Pyrolysis process for boron nitride fiber derived from tris(methylamino)borane: Evolution of the molecular structure

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 degrees C and then treats to 400, 600, 800, and 1000 degrees 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 degrees 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.

Automated summary

In order to investigate the structural evolution of tris(methylamino)borane during the thermal-treating process, polymeric green fiber is cured and treated in different temperatures. The composition and structure of volatile compounds and residual products are analyzed during the pyrolysis process. It is found that oxygen can be used to rapidly cure the fiber while maintaining its tensile strength. The evolution of molecular structure during the pyrolysis process is determined.