New insights into synthesis of nanocrystalline hexagonal BN

Uncovering the mechanism behind nanocrystalline hexagonal boron nitride (h-BN) formation at relatively low temperatures is of great scientific and practical interest. Herein, the sequence of phase transformations occurring during the interaction of boric acid with ammonia in a temperature range of 25–1000 °C has been studied in detail by means of thermo-gravimetric analysis, X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The results indicate that at room temperature boric acid reacts with ammonia to form an ammonium borate hydrate (NH₄)₂B₄O₇x4H₂O. Its interaction with ammonia upon further heating at 550 °C for 1 h leads to the formation of turbostratic BN. Nanocrystalline h-BN is obtained either during heating in ammonia at 550 °C for 24 h or at 1000 °C for 1 h. This result is important for the development of novel cost-effective and scalable syntheses of h-BN nanostructures, such as nanosheets, nanoparticles, nanofibers, and nanofilms, as well as for sintering h-BN ceramic materials.     

Nucleation and growth of single crystal graphene on hexagonal boron nitride

Direct graphene growth was demonstrated on exfoliated hexagonal boron nitride (h-BN) single crystal flakes by low pressure CVD. The size of the hexagonal single crystal graphene domain increases with deposition time, with maximum size of ～270 nm. Most domains were found to nucleate at screw dislocation sites, and a step-flow growth mechanism was observed at atomic steps on the h-BN surface. Understanding the nucleation and growth mechanisms is an important step towards the synthesis of large single crystal graphene on h-BN substrates.     

Synthesis and characterization of exfoliated polystyrene grafted hexagonal boron nitride nanosheets and their potential application in heat transfer nanofluids

Three different methods were used to develop surface-modified hexagonal boron nitride (h-BN) nanosheets, and polystyrene grafting was performed by an indirect covalent bond formation between modified h-BN nanosheets and styrene molecules through surface initiated atom transfer radical polymerization (SI-ATRP) approach. In all methods, an alkyl bromide as the ATRP-initiating site was first introduced on h-BN nanosheets and an SI-ATRP reaction of styrene from the initiator immobilized h-BN surface was achieved. The structure of synthesized PS grafted h-BN nanosheets (PS-g-h-BN) was identified and characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, field emission scanning electron microscopy, transmission electron microscopy, and atomic force microscopy methods. The functionalization promoted the exfoliation of h-BN layered structure into few layer sheets where the thickness of the sheets was dependent on the modification technique and the content of polymer grafted on nanosheets. The highest grafting content of PS-g-h-BN nanosheets was obtained around 20% which could enhance thermal conductivity of mineral oil-based nanofluids with the minimum concentration of the nanofiller (0.01 wt%). The electrical and physical properties of the nanofluid were also investigated. According to the results, the dielectric loss reduced by increase in nanofiller concentration was an indication of the enhanced dielectric nature of nanofluid. In addition, exfoliated PS-g-h-BN nanosheets dispersions were shown to be stable in mineral oil up to 2 months and this stability was linked to the presence of polymer chains followed by the formation of van der Walls interactions between the grafted polymer and the fluid.     

Electromagnetic wave absorption properties of graphene modified with carbon nanotube/poly(dimethyl siloxane) composites

By using a catalytic growth procedure, carbon nanotubes (CNTs) are in situ formed on reduced graphene oxide (RGO) sheet at 600 °C. CNTs growing on RGO planes through covalent C–C bond possess lower interfacial contact electrical resistance. As a hybrid structure, the CNTs/graphene (CNT/G) are well dispersed into poly (dimethyl siloxane). The hybrid combining electrically lossy CNTs and RGO, which disperses in electrically insulating matrix, constructs an electromagnetic wave (EM) absorbing material with ternary hierarchical architecture. The interfacial polarization in heterogeneous interface plays an important role in absorbing EM power. When the filler loading is 5 wt.% and thickness of absorber is 2.75 mm, the minimum value of reflection coefficient and the corresponding frequency are −55 dB and 10.1 GHz, and the effective absorption bandwidth reaches 3.5 GHz. Therefore, combining the CNTs and graphene sheet into three-dimensional structures produces CNT/G hybrids that can be considered as an effective route to design light weight and high-performance EM absorbing material, while the effective EM absorption frequency can be designed.     

