Necessity of base fixation for helical growth of carbon nanocoils
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e role played by catalyst aggregates in the growth of carbon nanocoils (CNCs) by a chemical vapor deposition (CVD) method has been studied. The experimental results show that CNCs can be grown from the discrete aggregates on a substrate with a porous surface, while only some irregular carbon nanofibers are grown from those on a flat substrate. It is accepted from the viewpoint of mechanics that the spiral motion of a CNC should generate a torsional moment on its base that attaches to an aggregate. The catalyst particles readily expand on the flat substrate during the CVD process and form a loose aggregate, which cannot provide a strong interaction between the aggregate and the base of a carbon fiber grown from there. On the contrary, the expansion of catalyst particles in a microsized hole is restricted by the surrounding wall of the hole, leading to the formation of a compact aggregate that fixes the base of the grown fiber. A perfect CNC can be grown only under the condition that its base is firmly fixed by an aggregate that can balance the torsional moment of the CNC during its spiral growth.
I. INTRODUCTION
Three-dimensional (3D) helical materials have stimulated worldwide enthusiasm due to their peculiar morphologies, unique properties, and interesting growth mechanisms.1–5 Carbon nanocoils (CNCs), as a kind of important 3D materials, have attracted much attention due to their outstanding mechanical6 and electromagnetic7,8 properties, which are expected to have wide applications, such as high-performance electromagnetic wave absorbers, field emitters, nanosprings, microsensors, etc.9–13 To realize these applications, it is necessary to control the morphology of the grown CNCs and understand their growth mechanisms. It is generally recognized that the morphology of the grown CNCs are mainly determined by structures and compositions of the used metal or alloy catalysts.12–16 So far, CNCs have been successfully synthesized by using iron-coated indium tin oxide (ITO) catalyst,13 Ni–P-based binary alloys catalysts,17 sputtered films of Au metal or Au–Ni alloys catalysts,18 etc. Most of the catalyst particles are observed at the tips of the coils indicating a tip growth mechanism. In addition, several proposals have been made that the growth of a CNC or a carbon microcoil (CMC) is due to the nonuniformity of the carbon extrusion speed at different parts of the catalyst particle at the coil tip.10,13,19 Amelinckx et al.10 proposed a model of spatial velocity hodograph to describe a formation mechanism for a catalytically grown CNC. Pan et al. a)
Address all correspondence to this author. e-mail: lpan@dlut.edu.cn DOI: 10.1557/jmr.2011.401 J. Mater. Res., Vol. 27, No. 2, Jan 28, 2012
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developed Fe–In–Sn–O alloy catalysts for synthesizing CNCs and considered that the mechanism of CNC formation is due to that the catalyst particle at the tip of the CNC is not uniform and thus causes the difference in the extrusion velocities of carbon network between inner and outer sides of
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