A modeling framework for the synthesis of carbon nanotubes by RF plasma technology.

摘要

The novel, cost and energy-efficient synthesis of carbon nanotubes (CNTs) by radio-frequency (RF) induction thermal plasma is a promising process for large scale production of CNTs for industrial and commercial applications. Techniques and conditions for producing larger quantities of CNTs have mainly depended on trial-and-error empirical variations of several operating parameters. Therefore, a detailed kinetic mechanism for CNT production in numerical simulations of an RF plasma system is required for understanding the process and identifying the parameters that mainly influence nanotube production. Such a model could also be used to enhance and optimize the design of synthesis systems.;In this work, a two-dimensional axisymmetric model was constructed for a RF processing system for the production of CNTs. The plasma field was solved using a computational fluid dynamics (CFD) Eulerian frame of reference. The interactions between the plasma and injected feed particles were considered using momentum, heat and mass transfer source terms in the plasma field governing equations. The trajectory and temperature history of the injected particles into the plasma were computed using a Lagrangian method. The effects of plasma gas composition and the raw material feed rate in the system on the plasma thermo-fluid fields were studied. It was found that when 100% He sheath gas was employed, compared to an Ar-He mixture, the evaporation rate of the injected feedstock particles in the plasma increased due to the higher thermal conductivity of the He gas. Based on the raw material feed rate analysis, a higher loading rate of feedstock reduces down the plasma temperature along the injection zone and increases the average evaporation time of the feedstock in the plasma.;A chemistry model for the formation of CNTs from feedstock material was implemented into an in-house two-dimensional axisymmetric parallelized CFD code. 36 elementary chemical reactions representing the formation of CNTs were numerically solved in the computational domain parallelized for execution on 64 Central Processing Units (CPUs). It was shown that the reaction rates of key reactions could be adjusted within their uncertainty range to improve the predicted yield of CNTs to within the yield rate reported from the experiments.