Electromagnetic wave absorbing properties of In–Sn/rGO composites supported by impurity defect

With the fast development of E-communication technology, effective electromagnetic wave absorbing materials are highly needed to address the growing electromagnetic pollution. Herein, Indium doped tin microsphere/reduced graphene oxide (In–Sn/rGO) composites with rich impurity defects were synthesized via the sol-gel and hydrothermal method. The excellent microwave absorption of In–Sn/rGO composites can be attributed to the modifications of electronics status and Fermi energy level after In doping. This can significantly increase the carrier mobility between In–Sn microspheres and rGO sheets to strike a superior interfacial polarization loss. As a result, the maximum absorptivity can reach ?51.16 dB at 8.73 GHz (thickness: 3.5 mm) with a lower filler loading of 10 wt%. Meanwhile, the synthesized In–Sn/rGO composites also exhibit an ultra-wide absorbing frequency range of 13.84 GHz (within the X band, Ku band, and most of the C band). This research provides a new idea for the synthesis of effective microwave absorbing material by introducing impurity defects.     

Preparation and mechanical properties of laminated B4C–TiB2/BN ceramics

Laminated B₄C–TiB₂ ceramics with h-BN interface layers were successfully prepared by roll forming and tape casting, and samples with different numbers of stacked layers were obtained. Scanning electron microscopy and X-ray diffraction were used to analyze the microstructure and interlayer crystal phases of the composites, and the bending strength, fracture toughness, and work of fracture were measured. As the number of h-BN layers increased, the fracture toughness increased from 7.38 ± 0.5 MPa m^1/2 to 9.01 ± 0.61 MPa m^1/2, which is 2–3 times higher than that of monolithic B₄C ceramics. As the fracture toughness increased, the hardness remained at a high level (31.67 GPa). Bending tests showed that cracks deflected when they encountered the h-BN interfacial layers. The toughening mechanisms included the deflection and branching of cracks and generation of new microcracks, which increased the length of the propagation path and work of fracture.     

Enhanced dielectric and microwave absorption properties of Cr/Al2O3 coatings deposited by low‐power plasma spraying

Low‐power plasma‐sprayed Cr/Al₂O₃ coatings have been developed for their potential application as broad bandwidth, thin thickness, lightweight, and strong microwave‐absorbing materials. The dielectric and microwave absorption properties of the as‐sprayed coatings were studied in the X‐band (from 8.2 to 12.4 GHz). High complex permittivity of the coatings was obtained because of a large number of internal boundaries and the conductive networks. Meanwhile, a significant enhancement of microwave absorption properties of the coating was achieved due to the enhanced interfacial polarization and conductance loss. The reflection loss (RL) <?10 dB of the Al₂O₃–15Cr coating was obtained from 9.8 to 11.4 GHz by choosing an appropriate coating thickness, and an optimal minimum reflection loss (RL_min) of ?45.35 dB was achieved at 10.3 GHz with a thin thickness of 1.32 mm.     

Monolayer graphene/hexagonal boron nitride heterostructure

A buried metal-gate field-effect transistor (FET) using a stacked hexagonal boron nitride (h-BN) and chemically vapor deposited (CVD) graphene heterostructure is demonstrated. A thin h-BN multilayer serves as both gate dielectric and supporting layer for the monolayer graphene channel. It is observed that electrical stressing could significantly improve graphene conduction, similar to the effect reported in the graphene/SiO₂ system. In the graphene/h-BN/TiN FET structure, p-type doping behavior in graphene is observed, possibly attributed to spontaneous doping due to the work function difference between the graphene channel and the metal gate electrode. At a high-level of stress, graphene exhibits n-type doping behavior due to charge transfer across the thin h-BN multilayer. The dielectric strength and tunneling behavior of h-BN are investigated, showing the robust nature of the layer-structured insulator.     

Effect of h-BN on the microstructure and fracture behavior of low carbon Al2O3-C refractories

h-BN is an ideal substitution candidate for graphite due to its similar crystal structure, better oxidation resistance. In this work, the effect of h-BN on microstructure and comprehensive properties of Al₂O₃-C refractories are investigated, and the specimen containing 0.5 wt% h-BN (G0.5N0.5) possesses the best comprehensive properties. The addition of h-BN could reduce the diameter of SiC whiskers, which leads to the highest strength of specimen G0.5N0.5 (42.63 ± 3.10 MPa). Moreover, the fracture behavior of the specimens is demonstrated using wedge splitting test. The results show that the specimen G0.5N0.5 possesses the highest crack initiation and propagation resistance, which could be attributed to the collaborative effect of h-BN and SiC whiskers. Noteworthily, the addition h-BN could improve the thermal shock resistance. The specimens containing h-BN possess the higher residual ratio, compared with the specimen containing no h-BN (G1N0), and the specimen G0.5N0.5 shows the highest residual strength (14.12 ± 0.67 MPa). Furthermore, the oxidation resistance could be enhanced with introducing the h-BN.     

