Hexagonal Boron Nitride Powder Research Articles

High hardness cubic boron nitride with nanograin microstructure produced by high‐energy milling

AbstractHigh‐energy shaker milling of hexagonal boron nitride (hBN) powders was used to produce powders rich in sp3 bonding. The powders contained up to 68% sp3 bonding and were found to nucleate nanosize cBN grains during consolidation at 5.5 GPa and 1400°C. The effect of hBN starting particle size, milling time, and powder‐to‐milling ball ratio were studied. The amount of sp3 bonding for milled hBN powders was determined, using 11B solid‐state NMR. The milled material was also analyzed by XRD, Raman spectroscopy, and HRTEM. The results indicate that the material has a nanosized microstructure comprised of a disordered hBN matrix and cBN nuclei in the form of sp3‐rich domains. Eight different milled powders were produced and consolidated at pressures of either 5.5 or 6.5 GPa and temperatures of either 1400°C or 1450°C into 12 mm diameter and 5 mm thick pellets. Consolidated pellets formed from milled hBN with 68% sp3 bonding had Vickers hardness of 42 ± 1 GPa and fracture toughness 3.8 ± 0.1 MPa.m1/2. Vickers hardness of 49 ± 1 GPa and fracture toughness of 4.6 ± 0.1 MPa.m1/2 was achieved with a precursor that contained milled hBN and 50 vol. % of 0.5 μm diameter cBN crystals.

Low temperature synthesis of hexagonal boron nitride by solid state reaction in the presence of lithium salts

In the present study, high crystalline nano sized hexagonal boron nitride powder (h-BN) was synthesized by classical solid-state reaction at 1100 °C using a mixture of boron oxide, urea and lithium salt (Li2O, LiCl, LiNO3). The characterization of samples was done by Fourier transformation infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Lithium salts played a positive role by lowering the formation temperature. The average grain sizes (22–26 nm) and graphitization index (1.7–2.9) of h-BN samples were calculated from XRD pattern. TEM images illustrated that the products were needle-like and multi-layered nano-crystalline h-BN powders.

Enhanced thermal conductivity of poly(vinylidene fluoride)/boron nitride nanosheet composites at low filler content

Due to the growing needs of thermal management in modern electronics, high thermal conductive polymer composites are increasingly demanded. Boron nitride nanosheet (BNNS) was prepared through molten hydroxide assisted liquid exfoliation of hexagonal boron nitride (h-BN) powder and used as thermally conductive filler. The poly(vinylidene fluoride) (PVDF)/BNNS films were obtained through solution blend and hot pressing. With only 4 wt% BNNS, the in-plane thermal conductivity of PVDF/BNNS composite achieved 4.69 W/mK, with a thermal conductivity enhancement of 2297% compared to neat PVDF. However, the through-plane thermal conductivity of the composites is only 0.23 W/mK, which shows a high thermal conductive anisotropy over 20. The thermal conductive anisotropy and the high in-plane thermal conductivity can be attributed to the formation of thermally conductive network in PVDF matrix. Thus, the BNNS reinforced PVDF films are promising for use as an efficient heat spreader for electronic cooling applications.

Large-area few-layer hexagonal boron nitride prepared by quadrupole field aided exfoliation

A quadrupole electric field-mediated exfoliation method is proposed to convert micron-sized hexagonal boron nitride (h-BN) powder into few-layer hexagonal boron nitride nanosheets (h-BNNS). Under optimum conditions (400 Hz, 40 V, 32 μg ml−1, sodium deoxycholate, TAE medium), the h-BN powders (thickness >200 nm, horizontal scale ∼10 μm) are successfully exfoliated into 0.5–4 nm (1–10 layers) thick h-BNNS with the same horizontal scale. Dynamic laser scattering and atomic force microscope data show that the yield is 47.6% (for the portion with the thickness of 0.5–6 nm), and all of the vertical sizes are reduced to smaller than 18 nm (45 layers).

