was very high.The yield of CNTS-Fe-5 was only 0.85%when Fe was added at 5 wt%,while the ratio of Ig/ID in Raman spectra was 2.46.When only 5 wt%Ni-Mo-Mg catalyst was added,the yield was 12.71%,and the ratio of I_G/I_D in Raman spectra was 1.33.However,when Fe/Ni-Mo-Mg mixed catalyst was used,both the yield and graphitization degree could reach a high level.When 2.5wt%Fe catalyst and 2.5 wt%Ni-Mo-Mg catalyst were added at the same time,the yield of CNTS-Fe/Ni-2.5/2.5 could be increased to 14.47%,and the ratio of I_G/I_D in Raman spectra was 1.40.The carbonyl iron has a synergistic effect on the Ni-Mo-Mg catalyst prepared by us.The addition of carbonyl iron can further improve the yield and graphitization degree of carbon nanotubes.(4)Study on the structure and properties of PVA/CNTs composite fiber membranes.Using CNTs as filler,PVA/CNTs composite fiber membrane was prepared by electrospinning.The effects of spinning voltage and the amount of CNTs on the structure and properties of the composite fiber membrane were studied.The experimental results showed that the amount of CNTs fixed was 0.5wt%,and the spinning voltage increased from 9 k V to 17 k V.PVA/CNTs composite fiber films with less defects were obtained at 17 k V,and the average diameter of the fibers was about 0.2?m.When the spinning voltage was increased to 25 k V,the structure and morphology of the composite fiber films did not change significantly.The fixed spinning voltage was 17 k V.With the addition of CNTs increasing from 0.25 wt%to 1 wt%,the number of defects such as beads and adhesions in the fibers became more and more.When the addition of CNTs increased to 2 wt%,the spinning process could not be carried out smoothly because the agglomeration of carbon nanotubes was too serious.Therefore,the amount of CNTs should not exceed 1 wt%in the preparation of PVA/CNTs composite fiber membrane.The addition of CNTs can improve the thermal properties and bonding ability of PVA fiber.Compared with pure PVA fiber film,the addition of 1 wt%carbon nanotubes can increase the carbon residue rate from 4.61%to 10.57%,and increase the bonding shear strength from 126.1k Pa to 546.6 k Pa.These properties indicate that the PVA/CNTs composite fiber membrane can be used as a binder for carbon paper or carbon felt. |
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