Formation of monolayer and few-layer hexagonal boron nitride nanosheets via surface segregation

We report that few-layer hexagonal boron nitride (h-BN) nanosheets can be produced by using a surface segregation method. The formation of h-BN sheets is via an intermediate boron-nitrogen buffer layer. Our results suggest that surface segregation of boron and nitrogen from a solid source is an alternative approach to tailoring synthesis of h-BN sheets for potential applications such as in graphene electronics.     

Scalable production of boron nitride nanosheets in ionic liquids by shear-assisted thermal treatment

Due to their intriguing properties, boron nitride nanosheets (BNNSs) with large lateral size and high crystallinity have great promise for many applications. However, the quantitative exfoliation of hexagonal boron nitride (h-BN) into good quality BNNSs still remains a key challenge. Herein, we report a scalable method to exfoliate BNNSs in ionic liquids (ILs) via shear-assisted thermal treatment. Few-layer BNNSs with well-preserved structural integrity are successfully prepared by this method. The synergistic effects of strong physical adsorption and intercalation of IL molecules, chemical interactions between hydrogen fluoride (HF) and h-BN, activation energy provided by heat treatment, and shear forces generated by repetitive stirring effect contribute to the exfoliation of BNNSs.     

Novel Poly(m‐phenyleneisophthalamide) Dielectric Composites with Enhanced Thermal Conductivity and Breakdown Strength Utilizing Functionalized Boron Nitride Nanosheets

Dielectric polymer‐based composites with high breakdown strengths and thermal conductivities have attracted considerable attention when applied in electronic devices. In this study, a novel poly(m‐phenyleneisophthalamide) (PMIA) dielectric nanocomposite is successfully fabricated by introducing functionalized hexagonal boron nitride nanosheet (fBNNS) fillers. Due to effective functionalization of hexagonal boron nitride nanosheets (BNNSs), fBNNSs fillers are homogeneously dispersed in the PMIA matrix. The breakdown strength and thermal conductivity of PMIA/fBNNSs dielectric nanocomposite are investigated. Research results indicate that the breakdown strength of fBNNSs‐12 reaches 105.6 MV m^?1, which is 1.34 times that of pure PMIA. Moreover, owing to high thermal conductivity of fBNNSs, the thermal conductivities of fBNNSs‐12 are observably increased to 8.06 W m^?1 K of in‐plane direction and 0.84 W m^?1 K of through‐plane direction, respectively. Considering these properties, the manufactured PMIA/fBNNSs dielectric nanocomposites show potential applications in field of electronics.     

Large-scale synthesis and growth mechanism of boron nitride nanocomposite assembled by nanosheets and nanotubes

Boron nitride nanocomposites assembled by nanosheets and nanotubes can exert multi-dimensional synergistic toughening and strengthening effects. This material is expected to be a high-efficiency reinforcement additive in advanced structural ceramics. In this study, we designed a universal method for synthesizing gram-scale boron nitride nanocomposites by annealing the precursor containing catalyst in chemical vapor deposition equipment under flowing ammonia, and a combined growth mechanism of surface-diffusion and solid-liquid-solid is proposed. The boron nitride nanosheets were initially formed by a surface-diffusion reaction between boron trioxide and ammonia at 1300°C. At elevated temperatures (1400°C-1500°C), the boron nitride nanotubes grew in-situ from the nanosheets in the presence of catalysts through a solid-liquid-solid mechanism, forming the desired boron nitride nanocomposite.     

One-step environmentally friendly exfoliation and functionalization of hexagonal boron nitride by β-cyclodextrin-assisted ball milling

Hexagonal boron nitride nanosheets (BNNs) are gaining much attention owing to their extraordinary performance; however, difficulties remain in achieving their large-scale preparation and dispersion at high concentration in solvents. A facile yet efficient β-cyclodextrin-assisted mechanochemical exfoliation method is proposed to exfoliate and functionalize hexagonal boron nitride (h-BN). The β-cyclodextrin-assisted ball milling process gave a high yield of 60%, with the resultant BNNs being covalently grafted with hydroxyl, and well dispersed in water and other solvents. The concentration of the hydroxyl-functionalized boron nitride nanosheets (OH-BNNs) reached 3 mg/mL, and was stable for several weeks. Next, we fabricated a membrane based on the resulting OH-BNNs to effectively capture Congo red dye. This work not only offers an environmentally friendly, efficient, solvent-less and extremely cost-effective scalable production method, but also assists in our understanding of the mechanism of exfoliation and functionalization in other layered materials.     

