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.     

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.     

Sustainable preparation of graphene-analogue boron nitride by ball-milling for adsorption of organic pollutants

The method of fabricating low-cost adsorbents with high activity and durability via a convenient and eco-friendly procedure is of great importance to wastewater treatment. Herein, a high-efficient mechanical exfoliation strategy was proposed to facilely prepare few-layered graphene-analogue boron nitride (BN) via a one-step non-organic solvent assisted wet ball mill procedure. Ball-milling treatment increased the specific surface area of BN 3.5-fold by reducing the thickness to ~3 layers with 45 min. The exfoliated BN exhibited strikingly improved sorption performance to organic contaminants with around 124% and 116% increased removal efficiency respectively for oxytetracycline (OTC) and Rhodamine B (RhB) as compared to the bulk BN. Batches sorption experiments showed that the sorption processes were thermodynamic endothermic, and well fitted to pseudo-second-order kinetic model and Freundlich isotherm equation. The π-π stacking interaction, hydrophobic effect and electrostatic interaction were proposed as the dominated sorption mechanism. In addition, no significant decline in adsorptive removal ability for the sorbent after 5 times recycling. The results indicate that the ball-milling exfoliation is a fast, green, sustainable and promising strategy for synthesis of highly potent BN based two-dimensional layered adsorbents.     

Fabrication and mechanical properties of boron nitride nanotube reinforced silicon nitride ceramics

In this study, silicon nitride (Si₃N₄) ceramics added with and without boron nitride nanotubes (BNNTs) were fabricated by hot-pressing method. The influence of sintering temperature and BNNTs content on the microstructures and mechanical properties of Si₃N₄ ceramics were investigated. It was found that both flexural strength and fracture toughness of Si₃N₄ were improved when sintering temperature increases. Moreover, α-Si₃N₄ phase could transform into β-Si₃N₄ phase completely when sintering temperature rises to 1800 °C and above. BNNTs can enhance the fracture toughness of Si₃N₄ dramatically, which increases from 7.2 MPa m^1/2 (no BNNTs) to 10.4 MPa m^1/2 (0.8 wt% BNNTs). However, excessive addition of BNNTs would reduce the fracture toughness of Si₃N₄. Meanwhile, the flexural strength and relative density of Si₃N₄ decreased slightly when BNNTs were added. The related toughening mechanism was also discussed.     

Influence of boron nitride nanotubes on the damage evolution of SiCf/SiC composites

As-grown and BN-coated boron nitride nanotubes (BNNTs) were incorporated into SiC_f/SiC composites to produce nanotube-based hierarchical composites. In-depth studies on damage evolution reveal that early damage development are delayed owing to the restriction effects on crack propagations from as-grown and BN-coated BNNTs. Moreover, this delay effect is more pronounced from BN-coated BNNTs because BN-coated BNNTs/matrix interfacial bonding strength is low. Final failure of composites with as-grown BNNTs still comes much earlier compared with virgin composite due to strong fibers/matrix bonding enhanced by as-grown BNNTs. This premature final failure is remedied in large part in composites with BN-coated BNNTs because fibers/matrix bonding enhanced by as-grown BNNTs is weaken after the deposition of an interphase on nanotube surface. Additionally, the type, the number and the released energy level of damage mechanisms during the whole damage evolution after the incorporation of as-grown and BN-coated BNNTs were also discussed elaborately compared with virgin composite.     

Enhancing mechanical properties of fused silica composites by introducing well-dispersed boron nitride nanosheets

Well-dispersed boron nitride nanosheets (BNNSs) reinforced fused silica composites were successfully fabricated by surface modification assisted flocculation method. Surface modification can enhance the performance of flocculation process. BNNSs were homogeneously mixed with fused silica through the electrostatic interaction between hydroxylated BNNSs with negative charge and amino-modified fused silica with positive charge. The BNNSs can act as excellent nanofillers for enhancing the mechanical properties of fused silica composites. Approximately 74% and 48% increases in flexure strength and fracture toughness can be achieved for the 1.5 wt% BNNSs/fused silica composite, respectively. The toughening mechanisms were analyzed by microstructural characterization, especially for pull-out mechanism.     

Mechanical and dielectric properties of functionalized boron nitride nanosheets/silicon nitride composites

Uniformly dispersed boron nitride nanosheets (BNNSs) reinforced silicon nitride (Si₃N₄) composites were prepared by surface modification assisted flocculation combined with SPS sintering. In order to improve the dispersibility of the BNNSs in the composites, the liquid phase stripped BNNSs are surface functionalized by a two-step covalently modification. The amino-modified BNNSs (NH₂-BNNSs) and Si₃N₄ powders have opposite surface potential, mixed evenly by electrostatic interaction during flocculation. The results showed that mechanical properties of Si₃N₄ composites were obviously enhanced by adding NH₂-BNNSs. The fracture toughness and bending strength of Si₃N₄ composites added 0.75 wt% NH₂-BNNSs were increased by 34% and 28%, respectively, compared with monolithic Si₃N₄. Toughening mechanisms are synergistic action of the torn, pull-out or bridging of BNNSs and crack deflection mechanisms with microstructural analyzes. The dielectric properties of the Si₃N₄ ceramics are also improved after the addition of NH₂-BNNSs.     

