**4. Conclusions**

BN/Si3N<sup>4</sup> composite ceramics and ceramic fibers were obtained through the precursorderived ceramic route using the hybrid polymers of poly(tri(methylamino)borazine) (PBN) and polycarbosilane (PCS). When the Si3N<sup>4</sup> content of the composite ceramics was below 25 wt%, no Si3N<sup>4</sup> crystals were found at 1600 ◦C, and the BN/Si3N<sup>4</sup> composite ceramic containing 25 wt% Si3N<sup>4</sup> was totally amorphous. Three kinds of BN/Si3N<sup>4</sup> composite fibers containing 5 wt%, 15 wt% and 25 wt% Si3N<sup>4</sup> were fabricated successfully, showing the nanocrystallines of BN and amorphous Si3N4, of which the mean tensile strength and Young's modulus was enhanced with the increasing of the Si3N<sup>4</sup> mass content. Additionally, the composite fibers (P3-F) showed a unique BN (shell)/Si3N<sup>4</sup> (core) structure, with average tensile strength of 1.36 GPa and Young's modulus up to 117 GPa. Moreover, the composite fiber (P3-F) exhibited excellent dielectric properties, with a dielectric constant of 3.34 and a dielectric loss tangent of 0.0047 at 10 GHz. Further research is in progress to optimize the oxygen content to improve the mechanical properties of the composite ceramic fibers.

**Author Contributions:** Formal analysis, Y.L.; investigation, Y.L. and M.G.; methodology, H.Z. and Z.W.; resources, S.Y., C.H. and W.K.; writing—original draft, Y.L.; writing—review and editing, M.G. and W.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Key Laboratory of Science and Technology on Particle Materials (Grant No. CXJJ-21S043), Key Laboratory of Multiphase Complex Systems (Grant No. MPCS-2021-a-02) and Innovation Academy for Green Manufacture (Grant No. IAGM2020C22), Chinese Academy of Sciences.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
