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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.

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BN fibers are one of the most potential reinforcements in high-temperature wave transparent composite. Polyborazane is an essential precursor of the polymerderived ceramics (PDCs) route to prepare BN continuous fibers. 1 The composition and structure of polyborazane not only make a significant impact on the melting spinning process but also determine the properties of BN continuous fiber to a great extent. 2,3 The synthesis, structure characterization, and melt spinning properties of the BN precursors create substantial interest, both scientifically and for practical purposes. 4,5 Presently, BN precursors can be generally divided into three types, which are derived from borazine, alkylamino borane, and tri-chloroborazine (TCB), respectively. Precursors derived from borazine were, firstly, investigated for BN fiber preparation. [6][7][8] Although the precursors have excellent ceramic yields of 81%-91%, the poor spinnability caused by the excessive reactivity and overlinking restricts further development. 9,10 The route based on alkylamino borane was also explored sufficiently. 11 Derivatives from alkylamino borane

Boron nitride nanosheets (BNNSs) have attracted more and more attention in the field of thermal management due to their excellent thermal and electrical properties. However, the low dispersity and low production yield are still a great baffle for its application. Herein, high-performance hydroxyl and amino bifunctionalized BNNSs (HA-BNNSs) were successfully prepared by the citric acid-assisted ball milling one-step method. Based on the additional friction provided by citric acid during ball milling, the aspect ratio of HA-BNNSs was up to 499 and the production yield was as high as 89.7%. Simultaneously, arising from the high functionalization rate, the HA-BNNSs show excellent dispersity in a variety of polar solvents, particularly in 50% aqueous ethanol solution. The dispersibility of HA-BNNSs was much improved to 39.8 mg/mL. Based on the good dispersibility of HA-BNNSs and strong interface interactions between HA-BNNSs and the matrix, HA-BNNSs have been introduced to aramid nanofibers (ANFs) to fabricate layered HA-BNNSs/ANF composite films for thermal management applications. In comparison with the pure BNNSs/ ANF, the HA-BNNSS/ANF composite exhibits improved mechanical strength (85.6−175.6 MPa) and better thermal conductivity (8−39.5 W/m•K) at a relatively low filler content (≤50%). In summary, benefiting from the bifunctional group, improved dispersibility, and high aspect ratio, the HA-BNNSs have great application potential in the fields of composite reinforcement, drug delivery, and photocatalysis.