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Technical Briefs

Impact of Thermodiffusion on Carbon Nanotube Growth by Chemical Vapor Deposition

[+] Author and Article Information
Andrew C. Lysaght

Department of Mechanical Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269-3139

Wilson K. S. Chiu1

Department of Mechanical Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269-3139wchiu@engr.uconn.edu

1

Corresponding author.

J. Heat Transfer 132(8), 084501 (Jun 04, 2010) (4 pages) doi:10.1115/1.4001099 History: Received April 18, 2008; Revised December 28, 2009; Published June 04, 2010; Online June 04, 2010

Thermal diffusion, the process by which a multicomponent mixture develops a concentration gradient when exposed to a temperature gradient, has been studied in order to understand if its inclusion is warranted in the modeling of single-wall carbon nanotubes (SWNTs) synthesis by thermal chemical vapor deposition (CVD). A fully coupled reactor-scale model employing conservation of mass, momentum, species, and energy equations with detailed gas phase and surface reaction mechanisms has been utilized to describe the evolution of hydrogen and hydrocarbon feed streams as they undergo transport, as well as homogeneous and heterogeneous chemical reaction within a CVD reactor. Steady state velocity, temperature, and concentration fields within the reactor volume are determined, as well as concentrations of adsorbed species and SWNT growth rates. The effect of thermodiffusion in differing reactor conditions has been investigated to understand the impact on SWNT growth. Thermal diffusion can have a significant impact on SWNT growth, and the first approximation of the thermal diffusion factor, based on the Chapman–Enskog molecular theory, is sufficient for modeling thermophoretic behavior within a CVD reactor. This effect can be facilitatory or inhibitory, based on the thermal and mass flux conditions. The results of this investigation are useful in order to optimize model and reactor designs to promote optimal SWNT deposition rates.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of horizontal tube flow chemical vapor deposition reactor for carbon nanotube synthesis

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Figure 2

Percent separation of a binary mixture of H2 and N2 (a) as a function of initial mole fraction when TC=284 K and log10(TH/TC)=0.2; (b) presents separation percentage for three mixtures as a function of temperature ratio. In both plots, the data points represent experimental data reported by Ibbs (25) and the curves represent diffusion model predictions.

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Figure 3

Percent change in predicted carbon nanotube growth rate (averaged along the reactor length) when the thermophoretic effect is incorporated in the model

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Figure 4

Percent surface coverage (averaged along reactor length) for a surface bound species as a function of the reactor wall temperature (left-axis). The right y-axis presents the percent change in the surface coverage when thermal diffusion is considered. All data are for carbon nanotube deposition with 20% methane.

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