Synthesis And Characterization Of High Ceramic Yield Poly(boro)silazane

Compared with traditional ceramics preparation methods,Polymer derived ceramics（PDCs）could adjust and modify the macroscopical chemical and physical properties of ceramic through design and control the molecular structure of precursors.Thus,PDCs has been widely focused and researched.Nitride silicon ceramics and SiBCN ceramics are important high temperature structural and functional materials due to their excellent high temperature stability,mechanical properties and special functionality.In this paper,the low polymerization silazane OLPS was prepared by using dichloromethylsilane and hexamethyldisilazane as the raw material.And with the OLPS as basis,by using thermal cross-linking method,amide modified method and boron source introduced method,polysilazane PSZ-N,PSZ-C,PSZ-M and polyborosilazane PBSZ-P,PBSZ-T can be synthesized respectively.OLPS is a type of small molecular silazane with the Si-N-Si as the molecular skeleton and numerous Si-H and N-H as the active group.The ceramic yield of OLPS was 29.6wt%after pyrolyzed 800℃under N₂.PSZ-N was prepared through thermal cross-linking of OLPS,it can dissolve in xylene,THF and trichloromethane and the soft point of it is 114.4℃.The molecular structure of PSZ-N was certain branched with Si-N-Si as the skeleton and-Si（CH₃）₃ as the terminal group.The ceramics from PSZ-N can be obtained at 1000℃under N₂,the ceramic yield is 56.4wt%.The ceramics pyrolyzed from PSZ-N stay amorphous below1400℃and appearedα-Si₃N₄ crystal when temperature went to 1400℃.PSZ-C and PSZ-M can be synthesized by adding cyanamide and melamine to OLPS respectively.PSZ-C can’t dissolve in xylene,THF and trichloromethane and its soft point is 203.3℃.PSZ-M exhibit good solubility in xylene,THF and trichloromethane and its soft point is 122.1℃.PSZ-C is a linear molecular with Si-N-Si as mainchain.PSZ-M is a retiform branched molecular with the structure of melamine as the center and Si N₃C,Si HN₂C as the repeated units.Both of PSZ-C and PSZ-M totally transform to inorganic ceramics when temperature increased to 1000℃under N₂.And the ceramics can keep amorphous below 1400℃and appearedα-Si₃N₄ crystal when temperature went to 1400℃.Through introducing pinacolborane and trimethyl borate to OLPS,polyborosilazane PBSZ-P and PBSZ-T can be obtained respectively.Both of them can dissolve in xylene,THF and trichloromethane and soft point of them is 74.9℃and77.1℃.The molecular structure of PBSZ-P was Si-N-Si as the mainchain,mainchains bridged with Si-O-B,further introduced the five-member ring of pinacolborane.PBSZ-T molecular consist with Si-N-Si,Si-N-B and Si-O-B as the repeated units.Two polyborosilazane conversed to ceramic at 1000℃under N₂.The ceramic of them is72.0wt%and 68.4wt%.both of them is amorphous when temperature below 1600℃and showα-Si₃N₄ and h-BN crystal at 1600℃.