Investigation On Preparation And Properties Of Pitch-Based Carbon Materials

Advanced carbon functional materials, such as vapor grown carbon fibers (VGCFs), carbon nanotubes (CNTs) and nano onion-like fullerenes (NOLFs), have achieved extensive application in many areas because of their excellent properties, including low density, high electrical conductivity and high thermal conductivity. Coal pitch (Coal Tar Pitch, CTP) is a carbon-rich residue of coal tar pitch after fraction separation. Vapor grown carbon materials can be synthesized from synergic effects of gaseous species, consisting of CH₄, CO, H₂, N₂, H₂S and low molecular hydrocarbons, which are released during the pyrolysis of CTP. So, it is of significance to synthesize high added value carbon functional materials from CTP.In this paper, carbon materials with different morphologies were achieved from CTP by chemical vapor deposition (CVD) using ferrocene as catalyst precursors (mass ratio of CTP/ferrocene 85/15), argon (150ml/min) and hydrogen as carrier gas by adjusting flowing of hydrogen at 1000℃. Field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy and thermalgravimetric analysis were employed to characterize the morphology and structure of the products. The resistance, ferromagnetic property and complex permittivity and permeability of the products were measured. The results are as follows:1. The preparation of long, oriented vapor-grown carbon fibers (LOVGCFs) was achieved when hydrogen was flowed at a constant rate of 80ml/min. The LOVGCFs having uniform diameter distribution (ranging from 0.8 to 1.2μm) are pure and solid, with a bundle length of up to 6.0cm. The resistance and thermal stability of LOVGCFs were measured. Results reveal that: 1) The products have good thermal stability below 520℃; 2) The Current-Voltage (I-V) curve (1-25V) shows a non-linear behavior, suggesting a semi-conductive property, which was mainly attributed to structure defects.2. The clew-like carbon materials consisting of curly carbon nanotubes were achieved when hydrogen was flowed at a constant rate of 300ml/min and other parameters were kept unchanged. The obtained products are pure, having good dispersion and a narrow-distributed diameter of 0.5μm. The ferromagnetic property and the complex permittivity and permeability of the as-synthesized products were measured. Results reveal that: 1) The ferromagnetic property was measured in magnetic fields within±10000Oe. The large coercivity value (446.13Oe) shows an obvious ferromagnetic behavior. 2) The complex permittivity and permeability of the products were measured in the frequency range of 2 GHz to 18 GHz and the dielectric loss and magnetic loss were calculated. The results indicate that as-synthesized products have certain dielectric/magnetic loss.3. To our surprise, the structure of LOVGCFs in our study was verified as turbostratic structure, not graphite structure, which activated our research interest in the structures of other carbon materials such as CNTs and NOLFs. Based on the experimental results, the criterion of turbostratic structure in VGCFs, CNTs and NOLFs are suggested as follows:1) For VGCFs: According to XRD, Raman spectroscopy and HRTEM images, if Lc is less than 10nm, the samples do not have graphitized structure, but have turbostratic structure or amorphous structure.2) For CNTs: According to XRD, Raman spectroscopy and HRTEM images, if Lc is less than 5nm, the samples do not have graphitized structure, but have turbostratic structure or amorphous structure.3) For NOLFs: According to XRD, Raman spectroscopy and HRTEM images, if Lc is less than 10nm, the samples do not have graphitized structure, but have turbostratic structure; if Lc is less than 2nm, the samples show amorphous structure.