Exfoliation Of Hexagonal Boron Nitride Research Articles

Salting-Out Effect Protected Ball-Milling Exfoliation of Hexagonal Boron Nitride and the Scale Laws on the Thermal Conductivities of the Exfoliated Nanosheet Assembled Composite Films

Salting-Out Effect Protected Ball-Milling Exfoliation of Hexagonal Boron Nitride and the Scale Laws on the Thermal Conductivities of the Exfoliated Nanosheet Assembled Composite Films

Phonon Polaritons Launched by Natural Boron Nitride Wrinkles Probed with Nano‐Fourier Transform Infrared Spectroscopy

Nano‐fourier transform infrared spectroscopy (FTIR) is a powerful tool to measure optical and electronic properties of materials at the nanoscale. It is especially useful for visualizing plasmon and phonon polaritons launched from edges of a sample or structures on top of it. Herein, an exfoliated hexagonal boron nitride flake with a thickness of ≈16 nm is transferred onto a gold substrate. The flake is characterized by micro‐Raman and nano‐FTIR spectroscopy. The Raman spectra show no difference between points on the flat surface and points on the wrinkles of the flake. Nano‐FTIR spectra, while comparable to conventional infrared spectra on the flat surface, show a strong change in the form of a second absorption peak appearing near a wrinkle in the flake. This second absorption peak shifts to higher wavenumber and becomes more intense as the probed spot gets closer to the wrinkle. This is consistent with the behavior of phonon polaritons when approaching the scattering point that is reflecting them.

SPI-Modified h-BN Nanosheets-Based Thermal Interface Materials for Thermal Management Applications.

The rising concern over the usage of electronic devices and the operating environment requires efficient thermal interface materials (TIMs) to take away the excess heat generated from hotspots. TIMs are crucial in dissipating undesired heat by transferring energy from the source to the heat sink. Silicone oil (SO)-based composites are the most used TIMs due to their strong bonding and oxidation resistance. However, thermal grease performance is unreliable due to aging effects, toxic chemicals, and a higher percentage of fillers. In this work, TIMs are prepared using exfoliated hexagonal boron nitride nanosheets (h-BNNS) as a nanofiller, and they were functionalized by ecofriendly natural biopolymer soy protein isolate (SPI). The exfoliated h-BNNS has an average lateral size of ∼266 nm. The functionalized h-BNNS/SPI are used as fillers in the SO matrix, and composites are prepared using solution mixing. Hydrogen bonding is present between the organic chain/oxygen in silicone polymer, and the functionalized h-BNNS are evident from the FTIR measurements. The thermal conductivity of h-BNNS/SPI/SO was measured using the modified transient plane source (MTPS) method. At room temperature, the maximum thermal conductivity is 1.162 Wm-1K-1 (833% enhancement) at 50 wt % of 3:1 ratio of h-BNNS:SPI, and the thermal resistance (TR) of the composite is 5.249 × 106 K/W which is calculated using the Foygel nonlinear model. The heat management application was demonstrated by applying TIM on a 10 W LED bulb. It was found that during heating, the 50 wt % TIM decreases the surface temperature of LED by ∼6 °C compared with the pure SO-based TIM after 10 min of ON condition. During cooling, the modified TIM reduces the surface temperature by ∼8 °C under OFF conditions within 1 min. The results indicate that natural polymers can effectively stabilize and link layered materials, enhancing the efficiency of TIMs for cooling electronics and LEDs.

Homoepitaxy of Boron Nitride on Exfoliated Hexagonal Boron Nitride Flakes.

Although large efforts have been made to improve the growth of hexagonal boron nitride (hBN) by heteroepitaxy, the non-native substrates remain a fundamental factor that limits the quality. This problem can be solved by homoepitaxy, which is the growth of hBN on hBN substrates. In this report, we demonstrate the homoepitaxial growth of triangular BN grains on exfoliated hBN flakes by Metal-Organic Vapor Phase Epitaxy and show by atomic force microscopy and photoluminescence that the stacking of these triangular islands can deviate from the AA' stacking of hBN. We show that the stacking order is enforced by the crystallographic direction of the edge of the exfoliated hBN flakes, with armchair edges allowing for centrosymmetric stacking, whereas zigzag edges lead to the growth of noncentrosymmetric BN polytypes. Our results indicate pathways to grow homoepitaxial BN with tunable layer stacking, which is required to induce piezoelectricity or ferroelectricity.

