Study On The Curing And Pyrolysis Of Polyborosilazane Precursor For Sibn Ceramic Fibers

Along with the rapid development of aerospace and military technology,near-space vehicle,precision-guided missile and other advanced weaponry set a higher request to the novel ultra-temperature structural and functional material.Ceramic is one of the preferred materials for ultra-temperature material because of its high temperature oxidation resistance,high strength and !good chemical stability while its brittleness limits applications.Using high-performance ceramic fibers as reinforcement materials to modify the ceramic matrix is a better way to solve the problem.SiBN ceramic fibers have become the current research interest due to its combination of high-temperature resistance,high-temperature dielectric properties,high temperature creep resistance and high temperature anti-oxidation,can be used as the reinforce fibers to the Ceramic Composite Material to meet the radome requirement of the hypersonic vehicle radome.The mainly technogical processes of preparing SiBN ceramic fibers including the synthesis of polyborosilazane precursor,curing and pyrolysis of green fibers which were obtained from precursor by melting spinning.During the curing process,a cross-linked structure was further formed,and the green fibers became insoluble and infusible,to ensure the fiber shape in the high-temperature process.Pyrolysis process is the transformation of organic fiber to inorganic fibers,which is very important to the performance of SiBN ceramic fibers.So it is important to study the curing and pyrolysis processes of green fiber.TGA,TG-FTIR,1H NMR,EA,SEM,XRD et al were used to study the process of curing and pyrolysis of SiBN green fibers.High-temperature TG,SEM,EDS et al were used to study the high-temperature resistance,high temperature oxidation resistance and dielectric properties of SiBN ceramic fibers.The major conclusions are as follows:1.Curning:The optical curing for green fibers were obtained by studying of structural and compositional changes,preferable fibers were achieved by pre-crosslinking at 80 ? for 4h,and treat at 310? for 7h in ammonia atmosphere.Dense and smooth fibers with diameter of about 30?35?m and Gel%of 90.5 wt%,carbon content of 14.89 wt%were obtained.During the curing process,the polycondensation and copolycondensation between B-NHCH3 and Si-NHCH3 were happened,with the release of CH4,Si-H transformed to Si-N via dehydrogenation reaction simultaneously.2.Pyrolysis:The favorable pyrolysis conditions for cured fibers were obtained by pyrolysised in ammonia atmosphere to 1000 0C at a heating rate of 1?20C/min,then heat up to higher temperature(less than 1600?)at a heat rate of 4 ?/min,cooling at 3 ?/min to room temperature.During the pyrolysis process,the CH4 gas was released,the content of carbon and hydrogen reduced obviously,the conversion of polymer-to-ceramic accomplished,the preceramic fibers retained the amorphous structure upon pyrolysis to 1400 ?.3.Characterization of SiBN ceramic fibers:The Si3.0B2.1N5 8 ceramic fibers which obtained at 1400 ? are dense and uniformed,free of apparent defects,with the diameter of 30?35 ?m,structure of Si3N4,B-N,Si-C and SiO2,carbon content of 0.12 wt%.The low carbon content endows SiBN fiber excellent wave-transparent property with the dielectric constant and dielectric loss tangent of about 2.5 and 0.001,respectively.After treated in Nitrogen atmosphere for 5h under 1400?,ceramic fibers were remained amorphous,After 4?8h oxidation treatment under 1300?,a multi-layer structure was formed on the ceramic fiber surfaces,which effectively protect SiBN ceramic fibers from oxidation.