BN Powders Research Articles

Combustion synthesis of Ti(C, N)–TiB 2 from a Ti–C–BN system

In this paper, a simple way of fabricating TiC x N y –TiB 2 ceramics through the combustion reaction of Ti, C and BN powder mixtures in an argon atmosphere is reported with an emphasis on the effects of the C/(C + N) ratio on the SHS reaction behaviors and mechanism. With the increase in the C/(C + N) ratio, the combustion temperature shows a zigzag variation behavior; the combustion wave velocity displays a similar variation tendency as did in the combustion temperature while the ignition delay time increases progressively. XRD results confirmed that TiC x N y –TiB 2 could form in all the samples. Microstructural observations revealed that both TiC x N y and TiB 2 grains had fine sizes of less than 1 μm in the products when the C/(C + N) ratio was lower than 0.5. Based on the characterization of quenched samples, the formation mechanism of the titanium carbonitride is proposed. Namely, the formations of TiN 0.3 and TiN are followed by the incorporation of C in TiN x to form the titanium carbonitride solid solution.

Low-Temperature Densification of TiN-TiB2Composites Through Reactive Hot Pressing with Excess Ti Additions

Reactive hot pressing of Ti and BN powder mixtures is used to produce dense TiNx–TiB2 composites. The effect of excess Ti along with a small addition, ∼1 wt% Ni, on the reaction and densification of the composite was investigated. A composite of ∼99.9% relative density (RD) was produced at 1200°C at 40 MPa for 30 min with 1 wt% Ni, whereas composites produced without Ni are porous and contain residual reactants. The microstructural studies on composite samples with excess Ti produced at short durations indicate the presence of a transient (Ni–Ti) phase from which Ti is finally removed to form substoichiometric TiNx. The hardness of the dense TiNx–TiB2 composite is ∼22 GPa. The densification mechanism in this system is contrasted with the role of nonstoichiometry in the Zr–B4C system.

Silicon nitride/boron nitride ceramic composites fabricated by reactive pressureless sintering

Using Si and BN powders as raw materials, silicon nitride/hexagonal boron nitride (Si 3N 4/BN) ceramic composites were fabricated at a relatively low temperature of 1450 °C by using the reaction bonding technology. The density and the nitridation rate, as well as the dimensional changes of the specimens before and after nitridation were discussed based on weight and dimension measurements. Phase analysis by X-ray diffraction (XRD) indicated that BN could promote the nitridation process of silicon powder. Morphologies of the fracture surfaces observed by scanning electron microscopy (SEM) revealed the fracture mode for Si 3N 4/BN ceramic composites to be intergranular. The flexural strength and Young's modulus decreased with the increasing BN content. The reaction-bonded Si 3N 4/BN ceramic composites showed better machinability compared with RBSN ceramics without BN addition.

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.

Consolidation of SiC/BN composite through MA-SPS method

Hexagonal-BN has been selected as a second phase for SiC/BN composite to improve SiC’s machinability and thermal shock resistance. In this research, nano-metric SiC/BN was prepared through mechanical alloying (MA) from Si + C + BN powder and then consolidated by SPS without any sintering aids. XRD results after MA revealed the absence of sharp peaks corresponding to SiC and BN. The density and the intensity of the SiC and BN peaks on XRD increased with temperature during SPS. The final density of the composite reached approximately 90–99% with 50/50 of SiC/BN to 100/0. During the consolidation process, crystallization, phase separation, and ordering were observed simultaneously. This phenomenon could accelerate the mass transfer for the consolidation and the preparation of bulk SiC/BN composite without any sintering aids. In a 50/50 SiC/BN ratio, the Vickers hardness of the nano-structured reference sample prepared by the conventional method with sintering aids could not be measured due to high porosity. However, the well-consolidated sample prepared in our research showed a hardness of approximately 3 GPa.

