Eco-Friendly, Dispersant-Free Purification of Boron Nitride Nanotubes (BNNTs): Controlling the Dispersibility of BNNTs

Abstract

The Boron nitride nanotubes (BNNTs) share a tubular structure similar to carbon nanotubes, but they are composed of boron instead of carbon. BNNTs are known for their exceptional thermal and chemical stability and remarkable neutron absorption capabilities, thanks to the presence of boron. This unique combination of properties has made BNNTs a subject of increasing interest and research in diverse scientific and industrial fields, leading to various potential applications. However, applying BNNTs has faced challenges due to the presence of various impurities. To address this problem, several purification methods have been developed, which include the utilization of polymers or dispersants, etching methods involving chlorine gas, or strong acids. However, these methods come with a significant drawback: either heat treatment is necessary to eliminate the remaining organic substances used in the purification process, or the chemicals employed can pose environmental or human health risks. This thesis introduces a novel approach to achieve high-purity BNNTs by modifying the surface of these nanotubes. This method aims to overcome the shortcomings of traditional purification techniques and elucidates its underlying mechanism. This research demonstrates the potential for obtaining highly purified BNNTs. Firstly, to functionalize and purify the surface of BNNTs, pyridine-based molecules like poly(4-vinylpyridine) (P4vP) are employed. The nitrogen atoms in pyridine molecules can interact with the boron atoms on the surface of BNNTs. Subsequently, a strong acid is introduced to remove P4vP present on the BNNTs surface, and Lewis acid-base interactions are used to inhibit the interaction between pyridine and boron. This process allows for the removal of most of the polymer from the BNNTs surface. Secondly, the surface tension was adjusted, and the steric repulsion effect of solvent molecules was utilized for BNNTs purification without the need for dispersants. Controlling the solution's surface tension based on the tert-butanol and water mixing ratio, which is similar to the surface tension of BNNTs, maximizes the dispersibility difference between impurities and BNNTs through the steric repulsion effect of tert-butanol's molecular structure. This mechanism enables the environmentally friendly purification of high-purity BNNTs without the use of dispersants. Finally, both the P4vP method and the tert-butanol and water co-solvent system successfully yielded high-purity BNNTs. Nevertheless, impurities smaller than 100 nm in size easily disperse in various solvents, resulting in their presence in BNNTs purified by both methods. To eliminate these small-sized impurities, shear force was employed to selectively separate BNNTs from the impurities. When shear force is applied to the impurities and BNNTs dispersed in the solution, the electrostatic interaction between the nanotubes causes them to aggregate and precipitate as the distance between the nanotubes becomes close due to shear force. In contrast, small-sized impurities maintain a stable dispersed state. Leveraging these mechanisms, high-purity BNNTs were obtained. Interestingly, these purification mechanisms result in the formation of bundled BNNTs. Because these bundles are densely packed, they have potential applications in neutron shielding effects or high thermal conductivity research. To develop neutron shielding materials using BNNTs known for their excellent neutron absorption capabilities, the five BNNT-epoxy composites (0 wt, 7 wt%, 10 wt%, 20 wt%, and 30 wt%) were prepared by mixing them with liquid epoxy resin (YD-128). As the content of BNNTs in the BNNTs-epoxy composites increased, there was an observed nearly proportional increase in neutron shielding effectiveness. An intriguing aspect is the relationship between neutron shielding and the formation of liquid crystals in BNNTs. A remarkable enhancement in neutron shielding performance was observed in BNNTs-epoxy composites with 20 wt% or more BNNTs content, where liquid crystal formation began, and birefringence images were observed. The formation of these liquid crystals suggests various potential applications, such as dielectric or radiation-shielding materials. Through this study, we have developed an eco-friendly and dispersant-free purification method for BNNTs using a mixture of tert-butanol and water, as well as a shear force-based removal method for impurities smaller than 100 nm that are easily dispersed in various solvents. This demonstrates the potential for obtaining high-purity BNNTs. Furthermore, we have evaluated the neutron shielding performance of BNNTs-epoxy composites. Particularly, the importance of nanotube alignment for superior shielding performance has been confirmed. This phenomenon is believed to have applications in various areas, including dielectric materials, high thermal conductivity, and neutron shielding.