  We performed integrated modelling of the chemical pathways of formation for
boron nitride nanotube (BNNT) precursors during high-temperature synthesis in a
B/N2 mixture. Modelling includes quantum chemistry, quantum-classical molecular
dynamics, thermodynamic, and kinetic approaches. It is shown that BN compounds
are formed in the interaction of N2 molecules with small boron clusters (N2
molecule fixation) rather than with less reactive liquid boron. We demonstrate
that the transformation and consumption of liquid boron proceeds through the
evaporation of clusters, Bm with m less than or equal to 5 and their subsequent
conversion into BmNn chains. The production of such chains is crucial to the
growth of BNNTs because these chains form the building blocks of bigger and
longer BN chains and rings, which are themselves the building blocks of
fullborenes and BNNTs. Moreover, kinetic modelling revealed that B4N4 and B5N4
species play a major role in the N2 molecule fixation process. The formation of
these species via reactions with B4 and B5 clusters is not adequately described
under the assumption of thermodynamic equilibrium because the accumulation of
both B4N4 and B5N4 depends on the background gas pressure and the gas cooling
rate. Long BN chains and rings, which are precursors of the fullborene and BNNT
growth, form via self-assembly of component B4N4 and B5N4. Our modelling
results (particularly the increased densities of B4N4 and B5N4 species at
higher gas pressures) explain the experimentally observed effect of gas
pressure on the yield of high-quality BNNTs. The catalytic role of hydrogen was
also studied; it is shown that HBNH molecules can be the main precursor of BNNT
synthesis in the presence of hydrogen.