An in situ strategy to produce high-performance composite thermal adhesives with excellent bonding, thermal transfer and adjustable electrical performance based on a designer hyperbranched epoxy copolymer

Thermally conductive adhesives (TCAs) have attracted considerable attentions in recent years as they can simultaneously promote interfacial bonding and thermal transfer. So far, a lot of strategies have been explored for producing them. Among them, modifying adhesive resins with thermally conductive nanofillers is one of the most frequently reported methodologies, which often involves complicated process and tedious procedures. We herein report a facile strategy for producing TCAs via an in situ process with the assistance of a designer hyperbranched epoxy copolymer, HBPE@PGMA. This copolymer consists of a hyperbranched polyethylene (HBPE) core and multiple poly (glycidyl methacrylate) (PGMA) side chains, which can be synthesized under mild conditions by a two-step polymerization process. As a stabilizer, this copolymer is found to effectively promote the exfoliation of hexagonal boron nitride (h-BN), natural graphite or multi-walled carbon nanotubes (MWCNTs) in chloroform under sonication, rendering boron nitride nanosheets (BNNS), graphene and individually dispersed MWCNT, respectively. Meanwhile, some of the copolymer can be irreversibly adsorbed on the resultant nanofiller surface based on the noncovalent CH-π interactions. This makes them well dispersible in chloroform and allows us to obtain high-performance TCAs from their dispersions via in situ process directly using the copolymer as matrix. The resultant TCAs simultaneously exhibit excellent interfacial bonding, thermal transfer and adjustable electrical performance. This strategy paves a way for producing high-performance TCAs through in situ process with relatively simple process.

Electron beam-assisted exfoliation of boron nitride and covalent functionalization

A facile, scalable exfoliation technique is still required for further modification to cater to any potential application for two-dimensional boron nitride. This paper discusses the radiation-assisted exfoliation of hexagonal boron nitride (h-BN) using electron beam irradiation and followed by functionalization with phosphoric acid 2-hydroxyethyl methacrylate ester (PA) to facilitate targeted applications. h-BN is dispersed in NaOH solution and subjected to mild sonication to facilitate ion intercalation between the layers. The sonicated h-BN solution was exposed to electron beam irradiation for BNNS exfoliation. The degree of exfoliation was measured in terms of d-spacing derived from XRD using Bragg's law. The effect of irradiation dose, energy, NaOH concentration, and mass of h-BN, on exfoliation degree was investigated. XRD analysis was used to compute the d-spacing, crystallinity, and crystal size of h-BN after irradiation. The d-spacing attained for pristine h- BN was 0.334 nm, however it increases to 0.337 nm upon irradiation at 100 kGy doses, 3 MeV of energy, and 0.1 g of h-BN in 80 mL of 30 % NaOH solution. The occurrence of radiation-assisted exfoliation and its mechanism were supported by FTIR, TGA, Raman, FESEM, XPS, AFM, and TEM analyses. The exfoliated BN was functionalized with PA. The degree of functionalization was measured by phosphoric density (mmol/g) via the acid-base titration method. The effect of PA concentration, reaction temperature and mass on phosphoric density was also investigated. The highest phosphoric density attained was approximately 13.32 mmol/g at 10 % PA, 0.1 g of exfoliated BN, 80 °C, and 5 h. The incorporation of PA onto exfoliated BN was further confirmed by XRD, FTIR, TGA, Raman, FESEM-EDX, and XPS analyses. Conclusively, this study reveals a facile dual-step exfoliation and functionalization technique of h-BN nanosheet through radiation assisted exfoliation technique.

In-situ exfoliation of hexagonal boron nitride during the forced flow of highly elastic polylactide to fabricate electrospun fibrous film with high thermal conductivity and low dielectric loss

Boron nitride nanosheets (BNNS) have promising applications in thermal management materials, due to their high thermal conductivity and low dielectric loss. However, it is still challenging to fabricate BNNS with large lateral size and structural integrity via solvent-free methods. Here, we report a novel processing method for in-situ exfoliation of hexagonal boron nitride (h-BN) to fabricate BNNS with an average lateral size of 1.45 μm, average thickness of 4.88 nm and few lattice defects during the forced flow of polylactide (PLA) in a highly elastic state. We further fabricate flexible and bio-degradable PLA/BNNS electrospun fibrous film for electronic devices to replace petroleum-based composites. The PLA/BNNS fibrous film exhibits an in-plane thermal conductivity of 4.9 W/(m·K) at the BNNS loading of 20 wt%. Moreover, the fibrous film also has a low dielectric loss (0.007 at 1 kHz) and good electrical insulation. The in-situ exfoliation strategy is simple, economical and environmentally friendly, providing an effective route to design thermal management materials with high thermal conductivity and low dielectric loss in the future of electronic devices.

