Feedstock Diffusion and Decomposition in Aligned Carbon Nanotube Arrays

Abstract

Feedstock diffusion and decomposition in the root growth of aligned carbon nanotube (CNT) arrays is discussed. A nondimensional modulus is proposed to differentiate catalystpoisoning controlled growth deceleration from one which is diffusion controlled. It is found that, at current stage, aligned multi-walled carbon nanotube (MWNT) arrays are usually free of feedstock diffusion resistance. However, for single-walled carbon nanotube (SWNT) arrays, since the inter-tube distance is much smaller than the mean free path of carbon source (ethanol here), high diffusion resistance is significantly limiting the growth rate. The method presented here is also able to predict the critical lengths in different chemical vapor deposition (CVD) processes from which CNT arrays begin to meet this diffusion limit, as well as the possible solutions to this diffusion caused growth deceleration. The diffusion of carbon source inside of an array becomes more important when we found ethanol undergoes severe thermal decomposition at the reaction temperature. This means, in a typical alochol CVD, hydrocarbons and radicals decomposed from ethanol may collide and react with the outer walls of SWNTs before reaching catalyst particles. We found when flow rate is low and ethanol is thoroughly decomposed, the produced SWNTs contain more soot structures than the SWNTs obtained at higher ethanol flow rate. Understanding the mass transport and reaction inside a SWNT array is helpful to synthesize longer and cleaner SWNTs.

Low-Cost Sputtering Process for Carbon Nanotubes Synthesis

“…This is also plausibly due to incompatible of catalysts deposition in our system by means of oxidized metal due to the presence of O2 in plasma, and low amount of ethanol introduction. Since ethanol could be decomposed at high temperature into acetylene, water and some other molecule species [10,11], the role of acetylene on nanotube growth has been then demonstrated [12]. Meaning that acetylene could be used to efficiently grow CNTs, whereas water could prolong the catalyst lifetime.…”