Dense, fine-grain Mo-Si-B alloys from nitride-based reactions

Mo-Si-B intermetallic alloys have the potential to possess the oxidation resistance and mechanical properties required for the next generation of jet turbine engine blades. In a novel approach, three-phase alloys of α-Mo, Mo3Si (A15), and Mo5SiB2 (T2) have been produced through the reaction of molybdenum, Si3N4, and BN powders. Formation of the intermetallics was examined by thermo-gravimetric and x-ray diffraction analysis. Densities of 94% of theoretical were achieved from pressureless sintering at 1,600°C. Microstructure examination using scanning electron microscopy and electron backscatter diffraction analysis shows that this powder metallurgy approach yields a fine dispersion of intermetallics in a molybdenum matrix with grain sizes on the order of 1–4 µm. The formation of Mo-Si-B alloys from the nitride-based reactions uses standard powder processing methods and provides a method for creating these materials in a much less complex and expensive manner than has been previously demonstrated.

Influence of the Support Surface Chemistry on the Catalytic Performances of PdO/BN Catalysts

Two high surface areas BN powders have been prepared to be used as supports for palladium catalysts in propylene oxidation. Two different precursors originating from borazines were used for BN preparation. The BN prepared from trichloroborazine (TCB) had a surface area of 184 m 2 g −1 and micrometric spherical (ball-shaped) texture. When polytrimethylaminoborazine was used as a precursor, the support obtained (BN 1− x O x ) presented an oxi-nitride structure with a surface area of 115 m 2 g −1 and a glassy aspect. Palladium oxide catalysts were prepared by impregnation of these two supports. Characterization techniques, such as BET, XRD, TEM, XPS, TG, and electrical properties measurements, have been used to study the influence of the surface chemistry of the support on the catalytic performances of the prepared catalysts. The acidic and redox properties of the resulting samples were characterized by TPR and adsorption microcalorimetry. The presence of oxygen in the structure of BN 1− x O x support induced different behaviors when compared with pure BN and played an important role in the catalytic activity of PdO/BN 1− x O x sample.

Synthesis of SiC/BN nanocomposite powders by carbothermal reduction and nitridation ofborosilicate glass, and the properties of their sintered composites

A new synthetic method for the fabrication of SiC/BN nanocomposites was devisedto attain strong machinable ceramics. SiC/BN nanocomposites that contained10, 20, and 30 vol% hexagonal BN (h-BN) were successfully fabricated bysintering SiC–BN nanocomposite powders by carbothermal reduction andnitridation of borosilicate glass powders. Homogeneous mixtures of silica(SiO2), boricacid (H3BO3), and carbon powder were heated in a nitrogen atmosphere to synthesizeSiC–BN nanocomposite powders. Borosilicate glass was obtained by reactingSiO2 andB2O3 above800 °C, and SiCand turbostratic BN (t-BN) were obtained by reacting borosilicate glass with carbon powder and nitrogengas at 1500 °C. Carbothermal reduction followed by nitridation yielded SiC–BN nanocomposite powder composedof nanosized SiC and t-BN. By hot-pressing nanocomposite SiC–BN powders containing 7 wt%Al2O3 and2 wt% Y2O3, machinable SiC/BN nanocomposites were obtained without a significant decrease in theirfracture strength.

Preparation and Properties of Hot Pressed 2D C/SiC Composites

In this paper a pseudo-HP process, which uses solid powder to transfer pressure to prepare complex shape articles, was adopted to prepare 2D C/SiC composites. Nano-SiC powder was used to lower the sintering temperature, thus decreasing the damage to carbon fibers. The optimum processing parameters are as follows: sintering temperature 1850°C, holding time 60min, sintering pressure 10MPa. The maximum strength is over 300MPa, and toughness is around 8MPa·m1/2. BN powder is more lubricous than carbon powder, and thus is more convenient for demoulding and pressure transferring. Precursor Infiltration and Pyrolysis (PIP) was used to further densify the composites after HP process, and strength and toughness of the samples were slightly enhanced.

