Spectral properties of spherical boron nitride prepared using carbon spheres as template

Spherical boron nitride nanoparticles have been successfully fabricated by temperature-controlled pyrolysis procedure in a N₂ atmosphere, using boron acid and urea as the precursors. The carbon spheres were prepared from glucose (C₆H₁₂O₆) by a hydrothermal method as a template to be used. Comprehensive scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier infrared spectrum (IR) characterizations all confirm that the obtained products are spherical boron nitride. The amount of C₆H₁₂O₆ and reaction time were found to affect the morphology and structure of the as-prepared products. The average diameter of the spherical boron nitride nanoparticles synthesized with the addition of C₆H₁₂O₆ is about 0.3–1 µm. The spherical boron nitride has a high surface area of 176.78 m²g⁻¹ and ~3.5 nm pore size. The as-synthesized nanospheres also exhibit strong photoluminescence (PL) bands at 436, 454, 486, and 616 nm under 312 nm excitation, indicating that they could have potential application in novel optical devices.     

Corrosion behavior of pulse laser deposited 2D nanostructured coating prepared by self-made h-BN target in salinity environment

Hexagonal boron nitride (h-BN) target was prepared by two step wet chemical reaction method using a nontoxic starting materials (urea and boric acid). Optimized annealing parameters (1600 °C for 2 h) and N₂ environment are applied for pertinent growth of boron nitride nano powder. A mechanical procedure was acquired to convert this powder into pallet (target) for further analysis. The prepared target (white pallet) was used to fabricate the corrosion resisting h-BN nano-sheet coating using pulse laser deposition technique. A thick h-BN coating was deposited on SS 304 and Si substrates. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were employed to characterize the coating for structural and morphological purpose. X-ray photoelectron spectroscopy (XPS), Atomic force microscopy (AFM), and energy-dispersive X-ray spectroscopy (EDXA) were employed to characterize the coating for surface properties and chemical composition purpose. However, Contact angle and electrochemical work station were employed for wetting and corrosion analysis tests. We find that pulsed laser deposition (PLD) grown h-BN coating shows the non-wetting (134.2°) and reduce corrosion rate by one order of magnitude compared to bare SS. On the basis of these results, the h-BN nano-sheet coating may be a promising candidate for corrosion resist application in (3.5% NaCl solution) salinity environment.     

Ultrasound exfoliation of inorganic analogues of graphene

High-intensity ultrasound exfoliation of a bulk-layered material is an attractive route for large-scale preparation of monolayers. The monolayer slices could potentially be prepared with a high yield (up to 100%) in a few minutes. Exfoliation of natural minerals (such as tungstenite and molybdenite) or bulk synthetic materials (including hexagonal boron nitride (h-BN), hexagonal boron carbon nitride (h-BCN), and graphitic carbon nitride (g-C₃N₄)) in liquids leads to the breakdown of the 3D graphitic structure into a 2D structure; the efficiency of this process is highly dependent upon the physical effects of the ultrasound. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) were employed to verify the quality of the exfoliation. Herein, this new method of exfoliation with ultrasound assistance for application to mono- and bilayered materials in hydrophobic and hydrophilic environments is presented.     

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.     

Facile synthesis of hexagonal boron nitride fibers with uniform morphology

Hexagonal boron nitride (h-BN) fibers were synthesized via the polymeric precursor method using boric acid (H₃BO₃) and melamine (C₃H₆N₆) as raw materials. The precursor fibers were synthesized by a water bath and BN fibers were prepared from the precursor at 1600 °C for 3 h in flowing nitrogen atmosphere. The products were characterized by X-ray powder diffraction, Fourier transformation infrared spectroscopy, thermogravimetry and scanning electron microscopy. The results showed that h-BN fibers with uniform morphology were successfully fabricated. The well-synthesized fibers were 1–2 μm in diameter and 200–500 μm in length.     

Synthesis and characterization of nanocrystaline hexagonal boron nitride powders: XRD and luminescence properties

Nanocrystalline hexagonal boron nitride powders (h-BN) were synthesized from urea and boric acid followed by pirolysis and subsequent heat treatment in nitrogen atmosphere. Materials have been analyzed by means of X-ray diffraction, Photoluminescence and Field emission electron microscopy methods. Obtained results show that starting h-BN powder, synthesized at 750 °C, is composed of ~11 layer crystallites with average crystallite thickness and crystallite lateral size of 3.94 and 10.4 nm, respectively. A broad emission and intense luminescence intensity were observed due to the large atomic disorder. Higher annealing temperature increases crystallite size and turbostratic h-BN transforms to well crystallized h-BN at 1500 °C.     