Role of the particle size of graphite-like boron nitride in nucleation of cubic boron nitride

The influence of the reduced radius of grains of the graphite-like hexagonal boron nitride (h-BN) on the nucleation of the cubic boron nitride (c-BN) during synthesis from an initiator solution at a high pressure is analyzed. The colloidal mechanism of nucleation is confirmed experimentally. It is shown that there is a correlation between the nucleus sizes of the hexagonal boron nitride and the pressure of the onset of nucleation of the cubic boron nitride. The effect of these sizes of the hexagonal boron nitride on the concentration of crystal nuclei of the cubic boron nitride is studied. The kinetic nucleation curves are obtained. It is demonstrated that the concentration of crystallization centers depends on the thermodynamic and kinetic parameters, as well as on the particle size of the graphite-like hexagonal boron nitride. Original Russian Text ? S.P. Bogdanov, 2008, published in Fizika i Khimiya Stekla.     

Fabricating boron nitride nanosheets from hexagonal BN in water solution by a combined sonication and thermal-assisted hydrolysis method

Boron nitride nanosheets (BNNSs) have unique and excellent thermal, electrical, and mechanical properties. But, their low efficiency in the production methods restricts their applications in the study of researchers. Hence, we reported a study in which BNNSs were successfully produced through a simple, affordable, and high-efficiency method. In this approach, hexagonal boron nitride block (h-BN) was firstly examined in aqueous solution under sonication (120 min) to produce hydroxyl functionalization and; furthermore, to penetrate water molecules between the layers. Then, with the explosion of the water molecules into the h-BN layers in the presence of heat, the space between the layers was increased. Finally, with the exfoliation of bulk h-BN layers through the ultrasonic irradiation, the ultrathin BNNSs were produced with an efficiency of 37%. The TEM and AFM images confirmed that the obtained BNNSs have mainly consisted of nanosheets with a thickness in the range of 3–8 nm. The results of UV–Vis analysis of BNNSs compared to h-BN showed a strong absorption peak at 204 nm^?1, which confirmed the presence of nanosheets. Also, other analyses, including XRD, BET, and FT-IR, confirmed the structure of BNNSs.     

Alkylated modified boron nitride nanosheets/polyimide composite films with advanced thermal conductivity and low dielectric constant

Owing to the rapid development of the latest micro-electronic devices, polymer composite materials that combine high thermal conductivity and low permittivity have aroused the interest of researchers. However, it is a huge challenge to balance the above parameters. In this work, hexagonal boron nitride (h-BN) powder was ultrasonically exfoliated to obtain alkylated boron nitride nanosheets (Alkyl-BNNS). Then, a series of polyimide (PI) composites were synthesized with different amounts of Alkyl-BNNS. Attributed to more robust interfacial non-covalent interactions between Alkyl-BNNS and polymer chains to inhibit interfacial polarization, Alkyl-BNNS can be scattered well in PI substrate. Thus, the obtained PI composite behaved a high thermal conductivity of 6.21 W/(mK) and a low dielectric constant (3.23) under the load of 20 wt%. Besides, Alkyl-BNNS/PI composites have efficient thermal management capability, low water absorption, favorable electrical resistance, and prominent tensile strength. Importantly, these composite films are expected to be excellent candidates in the field of microelectronics.     

Properties and applications of some structural refractory ceramic materials based on nonmetal nitrides

New composite ceramic materials developed at the Central Materials Research Institute, including nitride-oxide composites based on hexagonal boron nitride and nitride composites based on silicon nitride are described. Some important technological aspects of the production of articles and parts from these materials are considered. Data on the properties and main applications of the composites are reported. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 3, pp. 34–35, March, 1997.     

Green and efficient production of boron nitride nanosheets via oxygen doping-facilitated liquid exfoliation

As the structural analogue of graphene, boron nitride nanosheets (BNNSs) are anticipated to have a wide range of potential applications. BNNSs exhibit good mechanical properties, outstanding thermal conductivity, oxidation and chemical stability and are excellent electrical insulators. While BNNSs have gained recognition as one of the most versatile 2D materials in recent years, their application in research and industry is still hampered by the lack of methods to produce BNNSs in large quantity and a cost-effective way. In this study, we report highly efficient h-BN exfoliation via the oxygen doping-facilitated liquid exfoliation. Oxygen atoms are introduced into the hexagonal boron nitride (h-BN) structure via a facile thermal treatment. The relationship of thermal treatment, structural changes and h-BN exfoliation are studied to elucidate the key factor for advancing the BNNS production. The optimum concentration of hydroxyl groups and weakening of interlayer interactions have synergistically facilitated the delamination of h-BN in water under mild exfoliation conditions, resulting in up to 1255% yield increment and without noticeable new defects in the BNNS structure as compared with the untreated control. An efficient and environmentally friendly exfoliation process of h-BN is a crucial starting point towards the cost-effective and mass production of BNNSs which is needed for the currently identified and myriad future applications of BNNSs.     

