Boron Nitride Nanotubes – Exfoliated and Dispersed using Sonication

Ultrasonication is successfully applied to the processing and dispersion of boron nitride nanotubes (BNNTs). High-intensity sonication provides homogeneous detangling and distribution in various solutions and is thereby a crucial processing technique to incorporate BNNTs into solutions and matrices.

Ultrasonic Processing of Boron Nitride Nanotubes

In order to incorporate boron nitride nanotubes (BNNTs) or boron nitride nanostructures (BNNs) such as nanosheets and nanoribbons into liquid solutions or polymeric matrices, an efficient and reliable dispersion technique is required. Ultrasonic dispersion provides the required energy to exfoliate, detangle, disperse, and functionalize boron nitride nanotubes and boron nitride nanostructures with high efficiency. The precisely controllable processing parameters of high-intensity ultrasound (i.e. energy, amplitude, time, temperature, and pressure) allow to individually adjust the processing conditions to the targeted process goal. This means ultrasonic intensity can be adjusted in regards to the specific formulation (quality of BNNTs, solvent, solid-liquid concentration etc.), thereby obtaining optimum results.

The applications of ultrasonic BNNT and BNN processing cover the full range from the homogeneous dispersion of two-dimensional boron nitride nanostructures (2D-BNNs), to their functionalization and chemical exfoliation of mono-layer hexagonal boron nitride. Below, we present to the details on ultrasonic dispersion, exfoliation, and functionalization of BNNTs and BNNs.

Ultrasonic Dispersion of Boron Nitride Nanotubes

When boron nitride nanotubes (BNNTs) are used to reinforce polymers or to synthesize new materials, a uniform and reliable dispersion into the matrix is required. Ultrasonic dispersers are widely used to disperse nano materials such as CNTs, metallic nanoparticles, core-shell particles and other types of nano particles into a second phase.
Ultrasonic dispersion has been successfully applied to detangle and distribute BNNTs uniformly in aqueous and non-aqueous solutions including ethanol, PVP ethanol, TX100 ethanol as well as various polymers (e.g. polyurethane).
A common used surfactant to stabilize an ultrasonically prepared BNNT dispersion is a 1%wt sodium dodecyl sulfate (SDS) solution. For instance, 5 mg BNNTs are ultrasonically dispersed in a vial with 5 mL of 1%wt. SDS solution using an ultrasonic probe-type disperser such as the UP200St (26kHz, 200W).

Aqueous Dispersion of BNNTs using Ultrasound

Due to their strong van der Waals interactions and hydrophobic surface, boron nitride nanotubes are poorly dispersible in water-based solutions. To solve these problems, Jeon et al. (2019) used Pluronic P85 and F127, which have both hydrophilic groups and hydrophobic groups to functionalize BNNT under sonication.

Surfactant-Free Exfoliation of Boron Nitride Nanosheets using Sonication

Lin et al. (2011) present a clean method of exfoliation and dispersion of hexagonal boron nitride (h-BN). Hexagonal boron nitride is traditionally considered to be insoluble in water. However, they were able to demonstrate that water is effective to exfoliate the layered h-BN structures using ultrasonication, forming “clean” aqueous dispersions of h-BN nanosheets without the use of surfactants or organic functionalization. This ultrasonic exfoliation process produced few-layered h-BN nanosheets as well as monolayered nanosheet and nanoribbon species. Most nanosheets were of reduced lateral sizes, which was attributed to the cutting of parent h-BN sheets induced by the sonication-assisted hydrolysis (corroborated by the ammonia test and spectroscopy results). The ultrasonically induced hydrolysis also promoted the exfoliation of h-BN nanosheets in assistance to the polarity effect of the solvent. The h-BN nanosheets in these “clean” aqueous dispersions exhibited good processability via solution methods retaining their physical characteristics. The dispersed h-BN nanosheets in water also exhibited strong affinity toward proteins such as ferritin, suggesting that the nanosheet surfaces were available for further bio-conjugations.

The sono-mechanical dispersion method using ultrasonic cavitation and shear forces is a purely physical blending approach, which has been proven capable of debundling BNNTs and stabilizing individual BNNTs while maintaining their integrity and intrinsic properties. Applying suitable ultrasonic energy (Ws/mL), i.e. adjusted amplitude and sonication duration, ultrasonic dispersion can detangle and disperse BNNTS uniformly. Higher amplitudes and longer sonication can be applied if the length of the boron nitride nanotubes should be reduced. Read more about ultrasonic size reduction and length cutting of BNNTs in the next section.

Ultrasonic Size Reduction and Cutting of Boron Nitride Nanotubes

The length of boron nitride nanotubes plays a crucial role when it comes to the subsequent processing of BNNTs into polymers and other functionalized materials. Therefore it is an important fact that sonication of the BNNTs in solvent could not only separate BNNTs individually, but also shorten the bamboo structured BNNTs under controlled conditions. The shortened BNNTs have a much lower chance of bundling during composite preparation.Lee at al. (2012) demonstrated that the lengths of functionalized BNNTs can be efficiently shortened from >10µm to ∼500nm by ultrasonication. Their experiments suggest that effective ultrasonic dispersion of BNNT in solution is necessary for such cutting of BNNT size reduction and cutting.

High-Performance Ultrasonicators for BNNT Processing

The smart features of Hielscher ultrasonicators are designed to guarantee reliable operation, reproducible outcomes and user-friendliness. Operational settings can be easily accessed and dialled via intuitive menu, which can be accessed via digital colour touch-display and browser remote control. Therefore, all processing conditions such as net energy, total energy, amplitude, time, pressure and temperature are automatically recorded on a built-in SD-card. This allows you to revise and compare previous sonication runs and to optimize the exfoliation and dispersion process of boron nitride nanotubes and nanomaterials to highest efficiency.
Hielscher Ultrasonics systems are used worldwide for the manufacturing of high-quality BNNTs. Hielscher industrial ultrasonicators can easily run high amplitudes in continuous operation (24/7/365). Amplitudes of up to 200µm can be easily continuously generated with standard sonotrodes (ultrasonic probes / horns). For even higher amplitudes, customized ultrasonic sonotrodes are available. Due to their robustness and low maintenance, our ultrasonic exfoliation and dispersion systems are commonly installed for heavy duty applications and in demanding environments.
Hielscher Ultrasonics’ industrial ultrasonic processors can deliver very high amplitudes. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation. For even higher amplitudes, customized ultrasonic sonotrodes are available.
Hielscher ultrasonic processors for the dispersion and exfoliation of boron nitride nanotubes as well as CNTs and graphene are already installed worldwide on commercial scale. Contact us now to discuss your BNNT manufacturing process! Our well-experienced staff will be glad to share more information on the exfoliation process, ultrasonic systems and pricing!
The table below gives you an indication of the approximate processing capacity of our ultrasonicators: