Boron nitride nanotube precursor formation during high-temperature synthesis: kinetic and thermodynamic modelling

TL;DR

This study models the chemical pathways and kinetics of boron nitride nanotube precursor formation during high-temperature synthesis, revealing key species and conditions that influence BNNT growth.

Contribution

It introduces integrated quantum chemistry, molecular dynamics, and kinetic modelling to elucidate BNNT precursor formation mechanisms, highlighting the roles of specific boron-nitrogen species and gas pressure effects.

Findings

B2N2 formation occurs via N2 interaction with small boron clusters.

B4N4 and B5N4 are crucial in N2 fixation and BN chain formation.

Higher gas pressure increases B4N4 and B5N4 densities, enhancing BNNT yield.

Abstract

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.