Numerical Simulation Of DC Arc Discharge And Growth Mechanism Of Single-walled Carbon Nanohorn

Single-walled carbon nanohorns（SWCNHs）is a kind of carbon nanomaterials with unique conical structure,which has attracted the attention of researchers around the world for its excellent physicochemical properties.With the advantages of fast synthesis,high product purity,and environmental friendliness,the direct current（DC）arc discharge method is regarded as one of the most effective methods to prepare SWCNHs.Because the plasma atmosphere generated by arc discharge involves complex multi-physical fields coupling effects,it is difficult to obtain the plasma parameters by conventional experimental means.This restricts the development of the growth mechanism of SWCNHs,and there is no completely unified understanding so far.In order to solve the above problems,in this paper,SWCNHs is prepared by DC arc discharge under the condition of nitrogen without catalyst.The experimental process is numerically simulated,and the growth mechanism of SWCNHs is explored in combination with homogeneous nucleation theory.Firstly,SWCNHs is prepared and characterized under different current and gas pressure conditions,and the experimental results show that the average particle size of SWCNHs increases from 42.72 nm to 47.07nm when the current increases from 100 A to 250 A at 70 KPa nitrogen.The average particle size of SWCNHs increases from 39.17 nm to 45.61 nm when the gas pressure increases from 40 KPa to 70 KPa at 200 A current,and nitrogen is successfully doped into SWCNHs,which improves the defect degree.Subsequently,COMSOL software is used to establish a two-dimensional mathematical and physical model of electromagnetic-magnetic-thermal-current coupling of arc plasma.The current density distribution,temperature field distribution,and flow field distribution in the plasma region are obtained by steady-state calculation of the magnetohydrodynamic equations.Based on the steady-state calculation results,the transient calculation of the carbon vapor mass conservation equation is carried out.The distribution of diffusion coefficient,carbon mass fraction,and carbon cluster number density in the plasma region are obtained.It can be seen from the simulated cloud image that with the increase of current,the same position in the plasma region has larger current density,temperature,velocity,diffusion coefficient,and carbon mass fraction.At the same time,the higher current corresponds to the higher anode surface temperature,and the higher temperature makes the evaporation process on the anode surface become more intense,and the mass flux of carbon evaporation also increases,which is consistent with the trend of the anode surface evaporation rate from experiment.The region corresponding to the temperature range between the maximum nucleation temperature of carbon nanoparticles（4900 K）and the initial ionization temperature of nitrogen（3300 K）is considered as the growth region for the formation of single-walled carbon nanoconical structures.In this region,the carbon cluster number density showes an increasing trend with the increase of current and gas pressure.Finally,the relationship between the growth process of SWCNHs and the numerical simulation results is established by combining the homogeneous nucleation theory.The nucleation temperature and condensation coefficients of carbon nanoparticles increase with the increase of current and gas pressure,resulting in a larger average particle size of SWCNHs.In addition,the growth mechanism of SWCNHs prepared by DC arc discharge is proposed.A large number of C atoms,C₂molecules,and C₃ molecules are evaporated from the surface of the anode graphite rod,which combine with each other to form linear carbon chains.Nitrogen doping reduces the energy barrier,which is conducive to the formation of carbon five-membered rings and seven-membered rings.The carbon chains become longer and gradually close to the ring structure,and further develop into graphite lamellae structure.The outer structure of the spherical particles is curled into angular structure after high temperature quenching,and SWCNHs is obtained.The above work has laid a good theoretical foundation for the controlled preparation and large-scale production of SWCNHs,which can help to explore the potential of SWCNHs in related fields.