Oxidation behavior of carbon/carbon-boron nitride composites fabricated by additives and chemical vapor infiltration

Carbon/carbon-boron nitride (C/C-BN) composites were manufactured by adding hexagonal boron nitride (h-BN) powders into carbon fiber preform and a subsequent chemical vapor infiltration (CVI) process for deposition of pyrolytic carbon (PyC). Microstructure and oxidation behavior of carbon/carbon composites with 9?vol% h-BN (C/C-BN9) were studied in comparison to carbon/carbon (C/C) composites. Results showed that with the addition of h-BN powders, a regenerative laminar (ReL) PyC with higher texture was achieved. Note that the introduction of h-BN powder make great contributes to graphitization degree of PyC, leading to larger oxidation activation energy. Moreover, under an air atmosphere, h-BN started to oxidize above 800?°C, and generated molten boron oxide (B₂O₃) which prohibited oxygen diffusion by filling in pores, cracks and other defects. As these reasons mentioned above, after oxidation tests under an air atmosphere, mass losses of C/C-BN9 composites were lower than that of C/C composites at all test temperatures (600–900?°C), indicating that the oxidation resistance of C/C-BN9 composites is better than that of C/C composites.     

Selective etching of boron nitride phases

Cubic boron nitride (c-BN) films can be used as hard coatings and for electronic devices due to their outstanding material properties, but the gas phase deposition of c-BN is still a challenging task. Until now it has only been possible to achieve nanocrystalline c-BN layers via physical vapor deposition (PVD) methods with rather weak film qualities. Only a chemical vapor deposition (CVD) process for c-BN can produce high quality films with material properties similar to those of the product achieved by high pressure, high temperature processes (HPHT) conventional routes. Therefore it is essential to tune the individual steps in the CVD process (nucleation, growth and selective etching) in a similar manner to that for diamond CVD to enable continuous growth of c-BN.Since selective etching of hexagonal boron nitride (h-BN) and sp² phases is still a major problem, we investigated the interaction of h-BN and c-BN with different reactive gases — ammonia (NH₃), chlorine (Cl₂), hydrogen chloride (HCl) and boron trifluoride (BF₃) — regarding their etching behaviour and surface stabilisation properties. Etching ratios from ≈10:1 up to 450:1 were found in the temperature range 600–1300°C for the h-BN/c-BN system, clearly indicating a high selectivity due to kinetic effects.The reaction mechanisms will be discussed with respect to the kinetic differentiation of the degradation of c-BN and h-BN (selective etching). The morphological changes and the quality of the remaining BN phases was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and infrared and Raman spectroscopy and these indicated a homogeneous decay of the individual phases. Since a homogeneous decay of c-BN resembles the reversed growth, the study of the interaction of both BN phases with reactive gases allowed us to collect more detailed information of the molecular mechanisms involved in the formation of the individual phases. These results will provide new routes for growing c-BN in a CVD process.     

Influence of the thermal process of carbon template removal in the mesoporous boron nitride synthesis

Influence of the thermal process involved in the carbon template elimination during the synthesis of mesoporous boron nitride by using nanocasting process of a mesoporous CMK-3 carbon with a borazinic precursor is presented. The borazinic precursor, the tri(methylamino)borazine (MAB), is converted to boron nitride (BN) inside the mesopores of a CMK-3 mesoporous carbon template by ceramization under nitrogen or under ammonia. The carbon template elimination is carried out by thermal treatment under air or under ammonia. The X-ray diffraction, TEM and pore size analysis are used to study the texture of the boron nitride synthesized from the carbon template. A template elimination performed by hydrogenation with an ammonia treatment allows to obtain an organized porous structure, which is not possible by using an oxidation treatment. In order to preserve the mesoporous organization of boron nitride, a two steps procedure (ceramization followed with template elimination by hydrogenation) is more efficient than a one step procedure (ceramization and template hydrogenation simultaneously).     

