Study On Preparation Of Carbon Nanomaterials By Thermal Plasmas And Its Formation Pathway

Carbon nanomaterials have attracted extensive interests due to their excellent mechanical,electrical and chemical properties.In this paper,magnetically dispersed arc plasmas are adopted to prepare carbon nanomaterials by cracking the gas-phase slight hydrocarbons(CH4,C2H2,C2H4).The spherical/nearly spherical carbonaceous nanospheres(CNSs)and graphene nanoflakes(GNFs)are obtained,and CNSs include two basic forms of carbon of the amorphous carbon spherical particles and onion-like carbon particles.The carbon nanomaterials synthesis and its formation mechanism are studied by experiments combined with chemical kinetics calculation and computational fluid dynamics numerical simulation.The characterization methods of carbon nanomaterials include transmission electron microscopy,X ray diffraction,Raman spectroscopy,thermogravimetric analysis,and X-ray photoelectron spectroscopy.The control factors for the synthesis of carbon nanomaterials are clarified to understand the relationship among reaction conditions,formation mechanism and morphology of carbon materials,so as to provide a theoretical guidance for the industrial application of carbon nanomaterial production by thermal plasmas.1)First,CH4 and C2H2 are chosen as feedstocks,which have a large difference in the bond energy and the hydrogen/carbon mole ratio.The range of the initial reaction temperature is from 1800 K to 2500 K.Experimental results indicate that high reaction temperature can promote the CH4 pyrolysis products from amorphous carbon particles to graphene nanoflakes(GNFs),but for C2H2 to onion-like carbon particles.The calculation of gas-phase chemical reaction dynamics and particle dynamics are applied to analyze the influence of the types of feed gas,reaction temperature on nucleation,surface growth and collision,so as to establish the relation between the forming mechanism and the particle morphology of the carbon black.The type of feedstock and reaction temperature can impact the formation of polycyclic aromatic hydrocarbons(PAHs),which are assumed to be the nucleation precursors.changing the nucleation rate of carbon particles.There is a competitive relationship between the formation pathways of carbon nuclei and surface reaction on carbon source consumption.When a large amount of carbon source is consumed during nucleation,the surface growth of particles is inhibited and the average initial particle size of carbon black decreases.The number density and high temperature of particles can improve the efficiency of particle-particle collisions,resulting in the complex structure of aggregates.In terms of the internal nanostructure,at low temperature,the structure tends to be physically balanced,resulting in the random orientation of the self-organization of PAHs within the primary particle.At high temperature,the structure tends to be chemically balanced,resulting in the parallel arrangement of the PAH cluster around the center of the primary particle.Compared with C2H2,GNFs was obtained in the high-temperature pyrolysis of methane.It is speculated that high H/C ratio of methane and high chemical activity environment are the main causes of GNFs formation.2)Then,The effects of hydrogen addition,gas injection position,acetylene concentration,and reaction temperature on the morphology of pyrolysis products are evaluated.Carbon spherical particles and graphene nanoflakes are also obtained.Results indicate that a large amount of hydrogen,a low carbon flow,and high temperatures can promote the morphological transformation of carbon materials from nanospheres to graphene nanoflakes.Combined with a gas-phase kinetic model and a simulation of computational fluid dynamics,the reaction zone with a critical temperature of 2500 K can be divided basically into nucleation and surface growth,which correspond to the temperature-rise zone and high-temperature maintenance zone,respectively.Essential factors affecting graphene formation may include the formation of sheet-like nuclei and continuous planar growth at the side active sites of the structure.A low collision frequency of precursors and the high temperatures favor the formation of sheets-like nuclei.Planar surface growth requires hydrogen to terminate dangling bonds at edges and a high temperature to induce growth without a curvature.3)C2H4 is used as the precursor for the synthesis of carbon nanomaterials.the effects of doping with H2,CO2 and N2are studied on the morphology,structure and chemical properties of the products under different reaction temperatures,so as to further verify the rationality of the formation path of graphene and obtain nitrogen-doped graphene at the same time.Results show that the products exhibite porosity at low temperature.In general,with the reaction temperature increase,CNS formation is inhibited,and the lamellar graphene structure tends to be flat and less agglomeration.The fine structure is characterized by reduced defect density,defect types approaching marginal defects,and the decreasing number of graphene layers.The relative content of sp2 hybridized carbon increased and the relative content of oxygen element decreased.The effect of gas doping at low temperature(?2500K)is mainly to change the enthalpy of plasma.The defect density presents a reverse trend with specific enthalpy hH2>hN2>hCO2>hAr.The gas chemical properties show obvious differences at high temperature(?3500 K).The high temperature promotes C2H4 and C2H4-H2 to produce abundant hydrogen atoms,to etch the edge of graphene,resulting in an increase in the defect density of 4500 K.Under the high temperature of C2H4-CO2,the forward reaction of CO2+H=CO+OH is favorable,which can consume hydrogen atoms to some extent and slow down the edge etching process.Doping N2 at ?2500 K,the high enthalpy can maintain high energy environment,promote the H atoms are generated by dissociation,the graphene sheets with little defect density and high crystallinity.At?3500 K,due to nitrogen groups(CN,HCN and so on)to make graphene N elements doping,its defect density increase and the crystallinity decreased.The type of feed gas,H/C ratio,reaction temperature and doping all can affect the carbon nanomaterials properties.High H/C ratio,low carbon concentration and high temperature promote the morphology transformation.In terms of formation mechanism,the factors controlling carbon black morphology depend on the competition of nucleation and surface growth to carbon source and the efficiency of collision and coagulation.The controlling factor in the synthesis of graphene is the formation and maintenance of planar growth of the two-dimensional sheet-like carbon core.