Crosslinking Kinetics Of Polyalumi-nocarbosilane Fiber In Ozone Atmosphere And The Formation Mechanism Of Hollow SiC Fiber

Silicon carbide fiber is widely used in a such high temperature and harsh environment of aerospace engines,nuclear fuel cladding materials and the automobile industry because of its excellent high temperature resistance,oxidation resistance,corrosion resistance,high modulus,high tensile strength,high thermal conductivity and low thermal expansion coefficient.Compared with SiC fiber,hollow SiC fiber with special cavity structure,light weight and larger specific surface area,has the characteristics of adsorption,filtration,wave absorption,etc.,presenting advantages and application prospects in thermal insulation,corrosive fluid filtration,high temperature gas separation,micro-reactor,catalyst support,solid oxide fuel cell?SOFC?,electromagnetic shielding and other applications.However,there are still some challenges in the preparation of hollow fiber:?1?the whole preparation process is complex;?2?the cost is high,it is difficult to be commercialized;?3?the curing process is of high temperature and time-consuming;?4?the shape of nano-hollow fiber is irregular;?5?the preparation of ceramic slurry requires various additives;?6?the hollow structure of the fiber is formed by solvent dissolution.In this paper,the precursor-derived most method was used to prepare hollow SiC fiber.Different from the traditional air curing process,we applied ozone gas to oxidize polyaluminocarbosilane?PACS?fiber,and then inorganic hollow SiC fiber was fabricated through the pyrolysis at high temperature.This process is relatively simple and fast,does not use organic solvent and the hollow shape of the fiber is regular.In this paper,infrared spectrometer?FT-IR?,scanning electron microscopy?SEM?,thermogravimetric analyzer?TGA?,gel permeation chromatography?GPC?,specific surface area and porosity analyzer,etc.,are utilized to characterize and study the activation energy in the oxidation process,oxidation mechanism and formation mechanism of the hollow fiber.As a result,the gradient porous hollow SiC fiber and SiC honeycomb ceramics were made.The reaction activation energy?17.79 kJ/mol?of ozone is obviously lower than the activation energy of air oxidation?75.30 kJ/mol?,and from the characterization by IR,EDS,SEM,TGA,GPC,we found that the oxidation cross-linking by ozone at low temperature and for a short time formed a"core-shell"structure,the oxygen elements are mainly distributed in the outer layer of the fiber,and the subsequent oxygen slowly enters into the inner part of the fiber,which is a cross-linking reaction dominated by diffusion control.The long air oxidation reaction is a more uniform reaction control process.Through the experiments of ozone at different temperature and for various oxidation time,dynamic sintering,vacuum sintering,specific surface area and pore size analysis,it was shown that cross-linking fibers with different thickness can be prepared by adjusting the temperature and time of ozone oxidation.The dynamic sintering results demonstrate that the hollow structure of the fiber appears at⁶00oC,which is consistent with the curve of TGA.The adsorption-desorption test of N₂shows that a large number of micropores and mesoporous are the main escape pathways of the partially degraded products with insufficient cross-linking in the center of the fibers,continuous hollow SiC fibers were formed along the direction of fibers during the pyrolysis.However,these pores disappear when the pyrolysis temperature increases.In addition,it is proved that the formation of hollow SiC fibers is due to the escape of molecules with insufficient cross-linking in the inner part of the fibers during the pyrolysis in vacuum pyrolysis.By means of higher temperature sintering,SEM and N₂ adsorption-desorption test,we obtained the porous hollow SiC fiber with larger specific surface area?>10m²/g?,the outer pore is the largest,and the inner pore decreases gradually.There is obvious stratification phenomenon.This phenomenon proves that the"shell"of ozone oxidation cross-linking is not only a cross-linking structure of a single layer,but also a gradient oxygen permeation mode.This unique gradient hollow porous SiC fiber may have great advantages in the separation,adsorption or other applications of some molecules.We combined ozone-preoxidized fiber and liquid hyperbranched polycarbosilane?LHBPCS?to make honeycomb ceramics,which micrometer-level aperture resulted from the hollow SiC fiber.The micrometer-level honeycomb ceramic has potential application prospects in the fields of material transfer,regenerator,micro-reactor and so on.