Controllable exfoliation of hexagonal boron nitride and tailored three‐dimensional network for highly thermally conductive polymer composites

AbstractThermally conductive but electrically insulating composites have become significant components for efficient thermal dissipation in electronic devices due to their miniaturization, integration, and functionalization. Over the past decade, extensive efforts have been dedicated to improving the thermal conductivity of polymer‐based composites through incorporating various thermally conductive fillers. In particular, boron nitride nanosheets (BNNS) with outstanding intrinsic thermal conductivity, electrical insulation, and chemical stability, have sparked intensive interest from researchers. However, BNNS usually suffers from severe aggregation in polymer matrices, leading to high interfacial thermal resistance. Rational construction of three‐dimensional (3D) BNNS continuous networks is crucial for efficient thermal transport and has also been established as a feasible and effective approach to achieve significant enhancement of thermal conductivity for polymer/BNNS composites. In this brief review, we summarized and introduced recent progress on the controllable exfoliation of h‐BN and tailored three‐dimensional networks for highly thermally conductive polymer composites. The fabrication strategies for 3D BNNS networks and the structure–properties relationships of polymer/BNNS composites are highlighted. Finally, the opportunities and challenges for polymer/BNNS composites based on 3D thermally conductive networks are prospected.

Liquid Exfoliation of Hexagonal Boron Nitride

Liquid Exfoliation of Hexagonal Boron Nitride

Polydopamine-coated hexagonal boron nitride-based electrochemical immunosensing of T-Tau as a marker of Alzheimer's disease

Alzheimer's disease (AD) is a complex pathological process that is one of the leading causes of dementia globally. The demand for diagnostic tools that are minimally invasive, timely, and accurate is on the rise. Total tau (T-Tau) protein in blood serum is a promising biomarker for predicting early-stage AD diagnosis. In this study, the hexagonal boron nitride (HBN) based immunosensor platform was developed to detect T-Tau in artificial blood serum. After the exfoliation of HBN, its surface was coated with polydopamine (PDA) in alkaline conditions. The Anti-T-Tau was immobilized on a hydrophilic nanocomposite surface using PDA's reactive catechol and quinone groups, eliminating the need for extra crosslinkers. The working electrode surface of the screen-printed carbon electrode (SPCE) was coated with HBN-PDA nanocomposite using the drop-casting method. The biofunctional surface was created by directly immobilizing Anti-T-Tau on the HBN-PDA nanocomposite-modified SPCE. The analytical performance of the HBN-PDA/Anti-T-Tau/T-Tau immunosensor in the presence of T-Tau isoforms was determined through electrochemical measurements. The linear detection range was 1–30 pg/mL with a detection limit of 0.42 pg/mL for T-Tau, which is suitable for detecting T-Tau in the blood serum.

High detectivity solar blind photodetector based on mechanical exfoliated hexagonal boron nitride films

As an ultra-wide bandgap semiconductor, hexagonal boron nitride (h-BN) has drawn great attention in solar-blind photodetection owing to its wide bandgap and high thermal conductivity. In this work, a metal-semiconductor-metal structural two-dimensional h-BN photodetector was fabricated by using mechanically exfoliated h-BN flakes. The device achieved an ultra-low dark current (16.4 fA), high rejection ratio (R 205nm/R 280nm = 235) and high detectivity up to 1.28 × 1011 Jones at room temperature. Moreover, due to the wide bandgap and high thermal conductivity, the h-BN photodetector showed good thermal stability up to 300 °C, which is hard to realize for common semiconductor materials. The high detectivity and thermal stability of h-BN photodetector in this work showed the potential applications of h-BN photodetectors working in solar-blind region at high temperature.

Chemical-free exfoliation of hexagonal boron nitride via cavitation-bubble plasma in water

Hexagonal boron nitride (h-BN) sheets have numerous applications, but current low-cost, environmentally friendly techniques for h-BN exfoliation yield undesirably small sheet sizes. We have developed a novel chemical-free method to prepare large h-BN nanosheets (BNNSs) by the exfoliation of h-BN using plasma with cavitation bubbles in water, named here as cavitation-bubble plasma. Unlike ordinary liquid exfoliation methods, which reduce the sheet size of BNNSs compared with that of the starting h-BN, the cavitation-bubble plasma process exfoliated h-BN without reducing the sheet size. Furthermore, the plasma dramatically improved the exfoliation yield.

