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Research Papers

Feedstock Diffusion and Decomposition in Aligned Carbon Nanotube Arrays

[+] Author and Article Information
Rong Xiang

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering,  Sun Yat-Sen University, Guangzhou 510275, Chinaxiangr2@mail.sysu.edu.cn

Erik Einarsson, Junichiro Shiomi

Department of Mechanical Engineering,  The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japanmaruyama@photon.t.u-tokyo.ac.jp

Shigeo Maruyama1

Department of Mechanical Engineering,  The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japanmaruyama@photon.t.u-tokyo.ac.jp

1

Corresponding author.

J. Heat Transfer 134(5), 051023 (Apr 13, 2012) (4 pages) doi:10.1115/1.4005703 History: Received April 26, 2010; Revised February 12, 2011; Published April 13, 2012; Online April 13, 2012

Feedstock diffusion and decomposition in the root growth of aligned carbon nanotube (CNT) arrays is discussed. A nondimensional modulus is proposed to differentiate catalyst poisoning controlled growth deceleration from one which is diffusion controlled. It is found that, at present, aligned multiwalled carbon nanotube (MWNT) arrays are usually free of feedstock diffusion resistance. However, for single-walled carbon nanotube (SWNT) arrays, since the intertube distance is much smaller than the mean free path of carbon source (ethanol here), high diffusion resistance in some currently available samples 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 alcohol CVD, hydrocarbons and radicals decomposed from ethanol may collide and react with the outer walls of SWNTs before reaching catalyst particles. 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.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) SEM image, (b) growth curve from in situ optical absorption, and (c) cross-sectional Raman spectra across a 12 C-13 C junction. We kept the growth conditions (temperature, pressure, etc.) of 13 C ethanol exactly same as 12 C ethanol, but the growth rate is slower. This is probably caused by some impurities in 13 C ethanol, because when the same process is performed with 12 C ethanol, SWNTs grow continuously and no obvious decay was observed. The arrow and spots in (a) represent the positions of the incident light. The inset schematic in (b) illustrates the junction structure in the case of root growth. Reproduced from Xiang (2008) [14].

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Figure 2

(a) SEM micrograph of vertically aligned SWNT arrays from ACCVD, inset at top-right is a schematic of a CNT film on a substate, suggesting the different dimensions of film size and thickness; (b) schematic representation describing the diffusion of feedstock as well as gas product during the root growth of CNT arrays. Reproduced from Xiang (2008) [18].

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Figure 3

(a) Temperature distribution inside the quartz tube during CVD. (b) Growth curves at 800 °C for different ethanol flow rates show a change for slow flow rates. (c) Ethanol decomposition curves calculated by CHEMKIN, and experimentally measured ethanol concentrations (circles) by FTIR spectroscopy. Reproduced from Xiang (2010) [19].

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