Surface-diffusion mechanism for synthesis of substrate-free and catalyst-free boron nitride nanosheets

This study presents an innovative surface-diffusion mechanism for the growth of substrate-free and catalyst-free boron nitride nanosheets (BNNSs) by annealing an ammonium borate hydroxide hydrate precursor in a NH₃ chemical vapor deposition system. At elevated temperatures, part of NH₄B₅O₈ slowly decomposes and forms B₂O₃, and flowing NH₃ gradually diffuses to the B₂O₃ surface to form vertically aligned BNNSs. The lateral dimension and crystallinity of the BNNSs increase, while their thickness decreases with the continuous surface-diffusion reaction. The residual NH₄B₅O₈ and B₂O₃ absorbs moisture in the air to constitute NH₄B₅O₈·4H₂O and H₃BO₃ substrates. With increasing annealing temperature and soaking time (at 1400 °C for 7 h), all NH₄B₅O₈ decomposes and the surface-diffusion reaction between B₂O₃ and NH₃ completely occurs, forming substrate-free BNNSs with a lateral dimension of 1 μm and a thickness of 10 nm. This reliable approach for synthesizing BNNSs paves the way for future applications in advanced ceramic composites.     

Hexagonal boron nitride nanosheets exfoliated by sodium hypochlorite ball mill and their potential application in catalysis

The unique physicochemical properties of two-dimensional (2D) h-BN and its promising applications in future optoelectronics have motivated an extensive study of its properties. However, a major limiting factor is its high quality and scalable preparation of few-layer h-BN. Herein, a facile, low cost, and high yield process is developed by using a sodium hypochlorite aqueous solution-assisted ball milling exfoliation process. The facile process results in scalable production of few-layer (2–4 sheets) h-BN from commercial BN powders, with little damage of its in-plane structure and high yield amounting to 21%. Furthermore, few-layer h-BN has been demonstrated to be good carrier to support and disperse Ag nanoparticles with high catalytic activity for the reduction of p-nitrophenol to p-aminophenol with NaBH₄. The pseudo-first-order reaction rate constant of the pre-prepared catalyst was calculated to be 7.13×10⁻³ s⁻¹, larger than that of pristine BN supported Ag nanoparticles. The results indicate that stable exfoliation process could open the way to a range of important applications of h-BN based materials.     

Characterization of boron nitride nanosheets synthesized by boron-ammonia reaction

Boron nitride nanosheets (BNNS) with thickness 5–11 nm were successfully produced when pure boron powder (1–2 μm) interacted with ammonia gas in chemical vapour deposition set up. Under the optimized parameters, at 1200 °C and for uninterrupted 1 h of reaction duration, 2D BNNS with thickness of ca.11 nm were synthesized. BNNS were characterized by X-ray diffraction (XRD) for crystal structure, scanning electron microscopy for dimensions and morphology, energy dispersive X-ray analysis for chemical composition and Fourier transform infrared spectroscopy for sp² BN bond detection. The thickness of BNNS determined from both XRD data (using Scherrer equation) and atomic force microscopic analysis confirmed the stated product thickness. The BNNS obtained at 1200 °C had high crystallinity, purity and yield.     

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.     

Silicon nitride/boron nitride ceramic composites fabricated by reactive pressureless sintering

Using Si and BN powders as raw materials, silicon nitride/hexagonal boron nitride (Si₃N₄/BN) ceramic composites were fabricated at a relatively low temperature of 1450 °C by using the reaction bonding technology. The density and the nitridation rate, as well as the dimensional changes of the specimens before and after nitridation were discussed based on weight and dimension measurements. Phase analysis by X-ray diffraction (XRD) indicated that BN could promote the nitridation process of silicon powder. Morphologies of the fracture surfaces observed by scanning electron microscopy (SEM) revealed the fracture mode for Si₃N₄/BN ceramic composites to be intergranular. The flexural strength and Young's modulus decreased with the increasing BN content. The reaction-bonded Si₃N₄/BN ceramic composites showed better machinability compared with RBSN ceramics without BN addition.     

Few-layer boron nitride nanosheets: Preparation,characterization and application in epoxy resin

Large-scale exfoliated boron nitride nanosheets were achieved via a liquid exfoliation sonication method by using sodium fluoride and ammonium hydroxide as Lewis base compounds. The crystal structure, surface functional groups, morphology and thickness of the as-prepared samples were characterized by X-ray diffraction, fourier transform infrared spectroscopy, transmission electron microscopy and atomic force microscopy, respectively. The as-prepared samples were introduced into epoxy resin to fabricate the polymer-based composites. Experimental results showed that the layer thickness of the as-prepared nanosheets was in the range of 1 to 3 layers. Moreover, it could improve the tensile properties of the matrix. When the loading of the as-prepared nanoparticles was 0.4 wt%, the tensile strength and elongation at break of the composites reached to their maximum values 65.6 MPa and 25.9%, which were increased by 118% and 192% more than that of pure resin. In addition, the as-prepared boron nitride samples could improve the thermostability and promote the curing of the matrix.     

