Structural Evolution Of PCS/PAN System During High Temperature Treatment

With the constant expansion of applications in the field of aerobic high temperature,carbon fiber reinforce ceramic matrix composites are gaining more and more attention due to their excellent adaptability and irreplaceability.Carbon fiber reinforce silicon carbide(Cf/SiC)composites are the typical representative among these composite materials..The preparation of Cf/SiC composites are generally fabricated by precursor infiltration pyrolysis prosess(PIP).The carbon fiber impregnated precursor polysilanane(PCS)followed by a cross-linking and ceramization process.During the preparation and using of Cf/SiC composites,the interfacial structure of the composites is destroyed and the performance is impaired due to the different thermal physical properties of carbon fibers and matrix ceramics and the chemical reactions occurring at the interface.Cf/SiC composites are often prepared by coating with free carbon on the surface of carbon fiber in order to mitigate this effect,but this also leads to a decrease in the properties of the composites.In this paper,PAN fiber was impregnated with PCS to partially infiltrate the surface of the PAN fiber and form a gradient structure.Then simultaneous pre-oxidation and cross-linking method was used to realize the ceramization of PCS at the same time while the PAN fiber carbonization.The structural evolution of the PCS/PAN system under high temperature treatment during the simultaneous reaction was explored.The fiber samples with different reaction stages were tested by FT-IR,DSC,13C-NMR,and Raman spectroscopy.The interaction between PAN fiber pre-oxidation and PCS cross-link curing process,and its effect on the final fiber properties were studied.The study results indicated that:(1)This paper proposed a design idea for the preparation of carbon fiber and silicon carbide composition on the surface of the carbon fiber and has a gradient feature.The formation of a gradient structure is achieved by impregnating polysilazane(PCS)with polyacrylonitrile(PAN)precursor.(2)In the process of preoxidation and crosslinking,the main reaction of PCS is the Si-H bond with C=C double bonds and oxygen reaction formed Si-O-Si and Si-CH2-Si structures.Then calculated the relative crosslinking degree.(3)In the simultaneous pre-oxidation and cross-linking process,PAN promoted the crosslinking curing reaction of PCS.The relative crosslinking degree of PCS in the PAN fiber impregnated PCS is greater than that of the pure PCS.With the increase of the concentration of PCS,the relative cross-linking degree of PCS in the system is almost constant.(4)During the simultaneous pre-oxidation and cross-linking process,the PCS also promoted the cyclization reaction of PAN fiber,the relative cyclization rate of impregnation PAN fibers were greater than pure PAN fiber.With the increase of concentration of PCS impregnated,the preoxidation degree of PAN fiber showed a trend that first increased and then decreased.The relative rate of cyclization and cyclization degree are maximum when concentration was 1%.(5)In the case of PAN fiber stage impregnation,the performance of C/SiC fiber will be affected to a certain extent.When the concentration was 2‰,the tensile strength was 2.57 GPa.When the concentration was 0.5‰,the tensile strength was 3.08 GPa.