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

We performed integrated modelling of the chemical pathways of formation for boron nitride nanotube (BNNT) precursors during high-temperature synthesis in a B/N-2 mixture. Integrated modelling includes quantum chemistry, Quantum-classical molecular dynamics, thermodynamic modelling, and kinetic approaches. We demonstrate that BN compounds are formed via the interaction of molecular nitrogen with small boron clusters, rather than through interactions with less reactive liquid boron. (This process can also be described as N-2 molecule fixation.) Liquid boron evaporates to produce these boron clusters (B (m) with m <= 5), which are subsequently converted into B (m) N (n) 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 in turn the building blocks of fullborenes and BNNTs. Additionally, kinetic modelling revealed that B4N4 and B5N4 species in particular play a major role in the N-2 molecule fixation process. The formation of these species via reactions with B-4 and B-5 clusters is not adequately described under the assumption of thermodynamic equilibrium, as is demonstrated in our kinetic modelling. Thus, 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 components 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.

Automated summary

Integrated modelling was used to study the chemical pathways of boron nitride nanotube (BNNT) precursor formation, showing that BN compounds are formed through the interaction of molecular nitrogen with small boron clusters. Kinetic modelling revealed the significant role of B4N4 and B5N4 species in the N-2 molecule fixation process. Gas pressure and cooling rate were shown to impact the accumulation of these species, with higher pressures leading to increased densities of B4N4 and B5N4, explaining the observed effect of gas pressure on high-quality BNNT yield. The catalytic role of hydrogen in BNNT synthesis was also investigated.