Synthesis of Boron Nitride Nanotubes using ammonia borane precursor

Abstract

Boron nitride nanotubes (BNNTs) share a similar structural framework with carbon nanotubes (CNTs), but they are composed of boron (B) and nitrogen (N) atoms arranged alternately, in contrast to CNTs. While CNTs exhibit outstanding mechanical strength and high thermal conductivity, BNNTs possess unique properties stemming from the electronegativity difference between B and N, resulting in piezoelectricity and a high band gap due to localized electrons. Additionally, BNNTs offer a high neutron cross section owing to the presence of B. Despite these remarkable attributes, the synthesis of BNNTs remains challenging due to the necessity of combining two different elements, unlike CNTs. Current BNNT synthesis approaches have their limitations, and it is of significant value to explore new methods for BNNT synthesis, especially considering the insufficient progress compared to CNTs. This study introduces a novel approach to BNNT synthesis using ammonia borane as a molecular precursor. We present our research findings, a comparison with existing methods, and an analysis of the underlying mechanism. In this study, the pivotal value of ammonia borane lies in its unique feature of housing a B-N unit within a single molecule. Analogous to the synthesis approach for carbon nanotubes (CNTs), where carbon serves as a fundamental building unit for CNT formation, we can regard a B-N unit as the fundamental building unit for the formation of boron nitride nanotubes (BNNTs). While there are compounds like borazine and h- BN that also contain both B and N simultaneously, ammonia borane stands out as the sole compound that possesses a B-N unit within a single molecule. In our experiments, we synthesized BNNTs using ammonia borane through laser ablation and thermal plasma methods. In contrast to conventional methods, which involve the reaction of solid boron and gaseous nitrogen molecules, we exclusively employed ammonia borane for BNNT synthesis. Experiments conducted under nitrogen and argon gas atmospheres yielded consistent results, confirming that ammonia borane alone is sufficient for BNNT synthesis. We observed a significant reduction in the impurity content, particularly amorphous boron impurities, in the reaction products. Our analysis revealed that BNNT fibers spontaneously formed due to convection currents resulting from temperature differences between the reaction zone and its surroundings, increasing van der Waals forces among BNNTs. The argument regarding gas flow near the reaction zone was supported by thermo-fluid simulations. Furthermore, we proposed a mechanism for BNNT growth via homogeneous nucleation through B-N radicals derived from ammonia borane, substantiating our claim by analyzing changes in reaction products at macro, micro, and nano scales under varying pressures. The reduction in amorphous boron impurity content aligns with the mechanism proposed in this study. In the case of thermal plasma synthesis using h-BN as a precursor, we achieved superior results compared to existing methods, including a decrease in average wall count, an increase in average length, and enhanced production rates. Notably, we observed a reduction in amorphous boron and h-BN impurity content. This study offers an in-depth investigation into BNNT synthesis using ammonia borane as a precursor, presenting innovative results and interpretations when compared to previous research. We hope that these findings will contribute to the advancement of future BNNT synthesis research. In future work, it is essential to clarify the underlying causes of these results and further elucidate the synthesis mechanisms.