Flame Synthesis Of Carbon Nanotube Based On Supported Catalysts And Its Growth Mechanism

Carbon nanotubes,with unique structure and excellent physical and chemical properties,have huge application prospects in many fields,such as nanoelectronic devices,composite materials,sensors and so on.At present,the industrial production of carbon nanotubes is dominated by chemical vapor deposition(CVD).Compared to the CVD method,the flame method as a rapid method for preparing carbon nanotubes,has the advantages of short synthesis time,simple equipment and continuous synthesizing process under normal pressure.The in-depth study of the flame method to prepare carbon nanotubes will lay the foundation for the ultimate realization of mass production of carbon nanotubes using existing small coal-fired boilers and biomass furnaces.In this thesis,based on the existing V-flame method,it realizes the direct growth of carbon nanotubes on bare metal substrates,and develops a series of Co and Fe supported catalysts to evaluate the performance of catalytic growth of carbon nanotubes.X-ray diffraction spectroscopy(XRD),X-ray photoelectron spectroscopy(XPS),temperature programmed reduction(TPR),scanning electron microscope(energy spectrum)(SEM-EDS)and other characterization methods were used to analyze the catalytic properties and product properties.Provide theoretical guidance for mass production of carbon nanotubes by flame method.The mechanism of direct growth of carbon nanotubes on stainless steel substrates is analyzed by studying the effects of surface roughness and annealing temperature on the microstructure of 304 stainless steel substrates and products properties to achieve the goal of growing carbon nanotube coatings directly on components.Through comparison of the experimental results,the microscopic surface structure is the main factor affecting the properties of carbon nanotubes grown on 304 stainless steel substrate.The products evenly and densely distributed on the surface of wiredrawing stainless steel substrate annealed at 600?.For the martensite phase in wiredrawing stainless steel is beneficial to form micro boundary.,and high temperature results in recrystallization to form nano-scale particles.The formation of small island-like particles with uniform size and smooth boundaries on the surface of the substrate is a necessary for the formation of carbon nanotubes.The microstructure of carbon nanotubes is analyzed by transmission electron microscopy(TEM)to reveal that iron nanoparticles provided active sites for the growth of carbon nanotubes on the stainless steel substrates.Surface elements are analysed by XPS,it is found that annealing temperature affects the diffusion of Fe through the Cr oxide layer.Cr inhibits the formation of large Fe particles,thus the suitable nano particles are formed for growth of carbon nanotubes.It is indicating that proper addition of hetero elements is beneficial for the formation of carbon nanotubesThe effects of different preparation conditions on the direct growth carbon nanotubes on the substrate are studied to obtaining the optimal synthesis conditions The weight percentage of carbon increases as the synthesis time increases;the weight percentage of carbon increases rapidly at 2 minutes,after that it increases slowly.The growth process of carbon nanotubes includes dissolution,diffusion and precipitation In this study,the carbon nanotubes begin to grow after 2 minutes.The product reaches its best at 7min.The products are studied on 304 stainless steel substrate annealed at different temperature using CO,C2H4 and C2H2 as carbon sources respectively,no matter which is carbon source,the surface density of products growth on substrate annealed at 600? is maximum.Different carbon sources only affect the properties(diameter and length)of the carbon nanotubes.Therefore,the particles structure is a key factor affecting product growth,and carbon source plays a role in optimizing product propertiesThe Fe/Ni/Co-Cr series catalysts are prepared and developed by the impregnation method based on the element ratio of the stainless steel substrate.Through comparison of the effects of different bimetals and coordination metals on properties of carbon nanotubes,it is found that both FeNiCr and CoFeCr catalysts can catalyze the growth of clustered carbon nanotubes,and CoFeCr can obtain carbon nanotube clusters with high graphitization degree and high density.The characterization results of XRD,XPS,TPR and nitrogen adsorption and desorption showed that FeNiCr and CoFeCr catalysts showed a flaky structure with high dispersibility of supported metals.For catalysts using Fe as the active metal,the addition of Cr and Ni changes the ratio of Fe2+/Fe3+in the metal catalysts,the catalytic activity of the catalyst can be improved by reducing the ratio of Fe2+/Fe3+Preparing of CoXCr(X:Zn/Zr/V/W/Mo)and CoMoM(M:Zn/Zr/V/W/Cr)catalysts by impregnation method,Co(X:V/W/Mo/Fe)Cr and CoMo(M:Zr/V/W/Cr)catalysts can produce clustered carbon nanotubes,and the products obtained from CoMoZr and CoMoCr are the best.It is found that the mixed oxides formed between the active metal oxide,the auxiliary agent,dispersant and carrier has a decisive effect on the growth of carbon nanotubes.If the dissociation temperature of the mixed oxides is within the reaction temperature,it is beneficial to provide sites for the growth of carbon nanotubes.Cr mainly plays a dispersing role in the catalyst.By forming a mixed oxide with the active metal,it occupies the active site of the active metal and slows down the reduction rate of the catalyst.Mo is used as an auxiliary agent to form mixed oxides with active metals and carriers to prevent the agglomeration of active metal.mixed oxides are easily reduced at lower temperatures to form active metal particles and cooperates with Co to promote carbon nanotube growth.According to the results of N2 adsorption and desorption and SEM,the catalyst has a lamellar structure,and the protrusions on the surface of the lamellar structure provide growth sites for the carbon nanotube clusters.The diameter of the protrusions is similar to that of the carbon nanotubes.Each particle corresponds to a carbon nanotube.Adjacent carbon nanotubes interact and grow together.Due to the strong interaction with the carrier,the growth of the carbon nanotube cluster is consistent with base growth mode.