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To provide molecular‐scale insight into the structural evolution from tris(methylamino)borane to boron nitride (BN) fiber during the chemical thermal‐treating process, polymeric green fiber is cured in hot synthetic air at 300°C and then treats to 400, 600, 800, and 1000°C in ammonia. The chemical composition and structure of the volatile compounds and residual products are analyzed during the pyrolysis process for the polymeric green fiber. It is demonstrated that oxygen can be used to cure polymeric green fiber rapidly under the premise of ensuring a final fiber content of less than 1 wt% carbon and 2 wt% oxygen while maintaining the fiber tensile strength at 1000°C. The molecular structure evolution during the pyrolysis process for polymeric green fiber after oxygen curing is determined. Specifically, in hot synthetic air, introducing oxygen and releasing methylamine generates a B–O six‐membered ring structure in the polymer in the first stage. Then, the removal of methyl results in the formation of a B–N–O network in hot ammonia. Afterward, nitridation of the B–O six‐membered ring promotes the evolution of the B–N six‐membered ring structure with the release of water and carbon dioxide. Finally, the growth and rearrangement of the BN structure are achieved. [ABSTRACT FROM AUTHOR]

Oxygen plays a crucial role in the synthesis of boron nitride (BN) and the preparation of BN ceramics. The effects of oxygen content on the microstructure and properties of BN fibers treated at 1600 °C have not been studied yet. In this work, polymer-derived BN fibers are cured at 300 °C under oxygen and pyrolyzed at 600 °C, 700 °C and 800 °C under ammonia as the pre-heated BN fibers. Then, these pre-heated BN fibers are treated at 1600 °C under nitrogen as the final BN fibers. The compositions and microstructures of the pre-heated and final BN fibers are investigated. As the pre-heated temperature increases, the oxygen contents of the pre-heated BN fibers decrease to 24 wt%, 17 wt% and 5 wt%, respectively. The structures of the pre-heated BN fibers transform from amorphous BN to turbostratic BN. The oxygen in the pre-heated BN fiber volatilizes as boron oxide during treatment at 1600 °C, promoting the formation and growth of h-BN in the final BN fiber, with the maximum grain size reaching 74 × 20 nm (La × Lc). As the grain size of the final BN fiber increases, the Young's modulus, density and oxidation resistance level of the BN fiber improve, and the dielectric properties decrease. The final BN fiber that underwent pre-heating at 700 °C has the best comprehensive properties, with a tensile strength of 1.11 GPa and a Young's modulus of 265 GPa. This study researches the relationship between composition, structure and performance, and provides a scientific basis for the preparation process of BN fibers. • The initial BN fibers with gradient oxygen content are obtained by controlling the nitridation temperature after air curing. • The higher the oxygen content in the initial BN fibers, the better the crystallinity of the BN fibers treating at 1600 °C. • The tensile strength of BN fiber with a grain size of 34 × 11 nm (La × Lc) is 1.11 GPa, and the Young's modulus is 265 GPa. [ABSTRACT FROM AUTHOR]

• A simple and efficient method for generating and characterizing the microstructure of a -BN is proposed based on the classical molecular dynamics simulation. • Various densities and microstructures of a -BN could be successfully generated, which is mainly composed of the B 3 N 3 hexagonal ring. • High-density a -BN obtained from either high annealing temperature or high initial density, possesses high Young's modulus and fracture strength. • During the tensile process, the micro-voids in high-density of a -BN gradually undergoes expansion, elongation, mergence stages, and the fracture of a -BN. In this paper, a simple and effective strategy for generating the microstructure of a -BN is firstly proposed based on classical molecular dynamics methods, and then is validated by characterizing the microstructure. The characterization analysis show that: i) a -BN with different densities and microstructures could be successfully generated mainly composed of the B 3 N 3 hexagonal ring; ii) a- BN constructed by the combinations of the basic structural units with small size is more reasonable; iii) high-density a -BN, obtained at either high annealing temperature or high initial density, has discretely distributed micro-voids with larger scattering size distribution. The microstructure of the generated a- BN is subjected to tensile loading simulation, and the results showed the Young's modulus and fracture strength increase with increasing annealing temperature or initial density. During the tensile process, the micro-voids in high-density of a -BN gradually undergoes expansion, elongation, and mergence stages, and the ultimate fracture of a -BN. [ABSTRACT FROM AUTHOR]

• The influence of various factors on mechanical properties of h -BNNS are systematically investigated via classical MD simulations. • The effect of temperature and the ratio of circular-region radius to nanoindenter radius are greater than other factors. • Failure mechanism of mono-layer h -BNNS starts with discrete point defects, gradually expands and merges to form hole defects, and causes structural fracture. • Fracture mode of multi-layer h -BNNS will translate from the bottom-top failure to the top–bottom failure accompanying with increasing thickness. Hexagonal boron nitride nanosheets (h -BNNS) is one kind of promising 2D materials due to its outstanding mechanical properties especially at elevated temperature. However, the experimentally measured mechanical properties of h -BNNS have great dispersions due to the intricate experimental measurement conditions. In the present study, the classical molecular dynamics simulations are performed to study the effect of various experimental factors on the mechanical properties of h -BNNS based on nanoindentation, including: size, temperature, loading rate of nanoindenter, ratio of circular-region radius to nanoindenter radius, and thickness (the number of layers). Meanwhile, the evolution of microstructure of h -BNNS up to fracture are investigated to elucidate its fracture mechanism during nanoindentation process. The results indicate that the Young's modulus and fracture strength of h -BNNS gradually decreases with the increasing temperature, and the mechanical properties of h -BNNS remain relatively stable when the ratio of circular-region radius to nanoindenter radius and the circular-region radius is bigger than 10 and 18 nm, respectively. Based on the evolutionary process of the microstructure of h -BNNS, the results not only show the failure process of h -BNNS, but also reveal the conversion of the fracture mode in multi-layer h -BNNS, that is, it will be translated from the bottom-top to the top–bottom fracture with increasing thickness. [ABSTRACT FROM AUTHOR]