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Due to the small size of commercial BN crystal and the unique interlayer interaction of BN, the traditional exfoliate method makes it difficult to fabricate BN nanosheets (BNNSs) with high aspect ratio (> 5000). High aspect ratio BNNSs were obtained by sintering ammonium borane and centrifugal separation of the sintering products. The synthesis mechanism and structure of BN nanosheets were characterized by FTIR, TG, TEM and XRD. The results show that the cross-linking and dehydrogenation of ammonium borane begin when the temperature is over 80 ℃. Hydrogen releasing results in the formation of foam structure during the cross link process of ammonium borune, and finally the inorganic process was completed at 1000 ℃ to obtain the BN foam containing BNNSs. The as-prepared BNNSs are hexagonal crystal structure, with a thickness of about 1.5 nm and an average width of 30 μm. The width-thickness ratio reaches 20000. The high aspect ratio BNNSs were introduced to polyvinyl alcohol (PVA) to prepare the BNNS/PVA composite film. The composite film shows significant improvement in thermal conductivity and mechanical properties, fully explains the application advantages of high aspect ratio BNNSs in the field of BN based composites.

Continuous SiC fibers with excellent mechanical properties and high‐temperature resistance possess immense potential as reinforcements for aeroengines. In this work, we prepared functionalized boron nitride nanosheet (F‐BNNS) composite SiC (F‐BNNS/SiC) fibers by polymer‐derived method. Due to the existence of F‐BNNS, the F‐BNNS/SiC fibers demonstrate an improved tensile strength up to 2300 MPa, representing a 25% increase compared to the original SiC fibers. Furthermore, owing to the chemical bonds established between the F‐BNNS and the SiCxOy network, the decomposition of the SiCxOy amorphous phase and the crystallization of SiC at high temperatures is effectively suppressed. The F‐BNNS/SiC fibers exhibit improved high‐temperature resistance, maintaining a tensile strength of 720 MPa up to 1500°C, while the SiC fibers completely degraded under the same conditions. The successful preparation of BNNS/SiC fibers may offer new insights into the performance enhancement of advanced ceramic fibers. [ABSTRACT FROM AUTHOR]

Given the superior thermal stability and microwave transparency features, continuous BN fibers have displayed great potential to fulfill the increasing demand for high‐temperature structural and functional materials. The performance of BN ceramic fiber depends heavily upon its composition and microstructure. Herein, nearly stoichiometric BN fibers with high crystallinity were achieved through a facile boron trichloride (BCl3) assisted curing process. The fibers achieved a boron/nitrogen element ratio of 0.996:1 with increasing the degree of curing. After subsequent heat treatment, the fibers had uniform elemental distribution and h‐BN crystal structure. Compared with pure NH3 curing, the BN fibers cured with BCl3 demonstrated higher crystallinity (92.35%) and consistent orientation (79.8%). The existing state changes of elements revealed BCl3 assisted curing process could evenly disperse B atoms into the precursor fibers to fill the vacancies caused by nitrogen enrichment. These insights improve our understanding of preceramic fibers treatment for exploiting high‐performance BN ceramic fibers. [ABSTRACT FROM AUTHOR]