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Polyborosilazane synthesized from trichlorosilane (SiHCl 3 ), boron trichloride (BCl 3 ) and methylamine (MeNH 2 ) is highly sensitive to moisture for hydrolysis of -NHMe unites and is easy to cross-link for dehydrogenation of Si-H, N-H, which affected the industrial scale production of SiBNC ceramic fibers. In this paper,a new nitrogen source(n-propylamine, C 3 H 7 NH 2 ) was used instead of MeNH 2 to synthesis polyborosilazane precursor. The influences of nitrogen sources on the structures and properties of SiBNC ceramic fiber precursors were investigated. As compared to SiHCl 3 /BCl 3 /CH 3 NH 2 system, polyborosilazane precursor synthesized from the SiHCl 3 /BCl 3 /C 3 H 7 NH 2 system exhibited lower crosslink density, lower sensitivity to moisture and good melt spinnability, and the obtained ceramic fibers showed higher tensile strength (1.03 GPa, an increase of about 37.3%). This synthesis route (SiHCl 3 /BCl 3 /C 3 H 7 NH 2 ) may serve as a suitable synthetic route for a large-scale production of SiBNC ceramic fibers. [ABSTRACT FROM AUTHOR]