Oxidative Stabilization、Carbonization Of Mesophase Pitch Fibers And The Characterization Of Carbonized Fibers

Two types of mesophase pitch were studied as fiber precursor. They arenaphthalene derived mesophase pitch (AR) and mesophase pitch obtained fromthermal treatment (MP). The effects of heating rate, final oxidation temperature andresidence time on the oxidative stabilization of mesophase pitch fiber wereinvestigated by thermogravimetric analysis (TGA). Based on the TG curve ofisothermal oxidative stabilization, the relationships between oxidation time,temperature and weight gain were examined for carbon fibers of15μm、22μm、29μm.The effects of stabilization, carbonization, graphitization conditions on themechanical strength, microsturcture, electrical resistivity and thermal conductivity ofmesophase pitch-based carbon fibers were examined by tensile strength instrument,scanning electron microscopy (SEM), Raman spectroscopy, micro-ohmmeter. Theresults are as follows:The oxidative weight gain at constant reaction temperature (230～290°C) wasproportional to a fractional exponention of oxidation time (t0.3). Taking intoconsideration of temperature effect by Arrhenius equation, a simple and accuratemathematical model has been established to describe the overall kinetics of oxidativestabilization for mesophase pitch fiber. Based on this model, for a given reactiontemperature, the time required to achieve desired oxidation weight gain can beaccurately predicted for isothermal oxidative stabilization. The activation energy ofthree fibers with different diameter had24kJ/mol,25kJ/mol,31.5kJ/mol,respectively. At the same time, the isothermal oxidative stabilization process of ARfiber was also studied, results show that AR fiber has higher oxidative reactivity, theoxidation weight gain was also proportional to oxidation time, but the relationshipbetween rate constant and reaction temperature was difficult to express by Arrheniusequation. AR and MP fibers have different oxidative reactivity and kinetics.The oxidative stabilization, carbonization process of MP fiber bundles were alsoinvestigated. When the oxidative weight gain was higher than4%, the pitch fibercould be fully stabilized; When the oxidative weight gain reached4.4%, the pitchfiber has the best mechanical strength; The tensile strength increased as the heattreatment temperature increased; the tensile strength of random transverse texture of carbon fiber was higher than the radial transverse texture of carbon fiber. But thermalconductivity of radial texture of carbon fiber was higher than the random texture ofcarbon fiber; The diameter of carbon fiber reduced as the heat treatment temperatureincreased, from28.6μm of green fiber to19.6μm after graphitization, the diameterreduced by32%; The open angle of the crack increased with successive heatments,from5°after stabilization to120°after graphitization.Because of the lower structural integrity of the fibers before graphitization,theRaman spectra of the surface and that of the cross section were similar.But theyshowed obvious difference after graphitization, the R value(0.46) of the surface wassignificantly smaller than the R value(1.62) of the cross section; At the same time, theR value(1.62) of the edge of the cross section was lower than the R value(1.85) of thecore of the cross section, results show that there was skin-core structure in the crosssection of the fiber; The R value(0.45) of the radial transverse texture of carbon fiberwas smaller than the R value(0.51) of the random transverse texture of carbon fibe.