Carbon Formation at High Temperatures (550–1400 °C): Kinetics, Alternative Mechanisms and Growth Modes

Abstract

This Note aims at clarifying the alternative mechanisms of carbon formation from gases at temperatures above 550 °C. Both the growth of carbon nanotubes (CNTs) by a hybrid route, and of graphene layers deposition by a pyrolytic route are analyzed: the transition had no influence in apparent kinetics, but the carbon structure was totally different. The transition temperature from hybrid to pyrolytic growth varies with the gas pressure: higher temperature transition was possible using lower active gas pressures. The rate-determining step concept is essential to understanding the behavior. In catalytic and hybrid carbon formation, the slower step controls and determines kinetics. In the pyrolytic region, the faster step dominates, and carbon bulk diffusion is blocked: layers of graphene cover the external catalyst surface. It is easier to optimize CNTs growth (rate, shape, properties) knowing the details of the alternative mechanisms operating.

Kinetics of Carbon Nanotubes and Graphene Growth on Iron and Steel: Evidencing the Mechanisms of Carbon Formation

“…These authors used multilayer metal films of 10nm Al and Fe or Mo as catalysts and a flow of C 2 H 2 (6 sccm) diluted in Ar (ex: 2000 sccm) and H 2 (ex: 400 sccm) [ 14 ]. The reason for this gas dilution can be understood by our recent analysis of high temperature carbon formation kinetics [ 58 ]. Low hydrocarbon partial pressure is the key to keep Route II operating at higher temperatures with a faster rate and avoiding pyrolytic graphene layers deposition (Route III, pyrolytic).…”

“…Overall knowledge of the CNTs and graphene alternative growth mechanisms [ 5 , 8 , 58 ] is important to optimize rate, structure, and desired properties. Route I operates with acetylene and C 2 to C 4 olefins and CO. At higher temperatures, CNTs can be formed by route II only.…”

“…Concerning graphene, the deposition of layers at high temperatures is a transition from the CNTs growth by the hybrid route to the pyrolytic route, but the deposition rate observed follows the same Arrhenius plot line [ 58 ]: Almost a paradox (check Figure 3 , (A)). This must be understood—it is a change of mechanism with a continuous line in the Arrhenius plot: At lower temperatures, C 2 /C 3 rate of deposition controls the rate; at higher temperatures, C 2 /C 3 rate of deposition dominates and covers the catalyst surface with graphene layers–pyrolytic route.…”

“…Diffusion of C, N, and O atoms in transition metals is interstitial, due to the ratio of the covalent radius of the metal solvent and dissolved atoms (AR sol /AR solv ) being less than 0.60 (0.50 for Fe) [ 51 , 58 ]. The solid-state phases operating during CNT growth are mostly Ni and a Ni 3 C layer at the gas phase reaction side.…”

“…An introduction to solid-state diffusion and alternative diffusion types is well summarized by Schmalzried [ 71 ]: Vacancy diffusion, interstitial diffusion, interstitialcy diffusion. An Arrhenius plot of interstitial diffusion of C in Fe was shown as evidence of the temperature dependence of interstitial diffusivity of atoms in solids [ 9 , 58 ]. The Tammann temperature of iron is 632 °C.…”