Al-BN 固液反応における AlN 生成過程に及ぼす B の効果

The AlN/Al base composite has been made through a solid-liquid reaction between molten Al and BN powder under either the atmosphere of N2 or Ar. However, the volume fraction of AlN in the composite is larger in the former case than in the latter case, when both compared under the same forming condition besides the difference in the atmosphere. This fact suggests that extra AlN may be formed under the N2 atmospheric condition through the direct reaction between Al and N, and brings forth the idea that the reactant B in molten Al may play some promoting role on the formation of AlN. To verify this hypothesis, such experiments as B containing Al base alloy is heated under various pressures of N2 have been performed. These experiments have shown some interesting results concerning the morphology of AlN particles formed, and have suggested a “promoting effect of B for the AlN formation”.

Tensile Properties of Forged Direct-Squeeze-Cast Al-(6.0∼8.0 mass%)Si-(0.3∼0.52 mass%)Mg Alloy

The effects of lubricants on the quality of squeeze-cast Al-(6:0� 8:0 mass%)Si-(0:3� 0:52 mass%)Mg alloys were studied. Different melts with varying amounts of Si and Mg were prepared and then directly squeeze-cast into cakes. Different lubricants and two different pressures, 60 and 120 MPa, were used in the production of the cakes. These squeeze-cast cakes were then forged and heat-treated. Tensile specimens prepared from the forged and heat-treated cakes were tested to determine their tensile properties. Among all the variables tested, the Weibull modulus (in ultimate tensile strength, UTS) was the largest (m ¼ 53:7) for the cakes squeeze-cast at 120 MPa using water-based BN powder (� 4 mm in size) as a lubricant. After polishing and ultrasonic-vibration treatment, oxide films appeared as foggy films on the chilled blocks and forged cakes. Reaction films were also observed on the tensile fracture surfaces of the tested bars. They were particularly apparent on the bars produced from cakes that had been prepared using an oil-based lubricant. The area ratios of the foggy films and reaction films on the samples were calculated. The Weibull modulus (in UTS) of the forged cakes after T6 treatment was significantly affected by both the pore count and the total area ratio covered by reaction films and foggy films. [doi:10.2320/matertrans.MRA2007132]

High Surface and High Nanoporosity Boron Nitride Adapted to Hydrogen Sequestration

High surface area nanoporous powders of hexagonal boron nitride (h-BN) have been prepared from molecular precursors to be used for hydrogen sequestration. The more promising samples were obtained using a precursor derived from trichloroborazine (TCB). The precursor was first reacted with ammonia at room temperature leading to the molecular complex Cl3B3N3H3, 6 NH3 which was heated up to 650 °C under ammonia and then up to 1000 °C under nitrogen, giving rise to a high reactive h-BN powder. This crude powder was stabilised by an annealing up to 1800 °C under nitrogen atmosphere leading to a very stable compound exhibiting a specific area of more than 300 m2·g-1 and presenting a very specific nanometric spherical texture. Some samples were doped with platinum (about 1 wt.%) to enhance the activity of pure h-BN using an original one step synthesis route starting from a mixture of BN and Pt precursors. Attempts to sequester hydrogen into these powders were made successfully at -196 °C under 10 MPa, but the stored amount was only about 0.3 wt.% and the platinum added BN powders did not lead to an enhancement of the storage capacity.

Interaction of titanium hydride and boron nitride at temperature gradient

The paper examines chemical transformation in the mixture of BN and TiH2 powders, which is compacted in a graphite die into cylinders, in a temperature-gradient system under the impact of solar energy on the edge of the sample. The research shows how starting particles transform in different temperature zones and at different temperature increase rates in reaction volumes. It is established that the temperature increase rate significantly contributes to chemical transformations when a gaseous phase forms. It determines the kinetic parameters and transformation mechanism for the starting compounds and the fraction of gas components that may leave the reaction volume with a gaseous flow because of the smaller duration and probability of their contact with the particles involved in chemical processes.