Enhanced Thermal Conductivity of Polyimide Composites with Boron Nitride Nanosheets

A strategy was reported to prepare boron nitride nanosheets (BNNSs) by a molten hydroxide assisted liquid exfoliation from hexagonal boron nitride (h-BN) powder. BNNSs with an average thickness of 3 nm were obtained by a facile, low-cost, and scalable exfoliation method. Highly thermally conductive polyimide (PI) composite films with BNNSs filler were prepared by solution-casting process. The in-plane thermal conductivity of PI composite films with 7 wt% BNNSs is up to 2.95 W/mK, which increased by 1,080% compared to the neat PI. In contrast, the out-of plane thermal conductivity of the composites is 0.44 W/mK, with an increase by only 76%. The high anisotropy of thermal conductivity was verified to be due to the high alignment of the BNNSs. The PI/BNNSs composite films are attractive for the thermal management applications in the field of next-generation electronic devices.

Enhanced tribo-chemical properties of oxygen functionalized mechanically exfoliated hexagonal boron nitride nanolubricant additives

Two dimensional (2D) materials with layered lattice structure as nanofluid additives are useful to improve the tribological properties of metallic sliding interfaces. To enhance the tribological efficiency, the bulk crystalline hexagonal boron nitride (h-BN) powder sample was mechanically exfoliated by ball milling, and further processed through ultrasonication for de-aggregation. High resolution X-ray diffraction (HR XRD) and high resolution transmission electron microscopy (HR TEM) results clearly indicate the exfoliation of bulk h-BN into thinner two-dimensional (2D) crystalline sheets without creating noticeable structural defects. The topography of exfoliated nanosheets is well confirmed by atomic force microscopy (AFM). Oxygen functionalization into the h-BN nanosheets after the mechanical exfoliation was investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Oxygen functionalization of h-BN improved the cohesive compatibility between h-BN and commercial 10W30 lubricant oil for stable dispersion. Friction coefficient and wear of sliding metallic interfaces were reduced significantly in the presence of few layered exfoliated h-BN nanofluid as compared to neat lubricated oil. Micro- XPS and energy-disperse X-ray spectroscopy (EDX) analysis demonstrated the presence of adsorbed h-BN tribofilm in the metallic wear track. Thin 2D sheets of h-BN nanofluid was effective as an additive for low shear resistance under the tribo stressed condition which is the main reason for significant reduction in friction coefficient. Moreover, the enhanced wear resistance of exfoliated h-BN additives was explained by low shear resistance and high compressive/tensile strength of planer sheets which restricted the mechanical damage and protected the metallic interfaces against deformation and wear.

Controlling thermal conductivity of high density polyethylene filled with modified hexagonal boron nitride (hBN)

This study is focused on finding a versatile approach for obtaining low cost, lightweight composites with high thermal conductivity and good mechanical properties at the same time. The most promising application for such material is to replace fabric reinforced laminates in circuit board production. We used high density polyethylene (HDPE) as a polymer matrix and two types of hexagonal boron nitride (hBN) powders with average particle size 2–5 μm and 120–140 nm as a functional filler. To improve both filler-matrix adhesion and mechanical properties of the composites we used one of two techniques: treating the filler with silane coupling agent before composite processing or using titanate coupling agent in the process of roller-mixing. The study showed, that while the best thermal conductivity of 2 W/m K was achieved on the sample prepared without any coupling agent the sample was too brittle for further processing. On the other hand the sample with KR TTS additive and HDPE to hBN ratio 1:1 showed quite high ultimate strength of 24.8 MPa coupled with high Young's modulus, good elongation at break and thermal conductivity of 0.9 W/m K. Usage of hBN nanopowder as a filler resulted in good mechanical properties and high elongation at break values with thermal conductivity values significantly lower than for micro-sized fillers (0.43 W/m K).

Al2O3-coated h-BN composite powders and as-prepared Si3N4-based self-lubricating ceramic cutting tool material

The surfaces of solid lubricant hexagonal boron nitride (h-BN) powders were coated with compacted and integrated Al(OH)3 by heterogeneous nucleation and then the h-BN@Al2O3 composite powders were prepared by calcination under vacuum at 1200°C for 2h, as determined by SEM, EDS and XRD. The Si3N4 self-lubricating ceramic cutting tool material with coated h-BN was prepared by vacuum hot pressing. Microstructure analysis showed that the h-BN@Al2O3 particles were dispersed homogeneously in the matrix and that the particles were tightly bound. Vickers hardness was 14.1GPa, fracture toughness was 7.2MPa·m1/2, and flexural strength was 732MPa, as determined by mechanical property testing. The hardness, fracture toughness, and flexural strength of the material were increased by 27.0%, 5.9% and 22.2%, respectively, relative to the material to which uncoated h-BN was added. Cutting tests showed that the Si3N4/TiC/(h-BN@Al2O3) self-lubricating ceramic cutting tool had better antifriction property and wear resistance than the corresponding cutting tool.

