Application of h-BN particles using Ca-assisted carbothermal reduction nitridation to high-thermal-conductive resin/h-BN composites

Hexagonal boron nitride (h-BN) particles exhibit high thermal conductivity and are promising fillers for resin filling. However, h-BN particles are plate-like particles with thermal anisotropy in the planar and thickness directions. Therefore, their applications are limited due to low thermal conductivity in the direction of the thickness of a resin sheet filled with h-BN particles. In this study, we control the size and thickness of h-BN particles using carbothermal reduction nitridation (CRN), which involves the carbothermic reduction of boric oxide in an N₂ gas atmosphere and develop them into resin sheets. In CRN using a CaO promoter, a novel method is developed to control the shape, size, and thickness of h-BN particles. Using h-BN particles grown in the thickness direction, we have successfully provided resin sheets with high thermal conductivity.     

Catalytic effect of alkaline earth oxides on carbothermic formation of hexagonal boron nitride

Effect of MgO, CaO and BaO on carbothermic formation of hexagonal boron nitride (h-BN) was investigated. B₂O₃–C mixtures containing alkaline earth oxide additives were reacted at 1500 °C for 30–120 min in nitrogen atmosphere. Formed phases in the reaction products were determined by powder-XRD analyses, and amounts of the constituents were determined by chemical analyses. Particle size and morphology of the formed h-BN powders were examined by FESEM and particle size distributions were determined by particle size analyzer. Addition of alkaline earth oxides was found to increase the amount and grain size of h-BN significantly and to decrease the amount of B₄C formed in the system. Investigated alkaline earth oxides presented similar catalytic effects according to chemical analyses and FESEM observations. While the average particle size of h-BN powder obtained from plain mixture was 149 nm, those obtained from MgO, CaO and BaO containing mixtures were 297, 367 and 429 nm, respectively.     

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.     

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.     

A simple,low cost,and template-free method for synthesis of boron nitride using different precursors

In this research, hexagonal boron nitride (h-BN) was synthesized using a simple, low cost, and template-free method with urea-boric acid (UB), melamine-boric acid (MB), and melamine-urea and boric acid (MUB) precursors, followed by the pyrolysis and heat treatment in a nitrogen atmosphere at 1050 °C. Samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Raman Spectroscopy, Fourier transform IR (FT-IR), and Brunauer–Emmett–Teller (BET) techniques. The specific surface areas obtained for h-BN synthesized by UB, MB, and MUB precursors were 87.43, 573.07, and 1005.7 m²/g, respectively. The average diameters of the pores using the Barrett, Joyner, and Halenda (BJH) model were 37.78, 3.68, and 2.13 nm, respectively. A thermogravimetric analysis showed a wider range of decomposition temperatures after using three precursors for synthesizing h-BN. Crumpled, whisker, and flower-like morphologies for UB, MB, and MUB precursors were respectively found using FESEM investigations. The formation of h-BN within the MUB sample was confirmed using the XPS analysis with measured peaks of 398.5 and 190.6 eV belonging to N 1s and B 1s, respectively. Raman spectroscopy revealed a high-intensity peak in 1366 cm^?1 related to the E_2g mode for h-BN synthesized with MUB. Therefore, the results demonstrate that the employed method can increase the potential of using the h-BN porous powder with a high specific surface area as a lubricant, thermal insulation filler, anti-corrosion filler in paint coatings, adsorption of various gas and hydrocarbon molecules as well as its application in drug-delivery nanocarriers.     

Synthesis and reaction sintering of YB4 ceramics

Within this work, the preparation of yttrium tetraboride (YB₄) in the form of powder as well as bulk material was investigated.Powders were synthesized via four different reaction methods, including direct synthesis from elemental powders, reduction of yttrium oxide with boron, boron carbide reduction, and combined boron carbide/carbothermal reduction at 1500 °C, 1700 °C and 1900 °C. Pure YB₄ powder was successfully synthesized using the combined boron carbide/carbothermal reduction method. Secondary phases, especially Y₂O₃, YB₂ or YBO₃, were found in powders prepared using the other three methods.Bulk material was prepared using direct synthesis from elements by reactive hot-pressing. Influence of temperature and boron content on densification and phase evolution of samples was studied. In situ reaction sintering was performed using conventional hot-pressing at temperatures from 1100 °C to 1800 °C in vacuum. The amount of boron varied from the stoichiometric content to 5 and 10 wt% excess (with respect to the reaction from elemental powders). Stoichiometric reactions led primarily to the formation of YB₂ and YB₄ and several secondary phases such as Y₂O₃, YBO₃ and Y_16.86B₈O₃₈. YB₄ as a main phase was formed only at elevated temperatures (1700 °C and 1800 °C) but certain content of impurities was still present. Excess of B resulted in the formation of YB₄ as a primary phase in all prepared samples with a small content of YBO₃ and/or Y_16.86B₈O₃₈. Moreover, SEM analysis revealed the presence of unreacted boron.     