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.

A co-precipitation technique to prepare BiNbO 4 powders

A simple co-precipitation technique was successfully used for the preparation of pure ultrafine single phase BiNbO 4. A standard ammonium hydroxide solution was used to precipitate Bi 3+ and Ta 5+ cations as hydroxides simultaneously under basic conditions. For comparison, BN powders were also prepared by the traditional solid-state method. It is observed that the co-precipitation technique produces BiNbO 4 on heating at 600 °C, whereas complete phase formation occurs only at 800 °C in the solid-state method. The phase contents and lattice parameters were studied by powder X-ray diffraction (XRD).

Preparation of Porous Si<sub>3</sub>N<sub>4</sub> Based Ceramics by Combustion Synthesis

This paper presents the results of combustion synthesis of porous Si3N4 based ceramics from silicon powders, with fine α-Si3N4 and BN powders as diluents, under high nitrogen pressure. The effect of addition of BN on the morphology is investigated. The results revealed that the strength decreased and the porosity increased with the addition of BN, which is strongly depended on the final morphology of the products.

Effects of different BN grades on the machinability of Si 3N 4–MoSi 2–BN composites

The effects of the addition of 20 vol% hexagonal boron nitride (h-BN) on the surface finishing and machinability of a Si 3N 4–5 vol% MoSi 2 hot-pressed matrix were studied using three commercial BN powders with different particle sizes and impurity contents. After preparation of the specimens, the surface finishing of the BN-containing composites was much lower than the starting matrix. The materials’ machinability was quantitatively assessed by turning and drilling tests. In the severe turning tests, the wear resistance of the BN-containing composites was three times lower than the starting matrix without any BN size effect. In the mild drilling tests, the resistance of the composites was influenced by the BN grade, but without any obvious relationship to the BN pockets size. The impurities content of the starting BN powder could be the main factor affecting the machinability of the composites.

Formation of diamond-like BN phases under shock compression of graphite-like BN with different degree of structural ordering

The formation of diamond-like modifications of BN under high-temperature shock compression ( P = 33 GPa and T up to 3500 K) has been studied. The powders of graphite-like BN with different degree of three-dimensional ordering of structure ( D 3) were used as starting materials. To increase shock temperature and preserve dense phase formed, boron nitride powder mixed with alkali halide salt was compressed in cylindrical ampoule. The recovered samples were studied by X-ray diffraction and electron microscopy. It was revealed that the phase composition of shock compression products depends on the degree of three-dimensional ordering of initial graphite-like structure. Turbostratic BN with disordered structure ( D 3 = 0) transformed mainly into cubic cBN by diffusion mechanism, whereas highly ordered graphite-like BN ( D 3 = 0.95) transformed only into wurtzite wBN by martensitic mechanism. Both cubic and wurtzite dense modifications are formed from partly ordered initial structures (for which D 3 was 0.45 ÷ 0.7). The both diamond-like phases have nanocrystalline structure.

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.

Aerosol Synthesis of Spherical Morphology Boron Nitride Powders from Organoborate Precursors

The reaction of aerosol droplets of the organoborate reagent (MeO)3B, suspended in nitrogen, with gaseous ammonia in a flight tube heated to 800−1500 °C was used to produce, in high yield, spherical morphology precursor powders BNxOyCz that contain decreasing amounts of oxygen and carbon as the process temperature and mole fraction of ammonia increased. Subsequent static nitridation of the precursor powder under ammonia gave spherical morphology BN powders with low (<1 wt %) oxygen and carbon contents. The powders were characterized by elemental analysis, DRIFT-IR, XRD, SEM, and TEM methods. A weak molecular complex (MeO)3B·NH3 has been previously reported to exist, and this species was isolated and structurally characterized by single-crystal X-ray diffraction methods. Aerosol conversions using (MeO)3B/MeOH, H3BO3/MeOH, and B2O3/MeOH solutions were also examined, and the results are briefly described.