Boron nitride fibers have attracted significant research interest due to their low density, high strength, high temperature resistance, strong insulation and wave transparent properties, making them promising high-performance wave transparent composites required for aerospace applications. The polymer-derived ceramics (PDCs) method is one of the most potential approaches for preparing boron nitride fibers. Currently, developing high performance boron nitride fibers remains a major challenge in the field. Controlling the structure of polyborazane precursor and structure-property relationships between polymer and ceramic have been gained widespread attention. Numerous studies were focused on optimizing the boron nitride fiber fabrication process, including precursor molecular structure design, thermal treatment process, and fiber microstructure regulation. However, reviews from the perspective of precursor molecular structures for boron nitride fiber preparation via PDCs were still rarely, and the relationship between precursor molecular structure and boron nitride fiber performance requires further in-depth analysis.

The molecular structure of precursors plays a critical role in determining its physicochemical properties. in this paper, authors firstly analyzed the different molecular structures of boron nitride precursors to elucidate the structure-property relationships between polymer and ceramic. Based on the flexibility and molecular weight of the chains, boron nitride precursors can be divided into rigid chain structures, flexible chain structures, and small molecule boron nitride precursors. The synthesis methods, properties, and applications of precursors with corresponding chain structures were summarized. Directly synthesizing a precursor that simultaneously satisfies all performance requirements of thermal stability, solubility, meltability, high ceramic yield, and spinnability was highly challenging. To overcome this challenge, this paper summarized common approaches for modulating the elemental composition and molecular structure of precursors to improve its overall properties. Methods for modulating precursor molecular structures primarily include substitution group modification, mixing/copolymerization modification, and bridge-linking structure modification. Through structural modulation of precursor molecules, spinnability and crosslinking characteristics could be realized.

The molecular structural transformation process from polymer to boron nitride ceramic during PDCs was rather complex and significantly impacts the properties of boron nitride fiber. This paper reviews the heat treatment and microstructural control techniques employed during boron nitride fiber fabrication. Fiber heat treatment currently included inert atmosphere thermal crosslinking, ammonia (NH₃) atmosphere thermal crosslinking, air heat treatment, and BCl₃-assisted heat treatment with different mechanisms for crosslinking and solidifying precursor. Methods for controlling microstructure mainly included molecular symmetry modulation, heat treatment, and secondary phase induction. The goal of these operations is to achieve an intimate structure and crystalline orientation within fibers for optimal mechanical performance. Overall, a systematic understanding of structure-property relationships and process-microstructure-property correlations of boron nitride precursors is important for continued advances in high-performance boron nitride ceramic fibers.