Sol–Gel Synthesis and Formation Mechanism of Ultrahigh Temperature Ceramic: HfB2

Hafnium diboride (HfB₂) powder has been synthesized via a sol–gel‐based route using phenolic resin, hafnium chloride, and boric acid as the source of carbon, hafnium, and boron, respectively, though a small number of comparative experiments involved amorphous boron as boron source. The effects of heat‐treatment dwell time and hafnium:carbon (Hf:C) and hafnium:boron (Hf:B) molar ratio on the purity and morphology of the final powder have been studied and the mechanism of HfB₂ formation investigated using several techniques. The results showed that while temperatures as low as 1300°C could be used to produce HfB₂ particles, the heat treatment needed to last for about 25 h. This in turn resulted in anisotropic particle growth along the c‐axis of the HfB₂ crystals yielding tube‐like structures of about 10 μm long. Equiaxed particles 1–2 μm in size were obtained when the precursor was heat treated at 1600°C for 2 h. The reaction mechanism involved boro/carbothermal reduction and the indications were that the formation of HfB₂ at 1300°C is through the intermediate formation of an amorphous B or boron suboxides, although at higher temperatures more than one reaction mechanism may be active.     

Reduced-graphene-oxide-reinforced boron carbide ceramics fabricated by spark plasma sintering from powder mixtures obtained by heterogeneous co-precipitation

Reduced graphene oxide (rGO) sheets were uniformly dispersed in boron carbide ceramics by a heterogeneous co-precipitation method. This approach was used to improve the fracture toughness of boron carbide ceramics and to address the problem of agglomeration of graphene in the boron carbide matrix. Cetyltrimethyl ammonium bromide was used as a heterogeneous co-precipitation reaction initiator to prepare a homogeneously dispersed graphene oxide/boron carbide (GO/B₄C) mixture. Reduced graphene oxide/boron carbide (rGO/B₄C) powder mixtures with good dispersion were obtained by high temperature heat treatment. Dense rGO/B₄C composite ceramics were fabricated by spark plasma sintering at 1800 °C and 50 MPa. The fracture toughness and flexural strength of the rGO/B₄C with an rGO content of 2 vol% composite increased by 42% (from 3.43 to 4.88 MPa·m^1/2) and 28% (from 372 to 476 MPa) compared with those of pure B₄C, respectively. The markedly improved fracture toughness and flexural strength of the boron carbide ceramics were attributed to the effect of crack bridging and crack deflection by graphene sheets, graphene interface sliding, and pulling out of graphene.     

Carbon-containing droplet combustion–carbothermal synthesis of well-distributed AlN nanometer powders

A novel three-step technique was employed to synthesize the well-distributed AlN nanopowders. First, the hollow spherical precursor particles with an average diameter of 2–5 μm, consisting of an amorphous structure mixture of Al₂O₃ and C, was prepared by carbon-containing droplet combustion method by using glucose, urea, and aluminum nitrate as starting materials. The carbothermal reduction and nitridation (CRN) was carried out at 1500°C under N₂ flow for 2 h and subsequently the CRN product was calcined at 700°C in air for 1 h to remove residual carbon and transform the CRN product to high-purity AlN powders consisting of nanostructured hollow spheres. The formation mechanism of precursor and AlN hollow spheres was discussed in detail. The AlN powder exhibited well-distributed spherical particles with a size of 30–50 nm and good sinterability. After additive-free and pressureless sintering at 1800°C for 2 h, the relative density of the sintered AlN sample was measured to be 99.02%.     

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.     

