Synthesis of boron carbide powder from hexagonal boron nitride

We report the synthesis of boron carbide powder via the reaction of hexagonal boron nitride with carbon black. The reaction between hexagonal boron nitride and carbon black completed at 1900 °C for 5 h in vacuum. The particle sizes of the synthesized boron carbide powder were about 100 nm from transmission electron microscopy. The possible reaction mechanism was that hexagonal boron nitride decomposed into elemental boron and nitrogen even when there was no carbon at a relatively low rate, and introduction of carbon into hexagonal boron nitride powder facilitated the decomposition process; the boron from the decomposition of boron nitride reacted with carbon to form boron carbide.     

Synthesis of boron nitride

The authors examine the boric oxide—ammonia route with special stress on the yield and composition of the intermediate addition compound (BN) _x (B₂O₃) _y (NH₃) _z . It has been concluded that B₂O₃ and NH₃ present in the addition compound formed between 350°C and 900°C cannot be further reacted to convert the B₂O₃ into BN and the BN yield remains at around 66%. A formula (BN)_12·7(B₂O₃)_7·5NH₃ has been suggested for the addition compound.     

Dissimilar laser brazing of boron nitride and tungsten carbide

Using laser brazing process, we made the dissimilar joint of the boron nitride and tungsten carbide with an excellent functionality. In order to investigate the characteristic of joint, observation and structural analysis of its interface by the electron probe micro-analysis (EPMA) and the scanning acoustic microscopy were performed. The wetting property between h-BN and Ag–Cu–Ti braze was excellent, therefore no gaps were seen between them. Moreover, it was suggested that the Ti element, which is the active ingredient in Ag–Cu–Ti braze, reacted with N in h-BN to generate Ti–N composite phase as an interfacial precipitation and the generation of Ti–N composite phase was affected by the concentration of Ti in Ag–Cu–Ti braze. All fracture was formed in h-BN body near the interface and it seemed that the distribution of shear strength of 1.25%Ti to 1.68%Ti was within the margin of variation of bulk strength of h-BN.     

Catalyst-free synthesis and characterization of boron nitride whiskers and nanotubes

Catalyst-free transformation of h-BN in an optical furnace in the flow of nitrogen results in formation of nanotubes, whiskers and negligible quantity of melted drops on the surface of heated samples. The thread-like structures and equiaxial particles were precipitated on a silicon substrate. The phase composition of the produced material is a mixture of h-BN, two boron-enriched tetragonal phases of BN (B_51,2N and B₂₅N), tetragonal and rhombohedric phases of pure boron and amorphous phase.An approximation of the spectral dependence of optical absorption versus photon energy of an incident light was explained in terms of absorption of the corresponding phases. Tetragonal phase of B_51,2N, tetragonal and rhombohedric phases of pure boron may correspond to band gap 3.5 eV, tetragonal phase of B₂₅N - 3.8 eV and hexagonal phase of BN - 4.8 eV. This fact confirms a theoretical assumption for the effect of boron excess in BN on a band gap.     

Structural evolution of boron nitride films grown on diamond buffer-layers

Boron nitride films on diamond buffer layers of varying grain size, surface roughness and crystallinity are deposited by the reaction of B₂H₆ and NH₃ in a mixture of H₂ and Ar via microwave plasma-assisted chemical vapor deposition. Various forms of boron nitride, including amorphous α-BN, hexagonal h-BN, turbostratic t-BN, rhombohedral r-BN, explosion E-BN, wurzitic w-BN and cubic c-BN, are detected in the BN films grown on different diamond buffer layers at varying distances from the interface of diamond and BN layers. The c-BN content in the BN films is inversely proportional to the surface roughness of the diamond buffer layers. Cubic boron nitride can directly grow on smooth nanocrystalline diamond films, while precursor layers consisting of various sp²-bonded BN phases are formed prior to the growth of c-BN film on rough microcrystalline diamond films.     

Formation and atomic structures of boron nitride nanohorns

Boron nitride (BN) nanohorns were synthesized by an arc-melting method, and atomic structure models for BN nanohorns with tetragonal BN rings were proposed from high-resolution electron microscopy. Stability and electronic structures of the BN nanohorns were investigated by molecular orbital/mechanics calculations. The calculation showed that multiwalled BN nanohorns would be stabilized by stacking of BN nanohorns. The energy gap of BN nanohorn was calculated to be 0.8 eV, which is lower compared to those of BN clusters and nanotubes.     

Formation and atomic structures of boron nitride nanohorns

Boron nitride (BN) nanohorns were synthesized by an arc-melting method, and atomic structure models for BN nanohorns with tetragonal BN rings were proposed from high-resolution electron microscopy. Stability and electronic structures of the BN nanohorns were investigated by molecular orbital/mechanics calculations. The calculation showed that multiwalled BN nanohorns would be stabilized by stacking of BN nanohorns. The energy gap of BN nanohorn was calculated to be 0.8 eV, which is lower compared to those of BN clusters and nanotubes.     