BN analogues of graphene: On the formation mechanism of boronitrene layers — solids with extreme structural anisotropy

Boron nitride is an extremely useful material for applications in material sciences and appears in a manifold of crystalline modifications, with hexagonal and cubic boron nitride as most prominent substances. In hexagonal boron nitride, threefold coordinated boron and nitrogen atoms form two-dimensional layers, which resemble the boron nitride analogue of graphene layers, and we refer to this two-dimensional network as boronitrene layers. So far, there is little knowledge about the elementary growth reactions of these boronitrene layers.Here we show that it is possible to regenerate a previously prepared 14 × 14 BN/13 × 13 Rh(111) superstructure [obtained by CVD of borazine (B₃N₃H₆)] after an oxidation step from an unordered monolayer of a boron–oxygen compound by chemical reactions induced by ammonia as nitrification agent on the surface. The individual steps of the experiment were controlled by a LEED- and XPS- analysis and indicate that the original BN superstructure can be recovered without traceable amounts of oxygen in the film. The presented results indicate that highly mobile species from the B–N–O–H system can be considered as surfactants in the elementary formation and degradation reactions of highly ordered boronitrene layers. An understanding of these reactions is a fundamental issue in tuning the crystal growth and quality of the BN phases.     

Solution-processed white graphene-reinforced ferroelectric polymer nanocomposites with improved thermal conductivity and dielectric properties for electronic encapsulation

The recent surge in graphene research has stimulated interest in the investigation of various two-dimensional (2D) nanomaterials, including 2D boron nitride (BN) nanostructures. Among these, hexagonal boron nitride nanosheets (h-BNNs; also known as white graphene, as their structure is similar to that of graphene) have emerged as potential nanofillers for preparing thermally conductive composites. In this work, hexagonal boron nitride nanoparticles (h-BNNPs) approximately 70 nm in size were incorporated into a polyvinylidene fluoride (PVDF) matrix with different loadings (0–25 wt.%). The PVDF/h-BNNP nanocomposites were prepared by a solution blending technique and characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), polarized optical microscopy (POM) and scanning electron microscopy (SEM). In addition, the thermal conductivity and dielectric properties of the nanocomposites were investigated. The incorporation of h-BNNPs in the PVDF matrix resulted in enhanced thermal conductivity. The highest value, obtained at 25 wt.% h-BNNP loading, was 2.33 W/mK, which was five times that of the neat PVDF (0.41 W/mK). The thermal enhancement factor (TEF) at 5 wt.% h-BNNP loading was 78%, increasing to 468% at 25 wt.% h-BNNP loading. The maximum dielectric constant of approximately 36.37 (50Hz, 150 °C) was obtained at 25 wt.% h-BNNP loading, which was three times that of neat PVDF (11.94) at the same frequency and temperature. The aforementioned results suggest that these multifunctional and high-performance nanocomposites hold great promise for application in electronic encapsulation.     

氮化硼纳米片改性复合材料导热性能的研究进展

氮化硼纳米片（BNNSs）是一种二维片状纳米材料，具有较高的导热性和热稳定性。将其作为填料加入聚合物中，可显著提高复合材料的导热性能。本文基于近年来对BNNSs改性复合材料的导热性能的研究进展，总结了BNNSs制备和改性的方法以及建立导热路径的方法，介绍了该体系复合材料的导热机理，分析了影响复合材料导热性能的因素，最后对提高复合材料的导热性能进行了展望。     

Electrical Resistivity and Microwave Transmission of Hexagonal Boron Nitride

The dc conductivity of hexagonal boron nitride (BN) and BN-containing composites was measured as a function of temperature up to 2400°C. The results confirm that at high temperatures BN is an intrinsic semiconductor with an energy gap of 0.99 ± 0.06 aJ (6.2 ± 0.4 eV) at T = 0 K. Extrapolated values for the resistivity of BN in the range 2600° to 3000°C are used to analyze the absorption, reflectivity, and transmissivity of a BN window when subjected to microwave radiation under atmospheric reentry conditions. It appears that the transmissivity is of the order of 1 to 10% at these temperatures due mainly to the high conductivity in a very thin, very hot surface layer. The transmissivity can be improved by using a composite made of boron nitride and silica.     

