Arc usage is introduced into CNT synthesis for the reason that it provides a simple and traditional tool for generating a high temperature, which is necessary for the vaporization of carbon atoms into a plasma. The traditional arc discharging experimental equipment of fabrication of carbon nanotubes as shown in the figure. Once the arc is in operation, a carbon deposit forms on the negative electrode. The key parameters that control the growth kinetics including the hydrocarbon carrier gas, the growth time and temperature, and the catalyst composition. The growth of CNTs occurs under stable conditions but over a limited range of He (or Ar) pressure, electric current and arc voltage.
For MWNTs, no catalyst is required while the fabrication of SWNTs requires a catalyst, including transition metals such as Co, Ni, Fe and rare earths such as Y and Gd. In addition, mixed catalysts, for instance, Fe/Ni, Co/Ni and Co/Pt have been used to synthesize ropes of SWNTs.
Sad to say, the overt defects of the arc method, comprising low yield of the product and the huge inconvenience in purification and separation, make this traditional method pale in comparison to other methods.
Other synthesis methods, such laser vaporization synthesis, physical vaporization synthesis are also developed and reader could refer to other materials for further understanding.
Chemical vapor deposition
Advantage of CVD
Compared to other methods like arc-discharge and laser-ablation, CVD is both simple as well as economical which can synthesize CNTs at comparable low temperature and ambient pressure. Although in crystallinity, arc and laser methods are somehow superior to CVD (although CVD-grown MWCNTs possess inferi...
... middle of paper ...
...ion will cease. (iii). A process known as “tip-growth model”.
When the reciprocity of catalyst–substrate is strong (there is an obtuse contact angle between the metal and the substrate), initial hydrocarbon decomposition and carbon diffusion take place similar to that in the tip-growth case, but the CNT precipitation cannot push the metal particle up; so the precipitation is needed to emerge out from the metal’s apex (farthest from the substrate, having minimum interaction with the substratum). First, carbon crystallizes out as a hemispherical dome which then extends up in the form of seamless graphitic cylinder. Next hydrocarbon deposition takes place on the lower external surface of the metal, and dissolved carbon begin to diffuses upward. Thus CNT grows up with the catalyst particle rooted on its base; therefore, this is called as “base-growth model.”
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