Improved thermal conductivity and mechanical property of mercapto group-activated boron nitride/elastomer composites for thermal management

Ionic liquid 1-Ethyl-3-methylimidazolium chloride was employed to promote the exfoliation of hexagonal boron nitride (BN) to fabricate the boron nitride nanosheets (BNNSs). The BNNSs were then surface activated by covalently attaching mercapto groups on their surfaces. The surface-activated BNNS nanoparticles (S@BNNSs) were incorporated in a carboxylated styrene-butadiene rubber matrix for preparing thermal management materials. The developed material was provided with superior mechanical properties, satisfied thermal conductivity and strong storage modulus attributing to the two-dimensional structure of the filler and the enhanced interfacial interaction between the filler and matrix. The filler-filler and filler-matrix networks in the composite system were comprehensively analyzed. The thermal management applications were also demonstrated using an infrared camera and a commercial thermoelectric generator. All the performances demonstrated their potential use as thermal management materials in electronic devices.

Exfoliation of hexagonal boron nitride assisted with hierarchical ionic fragments by ball‐milling for achieving high thermally conductive polymer nanocomposite

AbstractHexagonal boron nitride nanosheets (BNNSs) are considered as ideal candidates for preparing thermally and insulating composite because of its excellent thermal and electrical properties. Herein, we propose a facile and efficient ball‐milling way to exfoliate hexagonal boron nitride (h‐BN) with the assistance of coordination polymers (CPs). During the exfoliation process, the CPs occurs crystal to liquid/glass transition, forming diverse molecular fragments. The hierarchical ionic fragments can interact with and intercalate into the layered h‐BN, leading to a high efficiency in exfoliation and good dispersion. The exfoliated BNNSs reach a yield of up to 34% approximately. Furthermore, the BNNSs were incorporated to poly(vinyl alcohol) to prepare composite with thermal conductivity reaching up to 11.5 W m−1 K−1 at 30 wt% of BNNSs loading. The composite exhibits good performance on LED thermal management. This work demonstrates an efficient way to exfoliate BN, which can shed some light on other two dimensional materials exfoliation and application.

Ball-Milling Exfoliation of Hexagonal Boron Nitride in Viscous Hydroxyethyl Cellulose for Producing Nanosheet Films as Thermal Interface Materials

Boron nitride nanosheets (BNNS) exfoliated by physical or chemical methods frequently exhibit poor dispersity in common solvents due to incomplete exfoliation and a low degree of functionalization,...

Preparation of functionalized boron nitride nanosheets by high-gravity liquid phase exfoliation technology

There was great interest in the exfoliation of hexagonal boron nitride to prepare functionalized boron nitride nanosheets (f-BNNSs) directly with high efficiency and ecofriendliness. In this study, a novel method has been demonstrated for the fabrication of two types of f-BNNSs by high-gravity liquid phase exfoliation technology. First, hydroxylated boron nitride nanosheets (OH-BNNSs) were prepared by high-gravity liquid phase exfoliation technology assisted by polyvinyl alcohol as a surfactant. The yield of OH-BNNSs was 1.4%. The average size of OH-BNNSs is approximately 390 nm, and the number of layers is approximately 7−8 layers. Second, The 4−5 layers of aminated and hydroxylated boron nitride nanosheets (NH2-OH-BNNSs) with an average size of approximately 136 nm were obtained by combining ammonium bicarbonate-assisted ball-milling pretreatment and high-gravity liquid phase exfoliation technology. The yield of NH2-OH-BNNSs was 8.5%. Both types of f-BNNSs can be stably dispersed in water. The similarities and differences in morphology and structure between them were confirmed by characterizations. Both preparation processes do not involve hazardous chemicals and harsh reaction conditions. Therefore, this work provides good ideas for simple and efficient preparation of f-BNNSs, which is conducive to their selective applications according to their characteristics.