Synthesis of boron nitride nanotubes and its hydrogen uptake

Boron nitride nanotubes (BNNT) were synthesized by annealing ball-milled boron-nickel catalyst in nitrogen/hydrogen gas mixture at a temperature of 1025 °C. The BNNTs were characterized by TEM, XRD, FTIR and Raman spectroscopy. The pressure-composition isotherms (PCI) and temperature-programmed desorption (TPD) techniques were used to investigate the H₂ uptake properties of BN nanotubes. The BNNTs show a reproducible room temperature hydrogen sorption of 1.8–2.2 wt% under 6 MPa.     

Microstructure and mechanical properties of boron nitride nanosheets-reinforced fused silica composites

In order to overcome intrinsic brittleness and poor mechanical properties of fused silica (FS), boron nitride nanosheets (BNNSs) as a novel reinforcement were employed for fabrication of BNNSs/fused silica composites. BNNSs with micron lateral size were homogeneously dispersed with FS powder using a surfactant-free flocculation method and then consolidated by hot pressing. The flexural strength and fracture toughness of the composite with the addition of only 0.5 wt.% BNNSs increased by 53% and 32%, respectively, compared with those of pure FS. However, for higher BNNSs contents the improvement in mechanical properties was limited. Microstructural analyzes have shown that the toughening mechanisms are combinations of the pull-out, crack bridging, and crack deflection mechanisms.     

Template-directed synthesis of boron nitride nanotube arrays by microwave plasma chemical reaction

Highly-ordered boron nitride (BN) nanotube arrays have been synthesized by microwave plasma-enhanced chemical vapor deposition (MW-PECVD) below 520 °C under the confinement of anodic aluminum oxide (AAO) template with borane/argon and ammonia/nitrogen as precursors. The low growth temperature and aligned arrangement of the BN nanotubes are beneficial to practical applications despite of the amorphous nature of the product. Novel morphology of Y-branching and dendriform BN nanotubes were also observed when the branching AAO template was used.     

Influence of hexagonal boron nitride nanosheets on phase transformation,microstructure evolution and mechanical properties of Si3N4 ceramics

Fully dense Si₃N₄ materials with 1 wt.% (～ 1.5 vol.%) and 2 wt.% (～ 3.0 vol.%) h-BN nanosheets were prepared by spark plasma sintering at 1750 °C with the dwell of 7 min under a pressure of 50 MPa in a vacuum. BN nanosheets with different dimensions were prepared by ultrasound-assisted liquid phase exfoliation of h-BN powder, followed by centrifugation at different speeds (1000 rpm and 3000 rpm). The addition of BN nanosheets hindered the particle rearrangement stage of sintering, which resulted in the delayed α→β phase transformation of Si₃N₄. To study a direct effect of BN nanosheets on the mechanical properties of Si₃N₄, the results were compared to the monolithic Si₃N₄ with similar grain size and α/β-Si₃N₄ ratio. The addition of 2 wt.% h-BN nanosheets (1000 rpm) increased both the fracture toughness (～ 26 %) and the strength (～ 45 %) of Si₃N₄, when compared to the monolithic Si₃N₄ with similar microstructure.     

Large-scale fabrication of boron nitride nanotubes with high purity via solid-state reaction method

An effective solid-state reaction method is reported for synthesizing boron nitride nanotubes (BNNTs) in large scale and with high purity by annealing amorphous boron powder and ferric chloride (FeCl₃) catalyst in ammonia atmosphere at elevated temperatures. FeCl₃ that has rarely been utilized before is introduced not only as a catalyst but also as an efficient transforming agent which converts boron powder into boron chloride (BCl₃) vapor in situ. The nanotubes are bamboo in shape and have an average diameter of about 90 nm. The effect of synthetic temperatures on nanotube morphology and yield is investigated. The photoluminescence (PL) measurement shows emission bands of the nanotubes at 354, 423, 467, and 666 nm. A combined growth mechanism of vapor–liquid-solid (VLS) and solid–liquid-solid (SLS) model is proposed for the formation of the BNNTs.     

Synthesis of boron nitride nanotubes by combining citrate-nitrate combustion reaction and catalytic chemical vapor deposition

High-quality boron nitride nanotubes were successfully synthesized via a novel two-step method, including citrate-nitrate combustion reaction and catalytic chemical vapor deposition. The composition, bonding features and microstructures of as-synthesized sample were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, transmission electron microscopy and selected area electron diffraction techniques. The results show that the as-synthesized boron nitride nanotubes with smooth surface are relatively pure. The diameter ranges between 20 and 80?nm, while the length is about dozens of micrometers. During the synthesis process of boron nitride nanotubes, citric acid chelates the cobalt ions and reacts with nitrate to form the cobalt oxide, depositing on the surface of boron powder homogeneously. The catalyst content and annealing temperature have a significant impact on the composition and microstructures of the final products. Based on the experimental results and thermodynamic analysis, the possible chemical reactions are listed, and vapor-liquid-solid mechanism is proposed to be dominant for the formation of boron nitride nanotubes.