氮化硼纳米管的合成与表征

以氧化铁和无定型硼粉为原料,反应气氛为碳气氛,在1 400℃下利用化学气相沉积法（CVD）制备出氮化硼纳米管。X射线研究表明,对应着六方氮化硼晶面的特征衍射峰非常清晰。采用扫描电子显微镜（SEM）对样品的结构与形貌进行表征,结果表明,样品属于一端开口的竹节状BN纳米管。     

Construction and flame-retardant performance of layer-by-layer assembled hexagonal boron nitride coatings on flexible polyurethane foams

We first prepared a novel hexagonal boron nitride (h-BN) nanosheet filled flame-retardant coating on the surface of flexible polyurethane foams (FPUFs) by a layer-by-layer assembly method to improve the fire safety of the FPUFs. The microstructures and compositions of the coatings could be well tailored by the alteration of the concentrations of the h-BN dispersions; this resulted in obvious improvements in the thermal stability and flame-retardant properties for the FPUFs under optimal conditions. Particularly, cone calorimetry testing confirmed that the FPUFs coated with a proper amount of h-BN nanosheets effectively reduced the peak of the heat-release rate (50.1%) and CO production (53.8%) of the FPUFs, which demonstrated excellent flame-retardant performance. The excellent flame-retardant properties were attributed to the physical barrier and tortuous-path effects and anisotropic thermal conductivity of the h-BN nanosheets; these retarded the transfer and diffusion of heat toward the polymer matrix. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47839.     

A simple technique to synthesise vertically aligned boron nitride nanosheets at 1200°C

In order to make the synthesis of boron nitride nanosheets (BNNS) easier and safe, a very simple technique is introduced in the present study. In this technique BNNS are synthesised in a conventional horizontal dual zone quartz tube furnace at 1200°C. Field emission scanning electron microscopy image shows the morphology of synthesised BNNS like spread out cotton packs. Transmission electron microscopy (TEM) images show highly crystalline nature of synthesised nanosheets of boron nitride with an interlayer spacing of 0.34 nm. Raman spectrum shows a major peak at 1366 (cm^?1) that corresponds to E_2g mode of h-BN. X-ray photon spectroscopy survey shows B 1s (at 191 eV) and N 1s (at 398 eV) peaks that verify the boron and nitrogen contents in synthesised nanosheets.     

Formation of cBN nanocrystals by He+ implantation into hBN

The structural modifications of polycrystalline hexagonal boron nitride implanted with He⁺ beams at energies between 200 keV and 1.2 MeV to fluences of 1.0 × 10¹⁷ ions cm^? 2 were investigated using micro-Raman spectroscopy. The measured Raman spectra show evidence of implantation-induced structural transformations from the hexagonal phase to nanocrystalline cubic boron nitride, rhombohedral boron nitride and amorphous boron nitride phases. The first-order Longitudinal-Optical cubic boron nitride phonon was observed to be downshifted and asymmetrically broadened and this was explained using the spatial correlation model coupled with the high ion implantation-induced defect density.     

Synthesis of Aluminum Nitride/Boron Nitride Composite Materials

Aluminum nitride/boron nitride composite was synthesized by using boric acid, urea, and aluminum chloride (or aluminum lactate) as the starting compounds. The starting materials were dissolved in water and mixed homogeneously. Ammonolysis of this aqueous solution resulted in the formation of a precomposite gel, which converted into the aluminum nitride/boron nitride composite on further heat treatment. Characterization of both the precomposite and the composite powders included powder X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. Analysis of the composite revealed that the aluminum nitride phase had a hexagonal structure, and the boron nitride phase a turbostratic structures.     

Combustion synthesis of hexagonal boron nitride nanoplates with high aspect ratio

High crystalline hexagonal boron nitride nanoplates with high aspect ratio of ~400 have been synthesized by combustion synthesis method through magnesiothermic reduction reaction between B₂O₃ and Mg in N₂ pressure. The synthesized hexagonal boron nitride nanoplates were about 50 nm in thickness and larger than 20 μm in lateral size. The six-fold symmetric spots electron diffraction pattern of transmission electron microscopy shows that the nanoplate is well crystallized. Hexagonal boron nitride nanoplates grow via an Oswald ripening process and have larger lateral size when it was prepared with larger magnesium particles. High temperature liquid magnesium provides an important environment for the growth and crystallization of boron nitride. This work provides an effective way to achieve low-cost and large-scale preparation of high-quality boron nitride nanoplates.