Study On Catalytic Graphitization Of Carbon Materials With Ni-B Alloy Coating

Carbon materials have very good mechanical, electrical and thermophysical properties, and are used widely in printing, electronics, metallurgy, avigation, spaceflight, etc. The performances of carbon materials are closely associated with crystal degree, microstructure and surface characteristics. The graphitization is one of the most important structure parameters of carbon materials. Carbon can transform from disorder to order structure by heat treatment (graphitization), and the properties of carbon fibers can be greatly enhanced after graphitization. However, high heat treatment temperature for graphitization is the main factor leading to high price of graphite carbon materials. In order to decrease energy waste and accelerate graphitization of non-graphitization carbon, some catalysts were used to accelerate the graphitization of carbon, i.e., catalytic graphitization.In this thesis, the effects of electrodeposited and electroless Ni-B alloy coatings on the catalytic graphitization of polyacrylonitrile (PAN)-based carbon fibers and multiwalled carbon nanotubes (MWNTs) have been studied respectively. X-ray diffraction and Raman spectroscopy were used to examine the extent of graphitization of the carbon materials. The morphology and phase structure of the resulting carbon were analysed by scanning electron microscope (SEM), high resolution transmission electron microscope (HRTEM), energy dispersive spectrum (EDS) and inductively coupled plasma-atomic emission spectrometry (ICP-AES). The property and possible catalytic graphitization mechanism of carbon materials were also investigated. The main points in this thesis are summarized as follows:1) The process of the catalytic graphitization of the electrodeposited Ni-B alloy coated carbon fibers was investigated. SEM, HRTEM, Raman spectroscopy, EDS, ICP-AES and XRD techniques were used to analyze the morphology, microstructure and phase composition of the carbon fibers. The results show that nickel, rather than boron, has a significant effect on the catalytic graphitization. The graphitization phenomenon first takes place on outer surface of the carbon fiber, and then diffuses to the center with the ejection of Ni from the coating under heat treatment. The Ni and B element in the Ni-B alloy have synergic catalytic effect on the graphitization of the carbon fiber, and its catalytic graphitization follows the dissolution-precipitation mechanism. The process of catalytic graphitization was evidently affected by heat treatment temperature and heat treatment time.2) The catalytic effect of electroless Ni-B coatings on the graphitization of multiwalled carbon nanotubes was investigated. The morphology, structure and phase composition of carbon nanotubes were characterized by HRTEM, Raman spectroscopy, XRD and ICP-AES. The results show that MWNTs are one of the carbon materials that are hard to be graphitized even heat treated at 2400℃. And Ni-B catalyst has a remarkable catalytic effect on the graphitization of MWNTs, with a AD/AG value of 0.13 at 2400℃, much lower than 0.79 of the MWNTs without catalyst. Moreover, an obvious fusion and shaping effect of Ni-B alloy on walls of the carbon nanobues could be observed during the heat treatment. The Ni, B elements and defect of nanotube were also eliminated after graphitization.3) The specific surface area and capacitance characteristics of the graphitized MWNTs were analyzed by isothermal N2 adsorption Brunauer-Emmett-Teller (BET) method, cyclic voltammetry, constant current charge/discharge and AC impedance analysis. The results indicate that as the increase of graphitization extent of MWNTs, the specific surface area and pore volume of the MWNTs decrease, while the pore size increases. The graphitized MWNTs electrode, when evaluated in 1 mol/L H2SO4 exhibited good capacitance performance and high stability. Moreover, the specific capacitance of MWNTs increases with the increase of graphitization degree. The specific capacitance and specific capacitance per surface unit of the 2400℃sample are 22.4 F/g and 14.97μF/cm2, much higher then 7.2 F/g and 3.27μF/cm2 of the 1800℃sample. It indicates that increasing the graphitization degree of the MWNTs would be good for the capacitance properties.