Hexagonal BN Powder Research Articles

Wet-chemical coating of silicon carbide fibers with hexagonal boron nitride layers

Hexagonal BN fiber coatings and BN powders were prepared by pyrolysis of the raw materials boric acid and urea in an atmosphere consisting of hydrogen and nitrogen. The powders were used to determine the appropriate mixing ratio of the raw materials to produce BN with the desired composition and crystal structure. The pyrolysis of boric acid and urea in a molar mixing ratio of 1:2 resulted in a BN that was hexagonal and had a near-stoichiometric composition.To prepare a solution for the coating of fibers, boric acid and urea were dissolved in an ethanol-water mixture. The coating was then applied to SiC filaments using a continuous roll-to-roll dip-coating process. It could be shown by SEM/EDS that BN layers were applied to the fibers. No significant bridging in the fiber bundle was found. Furthermore, it could be demonstrated by grazing incidence x-ray diffraction that the layers were crystalline.

Synthesis and mechanism of TiB2–TiN ceramic composites by high-pressure method

Using pure Ti and hexagonal-BN powder as raw materials, TiB2–TiN ceramic composites were synthesized by high-pressure method at 5.0 GPa and 1400–1800 °C. The phase composition, microstructure, density and hardness of TiB2–TiN ceramic composites were analyzed. The results shown that when the pressure was constant, the synthesis temperature had an important effect on the phase composition of the samples. When TiB2–TiN ceramic composites synthesized at 1800 °C for 30 min, the relative density reached 96.2% and the maximum Vickers hardness reached 17.9 GPa with the load of 4.9 N. The grain size of the TiB2–TiN ceramic composites changed with the change of synthesis temperature, which resulted in the change of density and hardness.

Polycrystalline cubic boron nitride prepared with cubic-hexagonal boron nitride under high pressure and high temperature

Polycrystalline cubic boron nitride (PcBN) compacts, using the mixture of submicron cubic boron nitride (cBN) powder and hexagonal BN (hBN) powder as starting materials, were sintered at pressures of 6.5–10.0 GPa and temperature of 1750 °C without additives. In this paper, the sintering behavior and mechanical properties of samples were investigated. The XRD patterns of samples reveal that single cubic phase was observed when the sintering pressure exceeded 7.5 GPa and hBN contents ranged from 20 vol.% to 24 vol.%, which is ascribed to like-internal pressure generated at grain-to-grain contact under high pressure. Transmission electron microscopy (TEM) analysis shows that after high pressure and high temperature (HPHT) treatments, the submicron cBN grains abounded with high-density nanotwins and stacking faults, and this contributed to the outstanding mechanical properties of PcBN. The pure bulk PcBN that was obtained at 7.7 GPa/1750 °C possessed the outstanding properties, including a high Vickers hardness (∼61.5 GPa), thermal stability (∼1290 °C in air), and high density (∼3.46 g/cm3).

Understanding Aqueous Dispersibility of Boron Nitride Nanosheets from 1H Solid State NMR and Reactive Molecular Dynamics

Stable aqueous dispersions of BN nanosheets can be obtained by the sonication assisted exfoliation of hexagonal BN powders in water without the need of stabilizers: a phenomenon not observed for the isoelectronic graphite, of comparable hydrophobicity. We show here that the aqueous dispersions are stabilized by electrostatic repulsive interactions and establish from ζ potential measurements that the BN nanosheets are positively charged on delamination with the medium turning increasingly basic as sonication proceeds. We have investigated how charge develops on the sheets by reactive force-field (ReaxFF) molecular dynamics simulations of the interaction of water with BN nanosheets and independently identified the major chemical species present on the nanosheets from 1H and 11B solid-state NMR measurements. Charges develop on the sheets as a consequence of the dissociation of water molecules at the edges of the sheet that leave the nitrogen edge atoms protonated and the release of hydroxyl groups into the b...

Role of Hydrogen in High-Yield Growth of Boron Nitride Nanotubes at Atmospheric Pressure by Induction Thermal Plasma.

We recently demonstrated scalable manufacturing of boron nitride nanotubes (BNNTs) directly from hexagonal BN (hBN) powder by using induction thermal plasma, with a high-yield rate approaching 20 g/h. The main finding was that the presence of hydrogen is crucial for the high-yield growth of BNNTs. Here we investigate the detailed role of hydrogen by numerical modeling and in situ optical emission spectroscopy (OES) and reveal that both the thermofluidic fields and chemical pathways are significantly altered by hydrogen in favor of rapid growth of BNNTs. The numerical simulation indicated improved particle heating and quenching rates (∼105 K/s) due to the high thermal conductivity of hydrogen over the temperature range of 3500-4000 K. These are crucial for the complete vaporization of the hBN feedstock and rapid formation of nanosized B droplets for the subsequent BNNT growth. Hydrogen is also found to extend the active BNNT growth zone toward the reactor downstream, maintaining the gas temperature above the B solidification limit (∼2300 K) by releasing the recombination heat of H atoms, which starts at 3800 K. The OES study revealed that H radicals also stabilize B or N radicals from dissociation of the feedstock as BH and NH radicals while suppressing the formation of N2 or N2+ species. Our density functional theory calculations showed that such radicals can provide faster chemical pathways for the formation of BN compared with relatively inert N2.

