Catalytic Graphitization Of PAN-based Carbon Fibers Treated By Alternate Electrolysis

Carbon fiber(CF) is a fibrous material which with carbon as the main element. It is a new kind of materials with valuable electrical,thermal and mechanical properties.In order to obtain high-strain and high-modulus of carbon fiber, increasing 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 like polyacrylonitrile(PAN)-based carbon fibers,adding catalysts are usually used to accelerate the graphitization of non-graphitizing carbon,which is called catalytic graphitization.In the process of catalytic graphitization,loading and scattering of catalysts are very important.The conventional method contains mixing,impregnating and electrodepositing which usually are not effective to disperse the catalysts.In the present research we select the PAN-based carbon fibers as a model,and adopt the alternate electrolytic method to destroy the texture of the PAN-based carbon fibers,stimulatingly catalysts are impregnated into the matrix of fibers.Accelerate graphitization of the PAN-based carbon fibers is observed.(1) The PAN-based carbon fibers were treated by a simple alternate electrolytic process in 14 g/L Na₂SO₄ + 50 g/L H₃BO₃ + 20 g/L C₆H₈O₇·H₂O aqueous solution and then heat-treated at 2400℃for 1 h under an argon atmosphere.The changes of morphology,structure and graphitization of carbon fibers were investigated by scanning electron microscopy(SEM),X-ray diffraction (XRD) and Raman spectroscopy(Raman).The results indicate that the texture of the PAN-based carbon fibers is destroyed and boron is impregnated into the matrix of fibers during alternate electrolytic process.Compared with the PAN-based carbon fibers as-received and impregnated with boric acid,enhanced graphitization can be obtained for the fibers treated by alternate electrolysis.(2) Catalytic graphitization of PAN-based carbon fibers was improved by alternate DC electrolysis in H₃PO₄and Na₂MoO₄ aqueous solution.The effects of the heat treatment temperature and the concentration of Na₂MoO₄ in the electrolytes on catalytic graphitization of PAN-based carbon fibers were investigated.The changes of morphology,structure and the element composition of fibers were characterized by SEM,XRD,Raman spectroscopy and energy dispersive spectroscopy(EDS).The results indicate that the fiber texture is effectively destroyed and the catalysts are simultaneously dispersed into fibers through the alternate DC electrolysis,which obviously accelerates the graphitization of the fibers.After heat treated at 2400℃,the graphitization degree of the PAN-based carbon fibers is improved to 99%and Lc is about 380(?).(3) We adopted phosphomolybdic acid as catalysts which were dispersed into fibers through alternate DC electrolysis to accelerate the catalytic graphitization of PAN-based carbon fibers.The effects of the heat treatment temperature and the concentration of phosphomolybdic acid in the electrolyte on catalytic graphitization of PAN-based carbon fibers were investigated.The loading of catalysts and the changes of the fiber's morphology and structure were characterized by EDS,SEM and XRD.The results show that the fibers texture is effectively destroyed and the catalysts are simultaneously dispersed into fibers after treated by alternate DC electrolysis in H₃PO₄ and H₁₂PMo₁₂O₄₀ aqueous solution.The catalytic graphitization of fibers is obviously accelerated.After heat treated at 2400℃for 2h,the graphitization degree of PAN-based carbon fibers is improved to 95.3%and Lc is 250(?).