Investigation of non-isothermal crystallization,dynamic mechanical and dielectric properties of poly(ether-ketone) matrix composites

ABSTRACT

Poly(ether-ketone)/hexagonal boron nitride (h-BN) composites reinforced with micrometer-sized h-BN particles were investigated. The composites exhibited glass transition temperature (T_g) and thermal stability over 160°C and 560°C, respectively. The melting point and peak crystallization temperatures of the composites decreased up to 17°C and 12°C, respectively. The linear CTE of the composites decreased both below and above the T_g. The storage modulus increased with increasing h-BN content at all temperatures (50–250°C). The composites possessed excellent dielectric properties with insignificant dispersion with increasing frequency. Thus, resultant composites are promising candidates for the printed circuit boards/electronic substrates.     

Preparation of spherical AlN powders by combined microemulsion method and carbothermal method

In this study, a two-step strategy for the preparation of micron-sized spherical aluminium nitride (AlN) powder by the combined micro-emulsion method in conjunction with the carbothermal reduction nitridation (CRN) route was designed. The spherical AlN powder with perfect dispersibility was prepared after a heat treatment at 1550 °C for 2 h in flowing N₂. The effects of the aluminium fluoride (AlF₃) content, reaction temperature and the introduction of yttrium oxide (Y₂O₃) on the nitridation ratio and on the morphology of granules, in particular, were investigated by XRD analysis and SEM. Additionally, the promotion mechanism of AlF₃ and Y₂O₃ on the nitridation reaction was also discussed. Specifically, one of the underlying formation mechanisms of the spherical granules with the aid of AlF₃ and Y₂O₃, and suggestions on the selection of additives for the CRN synthesis of spherical AlN powder were logically proposed.     

A new approach to the synthesis of nanosized powder CaO and its application as precursor for the synthesis of calcium borates

A procedure for the synthesis of calcium oxide has been developed, which consists in heat treatment of an aqueous solution of calcium acetate and d-glucose at 350 and then at 700 °C. The process parameters have been determined. It has been shown that when d-glucose is used and the reaction mixture is heat treated at 700 °C, highly dispersed calcium oxide with an average particle size of 77 nm is formed. CaO formed has been used as a precursor for the synthesis of priceite (Ca₂(B₅O₇)(OH)₅ · H₂O) during hydrothermal treatment of CaO in an aqueous solution of boric acid. It has been found that the optimal conditions for the synthesis of monophase priceite under hydrothermal conditions include the temperature range of 170–200 °C and the heating time of 12 h. When heating priceite at 800 °C in air for 1 h, highly dispersed powder calcium bis(borate) Ca(BO₂)₂ has been isolated as the final product. It is shown that the use of synthesized powder CaO leads to the formation of fine-crystalline powders of calcium borates. Samples obtained at each stage of the proposed synthesis of highly dispersed calcium oxide and calcium borates, using the example of priceite and calcium bis(borate), have been studied by powder X-ray diffraction, SEM, TEM, BET, IR spectroscopy, and DTA.     

Fabrication of Si3N4 preforms from Si3N4 produced via CRN technique

Ceramic preforms with randomly distributed particles as reticulated porous structure which are generally used for metal infiltration as reinforcement, membranes, catalyst supports etc. Preforms are characterized by open porosity making possible their infiltration by liquid metal alloys. In this work, quartz powders using carbon black as a reducing agent were used for alpha Si₃N₄ powders synthesis through a carbothermal reduction and nitridation (CRN) process. The CRN process was carried out under nitrogen flow at 1,450 °C for 4 h. At high temperatures, carbon as reducing agent reacts with the oxygen of SiO₂, and the resulting metallic silicon compounds with nitrogen gas to obtain silicon nitride powder. The reacted powders were used to obtain reticulated ceramic by replica method. The powders containing various bentonite ratios were mixed in water to prepare slurry. The slurry was infiltrated into a polyurethane sponge. A high porous ceramic foam (preform) structure was achieved after burn out of the sponge. All ceramic preforms were sintered to increase stiffness (in the temperature range 900–1,350 °C). The sintered ceramic foams were subjected to compressive tests. The scanning electron microscopy was used to examine the reticulated ceramic foam structure, and X-ray diffraction analysis was performed to determine phases.     