Highly thermally conductive hexagonal boron nitride/alumina composite made from commercial hexagonal boron nitride

AbstractHexagonal BN is an unusual material in that it is both highly thermally conductive as well as an electrical insulator. Additionally, hBN is also thermally stable in air. This unusual combination of properties makes hBN of significant interest for thermal management. Unfortunately, hBN is not easily consolidated into substrates without the addition of second phases which generally result in poorer thermal performance. This research investigates the potential to utilize this material to dissipate heat from high‐voltage, high‐power electrical devices. Specifically, a process to coat individual platelets of commercial hexagonal BN powder with a layer of amorphous aluminum oxide was developed. The coated hexagonal BN was then hot‐pressed to form a highly thermally conductive substrate. The process to coat hexagonal BN platelets with aluminum oxide was accomplished by mixing hexagonal BN with AlCl3 containing some water, then evaporation of excess AlCl3 to form a Al, Cl, and O layer on hexagonal BN. This product was then heated in air to convert the surface layer into aluminum oxide. Following hot pressing to 1950°C and 10 ksi, the consolidated composite has through‐plane and in‐plane thermal conductivity of 14 and 157 W·(m·K)−1, respectively, at room temperature.

Effect of Temperature on Reaction of Hexagonal BN Powder in Wet Air Between 1073 and 1373 K

The reaction of hexagonal BN powder in wet air at 1073–1373 K for 18 h was investigated using TG, XRD, and SEM analysis. Initially, the oxidation occurred rapidly with simultaneous volatile reaction. At long times, a steady state was achieved and the degradation rate was determined by the volatilization reaction. Temperature affected the reaction behavior. At 1073 K, it consisted of two linear processes. With temperature increasing, initially, the paralinear kinetics was observed and then followed the linear rate. The sample shape also affected the reaction behavior. The reaction kinetics was dealt with Chou's model and gotten good agreement.

Effects of h-BN additive on microstructure, mechanical property and thermal shock resistance of Al2O3 microarc oxidation coating

Aluminium oxide ceramic composite coatings containing hexagonal BN (h-BN) powder were produced successfully by a two-step microarc oxidation process. The incorporation of h-BN powder was achieved by means of wiping an emulsion into the pretreated coatings between these two steps. The thermal shock resistance of the fabricated samples was tested via quenching method. The morphological characteristics and the distribution of elements were analysed by SEM, energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The microhardness of samples was determined by employing a Vickers indenter. The phase composition of composite ceramic coatings was investigated by X-ray diffraction. The results showed that the inclusion of h-BN powder has no influence on the growth and microhardness of alumina ceramic coatings. However, the modified coatings exhibit lower area and number of pores than conventional coatings. More significantly, the incorporation of a certain amount of h-BN powder improves the thermal shock resistance of aluminium oxide ceramic composite coatings even in a thick coating.

Effect of Water-Vapor Content on Reaction Rate of Hexagonal BN Powder at 1273 K

AbstractHexagonal BN powder with average particle size of 1.2 µm was exposed at 1273 K in H2O/air gas mixture with the range of mass ratio from 0:100 to 6.5:100 at a total pressure of 1 atm. Thermalgravimetry (TG) method was used to measure the reaction behavior of BN powder in water vapor. XRD and SEM were employed to analyze the phases and morphological revolution during reaction. The TG curves showed that the overall reaction process could be divided into two stages. In first stage, weight gained rapidly and the main reaction was the oxidation of BN. The weight gain turning point of the curves with different water-vapor contents varied, indicating that water-vapor had an impact on oxidation rate to some degree. In second stage, weight loss due to B2O3 reacting with H2O to form volatile product. The rate of weight loss increased with increasing the water vapor content. The reaction rate was described by a linear rate law, i.e., α = −klt. The relationship between kl and partial pressure of water vapor p was described by a function of kl = 0.4p. The determination of the activation energy was complicated due to variations in the reaction mechanism.