Morphological and phase evaluation of Al/15 wt.% BN nanocomposite synthesized by planetary ball mill and sintering

In this study, Al/15wt.% BN nanocomposite was fabricated by high energy ball milling, cold compaction, and sintering process. Then the effect of milling process on the morphology change and phase evaluation was studied. The Aluminum (45µm) and hexagonal boron nitride (70nm) powders were milled using planetary ball mill under a pure argon atmosphere for various times of 75, 150, 300 and 600min. The as-milled powders were consolidated at 400MPa pressure and sintered at 600°C for 60min. The morphology change evaluation studied by using scanning electron microscopy (SEM) showed that the equiaxed composite powders were obtained after a short time milling process of 150min. The steady state condition between welding and fracturing occurred by continuous milling up to 600min. The results of X-ray diffraction patterns and differential scanning calorimetric (DSC) revealed that the BN particles completely decomposed to boron and nitrogen and dissolved in the Al matrix after 300min of ball milling and the AlN and AlB2 as in-situ phases were formed after sintering at temperatures above 580°C.

An ultrathin high-performance heat spreader fabricated with hydroxylated boron nitride nanosheets

An ultrathin, highly thermally conductive heat spreader has been fabricated by layer-by-layer stacking of hydroxylated boron nitride nanosheets (HBNNS) for the first time. HBNNS were prepared by a molten hydroxide-assisted liquid exfoliation from hexagonal boron nitride powder. The as-prepared heat spreader of HBNNS exhibits a high thermal conductivity of 51.1 W m−1 K−1 along the in-plane direction, and can be further enhanced 14% by annealing for de-hydroxylation. This heat spreader with 10–30 µm in thickness possessed excellent thermal stability with negligible weight loss at wide temperature range up to 700 °C, resulting in promising applications for heat dissipation in electronic components operated at high working temperature.

Microstructures and tribological properties of carbon/carbon-boron nitride composites fabricated by powdered additives and chemical vapor infiltration

The carbon fiber reinforced/carbon-boron nitride (C/C-BN) dual matrix composites were fabricated via adding hexagonal boron nitride (h-BN) powders into the needled carbon felt and subsequent chemical vapor infiltration (CVI) process. An experimental investigation was performed to study the influences of BN volume content on the microstructures and tribological properties of C/C-BN composites. The results indicate that the pyrolytic carbon (PyC) in the C/C-BN composites is regenerative laminar (ReL) due to the inducement of BN powders during CVI process, whereas the PyC in the C/C composite is classic smooth laminar. Additionally, the friction coefficients of C/C-BN composites with three different BN contents in volume fractions (4.5, 9 and 13.5vol%) are all higher compared to the reference C/C composite (0.22). Note that the highest coefficient of friction (0.29) is obtained when the BN volume content in the C/C-BN composite is 9vol%. Moreover, the linear and mass wear rates of C/C-BN composites as well as the 30CrSiMoVA counterparts are significantly decreased with the increase of BN volume content. The favorable friction and wear properties of C/C-BN composites are attributed to the synergistic effect induced by the ReL PyC and BN. The microstructural variation of C/C composites modified by h-BN could improve the compatibility between the C/C-BN composites and 30CrSiMoVA counterpart, resulting in an enhanced adhesive attraction between the wear debris and the surface of 30CrSiMoVA counterpart. Furthermore, the investigations concerning the friction surfaces indicate that the formation of sheet-like friction films with large areas are more easily to occur on the surfaces of 30CrSiMoVA counterparts mating with the C/C-BN composites rather than mating with the C/C composite.

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 10.4nm, 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.

