Controlling the chemistry, morphology and structure of boron nitride-based ceramic fibers through a comprehensive mechanistic study of the reactivity of spinnable polymers with ammonia

The present paper describes an access to polycrystalline boron nitride fibers from poly[B(methylamino)borazine]. Solid-state NMR and IR spectroscopies, thermo-analytical experiments, SEM and XRD investigations were applied to provide a comprehensive mechanistic study of the fiber transformation and understand the role played by ammonia during the polymer-to-ceramic conversion. It was shown that a typical melt-spinnable poly[B-(methylamino)borazine] (Tsynthesis = 180 deg C) is composed of borazine rings connected via a majority of NCH3 bridges and a small proportion of NB3-containing motifs forming a cross-linked network. In addition, a low proportion of peripheral N(H)CH3 groups, which are present in the starting molecular precursor, B-tri(methylamino)borazine, is identified. The polymer is capable of melting without decomposition in flowing nitrogen to produce high quality green fibers at moderate temperature. A curing process of green fibers in flowing ammonia at 400 deg C through transamination and condensation forming cross-linked NB3 motifs in the polymer network is seen as the most appropriate way to retain the fiber integrity during the polymer-to-ceramic conversion. The use of ammonia during the subsequent pyrolysis from 400 to 1000 deg C allows the basal unit of the 'naphthalenic-type structure' of boron nitride to be established at 1000 deg C through important structural rearrangements and the crystallization tendency to be improved during further heating from 1000 to 1800 deg C. Finally, incorporation of nitrogen using ammonia allows the production of polycristalline fibers in which the stoichiometry approaches that of BN.