The influences of oxygen concentration and external heating on carbon nanotube growth in diffusion flame

Abstract

Tight control of the carbon nanotube (CNT) synthesis process in flames remains a challenge due to the highly non-uniform gradient of flame thermochemical properties. The present study aims to establish a baseline model for flame-enhanced chemical vapor deposition (FECVD) synthesis of CNT and to analyze the CNT growth region at varying flame and furnace conditions. The numerical model comprises a computational fluid dynamics (CFD) simulation that is coupled with the CNT growth rate model to simulate the flow field within the furnace and the CNT growth respectively. Validation of the flame shape, flame length, and temperature profile are carried with a reasonable comparison to experimental measurements. A parametric study on the effects of furnace heating capacity and oxidizer concentration is conducted. The results of the CNT growth rate model reveal that there is a positive correlation between the heater power and CNT length. Supplying a higher concentration oxidizer at a fixed furnace power is predicted to result in further improvement in CNT length and high yield region. Flame structure analysis showed that with the heater turned on at 750 W (corresponding to heat flux of 21,713W/m2), the growth region expands twofold when oxygen concentration is increased from 19% to 24%. However, the growth region shrinks when the oxygen concentration is further increased to 27% which indicates depletion of carbon source for CNT growth due to excess oxygen. The finding of this research could guide and optimize the experiment of the flame-assisted CNT production in the future.