**1. Introduction**

Advanced antenna radomes or windows are widely used in aerospace radar devices, and wave-transparent materials with high performance are urgently required [1–3]. Continuous ceramic fiber reinforced ceramic matrix composites are the most promising materials, and several inorganic continuous fibers, such as quartz, Si3N<sup>4</sup> and BN fibers, are suitable candidates as reinforcements [4,5]. Because of the occurrence of re-crystallization and particle coarsening, quartz fibers undergo serious degradation of mechanical strength at temperatures above 900 ◦C [6]. Although BN fibers show excellent high-temperature stability, with a melting point above 2900 ◦C [7–9], their low mechanical strength impedes any further development [10,11]. Generally, Si3N<sup>4</sup> fibers possess higher mechanical strength than BN fibers, while the thermal resistance and the dielectric properties of Si3N<sup>4</sup> fibers are inferior to BN fibers [12]. Therefore, it is urgent to develop new types of ceramic fibers containing B, N, and Si to combine the merits of both BN and Si3N<sup>4</sup> fibers in order to ensure excellent dielectric properties, high mechanical properties and good thermal resistance [13–16].

The polymer-derived ceramics (PDCs) method is the only feasible approach to prepare SiBN fibers; it can be used to design the atomic composition and the microstructure of fibers with low impurity [17–20]. Numerous researchers have synthesized different types of precursors for SiBN ceramic fibers. Tang et al. [21] obtained SiBN fibers using a novel precursor fabricated by the reaction of boron trichloride, dichloromethylsilane and hexamethyldisilazane and the obtained SiBN fibers had excellent mechanical strength

**Citation:** Li, Y.; Ge, M.; Yu, S.; Zhang, H.; Huang, C.; Kong, W.; Wang, Z.; Zhang, W. Characterization and Microstructural Evolution of Continuous BN Ceramic Fibers Containing Amorphous Silicon Nitride. *Materials* **2021**, *14*, 6194. https://doi.org/10.3390/ma14206194

Academic Editor: Dinesh Agrawal

Received: 3 October 2021 Accepted: 15 October 2021 Published: 18 October 2021

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up to 1.83 GPa as well as thermally stable dielectric properties. Liu et al. [22] prepared SiBN ternary ceramic fibers with a tensile strength of 0.87 GPa from a polymer precursor made from the reaction of hexamethyldisilazane, trichlorosilane, boron trichloride and methylamine. Peng et al. [23] prepared a precursor for SiBN ceramic fibers from the reaction of hexamethyldisilazane, silicon tetrachloride, boron trichloride and methylamine. In summary, a substantial amount of research has been focused on the fabrication of the spinnable polymer precursor containing the Si-N-B bridge bond, since the atomic ratio of Si:B:N of the preceramic polymer is difficult to adjust owing to its high activity [22]. In addition, the synthesis of single-source precursors usually needs multiple-stage processes, which results in significant yield loss and the unavoidable removal of by-products. Additionally, during the multi-step process, the polymeric intermediates are extremely sensitive to moisture, and the requirement for the large inert environment hampers its industrial applications [23].

In a previous study, Tan et al. [24] successfully synthesized BN/Si3N<sup>4</sup> composite fibers using a mixture of polymers of poly(tri(methylamino)borizane) (PBN) and polysilazane (PSZ), and the fibers showed a mean tensile strength of 1040 MPa. Difficulties in the industrialization of this process exist mainly due to the fact that the PSZ is very sensitive to moisture. Polycarbosilane (PCS) is an organosilicon polymer with a normal structure—(CH3HSiCH2)x—that contains a –Si–C– backbone [25] and has been widely used as the precursor for preparation of silicon carbide and silicon nitride ceramic fibers owing to its good solubility and fusibility [26]. Therefore, we adopted a new strategy to prepare BN/Si3N<sup>4</sup> ceramic fiber by using hybrid polymers of PBN and PCS. The proportions of BN and Si3N<sup>4</sup> in the final ceramics could be easily adjusted through changing the ratios of PBN/PCS in hybrid composite polymers. The microstructural evolution of the obtained BN/Si3N<sup>4</sup> composite ceramics and the properties of the composite fibers were investigated.