Photo- and cathodoluminescence of cubic boron nitride micropowders activated by Tm, Tb, and Eu rare-earth ions

This work is devoted to the production of cubic boron nitride (c-BN) micropowders that are activated by ions of rare-earth elements, such as europium (Eu), terbium (Tb), and thulium (Tm), as well as to the study of the structural properties and photo- and cathodoluminescence of these micropowders. The micropowders have been synthesized from a hexagonal boron nitride powder in the presence of a catalyst under pressures of 4–6 GPa at temperatures of 1800–1900 K. The activation of the micropowders by the rareearth elements (REEs) has been carried out by introducing the corresponding REE compounds into synthesis precursor. The efficiency of the introduction of an impurity into the c-BN lattice is ~5%. The composition and structure of the samples have been examined using X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive spectrometry. The results obtained during studying c-BN, c-BN:Tm, c-BN:Tb, and c-BN:Eu micropowders using color cathodoluminescence clearly demonstrate their ability to emit light in the wide spectral range, which is of interest for developing new light-emitting devices that are intended for operation in corrosive ambient. An analysis of the photoluminescence spectra of c-BN:REE micropowders has made it possible to find that the observed spectral bands belong to the corresponding transitions between the energy levels of the REEs, as well as to determine the probable positions of Tb3+ and Eu3+ ions in the cubic boron nitride lattice.

Synthesis and characterization of nano hexagonal boron nitride powder and evaluating the influence on aluminium alloy matrix

An attempt is made to synthesize the nano h-BN powders by high energy ball milling and also aluminium reinforced h-BN composite by powder processing route. The impact of ball milling time on microstructure and particle size reduction of h-BN powder was investigated by FESEM, FTIR, XRD, UV-vis diffuse reflectance spectroscopy and particle size analyser.It was indicated that high energy ball milling is effective up to 10 hours in reducing the particles size a further increase in time is resulting in agglomeration. Microhardness of the composite increases in proportion to increases in weight percentage of reinforcement, h-BN exhibited a poor sinterability without any accelerating agentand with tin addition up to 0.5% low melting point metal.TGA and DTA study shows that there is reduction in melting point from 641°C to 638 °C with addition of 2% weight fraction of h-BN.Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.].

Effect of hBN Solid Lubricant Concentration on Machinability of Titanium (Ti-6Al-4V) Alloy

The prime rationale for designers to choose titanium in their designs for aerospace applications is its relative low weight for a given strength level and its relative resistance to high temperature. Excellent biocompatibility makes titanium as ideal material for many biomedical applications. Even though the titanium products are either sintered or cast into required shape, there is a need for machining in order to produce intricate shapes. However machining of titanium alloys poses many serious problems owing to the reactivity of titanium at high cutting temperatures and rapid tool wear. An alternative method to overcome this is by reducing the cutting zone temperature. This can be achieved by the addition of solid lubricants to regular cutting liquids and using it as minimum quantity lubrication (MQL) strategy. In this study, hexagonal boron nitride (hBN) powder with different concentrations (5, 10, 15 wt %) was mixed with water and used as a lubricant. Turning experiments were performed with TiAlN coated Tungsten carbide insert for a constant speed and variable feed rates. For comparison purpose, machining was carried out under dry conditions. Results indicate that the cutting zone temperature reduced drastically on addition of solid lubricant hBN with water. MQL conditions showed that cutting zone temperature decreased by several folds when compared to dry machining. However there was no significant decrease in temperature between 10 and 15 wt% hBN additions which indicates that 10% hBN addition proves to be optimal. This type of machining thereby paves way for sustainable manufacturing.

The Influence of Surface-Active Agent on the Micro-Morphology and Crystallinity of Spherical Hexagonal Boron Nitride

This search used the low-cost boric acid and borax as a source of boron, urea as a nitrogen source, surface-active agent Dodecyl benzenesulfonic acid (SDBS) as a dispersant, and thus prepared different micro-morphology and crystallinity hexagonal boron nitride powders under a flowing ammonia atmosphere at a nitriding temperature of 900 °C. The effect of the surface-active agent (SDBS) content on the crystallinity and micro-morphology of hexagonal boron nitride powders was studied, and the corresponding relationship between micro-morphology and crystallinity was explored. The results showed that under a certain synthetic process (900 °C for 3 h), the surfactant content had affected the crystallinity and micro-morphology of h-BN powders. Without the added surfactant, the graphitization index (GI) was 3.61, and micro-morphology of h-BN powders. was spherical, the distribution of ball diameters was uneven, and there was also significant particle agglomeration, with some particles even exhibiting adhesion, sintering necks, and high sphericity and diameter. When the added SDBS percentages were 2%, 4% and 6%, the graphitization index (GI) decreased to 2.98, 2.58 and 2.41 respectively: the corresponding crystallinity improved significantly. When the surfactant SDBS content was higher (10%), the diameter distribution of the h-BN powders was even, but there was evidence of agglomeration of particles and particle adhesion. The crystallinity decreased when the GI value was increased to 4. When the surfactant SDBS content was 6%, the dispersion of h-BN powders was at its optimum, and the particle size distribution was at its most uniform. Meanwhile the GI value was at its lowest, and the crystallinity at its highest.

