Graphene is a two dimensional carbon material with a unique two dimensional structure, which exhibits excellent properties such as high thermal conductivity, negative thermal expansion coefficient, high electrical conductivity, good charge carrier mobility and outstanding mechanical strength at low densities, making the graphene a topic of interest in nano-science since its appearance.
For the purpose of mass and continuous productions of high-purity graphene, non-thermal and thermal plasmas at atmospheric pressure have been developed for continuous gas phase synthesis. This method had a relatively high production rate, nevertheless, the products were inevitably accompanied by other carbon allotropes like amorphous carbon and graphitized particles.
Recently, a research team led by Prof. XIA Weidong from University of Science and Technology of China (USTC) of the Chinese Academy of Sciences reported a novel approach for the preparation of few-layer graphene nano-flakes (GNFs). The GNFs are continuously synthesized by thermal decomposition of hydrocarbons using a magnetically rotating arc at atmospheric pressure. The study was published in Carbon.
The researchers first investigated and discussed the effects of magnetic field, arc current, feedstock gas flow rate, and feedstock gas type on the morphologies and microstructures of pyrolysis products. They then investigated systematically the effects of hydrogen addition, gas injection positions, acetylene concentration, and reaction temperature on the morphologies of products.
Carbon nanospheres (CNSs) or graphene nanoflakes (GNFs) can be obtained under varying conditions. The essential factors affecting the morphological transformation of carbon materials are summarized. Combining the gasephase kinetic model of precursor (C20) formation and simulations of computational fluid dynamics, the researchers ascertained a feasible graphene formation path in thermal plasmas.
The synthesized GNFs are agglomerative flakes, where each flake is between 50 and 300 nm. Material analyses indicates that the GNFs have excellent properties such as a good crystalline structure, a low number of layers, and a large specific surface area. This indicates that the GNFs could be applied in fuel cells and energy storages. This method is suitable for mass production of few-layer GNFs since it is a continuous process with a relatively high yield (～14%) and a relatively low energy cost (～0.4 kWh/g).
The researchers found that essential factors affecting graphene formation may include the formation of sheetelike nuclei and continuous planar growth at the side active sites of the structure. A low collision frequency of precursors and the high temperatures favor the formation of sheetselike nuclei. Planar surface growth requires hydrogen to terminate dangling bonds at edges and a high temperature to induce growth without a curvature.
This study provides a new insight into the gasephase synthesis of graphene by using arc plasmas. It may lead to the theoretical analysis of carbon material synthesis. It is anticipated that the method developed in this paper will be highly advantageous for the mass and continuous productions of GNFs. Additionally, the morphologies and microstructures of final products can be adjusted easily by controlling the operating parameters.
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