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

Abstract

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] (T_synthesis = 180 °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 °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 °C allows the basal unit of the “naphthalenic-type structure” of boron nitride to be established at 1000 °C through important structural rearrangements and the crystallization tendency to be improved during further heating from 1000 to 1800 °C. Finally, incorporation of nitrogen using ammonia allows the production of polycristalline fibers in which the stoichiometry approaches that of BN.