High-temperature carbothermal synthesis and characterization of graphite/h-BN bimaterials

Hexagonal boron nitride (h-BN) and graphite have similar crystal structures, comparable lattice parameters, and coefficients of thermal expansion, but vastly different electrical and thermal transport. Despite their key differences, it is possible to couple h-BN and graphite in a bimaterial system allowing the unique properties of both materials to be utilized in a single component. Through a carbothermal reduction of B₂O₃ in nitrogen, the surface of graphite can be converted to h-BN. This results in a layered system that is electrically insulating on the surface due to h-BN, and more compliant as well as conductive within the substrate due to the graphite structural body. We discuss the high-temperature synthesis and characterization of this layered material, focusing on the processing–microstructure relationship as well as the interface of graphite/h-BN to assess the chemical and mechanical adhesion of the layers, and to establish how such properties are contingent on the reacting phase of B₂O₃. This is achieved by investigating the origin of h-BN formation and the unwanted side reaction of boron carbide formation, through the evaluation of the thermochemistry and kinetics governing the carbothermic reactions. We establish that a reaction temperature and holding time of 1700°C for 18 h produced the thickest h-BN layers which exhibited the highest fracture toughness over all lower temperature synthesis conditions.     

Investigation on mechanochemical synthesis of Al2O3/BN nanocomposite by aluminothermic reaction

Alpha-alumina–boron nitride (α-Al₂O₃–BN) nanocomposite was synthesized using mixtures of aluminum nitride, boron oxide and pure aluminum as raw materials via mechanochemical process under a low pressure of nitrogen gas (0.5 MPa). The phase transformation and structural evaluation during mechanochemical process were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential thermal analysis (DTA) techniques. The results indicated that high exothermic reaction of Al–B₂O₃ systems under the nitrogen pressure produced alumina, aluminum nitride (AlN), and aluminum oxynitride (Al₅O₆N) depending on the Al value and milling time, but no trace of boron nitride (BN) phases could be identified. On the other hand, AlN addition as a solid nitrogen source was effective in fabricating in-situ BN phase after 4 h milling process. In Al–B₂O₃–AlN system, the aluminothermic reaction provided sufficient heat for activating reaction between B₂O₃ and AlN to form BN compound. DTA analysis results showed that by increasing the activation time to 3 h, the temperature of both thermite and synthesis reactions significantly decreased and occurred as a one-step reaction. SEM and TEM observations confirmed that the range of particle size was within 100 nm.     

Controllable preparation of boron nitride microspheres and their epoxy-based thermoconductive composites

How to prepare spherical boron nitride (BN) particle with different size is an extremely challenging work. In this paper, the controllable preparation of spherical BN particle from nanospheres to microsphere was realized by changing the synthesis temperature of trimethyl borate (B(OMe)₃) and ammonia. The spherical precursor (SP) with high oxygen content was obtained first, and then it was heated under flowing ammonia atmosphere to form stable boron nitride microspheres (BNMS). The BNMS exhibits onion-like cavitation structure with a diameter of 0.8–3.4 μm. The effects of the lower reaction temperature (700–825 °C) and gas flow rate on the spherical precursor are discussed. A possible mechanism is proposed to explain the formation of precursors and the appearance of onion-like structure. It is believed that the formation of microsphere is due to the deposition and growth of BO species during the flow process of nanosphere. In addition, the effect of the addition of BNMS on the thermal conductivity of epoxy resin (EP) composites was investigated.     

Hexagonal boron nitride nanosheets as metal-free electrochemical catalysts for oxygen reduction reactions

A high-performance ball milling technique was developed for synthesizing hexagonal boron nitride (h-BN) carbon paper (CP) electrodes as metal-free catalyst for the oxygen reduction reaction (ORR) and hydrogen storage (electrochemically) in acidic media. The h-BN nanosheets were functionalized with glycine to enhance the number of active sites and to boost the catalytic activity. The ball-milled h-BN catalytic electrodes demonstrated ultra-high catalytic activity toward electrochemical hydrogen adsorption/desorption (～3.5 times higher than pristine electrodes) as well as ORR in acidic electrolytes. Furthermore, in-situ durability analysis of the h-BN electrodes was performed via conducting a long-duration cycling experiment (>200 cycles). A mechanistic reaction pathway (sequential) including chemisorption and charge transfer reactions (four-electron and two-electron pathways) was also proposed for the ORR. Considering superior catalytic activity of as-prepared h-BN/CP electrodes, this class of metal-free nanostructured materials can be employed as inexpensive catalysts for the electrochemical H-storage and ORR within various energy storage/conversion devices (e.g., batteries, electrolyzers, and fuel cells).     

Nanosized and large crystals of hexagonal boron nitride by SHS under high pressures of nitrogen gas

Nanocrystalline powder of hexagonal boron nitride (h-BN) was prepared by SHS reaction of boron powder with nitrogen gas under pressures up to 150 MPa. At higher nitrogen pressures (up to 1000–2000 MPa), the reaction products were found to contain melted h-BN.     

