Carbon, filamentous growth mechanism

Figure 11 Tip-growth and extrusion mechanisms of carbon filament growth. (Ref. 58. Reproduced by permission of Elsevier)...

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.

Up to now, most of published work has been devoted to studies of mechanisms of carbon filament growth during conversion of carbon-containing compounds over metal catalysts [6,7]. Apparently, the properties of the filaments as adsorbents and supports have been neglected. Only a few studies on this matter may be mentioned [8,9]. The aim of this work is to bridge partially this gap and draw more attention to this promising new type of carbon materials. 

This process of filament growth is closely related to the synthesis of single walled carbon nano-tubes. Here the aim is to selectively produce a single layer of carbon in a tube that is as long as possible. Owing to their extreme mechanical strength and interesting electronic behavior these materials have recently attracted substantial interest in materials science. 

Figure 4. Schematic of growth mechanism of filamentous carbon on iron carbide. Key left, on rectangular carbide particle and right, on carbide particle with two active faces at oblique angles. (Reproduced, with permission, from Ref. 8.)...

In the present macro-scale experiments the filaments have been formed following reaction of ethane with iron and iron oxides. The decomposition of ethane to elemental carbon and hydrogen is endothermic (27) and so, at first sight, it appears that the experimental results are in conflict with the above mechanism of filament growth. However, earlier work (28) has shown that the majority of carbon formed from ethane arises from the decomposition product ethylene. The latter decomposes exothermically (27) (- A H for C2H4 at 725°C is 9.2 kcal. mole- ) so that this mechanism is not contravened. A similar rationale was used by Keep, Baker and France (29) to account for the formation of carbon filaments during the nickel catalyzed decomposition of propane. 

Fig. 12. Growth mechanism postulated by Baker et al. for the formation of carbon filaments by pyrolysis of acetylene (C2H2) on a metal particle (M) (C) denotes carbon. (From Baker et al.)...

Fig. 13. Schematic illustration of the growth mechanism proposed for carbon fibres and filaments formed by pyrolysing benzene over catalytic particles. (From Oberlin et al.)...

Figure 8.15 The most common mechanism of carbon filament formation from the pyrolysis of acetylene (CaHa) on a metal particle (M) where (C) denotes carbon. Source Reprinted with permission from Baker RTK, Electron microscopy studies of the catalytic growth of carbon filaments. Carbon Fibers Filaments and Composites. Copyright 1990, Springer. Figueiredo JL, Bernardo CA, Baker RTK, Hiittinger KJ eds., Kluwer, Dordrecht, 419, 1990, Baker RTK, Barber MA, Harris PS, Feates FS, Waite RJ, J Catal, 80, 86, 1972.

Figure 8.19 Mechanism of carbon filament formation in the PVFe system by extruded filament growth. Source Reprinted with permission from Baker RTK, Waite RJ, Formation of carbonaceous deposits from the platinum-iron catalyzed decomposition of acetylene, J Catal, 37, 101-105, 1975. Copyright 1975, Elsevier.

Figure 8.21 Schematic of ribbon and braided carbon filament morphologies. Source Reprinted with permission from Boehm HP, Carbon from carbon monoxide disproportionation on nickei and iron catatysts Morphoiogicai studies and possible growth mechanisms, Carbon, 11, 583-590, 1973. Copyright...

Figure 8.22 Proposed initiation mechanism for development of carbon filaments on an a-iron foil. Source Reprinted with permission from Sacco A Jr., Thacker P, Chang TN, Chiang ATS, The initiation and growth of filamentous carbon from ot-iron in Ha, CH4, HaO, COa and CO gas mixtures, J Catal, 85, 224-236, 1984. Copyright 1984, Elsevier.

Sacco A Jr., Carbon deposition and filament initiation and growth mechanisms on iron particles and foils, Figueiredo JL, Bernardo CA, Baker RTK, Huttinger KJ eds.. Carbon Fibers Filaments and Composites, Kluwer, Dordrecht, 459-505, 1990. 

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