Fabrication of hexagonal cerium oxide nanoparticles decorated nitrogen-doped reduced graphene oxide hybrid nanocomposite as high-performance microwave absorbers in the Ku band

With the aim to obtain microwave absorbers simultaneously possessing broad absorption bandwidth, strong absorption intensity and thin matching thickness, nitrogen-doped reduced graphene oxide decorated by cerium oxide particles (NRGO/CeO₂) hybrid nanocomposite was prepared through a hydrothermal and calcination two-step route. Results of micromorphology analysis showed that numerous hexagonal CeO₂ nanoparticles were evenly anchored on the crumpled surfaces of NRGO. Moreover, both nitrogen doping and hybridization with RGO could notably strengthen the microwave absorption capacity of CeO₂. Remarkably, the NRGO/CeO₂ hybrid nanocomposite exhibited the minimum reflection loss of ?57.2 dB at 13.4 GHz (Ku band) under a matching thickness of 1.66 mm and maximum absorption bandwidth of 4.6 GHz (from 13.2 to 17.8 GHz) at an ultrathin thickness of only 1.5 mm. Meanwhile, the hybrid nanocomposites displayed strong absorption intensity (≤-20 dB, 99% absorption) in almost the whole measured thicknesses range. Furthermore, the relationship between absorption intensity and filler loadings was uncovered. The potential microwave absorption mechanisms were further revealed. Therefore, this work opened a novel idea for designing RGO-based hybrid nanocomposites as high-performance microwave absorbers.     

Silver nanorods attached to graphene sheets as anode materials for lithium-ion batteries

We employed a homogenizing method to disperse silver nanorods onto graphene nanosheets (GNs) in liquid phase, forming a three-dimensional Ag/GN framework. The Ag/GN hybrid material was used to investigate Li-ion insertion/extraction properties. A microwave-assisted polyol approach was adopted to synthesize highly crystalline Ag nanorods. The as-prepared Ag/GN hybrid anode shows improved Li-storage capacity, excellent rate capability, and good cycling stability, as compared with the pure GN anode. The hybrid anode offers a reversible capacity of 1015 mA h g⁻¹ at 0.1 C and a high capacity retention of 64.1% based on the capacity ratio at 5 C–0.1C rates. The enhancement of anode performance can be attributed to an increased electrode conductivity and interlayer distance due to the insertion of metallic Ag nanorods onto a percolated GN network, thus preventing the re-stacking of graphene sheets.     

Increasing fracture toughness of zirconia-based composites as a synergistic effect of the introducing different inclusions

The effect of multi-walled carbon nanotubes (MWCNTs) and hexagonal boron nitride (h-BN) inclusions on the fracture toughness of yttria-stabilized zirconia (YSZ) ceramics has been studied. It is shown that an increase in the MWCNTs and h-BN content has a positive effect on the K_1C of zirconia ceramics. The greatest increase in the fracture toughness of YSZ ceramics was observed with the introduction of hexagonal boron nitride particles. For YSZ ceramics, the K_1C value was ≈6.1 MPa m^1/2, for ceramics with a 5 wt % of h-BN K_1C ≈ 9.2 MPa m^1/2. It was shown that an increase of the YSZ ceramics fracture toughness with the introduction of MWCNTs and h-BN, both and separately was provided by the combined action of several mechanisms of increasing the work of crack propagation. In addition, in all composites obtained in this work, the transformation of tetragonal ZrO₂ into monoclinic was observed.     