Synthesis of boron nitride nanotubes from boron oxide by ball milling and annealing process

Boron nitride nanotubes were synthesized from boron oxide by high-energy ball milling and annealing method. The diameter of the nanotubes is in the range of 20-200 nm. The nanotubes show a bamboo-like structure and cylindrical-like structure under low magnification. The shorter bamboo nodes with distinct knots were observed for the bamboo-like nanotubes with larger diameters and the knots can also occasionally be observed in the cylindrical-like BN nanotubes with smaller diameters under high magnification. Al and Si were found to be catalytic materials responsible for the formation of BN nanotubes besides the metallic particles containing Fe, Ni and Cr.     

Facile synthesis of boron nitride coating on carbon nanotubes

Boron nitride (BN) coating on the surface of carbon nanotubes (CNTs) was synthesized by the direct reaction of NaBH₄ and NH₄Cl in the temperature range of 500–600 °C. X-ray diffraction, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) confirm the formation of BN coating. It is revealed that the BN coating follows the shape of CNTS without damaging the surface of CNTs, and the elements B and N distribute homogenously along the whole CNTs without chemical bonds between carbon and BN layers. Besides, the oxidation resistance of the CNTs improved a lot after being coated with BN.     

Facile synthesis of cubic boron nitride nanoparticles from amorphous boron by triple thermal plasma jets at atmospheric pressure

Cubic boron nitride (c-BN) is a superhard material, with hardness value comparable to that of diamond. c-BN is used in a wide range of industrial applications, including tool, abrasives, and refractory. The hardness of c-BN can be improved by decreasing the particle size to the nanoscale; however, the simultaneous application of high pressure (～8 GPa) and temperature (>2,500 K) is required to synthesize the c-BN crystal structure. In this study, we effectively synthesized c-BN nanoparticles from amorphous boron using a triple direct current (DC) thermal plasma jet system at atmospheric pressure. The injection of nitrogen as plasma forming gas generated reactive nitridation species. The average particle size of the synthesized c-BN was 22 nm, and the major crystal structure is the (1 1 1) cubic phase. We carried out a numerical simulation for a thermal fluid, to confirm the high temperature and velocity fields of the plasma jets that formed inside the reactor as the flow rate of plasma forming gas was adjusted. A high production yield of 51% was achieved using amorphous boron at a feed rate of 190 mg/min and the c-BN nanoparticles exhibited high crystallinity without requiring pre-and post-processing.     

Catalyst-free synthesis of carbon and boron nitride nanoflakes using RF-magnetron sputtering

Catalyst-free boron nitride (BN) and carbon (C) nanoflakes have been produced by direct radio frequency (RF)-magnetron sputtering on molybdenum and tungsten substrates at or above temperatures of 1000 °C and 800 °C, respectively. Selected-area electron diffraction (SAED) shows that the films are polycrystalline and contain disordered graphite and hexagonal BN. Transmission electron microscopy (TEM) reveals curved or twisted flakes up to several hundred nanometres in length. High resolution transmission electron microscopy (HRTEM) confirms the nanoflake structure to be turbostratic, which is intermediate between an amorphous phase and the ordered layered phases of hexagonal BN or graphite.     

Modelling of boron nitride: Atomic scale simulations on thin film growth

Molecular-dynamics simulations on ion-beam deposition of boron nitride are presented. A realistic Tersoff-like potential energy functional for boron nitride, which was specially fitted to ab initio-data, has been used. The impact of energetic boron and nitrogen atoms on a c-BN target is simulated with energies ranging from 10 to 600 eV. The structural analysis of the grown films shows that a loose, dominantly sp²-bonded structure arises at high ion flux. In no case the formation of a sp³-bonded phase is observed, but the obtained films partially reveal textured basal planes as found in experiment. Two different growth regimes are identified for ion energies above and below 100 eV.     

Synthesis of multiwall boron nitride nanotubes dependent on crystallographic structure of boron

Synthesis and growth of multiwall boron nitride nanotubes (BNNTs) under the B and ZrO₂ seed system in the milling–annealing process were investigated. BNNTs were synthesized by annealing a mechanically activated boron powder under nitrogen environment. We explored the aspects of the mechanical activation energy transferred to milled crystalline boron powder producing structural disorder and borothermal reaction of the ZrO₂ seed particles on the synthesis of BNNTs during annealing. Under these circumstances, the chemical reaction of amorphous boron coated on the seed nanoparticles with nitrogen synthesizing amorphous BN could be enhanced. It was found that amorphous BN was crystallized to the layer structure and then grown to multiwall BNNTs during annealing. Especially, bamboo-type multiwall BNNTs were mostly produced and grown to the tail-side of the nanotube not to the round head-side. Open gaps with ∼0.3 nm of the bamboo side walls of BNNTs were also observed. Based on these understandings, it might be possible to produce bamboo-type multiwall BNNTs by optimization of the structure and shape of boron coat on the seed nanoparticles.     

