Mass Production and Growth Mechanism of Carbon Nanotubes in Optimized Mechanothermal Method

Abstract

There are a lot of major parameters in the mechanothermal approach which play a key role in the quality and quantity of the carbon nanotubes (CNTs). In this study, these factors were optimized to maximize the efficiency of the process, and also the growth mechanism of CNTs was investigated. For these purposes, the milling of graphite was performed in a high alloy steel vial for 330 h at the vial speed of 300 rpm in a planetary ball mill. The morphology and crystal structures of the graphite powder during the mechanical activation were studied by x-ray diffraction (XRD), Zeta-Sizer, and scanning electron microscope/energy-dispersive x-ray spectroscopy (SEM/EDX). After the heat treatment of amorphous carbon at 1400 ˚C, the CNTs were synthesized and their quality and quantity were analyzed by transmission electron microscopy (TEM), atomic force microscopy (AFM), XRD, Raman spectroscopy, and differential thermal analysis/thermogravimetric analysis (DTA/TGA). A special type of tip-growth mechanism based on the motion of the catalyst particles was proposed regarding the TEM images. According to this mechanism, the diameter, length, and shape of the CNT are completely dependent on a random motion of the catalyst particle at the tip of the nanotube. As a consequence, the growth mechanism in the mechanothermal process does not follow a certain pattern and this is the main reason for the spring-like and curved shape of the nanotubes. Furthermore, results of the differential thermal analysis revealed that the yield of fabricated multi-walled carbon nanotubes (MWCNTs) is more than 97% of the precursor.