Growth of oriented vapor grown carbon fibers on pyrolytic carbon films without catalyst

Abstract

Vapor-grown carbon fibers (VGCFs) have been of great interest to scientific and industrial communities for many years due to their unique crystalline structure as well as excellent physical and chemical properties [1–3]. They have potential applications as fillers in composites, superhigh hydrogen storage materials, and good electrode materials [4–8]. VGCFs are usually prepared by the decomposition of hydrocarbons, such as benzene and methane, using ultra-fine transition metal particles or their alloys as a catalyst at a growth temperature of 600–1300 C [9–11]. However, these methods sustain the disadvantage of introducing catalyst particles, which can make the synthesis process more complex, and the existence of catalyst must influence the properties of the products. Moreover, the mesoporous template or the d.c. bias was generally needed to enhance the growth of the oriented carbon nanotubes and carbon nanofibers [12, 13]. Recently, Lin et al. [14] prepared multi-walled carbon nanotubes on a porous carbon surface using carbon black as a substrate by the decomposition of diluted ethylene. Rantonen et al. [15] synthesized needle-shaped carbon filaments at low temperature by ultraviolet laser irradiation under nearand supercritical conditions without catalyst. These reports showed that the growth of carbon filaments without the aid of catalysts is possible. In this paper, microwave-assisted CVD method was used to prepare oriented VGCFs on pyrolitic carbon films by direct pyrolysis of methane without the use of any catalysts. The influence of reaction time (2, 10, 25, and 30 min) on the morphologies and structures of products at 1100 C was also discussed. The experiments were carried out in a vertical quartz tube heated in a home-made microwave furnace. The schematic of the experimental device is shown in Fig. 1. Carbon source gas is introduced into the gap between quartz tube and microwave absorbing materials, and then conducted out of microwave furnace via gas outlet. The silver-gray pyrolytic carbon films were deposited on the inner wall of quartz tube with a gas residence time of 0.1 s at 1000 C for 60 min by the microwave chemical vapor infiltration technique, and then used as the substrate. The reaction was conducted at 1100 C with the flow of mixture gas CH4/N2 (1:1 vol), for 2, 10, 25, and 30 min, respectively. After the reaction, the furnace was cooled down to room temperature in the atmosphere of N2. A layer of black filament-like substance deposited on the surfaces of the substrate was collected for analysis. The morphology and structure of products were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4700), and Raman spectroscopy (Renishaw Invia Reflex). The elemental composition of the materials was analyzed by an energy-dispersive X-ray (EDX) analysis technique attached to the scanning electron microscopy. Figure 2 shows typical FESEM images of oriented VGCFs with a uniform diameter of about 50 lm when the reaction time is 30 min. Figure 2a shows a side view of VGCFs formed on pyrolytic carbon film. It shows that the D. Fu F. Deng H. Sheng School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, Shannxi 710072, China