Synthesis of highly purified and crystallized h-BN using calcium-assisted carbothermal reduction nitridation

Highly purified and crystallized hexagonal boron nitride (h-BN) powder is suitable as thermally conductive filler in resins. To obtain h-BN powder with large particle size, as well as high purity and crystallinity, high-temperature heat treatment over 1800°C in a N₂ gas atmosphere is effective. The carbothermal reduction nitridation (CRN) involves the carbothermic reduction of boric oxide in a N₂ gas atmosphere. In CRN using a CaO promoter, h-BN particles with high crystallinity can be obtained by a simple heat treatment process. CaO prevents the evaporation of boron oxide and aids in h-BN particle growth at high temperatures. However, CaB₆ is formed as byproduct or impurity when CRN using the CaO promoter is performed at temperatures higher than 1800°C. In this study, the relationship between the products and the reaction temperature was clarified via thermodynamic considerations and experimentation. The results clarified the ideal reaction process of CRN using a CaO promoter to obtain highly purified and crystallized h-BN powder.     

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.     

Fabrication and characterization of amorphous SiBCN powders

A new approach in the synthesis of amorphous SiBCN has been suggested using high-energy shaker ball mill. Hexagonal boron nitride (h-BN), graphite (C) and amorphous silicon (Si) were blended according to the mole ratio of 1:1:1, and then ball milled by different ball-to-powder mass ratio, diameter of ball and milling time. The structural evolution at different stages of milling has been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution electron microscopy (HREM) and electron energy-loss spectroscopy (EELS). The results showed that the SiBCN powders were mainly amorphous with some nanocrystlline phases. The thermal stability of SiBCN powders has been analyzed by thermogravimetry in argon. Mass loss occurred at high temperature, especially above 1000 °C.     

Production and characterization of spark plasma sintered (Ti,Nb)B2 solid solutions with graphene nanoplatelets and hexagonal boron nitride

For this study, (Ti,Nb)B₂ solid solutions were consolidated by spark plasma sintering. In addition, (Ti,Nb)B₂ with graphene nanoplatelets (GNPs) and hexagonal boron nitride (h-BN) were produced to evaluate the potential of the new structural materials. The phase formation, microstructure, mechanical properties, oxidation resistance and room temperature reflectance, and absorbance features of (Ti,Nb)B₂ were investigated. X-ray diffraction and Transmission electron microscopy observations showed that a complete solid solution phase was formed when the samples were sintered at 1850 °C for 5 min under 50 MPa. Ti_0.75Nb_0.25B₂ exhibited a relative density of ～98.6%, a hardness of ～20.5 GPa, and an indentation fracture toughness of ～3.4 MPa·m^1/2. It was found that the presence of 1 vol% h-BN as an additive enhanced the hardness (～10%) and fracture toughness (～30%) of Ti_0.75Nb_0.25B₂ by activating toughening mechanisms. The GNP added Ti_0.75Nb_0.25B₂ proved to have better oxidation resistance and optical absorbance than the other materials used in the study.     

Solvothermal synthesis of hexagonal B–C–N compound at low temperature conditions

Hexagonal B–C–N crystals have been successfully synthesized by a solvothermal method from carbon tetrachloride and calcium boron nitride at 400 °C. The characterization of synthesized product was carried out using X-ray diffraction, scanning electron microscopy and transmission electron microscopy equipped with electronic energy loss spectrometer. The composition of the hexagonal B–C–N single crystal is detected by electronic energy loss spectrometer as B_0.52C_0.11N_0.37.     

Enhancing the Anhydrous Proton Conductivity of Sulfonated Polysulfone/Polyvinyl Phosphonic Acid Composite Membranes With Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) particles have attracted increasing interest due to mechanical properties, chemical stability, electrical features, thermal stability, and good lubrication property. In this work hexagonal boron nitride were used as inorganic fillers, which increase the mechanical and thermal stabilities of the membrane. The proton conducting polymer membranes were prepared by blending of sulfonated polysulfone, polyvinyl phosphonic acid, and boron nitride. Scanning electron microscopy indicated the homogeneous distribution of hBN nanoparticles in the polymer matrix. hBN increased the proton conductivity and in the anhydrous state the maximum proton conductivity was determined as 7.9 × 10^?3 S/cm at 150°C for PVPA-SPSU-5hBN.