Research On Catalytic Graphitization Of Polyacrylonitrile-Based Carbon Fibers

Carbon fibers are one of the most commonly used reinforcing fibers. They have not only exceptional mechanical properties but also chemical stability, which makes them ideal for high performance composites. Polyacrylonitrile (PAN)-based carbon fibers have been found to be most suitable for making high-performance carbon fibers. In order to obtain high-strain and high-modulus of carbon fiber, elevation of its graphitization degree has become necessary. High-temperature heat treatment is required to provide energy for atomic displacements and to develop graphitization structure in carbon materials. In order to improve the graphitization degree of non-graphitizing carbon materials, adding catalysts are usually used to accelerate the graphitization of non-graphitizing carbon, which is called catalytic graphitization. Here, taking PAN–based carbon fibers as model, we investigated the catalytic graphitization of PAN–based carbon fibers.(1) It is well known that the graphitization of carbon may be limited by some factors, such as the dispersion of the catalyst in carbon and the extent of interfacial contact between the carbon and the catalyst, which are related with the methods of addition of the catalyst to the carbon. In this thesis, we introduced a simple and efficient redox deposition method using KMnO4 as a precursor to coat closely PAN-based carbon fiber with a uniform layer of manganese oxides by a direct redox reaction between the carbons and permanganate ions, which provided an improvement of the catalyst dispersion. The catalytic graphitization of fibers coated with manganese oxidess was investigated by X-ray diffraction (XRD) and Raman spectroscopy. It indicates that the graphitization of PAN-based carbon fibers was accelerated in the present of the manganese oxides coating. High degree of graphitization in this manganese oxides coated fibers is observed clearly at HTT of 1600℃. The effects of the heat treatment temperatures and the deposition parameters of manganese oxides (temperature and time) on the graphitization process of PAN-based carbon fibers were also studied.(2) An extensive study has been made for the catalytic graphitization of carbon by various elements, such as Al, Cr, Mn, Fe, Co, Ni, Ca, Ti, V, Mo, W and B. However, the catalytic effects of rare-earth elements on the graphitization of nongraphitizing carbon have been seldom studied. In this thesis, we introduced the rare-earth catalysts, yttrium, by electrodeposition and lanthanum nitrate by liquid method, to catalyze the graphitization of non-graphitizing PAN-based carbon fibers. The effects of the yttrium (lanthanum nitrate) content and the HTT on the catalytic graphitization were investigated by X-ray diffraction and Raman spectroscopy. It indicats that rare-earth elements can accelerate the graphitization process, especially at temperatures of 2000~2400℃, which has been proved by the variation of crystallite parameters.(3) Carbon fibers were pretreated by plasma. Then the process of immersing fibers to HBO3 solution was adopted to disperse the B element. The effects of discharge power and plasma-tretment time on catalytic graphitization of PAN-based carbon fibers were studied by scanning electron microscopy (SEM), X-ray diffraction and Raman spectroscopy. The results from scanning electron microscopy show that the surface morphology of carbon fibers has changed after the pretreatment. X-ray diffraction and Raman spectroscopy results show that the catalytic graphitization of pretreated PAN-base carbon fibers can be improved at suitable discharge power treatment, which is beneficial for boron-doping. While discharge power increase further, the opposite effect can be observed because the structure of graphite crystal would be destroyed, which have negative influence on the catalytic graphitization of PAN-based carbon fibers.