Structures and magnetic properties of Fe–N alloys formed by ball milling of mixture of Fe and BN

Diffusion reaction between Fe and BN and structural and magnetic properties of Fe–N alloys are investigated by ball milling of mixture of the α-Fe and hexagonal BN powders with volume ratio of 1:12.5, 1:5.4, 1:1.2 and 1:0.38 up to 72h. It is found that Fe can react with N of the BN to form amorphous Fe–N alloy (α-Fe–N) when the grain size of the Fe decreases to 8nm and the hexagonal BN transforms into amorphous BN; further milling, α-Fe–N can crystallize to ε-FexN. The formation of ε-FexN is related to the volume ratio of Fe to BN, when the ratio is larger than 1:0.38, no ε-FexN forms. The N content of ε-FexN increases with milling time and the volume ratio increased, resulting in decrease in saturation magnetization of ε-FexN. ε-FexN transforms from ferromagnetism to paramagnetism, when the N content exceeds 43at%.

Microstructures and grain boundaries of cubic boron nitrides

Two types of chemically pristine polycrystalline cubic boron nitrides (c-BN) are sintered at different conditions using the starting cubic and hexagonal BN powders and their microstructures and grain boundaries are investigated systematically by transmission electron microscopy. The two c-BN samples are found to exhibit a number of twins inside their grains and have a similar grain size, despite their huge differences in the grain size of the starting powders and sintering conditions. Twin width for the c-BN sintered from hexagonal BN is significantly smaller than that for the c-BN sintered from c-BN. Grain boundaries in the two samples can be atomically abrupt without any amorphous or secondary layers and oxygen is detected merely at the grain boundaries of the c-BN sintered from the c-BN powders. Such microstructural differences have a direct impact on mechanical behaviors of the c-BN, as the Vickers hardness of the c-BN sintered from hexagonal BN powders is found to be higher than that of the c-BN sintered from the c-BN powders.

Preparation and wear performance of NiCr/Cr3C2–NiCr/hBN plasma sprayed composite coating

NiCr clad hexagonal BN powder (NiCr/hBN) was added to NiCr/Cr3C2 feedstock to improve the tribological properties of chromium carbide nichrome coating. The microstructure, flowability and apparent density of the composite powder, as well as the structure and mechanical properties of the plasma sprayed coating were characterized. The friction and wear behavior of the NiCr/Cr3C2–NiCr/hBN coating from ambient temperature up to 800°C was evaluated on a ball-on-disk wear tester and compared with that of NiCr/Cr3C2 coating and NiCr/Cr3C2–NiCr/BaF2·CaF2 coating. The results show that NiCr cladding can reduce the decarburization of Cr3C2 and oxidation of hBN during the thermal spray. The main wear mechanisms of the NiCr/Cr3C2–NiCr/hBN composite coating are ploughing and adhesive wear. Layered hexagonal BN particle reduce the direct contact and severe adhesion between friction pairs, thus decreasing the friction coefficient. The NiCr/Cr3C2–NiCr/hBN composite coating shows a promising application in the high temperature environment with the request of both wear resistance and friction reduction.

Photoluminescence of Self-Trapped Excitons in Boron Nitride Nanotubes

The excitation energy dependence and temperature dependence of photoluminescence from boron nitride nanotubes and hexagonal BN powder samples are reported. The results are discussed within a model attributing the broad 3.2 eV luminescence from these samples to self-trapped excitons in the low-dimensional structures of BN nanotubes and of nano-arch surface reconstructions on h-BN sheet edge faces in powder. An empirical model accounting for the unusual combination of excitation and temperature dependence of photoluminescence seen in these measurements is suggested. For the model to be consistent with the hypothesis of self-trapped excitons on BN nanotubes, it may be necessary to show that the cores of multiwall nanotubes are selectively probed by light tuned below the h-BN exciton.

Synthesis of nanocrystalline Zr(B1−xNx) by mechanical alloying

Abstract Investigation on the structural evolution of the mixture of Zr and hexagonal BN (h-BN) powders during mechanical alloying (MA) was performed by means of X-ray diffractometry, Fourier transform infrared spectroscopy and scanning electron microscopy. The grain size of h-BN decreases rapidly with increasing milling time, which makes the diffraction peaks disappear at the initial stage of MA. And then most of the N and B atoms diffuse into Zr grains to form a (Zr, B, N) solid solution. An fcc phase Zr(B1−xNx) was found in the sample milled for about 40 h, and all the sample transformed into nanocrystalline Zr(B1−xNx) when the mixture was ball milled for longer than 48 h. The (Zr, B, N) solid solution may transform into Zr(B1−xNx) during annealing at about 900 °C, which confirmed that the formation of Zr(B1−xNx) is a diffusion process.

Light emission and excitonic effect of boron nitride nanotubes observed by photoluminescent spectra

Photoluminescent (PL) and optical absorption spectra of high-yield multi-wall BN nanotubes (BNNTs) were systematically investigated at room temperature in comparison with commercial hexagonal BN (h-BN) powder. The direct band gap of the BNNTs was determined to be 5.75eV, just slightly narrower than that of h-BN powder (5.82eV). Two Frenkel excitons with the binding energy of 1.27 and 1.35eV were also determined. However, they were not a distinctive characteristic of the BNNTs as reported previously. Observed broad UV–visible–NIR light emission demonstrates the potential of the BNNTs as a nano light source.