Hexagonal boron nitride nanosheet/carbon nanocomposite as a high-performance cathode material towards aqueous asymmetric supercapacitors

The two-dimensional hexagonal boron nitride (h-BN) has garnered tremendous interest due to its unique mechanical, thermal and electronic properties. However, the application of h-BN has been restricted as electrode materials for supercapacitors because of its wide band gap and rather low conductivity. Herein, a carbon-modified hexagonal boron nitride nanosheet (h-BN/C) nanocomposite is prepared through a facile and scalable solid-state reaction. Interestingly, the h-BN/C nanocomposite as cathode material exhibits a pair of distinct and reversible redox peaks in 2?M KOH aqueous electrolyte. Because of the enhanced electrical conductivity derived from the modified carbon and the increased specific surface area, the h-BN/C nanocomposite presents a high specific capacitance of 250?F?g^?1 at the current density of 0.5?A?g^?1. More importantly, the fabricated aqueous asymmetric supercapacitor with the h-BN/C as cathode and activated carbon as anode displays an operating voltage of 1.45?V, an energy density of 17?Wh?kg^?1 at a power density of 245?W?kg^?1, and high stability up to 1000 cycles. Therefore, h-BN/C nanocomposite would promisingly be a cathode material for aqueous asymmetric supercapacitors.     

One-step hydrothermal synthesis and enhanced microwave absorption properties of Ni0.5Co0.5Fe2O4/graphene composites in low frequency band

Ni_0.5Co_0.5Fe₂O₄/graphene composites were synthesized successfully via one-step hydrothermal method. The crystal structure, morphology and corresponding elemental distribution, electromagnetic parameters and microwave absorption performances of the as-prepared composites were measured by XRD, SEM, TEM and VNA, respectively. The results indicated that the microwave absorbing performance can be obviously enhanced through the addition of graphene in a suitable range, the magnetic loss plays a dominant contribution for the microwave absorption of composites. The maximum reflection loss of ?30.92?dB at 0.84?GHz with a ?10?dB bandwidth over the frequency range of 0.58–1.19?GHz is obtained when the composite contains 12?wt% graphene and the thickness of sample is 4?mm. This investigation presents a simple method to prepare Ni_0.5Co_0.5Fe₂O₄/graphene composites with excellent microwave absorption performance in the low frequency band of 0.1–3?GHz.     

Design of artificial nacre-like hybrid films as shielding to mitigate electromagnetic pollution

Multifunctional designs of biomimetic layered materials are in great demand for broadening their applications. Artificial hybrid films are fabricated using a simple evaporation-induced assembly method, using nacre as the structural model, two-dimensional reduced graphene oxide (RGO) and magnetic graphene (MG) as inorganic building blocks and poly(vinyl alcohol) (PVA) as glue. The nacre-like films exhibit good mechanical performance, such as high stiffness, strength and toughness. The biomimetic materials possess the shielding properties of electromagnetic pollution. MG based nacre-like films present more significant electromagnetic interference (EMI) shielding performance than RGO film, because of a synergism between dielectric loss of graphene and magnetic loss of magnetic nanoparticles. Average EMI shielding effectiveness (SE) reaches ∼20.3 dB over the frequency range of 8.2–12.4 GHz (X band) for MG hybrid film only 0.36 mm thick. The lightweight, flexible and thin MG artificial hybrid films possess good potential for EMI shielding applications.     

Effects of Sr doping on the structure,magnetic properties and microwave absorption properties of LaFeO3 nanoparticles

Lightweight, broadband microwave absorbing materials, with strong absorption capacities, are an urgent demand for practical applications. The microstructural and microwave absorption properties of LaFeO₃ samples prepared by a sol-gel method using different amounts of Sr are investigated systematically. X-ray diffraction and Rietveld refinement studies showed that Sr²⁺ doping can distort the crystal structure of LaFeO₃, leading to lattice expansion and spin tilt of the Fe-O-Fe bond angle. The improvement of magnetic properties mainly originates from the synergistic effect of the bond angle spin tilt and crystal structure defects. Oxygen vacancies will be generated due to the fluctuations in the valence state of Fe³⁺ resulting from the substitution of La³⁺ by Sr²⁺ as deduced from the X-ray photoelectron spectroscopy analysis. The generation of oxygen vacancies, electronic hopping and polarization loss may be one of the main reasons for changes in the electromagnetic parameters. The minimum reflection loss (RL) of La_1–xSr_xFeO₃ nanoparticles with the Sr doping of 0.2 can reach approximately -39.3 dB at 10 GHz for the thickness of 2.2 mm, and the effective absorption bandwidth (RL ≤ -10 dB) can reach approximately 2.56 GHz. In addition, the La_1–xSr_xFeO₃ nanoparticles also can obtain better microwave absorbing performance in the C-band (4–8 GHz) with the minimum RL of -36.8 dB for the matching thickness of 3.0 mm and Sr content of 0.3. Consequently, La_1–xSr_xFeO₃ nanoparticles are promising materials for use in a high-performance and adjustable electromagnetic wave absorber, particularly in C-band and X-band.     