On the effect of water and oxygen in chemical vapor deposition of boron nitride

Growth studies of sp²-hybridized boron nitride (BN) phases by thermal chemical vapor deposition (CVD) are presented; of particular interest is the presence of oxygen and water during growth. While Fourier transform infrared spectroscopy reveals the presence of BN bonds and elemental analysis by elastic recoil detection analysis shows that the films are close to stoichiometric, although containing a few atomic percent oxygen and hydrogen, X-ray diffraction measurements show no indications for nucleation of any crystalline BN phases, despite change in N/B-ratio and/or process temperature. Thermodynamic modeling suggests that this is due to formation of strong BO bonds already in the gas phase in the presence of water or oxygen during growth. This growth behavior is believed to be caused by an uncontrolled release of water and/or oxygen in the deposition chamber and highlights the sensitivity of the BN CVD process towards oxygen and water.     

Molecular orbital calculations of hydrogen storage in carbon and boron nitride clusters

Hydrogen gas storage ability in carbon and boron nitride (BN) clusters was investigated by molecular orbital calculations. From single point energy calculations, H₂ molecules would enter from hexagonal rings of C₆₀ and B₃₆N₃₆ clusters and octagonal rings of B₂₄N₂₄ cluster because of lower energy barrier. Chemisorption calculation of hydrogen for BN clusters showed that hydrogen bonding with nitrogen atoms was more stable than that with boron atoms. Stability of H₂ molecules in BN clusters seems to be higher than that of carbon clusters.     

Structural models for epitaxy of hexagonal phases of boron nitride on Si(001)

Interface models for matching either w-BN or h-BN layers to an Si(001) substrate, are considered. Aligning the [2 − 1 − 10] direction of h-BN (or equivalently the [−1 − 120] direction in w-BN) along [1 − 10] the Si(001), chemical saturation of interface bonds can be achieved. For h-BN, orientations of the basal plane parallel and perpendicular to the surface are both considered. The total energy of atomic clusters constructed to represent these models, are calculated using the AM1 quantum chemical Hamiltonian, and atomic geometries optimised allowing full symmetric relaxation of interface atoms. Only the perpendicular h-BN basal plane model is found to be energetically favourable compared to earlier calculations for cubic boron nitride.     

Enhanced deposition of cubic boron nitride films on roughened silicon and tungsten carbide-cobalt surfaces

We report the influence of substrate surface roughness on cubic boron nitride (cBN) film deposition under low-energy ion bombardment in an inductively coupled plasma. Silicon and cemented tungsten carbide-cobalt (WC-Co) surfaces are roughened by low-energy ion-assisted etching in a hydrogen plasma, followed by deposition in a fluorine-containing plasma. Infrared absorption coefficients are measured to be 22,000 cm⁻¹ and 17,000 cm⁻¹ for sp²-bonded BN and cBN phases, respectively, for our films. For the silicon substrates, the film growth rate and the cBN content in the film increase with increasing the surface roughness, while the amount of sp²BN phase in the film shows only a small increase. A larger surface roughness of the substrate results in a smaller contact angle of water, indicating that a higher surface free energy of the substrate contributes to enhancing growth of the cBN film. For the WC-Co substrates, the film growth rate and the cBN content in the film increase similarly by roughening the surface.     

添加相变材料微胶囊的聚氨酯泡沫制备及表征

在聚氨酯发泡体系中加入正十八烷微胶囊,制备含有相变材料微胶囊的聚氨酯泡沫.相变材料微胶囊能很好的埋置于聚氨酯基体中,泡沫的储热量随微胶囊含量的增大而增大,舍有20%(wt)相变材料微胶囊的泡沫储热量为24.7J/g,该泡沫可用作保温材料.     

Fabrication of a polymer composite with high thermal conductivity based on sintered silicon nitride foam

A novel route was developed to fabricate Si₃N₄/epoxy composite. In this route, the Si₃N₄ particles were constructed into the foamed shape by using protein foaming method, firstly. Then the Si₃N₄ foams were sintered to bond these Si₃N₄ particles together. Finally, the Si₃N₄/epoxy composite was fabricated by infiltrating the epoxy resin solution into the sintered Si₃N₄ foams. This route was proved to be an efficient way in enhancing the thermal conductivity of epoxy matrix at a low loading fraction. For example, the thermal conductivity of the as-prepared Si₃N₄/epoxy composite with a loading fraction of 22.2 vol% was up to 3.89 W m⁻¹ K⁻¹, which was about 17 times higher than that of neat epoxy.     

Chemical stability of hydrogen-containing boron nitride films obtained by plasma enhanced chemical vapour deposition

The purpose of this paper is the investigation of the dehydrogenation kinetics of boron nitride films during thermal annealing. BN_x:H films on silicon substrates were prepared by remote plasma enhanced chemical vapour deposition at 473 K using a mixture of borazine and helium. IR spectroscopy and ellipsometry were used to characterize the film properties and composition. The films contain a certain amount of hydrogen in B---H and N---H bonds. The breakage kinetics of these bonds is different. The breakage of N---H bonds determines the hydrogen annealing kinetics at 973–1073 K. The low-temperature annealing (673–873 K) of B---H bonds is sensitive to the generation of hydrogen from N---H bonds. Heat treatment leads to ordering of the films.