Synthesis of Polyborosilazane and Composition and Structure of the Pyrolyzed Products

Polyborosilazane (PBSZ),a precursor to SiBNC ceramic,was synthesized via cocondensa- tion approach using methyldichlorosilane (MeHSiCl_2),boron trichloride (BCl_3) and hexamethydisilazane (HMDZ) as starting materials.SiBNC ceramics were obtained by pyrolysing the as-synthesized PBSZ in N_2 or NH_3/N_2 atmospheres.The chemical composition,structure and high temperature properties of the polymer and ceramics were investigated by using EA,XPS,FTIR,NMR,and XRD.The results indicate that the backbone of PBSZ is-Si-N-B-linkage in the form of borazine and C is in the form of Si-CH_3.The ceramic yield of the PBSZ pyrolysed at 1000℃in N_2 and NH_3/N_2 atmospheres is 63wt% and 61wt%,respectively.The products show excellent high temperature stability which are fully amorphous to 1700℃.Furthermore,only partial crystallization,giving a mixture of Si_3N_4,BN and SiC phases,is observed heating at 1850℃.The weight loss of SiBNC ceramics pyrolyzed in NH_3/N_2 and N_2 atmospheres from 1500℃to 1850℃is about 10.0wt% and 3.8wt%,respectively.The N_2 pyrolyzed products show better stability than the NH_3/N_2 pyrolyzed products and have less tendency to crystalline at higher temperature.