Silicon carbide nano-fibers in-situ grown on carbon fibers for enhanced microwave absorption properties

Silicon carbide nano-fibers (SiCNFs) were in-situ grown on the surface of carbon fibers by catalysis chemical vapor deposition (CCVD) with Ni nano-particles as catalyst at 1000 °C. The phase composition, microstructures, oxidation resistance and microwave absorption properties of the SiCNFs coated carbon fibers were investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Thermal gravity analysis (TGA) and Vector network analyzer, respectively. The results show that the as-grown nano-fibers which are mainly composed of β-SiC, present a withe-like morphology with diameter of 20–50 nm and aspect ratio of 100–150. Additionally, the TGA curves indicate that the oxidation resistance of the SiCNFs coated carbon fibers is significantly improved in comparison to the pure carbon fibers. Moreover, the investigation reveals that the microwave absorption properties of the SiCNFs coated carbon fibers are effectively enhanced. The reflectivity of the SiCNFs coated carbon fibers is less than −10 dB within the frequency ranging from 9.2 to 11.7 GHz and the lowest value of reflectivity can approach −19.9 dB when the thickness of specimen is 2 mm. While the reflection loss of the pure carbon fibers is higher than −2.1 dB within the whole band ranging from 2 and 18 GHz. The superior microwave absorbing performance of the SiCNFs coated carbon fibers is mainly attributed to the improved impedance matching as well as dissipation resulted from hopping migration. In conclusion, this study provides an effective modification approach to improve the microwave absorption properties of carbon fibers. Finally, the SiCNFs coated carbon fibers could be considered as a promising candidate in light-weight microwave absorbing materials.     

Fabrication of carbon encapsulated Co3O4 nanoparticles embedded in porous graphitic carbon nanosheets for microwave absorber

Carbon-encapsulated Co₃O₄ nanoparticles homogeneously embedded 2D (two-dimensional) porous graphitic carbon (PGC) nanosheets were prepared by a facile and scalable synthesis method. With assistance of sodium chloride, the Co₃O₄ nanoparticles (10–20 nm) with magnetic loss were well encapsulated by onion-like carbon shells homogeneously embedded porous graphitic carbon nanosheets (thickness of less than 50 nm) with dielectric loss. In the architecture, the well impedance matching for microwave absorption can be obtained by the synergetic effect between Co₃O₄ nanoparticles and encapsulated porous carbon nanosheets. The minimum reflection loss value of −32.3 dB was observed at 11.4 GHz with a matching thickness of 2.3 mm for 2D Co₃O₄@C@PGC nanosheets. The 2D Co₃O₄@C@PGC nanosheets can be used as a kind of candidate for microwave absorbing materials.     

Assembled porous Fe3O4@g-C3N4 hybrid nanocomposites with multiple interface polarization for stable microwave absorption

Magnetic/dielectric composites can offer good electromagnetic impendence. However, the strategy for embodying strong absorbing ability and broad effective absorption band simultaneously is a significant challenge. Therefore, assembled porous Fe₃O₄@g-C₃N₄ hybrid nanocomposites have been designed and synthesized, in which porous Fe₃O₄ nanospheres assembled by ~ 3?nm Fe₃O₄ nanoparticles are surrounded by g-C₃N₄. The introduction of g-C₃N₄ improves dielectric loss ability at 2–18?GHz and magnetic loss ability at 2–10?GHz, and enhances attenuation constant, and increases electromagnetic impedance degree. These merits ensure that assembled porous Fe₃O₄/g-C₃N₄ hybrid nanocomposites deliver superior microwave absorption performance, such as effective absorption bandwidth, f_E, (reflection loss less ??10?dB) exceeding 5?GHz at 2.0–2.3?mm, the maximal f_E of 5.76?GHz and minimal reflection loss of at least ??20?dB with thickness ranging from 2.3 to 10.0?mm, avoiding the sensitivity of absorption properties to absorbing layer thickness. Stable microwave absorbing performance originates from multi-interfacial polarization, multi-reflection, enhanced electromagnetic loss capability, and good electromagnetic impedance. Our study offers a new idea for stable microwave absorber at 2–18?GHz.     

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.