Size Effect of Nanosheet on BN Fibers Derived from BNNS/Polyborazine Hybrid Precursor

BN ceramic fibers exhibit significant potential for applications in high-temperature wave-transparent and semiconductor fields, due to their excellent resistance to high temperatures and thermal conductivity, as well as their outstanding wave-transparent performance. However, the low crystallinity observed in BN ceramic fibers inhibits complete realization of their potential superior properties associated with the h-BN crystal structure. In this work, based on the mechanism that inorganic nanofillers could act as heterogeneous nucleating agents to accelerate matrix crystallization, three lateral sizes (0.5, 2 and 4 μm) of amino-functionalized BN nanosheets (BNNSs) were prepared utilizing a one-step ball milling method. BNNSs were chemically bonded to molecular chain of polyborazine to synthesize hybrid BNNS/polyborazine precursors, which were finally derived into high-performance BN ceramic fibers with high crystallinity. This investigation thoroughly explored the scale effects of BNNS on the molecular structure of hybrid precursor, as well as their physicochemical properties and melt spinning performance. Relationship among the BNNSs’ size, microstructure and mechanical properties was elucidated. Increasing BNNSs size could enhance the ceramic yield of precursor (up to 64.1%), but destroy the viscosity-time stability. Moreover, it was demonstrated that BNNS scale could evidently regulate crystal structure of BN ceramic fibers. Relationship among the BNNSs’ lateral sizes, crystal structure and mechanical performance was determined to be non-linear. BN ceramic fibers containing 2 μm BNNS displayed the highest crystallinity (94%), grain size (12.5 nm) and density (2.00 g/cm3). However, surface defects associated with 2 μm BNNS resulted in a non-optimal average tensile strength (0.90 GPa). BN ceramic fibers doped with 0.5 μm BNNS exhibited the best average tensile strength (0.94 GPa), attributed to the favorable combination of high crystallinity and smooth surface. This work could provide crucial references for fine regulation of microstructure and preparation of high-performance BN ceramic fibers.