Chemical growth mechanisms, nanotube

Vinciguerra, V., et al. (2003), Growth mechanisms in chemical vapour deposited carbon nanotubes, Nanotechnology, 14(6), 655-660. 

Figure 1.6 Schematic representation of (a) the diffusion-precipitation mechanism of carbon filament growth from the gas phase [32], and (b) the carbon-fiber growth mechanism proposed by OberUn et al. [33]. Important details regarding the effects of metal particle size and shape on the chemical reactions occurring at the metal-carbon interface, and thus on the nature and size of the filaments or nanotubes produced, have yet to be sorted out.

Kumar M, Ando Y. Chemical vapor deposition of carbon nanotubes a review on growth mechanism and mass production. J Nanosci Nanotechnol 2010 10 3739-58. 

Huang S, Woodson M, Smalley R, Liu J. Growth mechanism of oriented long single walled carbon nanotubes using fast-heating chemical vapor deposition process. Nano Lett 2004 4 1025-8. 

Yun, Y.H., Shanov, V., Tu, Y., et al. Growth mechanism of long aligned multiwall carbon nanotube arrays by water-assisted chemical vapor deposition. J. Phys. Chem. B 1KK47),... 

The development of ways to make SWNTs and control their structure as well as ways to manipulate them and incorporate them into devices is at the forefront of research in nanotechnology. Recent developments in CVD growth of nanotubes have resulted in the ability to grow bundles of CNTs that can be harvested and spun into fibers to make a super thread. The individual CNTs in a super thread are bonded by van der Waals forces, but it is possible to alter their mechanical and electrical properties by heat treating and irradiation. The sides of CNTs have been functionalized to bind with epoxies and polymers to form composites and their tips have been functionalized to serve as chemical sensors, atomic force microscope (AFM) tips, ion and electron emitters, and for other novel applications. 

Figure 3.16 Different steps in the fabrication of MWNT nanoelectrode arrays, (a) metal film deposition, (b) catalyst deposition, (c) plasma-enhanced chemical vapor deposition for CNT growth, (d) dielectric encapsulation with Si02, (e) planarization with a chemical mechanical polishing to expose the ends of the carbon nanotubes, (f) electrochemical characterization. Readapted from Ref [6].

The mechanism of nanotube formation in chemical vapor deposition features characteristics rather distinct from those found for the synthesis by arc discharge or laser ablation. Contrary to the latter, a solution of small carbon clusters in and subsequent diffusion through catalyst particles play a minor role in the deposition from the gas phase. The employed hydrocarbons decompose directly on the surface of the catalytic particle. The carbon, therefore, becomes immediately available for nanotube growth. 

Therefore, the key step for the in-situ polymerization of PUCNs is the dispersion of CNTs in macromolecular polyols. In order to reduce the aggregations, it is necessary to physically or chemically modify the surface of CNTs to reduce the van der Waals force among the nanotubes. Strong mechanical tools such as ball milling and ultrasonic treatment can be used to help break down the aggregation of CNTs. The kinetics of PU chain growth should be taken into account although it is rarely reported in up-to-date publicahons. 

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