Effect of Y, Al doping on the microstructure and properties of h-BN fibers with low oxygen content

Hexagonal boron nitride (h-BN) particles present significant challenges in sintering and crystallization processes, necessitating the use of specific aid reagents. While Y or Al containing compounds are extensively utilized as additives in the hot-press sintering of monolithic h-BN ceramics to enhance density and strength, their application in the production of continuous h-BN fibers remains under-explored. In this study, polymer-derived h-BN fibers doped with Y and Al elements were prepared and systematically investigated. The effects of these additives on the crystallization and properties of the derived ceramic fiber were analyzed. The results demonstrate that Y, Al, and Y/Al doping promote the growth of h-BN grains. Additionally, the formation of borate and yttrium aluminate in the Y and Al co-doped fibers, which melt at high temperatures, creates a liquid phase that aids in the bonding of BN layers. This liquid phase promotes the closure of micropores and densifies the structure. Conversely, it was observed that the introduction of Y/Al hindered the formation of excessively large h-BN grains, with an Lc of only 11 nm and a La of 27 nm when treated at 2000 °C. Furthermore, the tensile strength of the Y/Al-doped fibers was found to peak at 860 MPa upon treatment at 2000 °C, while Young’s modulus reached its maximum value of 110 GPa at the comparatively lower temperature of 1800 °C. These findings underscore the pivotal role of Y and Al doping in optimizing the microstructure and mechanical properties of h-BN fibers, thereby establishing them as promising candidates for advanced applications.