Insight into BN Impurity Formation during Boron Nitride Nanotube Synthesis by High-Temperature Plasma

Abstract

The high-temperature plasma process has demonstrated great potential in growing high-quality boron nitride nanotubes (BNNTs) with small diameters (∼5 nm) and few walls (3–4 walls) and led to successful commercialization with a high production rate approaching 20 g/h. However, the process is still accompanied by the production of BN impurities (e.g., a-BN, BN shell, BN flakes) whose physicochemical properties are similar to those of BNNTs. This renders the post-purification process very challenging and thus hampers the development of their practical applications. In this study, we have employed both experimental and numerical approaches for a mechanistic understanding of BN impurity formation in the high-temperature plasma process. This study suggests that the flow structure of the plasma jet (e.g., laminar or turbulent) plays a key role in the formation of BN impurities by dictating the transport phenomena of BNNT seeds (e.g., B droplets), which play an important role in BNNT nucleation. We discussed that the turbulence enhances the radial diffusion of B droplets as well as their interparticle coagulation, which leads to a significant reduction in the population of effective BNNT seeds in the BNNT growth zone (T < 4000 K). This results in the generation of unreacted BN precursors (e.g., B-N-H species) in the BNNT growth zone that eventually self-assemble into BN impurities. Our numerical simulation also suggests that a higher thermal energy input makes the flow more turbulent in the BNNT growth zone due to the elevated velocity difference between the plasma jet and ambient cold gas. This finding provides critical insight into the process design that can suppress the BN impurity formation in the high-temperature plasma process.

Growth methodologies of boron nitride nanotubes and their neutron shielding applications: a review

“…BNNT growth typically occurs at temperatures of approximately 8000 K, without the use of metal catalysts, by flowing hydrogen gas along with the reactants. 37,38,40,46 The precursor was introduced into the high-temperature region, causing it to enter a plasma state and initiate reactions in the form of radicals containing B, N, H, and other species. Various radicals such as B, N, H, B-N, B-H, N-H, B-N-H, H are formed, and the growth proceeds from a boron nano-droplet as the starting nucleus.…”

“…The follow-up study 46 by the same author investigates the formation of BN impurities (such as a-BN, BN shell, BN flakes) during the high-temperature plasma process used for synthesizing BNNTs. Despite the success of this process in producing high-quality, small-diameter BNNTs with few walls at a high production rate (approximately 20 g h −1 ), the concurrent production of BN impurities poses a significant challenge for post-purification and practical application development.…”

“…The complex atomic structure of BNNTs necessitates precise control over the growth process, which is influenced by various factors, such as temperature, pressure, and choice of catalysts. Different growth methods, including laser ablation, [30][31][32][33][34][35][36] thermal plasma, [37][38][39][40][41][42][43][44][45][46] chemical vapor deposition (CVD), [47][48][49][50][51][52][53][54][55] and ball milling, [56][57][58][59][60][61][62][63][64][65][66][67] have intricacies and yield BNNTs of different qualities. This review examines these growth techniques and highlights their mechanisms, advantages, and limitations.…”

Poly(4-vinylpyridine) adsorption on boron nitride nanotubes and hexagonal boron nitride: A comparative molecular dynamics study

“…However, the as-produced materials still contain various hBN impurities, including hBN sheets and shells, requiring additional purification steps. To date, extensive efforts have been made to obtain highly pure BNNTs at large quantities from synthesis to post-synthesis processes [10][11][12][13].…”

Influence of Impurities from Manufacturing Process on the Toxicity Profile of Boron Nitride Nanotubes

“…such as ammonia borane. [7][8][9][10][11][12][13] Depending on the manufacturing method employed, other metal catalyst residuals like iron and nickel were also present. [14,15] Morphologically, apart from the tubular structures of BNNT, the impurities appear as a combination of fibrous, sheet-and powder-like structures.…”