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Research Papers: Micro/Nanoscale Heat Transfer

Carbon Nanotubes, Synthesis, Growth and Orientation Control in Opposed Flow Diffusion Flames

[+] Author and Article Information
Lawrence A. Kennedy

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, IL 60605-7043lkennedy@uic.edu

J. Heat Transfer 130(4), 042402 (Mar 17, 2008) (11 pages) doi:10.1115/1.2818751 History: Received August 21, 2006; Revised March 24, 2007; Published March 17, 2008

The combustion synthesis of carbon nanotubes is reviewed, examining their formation and control in diffusion flames. Much of the initial work in this area employed coflow diffusion flames and provided insight into carbon nanotube (CNT) formation. However, the inherent multidimensional nature of such coflow flames made the critical spatial location difficult to maintain. Among this early work, our UIC group demonstrated the superiority of the opposed flow diffusion flame configuration due to its uniform radial distribution that reduces such flow to a one-dimensional process. While a summary of the early coflow flame work is presented, the use of the opposed flow diffusion flame will be the focus of this review. The production of carbon nanostructures in the absence of a catalyst is discussed together with the range of morphology of nanostructures generated when a catalyst is employed. The important aspect of control of the growth and orientation of CNTs and generation of CNT arrays through the use of electric fields is examined as is the use of anodized aluminum oxide templates. Fruitful areas for further research such as the functional coating of CNTs with polymers and the application of these opposed flow flames to synthesis of other materials are discussed.

Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of counterflow burner

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Figure 2

TEM of single CNT

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Figure 3

TEM of captured nanotubes and nanopartcles

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Figure 4

Schematic of the experimental setup

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Figure 5

SEM image showing the catalyst substrate covered with high-density layer of carbon nanofibers and CNTs

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Figure 6

TEM image of CNTs transferred to the microscope grid

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Figure 7

TEM micrograph of well-aligned MWNT

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Figure 8

SEM image of regularly coiled spiral carbon nanofiber

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Figure 9

TEM image of coiled carbon nanofiber. The nanofiber has rectangular cross section with height of 300nm and thickness of 100nm.

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Figure 10

SEM image of long (∼0.2mm) uniform diameter tubular carbon nanofiber

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Figure 11

High-resolution TEM image of the wall of the long uniform diameter tubular carbon nanofiber uniform-diameter carbon nanofiber shown in Fig. 1. Regular structure and orientation of the carbon layers parallel to the fiber axis is observed.

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Figure 12

Temperature and major chemical species in opposed flow oxy-flame as a function of the distance from the fuel nozzle (Z), data of numerical model (24)

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Figure 13

SEM image of orderly VACNT layer covering the probe surface; floating potential mode, Z=8.5mm, T∼740C. The layer is partially removed revealing the bare catalytic surface.

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Figure 14

High-resolution SEM of the wall edge of the layer shown in Fig. 4 displays nanotube purity and alignment

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Figure 15

Low resolution SEM image shows the catalyst substrate coated with a layer of carbon nanotubes (FPM) Z=9.5mm.

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Figure 16

Higher resolution SEM of a micro area in Fig. 6. The CNT bundle mechanically removed from catalytic surface is reattached to the top of the originally uniform VACNT layer.

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Figure 17

Low- and high-resolution (inserts) SEM images of scanned probe surface coating layer, showing uniformity of VACNT

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Figure 18

Coating of the flame generated CNT: (a) not treated CNTs; (b) CNT coated with polystyrene-fluid bubbles in inner channels; (c) polystyrene coating

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Figure 19

Double coating of CNT

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Figure 20

Representative SEM images collected on the surface of a Mo probe inserted at the flame height of 12mm for 2min: (a) A low resolution SEM image collected on the Mo surface, (b) Higher-resolution imaging analysis shows the presence of rectangular (1), square (2), and circular fiberlike structures (3,4) (c) Some tips of the circular and rectangular structures are open showing the inner hollow cavities.

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Figure 21

Representative SEM images collected on a Mo probe inserted at the flame height of 11.0mm for sampling time of 2min: (a) SEM image shows the presence of channellike structures with rectangular and square morphologies, the structures appear to be completely hollow, and their corners are well defined, with lengths slightly longer at one of the corners, as shown by arrows; (b) High-resolution TEM image exhibiting the lattice structure on the edge of the channel with measured lattice distance of 0.36nm corresponding to 0̱111p plane of a monoclinic MoO2; (c) EDX of elemental spectrum acquired using SEM.

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