The Synthesis Of Precursors For SiBN(C) Ceramic Fibers And Its Modification By Chain-extending

Random inorganic networks ceramic fibers composed of Si,B,N and C exhibits a series of excellent properties,such as high-temperature stability,high strength,high modulus,outstanding resistance to creep as well as corrosion and wave transmittance or absorbing,makes it an ideal reinforcement for ceramic matrix composites and meets the needs in the field of national defense and aerospace.At present,the polymer-derived ceramic approach,which mainly contain two routes of co-condensation and single source precursors,is the most advantageous method to prepare SiBN(C)ceramic fibers and suitable for continuous preparation of the ceramic fibers.The mainly interest of the route lies in the feasibility of tailoring the structure of polymers with adjustable viscoelastic properties,then ceramic fibers can be obtained by subsequent melt or dry spinning,curing and pyrolysis processes.In the paper,molecular precursor was synthesized with BCl3,MeHSiCl2 and hexamethyldisilazane as starting materials and aminolysis with additional methylamine,Polyborosilazane(PBSZ)can be obtained by further polymerization.The effects of polymerization temperature and time on the structure and properties of PBSZ were investigated by FTIR,NMR,gel content test,EA,DSC and TGA testing.In addition,rheological properties of Polyborosilazane were studied.However,the follow-up preparation of continuous SiBN(C)ceramic fibers via continuous curing and pyrolysis was hindered by the brittleness of precursor polymer fibers.Therefore,Polyborosilazane was modified by chain-extending reaction using a flexible telechelic polymer,bis(3-aminopropyl)-terminated polydimethylsiloxane(BTA-PDMS),as chain extender.As a result,flexible molecular chains was introduced into the precursor along with higher molecular weight.The mothed is expected to improve the flexibility of the precursor fibers which could be suitable for preparation of continuous SiBN ceramic fibers.The results are as follows:(1)Polymeric precursors PBSZ prepared by different polymerization processes had similar chemical structures.Including skeleton structure,such as Si-N-B,BN3 and B3N3 hexagon rings.-N(CH3)-bridged linkages,formed in the polymerization process..And residual-NHCH3 active group.(2)Higher temperature can improve the efficiency of the polymerization,increase the polymerization degree,and beneficial to form B3N3 hexagon rings structure which is essential for higher-temperature properties of pyrolyzed ceramic fibers,glass transition temperature and thermal stability of the precursor were increased simultaneously.However,crosslinked structure was easier to form under excessively high reaction temperature.The polymerization process becomes difficult to control.150?170 ? was the optimum polymerization temperature.(3)The polymerization reaction can be promoted by the increase of polymerization time.Showed by more-N(CH3)-bridged linkages,higher Ceramic yielding,thermal stability,gel content and glass transition temperature.Interim polymerization after the gel point is the key to control the reaction.(4)Polyborosilazane is Pseudoplastic fluid,the viscous flow activation energy is between 81?135KJ/mol.PBSZ was proved had good melt spinnability by melt spinning experiment,and can be continuous spun and wound into a roll.(5)Molecular weight,crosslinking density and the rigid property of the molecular chain can be improved by the use of chain extenders,BTA-PDMS,without major structural changes.Better spinnability was confirmed by rheological analysis and spinning test.The flexibility of the green fibers was greatly improved.(6)The Si-N and B-N bond were easy to break and forming Si-O-Si structure in the air moisture.