Explaining Bamboo-Like Carbon Fiber Growth Mechanism: Catalyst Shape Adjustments above Tammann Temperature

Luís Sousa Lobo,Sónia A.C Carabineiro

doi:10.3390/c6020018

Abstract

The mechanism of bamboo-like growth behavior of carbon fibers is discussed. We propose that there is a requirement to have this type of growth: operation above the Tammann temperature of the catalyst (defined as half of the melting point). The metal nanoparticle shape can then change during reaction (sintering-like behavior) facilitating carbon nanotube (CNT) growth, adjusting geometry. Using metal nanoparticles with a diameter below 20 nm, some reduction of the melting point (mp) and Tammann temperature (TTa) is observed. Fick’s laws still apply at nano scale. In that range, distances are short and so bulk diffusion of carbon (C) atoms through metal nanoparticles is quick. Growth occurs under catalytic and hybrid carbon formation routes. Better knowledge of the mechanism is an important basis to optimize growth rates and the shape of bamboo-like C fibers. Bamboo-like growth, occurring under pyrolytic carbon formation, is excluded: the nano-catalyst surface in contact with the gas gets quickly “poisoned”, covered by graphene layers. The bamboo-like growth of boron nitride (BN) nanotubes is also briefly discussed.

Highlights

The mechanism of bamboo-like carbon nanotubes growth requires clarification
The access of C to catalyst bulk operates under catalytic and hybrid carbon formation routes but not in the pyrolytic route, because the whole catalyst surface is covered by graphene layers
They observed: “Lower decomposition rate of C2 H2 seems to prevent the passivation of the catalyst of the catalyst surface caused by excessive carbon deposition, which in turn enhances the growth of carbon nanotube (CNT)”

Summary

Introduction

The mechanism of bamboo-like carbon nanotubes growth requires clarification. Detailed kinetic studies are an efficient key to understand and support a mechanism This has been demonstrated in catalytic graphene growth [2]. The requirement to have a growth mechanism including carbon (C) atoms bulk diffusion through the metal catalyst nanoparticle excludes bamboo growth from occurring under pyrolytic carbon growth (check Figure 1 and Table 1). The ratios include C atoms diffusion and N and B atoms diffusion, of interest in boron nitride (BN) nanotubes catalytic growth (point 5, below).

C Growth Type

Bamboo‐like

TEM images:

Alternative

Opening and Filling Observed in MWCNTs

Conclusions