Synthesis, characterization and properties of novel BN nanocages from a single-source precursor

In this Letter, we report the synthesis of hexagonal crystalline BN powders with novel cage-like structures from a single-source precursor NH 4BF 4 at 400–600 °C for 12 h. The obtained samples are characterized by XRD, XPS, TEM and HRTEM. Experiments reveal that the as-prepared BN powders exist in multiwall nanocages with dimensions from about 20 to 200 nm and have very good anti-oxidation properties at high temperatures.

Influence of stearic acid on mechanochemical reaction between Ti and BN powders

The influence of stearic acid as a process control agent on the mechanochemical reaction between Ti and BN to form TiN/TiB 2 nanocomposite powder by high energy ball milling has been investigated. A powder mixture of pure Ti and hexagonal BN powders with a molar ratio of 3:2 was milled for up to 40 h with up to 1.75 wt.% of stearic acid. The XRD analysis shows that an intimate mixture of TiN and TiB 2 crystalline powders was formed during milling by a displacement reaction. It has been observed by monitoring the temperature of the vial surface during the milling process that the addition of over 1.5 wt.% of stearic acid changed the reaction mode from a mechanically induced self-propagating reaction (MSR) to a gradual reaction. It has also been found by XRD and TEM analyses that the MSR produces TiN and TiB 2 particles initially of sub-micron size, which are reduced gradually by further milling. However, there still remain some coarse TiB 2 particles larger than a few hundred nanometers even after 16-h milling due to their extremely high hardness. On the other hand, the gradual reaction induced by stearic acid successfully inhibits the formation of coarse TiB 2 particles and eventually decreases the crystallite size of the products below 40 nm after 16 h of milling.

Detailed Structures of Hexagonal Diamond (lonsdaleite) and Wurtzite-type BN

Hexagonal diamond (hDIA) and wurtzite-type BN (wBN) powders were synthesized using a Kawai-type high-pressure apparatus and the essential details of their structures were examined by Reitveld refinements in order to investigate their thermodynamic stability and transition mechanism. X-ray diffraction profiles of the products were well explained by a mixture of hDIA and cubic diamond (cDIA) with stacking faults. The mass fraction of hDIA and cDIA was 50:50 for the products annealed between 800 and 1400°C and it became 20:80 for the product annealed at 1600°C. Temperatures higher than 1600°C seem to favor the formation of cDIA or to induce the conversion from hDIA to cDIA. Structure refinement revealed that a decrease and an increase in the basal and apical distances of C–C and B–N bonds in hDIA and wBN, respectively, are introduced by lowering the symmetry from cubic to hexagonal. Since the relative stability of wurtzite-type compounds largely depends on the distortion of the tetrahedral bond angle, the deviation from the ideal tetrahedron in both hDIA and wBN was refined to discuss their stability. The transition mechanism from graphite and graphite-like BN to hDIA and wBN is discussed by comparing the present results and those of previous simulation studies. Based on analogous features observed in the synthetic hDIA and lonsdaleite (natural hDIA found in meteorites), the formation mechanism of hDIA in meteorites is proposed.

Preparation of Ultrafine Boron Nitride Powders by Self-propagating High-Temperature Synthesis

The possibility of preparing ultrafine boron nitride powders by self-propagating high-temperature synthesis (SHS) is examined. The results demonstrate that, by varying SHS conditions (starting-mixture composition, combustion rate, cooling rate, and intermediate grinding time), one can control the morphology and particle size of the resultant hexagonal BN powder. Systematic data are presented on the chemical dispersion of SHS products for separating nanometer-sized particles. The effect of chemical dispersion (composition of the dispersion medium, process temperature, and dispersion time) on the particle size, morphology, and chemical and phase compositions of boron nitride powders is analyzed. Hexagonal boron nitride powders are obtained with a purity of 99.5+%, particle size of 0.1–0.2 μm, and specific surface of up to 65 m2 /g.

The influence of the h-BN morphology and structure on the c-BN growth

The morphology and structure of hexagonal graphitic BN (h-BN) powders with graphitization indices GI < 5, used as precursors for the synthesis of cubic BN (c-BN) crystals, has been investigated by transmission electron microscopy in diffraction contrast and high resolution. We show that besides the GI, which is a general parameter for controlling the structural quality of h-BN powders, some other microstructural features strongly influence the synthesis of c-BN. In our opinion, the high reactivity of some h-BN powders results from the presence of some nucleation centers for c-BN, observed at the edges of the h-BN particles. They are formed by a rearrangement of the graphitic (0002) planes by bending back, joining in pairs and forming locally nanoarches (half nanotubes). In these particular places, the nature of bonding locally turns towards sp 3 , as in the case of c-BN.