Microwave Technique Allows Selective Drilling of Thermal Barrier Coatings
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isolation of a single CNT phase. However, the products of current synthetic schemes are either pure multiwalled CNTs (MWNTs) with a range of outer diameters or a mixture of single-walled CNTs (SWNTs) with different diameters and chirality. A detailed knowledge of the growth mechanisms for CNTs is currently lacking, although it has been found recently that catalytic chemical vapor deposition (CVD) provides a near-equilibrium route to both MWNTs and SWNTs. In order to gain a detailed understanding of the CNT growth process, which might lead to the ability to control the type of CNTs produced, R. Sharma of Arizona State University and Z. Iqbal of the New Jersey Institute of Technology, Newark, have used an environmental transmission electron microscope (ETEM) to observe in situ the growth of CNTs in various reaction conditions. As reported in the February 9 issue of Applied Physics Letters, Sharma and Iqbal used CVD to produce CNTs by flowing propylene or acetylene gas over Ni or Co catalysts. While the researchers observed only growth of graphitic filaments as propylene flowed over Ni-SiO2 at 400°C, the growth of well-defined CNT structures was observed as acetylene flowed over Ni-SO2 at 450°C. Sharma and Iqbal observed Ni particles at the tips of the CNTs as well as in the middle of the tubes. In addition, the researchers said that these CNTs grew in a zigzag manner, with cumulative turns of up to 360°. After finding that CNTs had formed all over the sample area, the researchers concluded that the electron beam did not induce the CNT growth process and, therefore, the observations in situ are directly comparable to the bulk synthesis conditions. Sharma and Iqbal said that ex situ, highresolution electron micrographs confirmed that MWNT growth is favored at 450°C. The inner diameters of these tubes ranged over 2–8 nm, while the outer diameters ranged over 8–17 nm. The researchers also observed several graphene sheets perpendicular to the length of a tube and said that a bubble seen within the wall of a tube indicates that the CNT was in the process of development. Sharma and Iqbal found that at 700–800°C, single- and double-walled CNTs formed predominantly but occasionally very thin graphene filaments were observed. They also often observed bun-
dles of SWNT sheets forming. The researchers said that, in contrast to MWNT growth, the SWNTs grew straight out from the catalyst with very uniform diameters, varying between 1 nm and 1.5 nm for different tubes. In addition, the researchers observed no catalyst at the tube tips or in the middle of the tubes, which is consistent with the proposal that SWNTs grow from the catalyst that is present at the tube root, although they admit that the roots of their SWNTs are hidden from view. Sharma and Iqbal used confocal micro-Raman spectroscopy to confirm that samples prepared with acetylene at 700°C were indeed SWNTs. They said that the diameters calculated from the Raman data agree well with the diameters measured from the ETEM images. In future work, Sharma plans to “observe the
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