Morphological and physical properties and friction/wear behavior of h-BN filled PEEK composite coatings

Hexagonal boron nitride (h-BN) powder with different particle sizes and contents was incorporated in polyether ether ketone (PEEK) under ultrasonic conditions and deposited using flame-spray coating technique. Specific wear rate and friction coefficient of the h-BN/PEEK composite coatings were determined using a ball-on-disk sliding wear tester under applied loads of 5–25N. Microstructure, microhardness, degree of crystallinity and surface roughness of the h-BN/PEEK composite coatings were investigated. The results showed that the microhardness and degree of crystallinity could be improved with h-BN particles. At an applied load of 5N, the addition of h-BN particles into PEEK gave a lower specific wear rate. A higher content of h-BN added to PEEK coating resulted in a decrease in the friction coefficient. At an applied load of 25N, 8wt.% h-BN/PEEK composite coatings provided good friction reduction while the composite coatings formed severe wear with grooves.

The influence of metal Mg on micro-morphology and crystallinity of spherical hexagonal boron nitride

This search used the boric acid and borax as a source of boron, urea as a nitrogen source, Mg as metal catalyst, and thus prepared different micro-morphology and crystallinity hexagonal boron nitride powders under a flowing ammonia atmosphere at a nitriding temperature of 750°C. The effect of Mg content on the crystallinity and micro-morphology of hexagonal boron nitride powders was studied, and the Mg action mechanism was explored. Without the added surfactant, the graphitization index (GI) was 6.87, and the diameter of the spherical h-BN was bigger. When the added Mg were 0.1g, 0.3g, 0.5g and 0.7g, the (GI) decreased to 6.04, 5.67, 4.62 and 4.84, respectively. When the Mg content was higher (0.9g), GI value increased rapidly, and the crystallinity became bad. When the Mg content was 0.5g, the dispersion of h-BN powders was at its optimum and refinement apparently, and the crystallinity at its highest.

Improvement of thermal propagation in carbon fiber/thermoplastic composite with hexagonal boron nitride powder

Improvement of thermal propagation in carbon fiber/thermoplastic composite with hexagonal boron nitride powder

The preparation of high-adsorption, spherical, hexagonal boron nitride by template method

This research used low-cost boric acid and borax as a source of boron, urea as a nitrogen source, dodecyl-trimethyl ammonium chloride (DTAC) as a template, and thus prepared different micro-morphology hexagonal boron nitride powders under a flowing ammonia atmosphere at different nitriding temperatures. The effects of the template content and nitriding temperature on the micro-morphology of hexagonal boron nitride were studied and the formation mechanism analysed. The influences of the template content and nitriding temperature on adsorption performance were also explored. The results showed that at a nitriding temperature of 675°C, the micro-morphologies of h-BN powder were orderly, inhomogeneous spherical, uniform spherical, beam, and pie-like with increasing template content. The micro-morphology was inhomogeneous spherical at a DTAC dose of 7.5%. The micro-morphology was uniform spherical at a DTAC dose of 10%. At a DTAC dose of 12%, the micro-morphology was a mixture of beam and pie-like shapes. At a certain template content (DTAC at 10%) and at lower nitriding temperatures (625°C and 650°C), spherical shell structures with surface subsidence began to form. The porous spheres would appear at a nitriding temperature of 675°C, and the ball diameter thus formed was approximately 500–600nm. The ball diameter was about 600–700nm when the nitriding temperature was 700°C. At a nitriding temperature of 725°C, the ball diameter was between 800 and 1000nm and sintering necking started to form. When the relative pressure was higher, previously closed pores opened and connected with the outside world: the adsorption then increased significantly. The adsorption increased gradually with increased nitriding temperature, and the adsorption was maximised at 276.02cm3/g under nitrogen at 675°C. The adsorption decreased when the nitriding temperature continued to increase to between 700°C and 725°C).