Characterization of boron nitride film synthesized by helicon wave plasma-assisted chemical vapor deposition

The microstructure of boron nitride film grown on Si (100) at various temperatures by a helicon wave plasma chemical vapor deposition using borazine as a precursor was investigated. The optimum substrate bias voltage for c-BN growth by the employed deposition process ranged from −200 to −400 V. HRTEM images revealed that the film included an interlayer of a-BN and h-BN followed by c-BN layer. A sufficient accumulation of compressive stress is required before c-BN growth. With increasing interlayer thickness and random orientation at high growth temperatures, residual compressive stress seems to decrease owing to an annealing effect. At the initial c-BN growth stage, the congruent growth of hexagonal and cubic phases occurs at low temperatures of 300 and 500°C; however, c-BN growth proceeds only after the formation of h-BN layer at the high temperature of 800°C. The hydrogen content in the BN films synthesized at lower temperatures was ∼8%, while that of the BN film synthesized at 800°C was ∼2.6%. In addition, with increasing the temperature, the decreasing tendency in c-BN IR mode FWHM indicates enhancement of c-BN crystallinity.     

Synthesis of HfB2 powders by mechanically activated borothermal reduction of HfCl4

HfB₂ powders were synthesized via a borothermal reduction route from mechanically activated HfCl₄ and B powder blends. Mechanical activation of the powder blends was carried out for 1 h in a high-energy ball mill using hardened steel vial and balls. Mechanically activated powders were subsequently annealed at 1100 °C for 1 h under Ar atmosphere. Then, purification processes such as washing with distilled water and leaching in HCl solution were applied for the elimination of the undesired boron oxide (B₂O₃) phase and the probable Fe impurity. The effect of boron amount on the microstructure of the resultant powders was investigated. The boron amount in the starting blends plays an important role in the formation of the HfO₂ phase. HfB₂ powders without any detectable HfO₂ were prepared by adding 20 wt% excess amount of boron. Microstructural analyses of the mechanically activated, annealed and purified powders were performed using X-ray diffractometer (XRD), particle size analyzer (PSA), stereomicroscope (SM), scanning electron microscope/energy dispersive spectrometer (SEM/EDS) and transmission electron microscope (TEM).     

Synthesis of cubic aluminum nitride by carbothermal nitridation reaction

Cubic aluminum nitride (AlN) was synthesized by the carbothermal nitridation reaction of aluminum oxide (Al₂O₃). The effects of Al₂O₃ particle size, reaction temperature and reaction time on the synthesis of cubic AlN were investigated, and the reaction mechanism was also analyzed. The results showed that cubic AlN could be formed at a lower temperature with fine Al₂O₃ powder than with coarse Al₂O₃ powder. The cubic AlN may be the product of Al₂₃O₂₇N₅ synthesized from Al₂O₃ and hexagonal AlN, and transforms into hexagonal AlN at temperatures above 1800°C.     

Emissionspectroscopy of Boron Ignition and Combustion in the range of 0.2 m̈m to 5.5 m̈m

Boron particle combustion is retarded by initial presence of an oxide coating. In current boron ignition models, oxygen is assumed to desolve in the oxide and diffuse to the B/B₂O₃ interface for reaction. One method to observe the reaction of boron with its surrounding atmosphere is the time resolved emission spectroscopy we applied in the range of 0.2 m̈m to 5.5 m̈m for different burning processes. In one experiment boron powder was burned in oxygen atmosphere initiated by an efficient pyrotechnic device. The energy transfer by the hot gases led to a glowing phase of the boron particles which then changed to a high temperature combustion and ended in a further glowing phase. The two glowing phases emitted continuous emission spectra, while the burning phase emitted the bands of BO and BO₂. Another experimental setup was used to feed boron particles in a hot oxidizing atmosphere provided by a propane/air flame which flew into a combustion chamber. Herein the reaction of boron was recorded with high speed cinematography and time resolved emission spectroscopy. The flame contained a small amount of background radiation and we could identify emitted bands of BO, BO₂, HBO₂, CO and CO₂.     

Synthesis of ultra‐fine TaB2 nano powders by liquid phase method

Ultra‐fine TaB₂ powders were synthesized by a liquid phase method using tantalum ethoxide, boric acid and sucrose as the sources of tantalum, boron, and carbon. The TaB₂ precursor powders is a Ta–B–C–O network system, which were heat‐treated at lower temperature (1500°C) in normal argon atmosphere to obtain the TaB₂ powders. XRD confirms the presence of only hexagonal TaB₂, while EDS and XPS spectrums confirm the composition and element chemical states of TaB₂. The TEM images show a platelet shape of the TaB₂ powder and the monocrystal SAED pattern confirms the presence of hexagonal TaB₂. Particle size distribution curves show that particle size of the TaB₂ powders distributes in the range of 30‐160 nm, whose mean particle size is 106 nm.