Enhanced Electrochemical Performance of Hydrothermally Exfoliated Hexagonal Boron Nitride Nanosheets for Applications in Electrochemistry

Two-dimensional (2D) morphology of hexagonal boron nitride (h-BN), owing to its peculiar characteristics of non-toxicity and uniquely featured oxidation resistance has attracted extensive attention in electrochemical applications. Here, we report a facile top-down approach for the successful synthesis of hexagonal boron nitride nanosheets (h-BNNS) through a low-temperature hydrothermal method. The structural and spectroscopic characterizations have been performed using XRD, FT-IR, Raman and UV-Visible Spectroscopy that reveals incorporation of maximum induced strain, multifunctional groups and formation of few layers h-BNNS. The nanosheets morphology is confirmed by series of characterizations (SEM, TEM, and AFM) revealing the large lateral size and relatively low surface roughness of h-BNNS. The electrodes for electrochemical characterizations are prepared using the electrophoretic deposition (EPD) technique onto ITO substrates with the help of magnesium nitrate [Mg(NO3)2·6H2O] as a mediator. Electrochemical Studies has been performed in phosphate buffer saline (PBS) with Ferro-ferricyanide [Fe(CN)6]3−/4− as a redox couple. Cyclic voltammogram of h-BNNS (Ipa = 1.29 mA and Ipc = −1.23 mA) indicates 69.96% enhancement in redox peaks current and 71.04% rise in electro-active surface area with respect to Bulk h-BN. The electrochemical studies uncover the potential of h-BNNS in the development of electroanalytical devices in applications such as sensing, anticorrosion, energy conversion and energy storage applications owing to their enhanced redox peaks, large electroactive surface area, and reduced charge transfer resistance as well as the admittance of Warburg element.

Improved thermal conductivity of polypropylene filled with exfoliated hexagonal boron nitride (hBN) particles

In this work, we show the route to obtain thermoplastic based composites with an enhanced thermal conductivity that was achieved by using exfoliated hexagonal boron nitride (hBN) particles as a filler material. Reports on composites with bulk or nano hBN showed, that while increasing the filler load improves thermal properties it could aggravate the composite melt rheology and mechanical properties. On the other hand, exfoliated hBN particles compared to bulk or nanopowder hBN could provide even better thermal properties with no degradation in composite mechanics, which was shown for epoxy. Taking into account the above, we obtained exfoliated hBN particles by ultrasound treatment in isopropyl alcohol and then used them as a filler in polypropylene based composites to increase the thermal conductivity. The composites with 29% wt. of exfoliated hBN showed a thermal conductivity of 0.721 W m−1 K−1 which is 4 times higher than for pristine polypropylene.

Exfoliation of hexagonal boron nitride nanosheets in low-modulus concentrated alkali silicate pastes

We herein demonstrated the exfoliation of hexagonal boron nitride (h-BN) into nanosheets (BNNSs) by using concentrated low-modulus water glass (WG, SiO2/Na2O ratio = 1.6) as high viscosity exfoliating medium. The process led to BNNSs yield of nearly 30%. As the sheared suspension was neutralized with HCl, BNNSs with lateral size of approximately 250 nm and narrow size distribution, were found to align into perfectly parallel rows. High-resolution transmission electron microscopy (HR-TEM) analysis confirmed how WG preserved the integrity of the exfoliated BNNS in comparison with an analogous route with sole hydroxide, resulting in a high amount of BNNSs with undamaged structure.

Printable hexagonal boron nitride ionogels.

Due to its excellent chemical/thermal stability and mechanical robustness, hexagonal boron nitride (hBN) is a promising solid matrix material for ionogels. While bulk hBN ionogels have been employed in macroscopic applications such as lithium-ion batteries, hBN ionogel inks that are compatible with high-resolution printing have not yet been realized. Here, we describe aerosol jet-printable ionogels using exfoliated hBN nanoplatelets as the solid matrix. The hBN nanoplatelets are produced from bulk hBN powders by liquid-phase exfoliation, allowing printable hBN ionogel inks to be formulated following the addition of an imidazolium ionic liquid and ethyl lactate. The resulting inks are reliably printed with variable patterns and controllable thicknesses by aerosol jet printing, resulting in hBN ionogels that possess high room-temperature ionic conductivities and storage moduli of >3 mS cm-1 and >1 MPa, respectively. By integrating the hBN ionogel with printed semiconductors and electrical contacts, fully-printed thin-film transistors with operating voltages below 1 V are demonstrated on polyimide films. These devices exhibit desirable electrical performance and robust mechanical tolerance against repeated bending cycles, thus confirming the suitability of hBN ionogels for printed and flexible electronics.