Boron Nitride Powders Research Articles

Reactive Mechanical Alloying Synthesis of Nanocrystalline Cubic Zirconium Nitride

Zirconium nitride powders with rock salt structure (γ-ZrNx) are prepared by mechanical milling of a mixture of Zirconium and hexagonal boron nitride (h-BN) powders. The products are analysed by x-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy (RS). The formation mechanism of γ-ZrNx by ball milling technique is investigated in detail. N atoms diffuse from amorphous BN (α-BN) into Zr to form Zr(N) solid solution alloy then the Zr(N) solid solution alloy decomposes into γ-ZrNx. No ZrB2 is observed in the as-milled samples or the samples annealed at 1050° C for 2h.

Effect of boron on the crystallization of cubic BN in the Li3N-BN system

The introduction of 1 to 5 wt % boron into the starting mixture has been shown to influence the crystallization behavior of cubic boron nitride in the Li3N-BN system. We have studied the effect of boron content on the conversion, particle size distribution, lattice parameter, and abrasive ability of cubic boron nitride powders and also the effect of neutron irradiation on the paramagnetic structure of the BN particles.

Performance of Graphite and Boron-Nitride-Silicone Based Lubricants and Associated Lubrication Mechanisms in Warm Forging of Aluminum

Although water/oil-graphite emulsions are widely used in warm forging processes, they carry environmental concerns. In an attempt to replace graphite-based lubricants in warm forging of aluminum alloys, two variants of boron-nitride-silicone lubricants were formulated. The two variants were made by dispersing boron nitride powder in polydimethyl siloxane oil at concentrations of 1% and 8%. The formulated lubricants were initially tested for their thermal degradation characteristics using a thermogravimetric analyzer and compared to the thermal degradation behavior of graphite and silicone oil lubricants. Ring compression tests were then carried out at 260°C and 370°C. Boron-nitride-silicone lubricant variants did not show significant difference in performance as die temperature was increased from 260°Cto370°C. This is in contrast to graphite, which performed much better at 260°C than at 370°C, due to thermal oxidation. On the other hand, silicone oil exhibited the worst performance at 260°C and the best performance at 370°C. In both boron nitride lubricant variants, the polydimethyl siloxane facilitated hydrostatic/hydrodynamic lubrication at 260°C, with boron nitride acting as a barrier film that reduced friction. However, the lubrication mechanisms changed at 370°C, where the depolymerization of polydimethyl siloxane led to formation of silica due to thermal oxidation. Silica, together with boron nitride, acted as a film barrier with low shear strength. The dual lubrication mechanisms make boron-nitride-silicone lubricants suitable for a wide range of aluminum forging temperatures.

Crystallization of cubic boron nitride in the system Li3N-BN (4–10 wt % Li3N)

The crystallization behavior of cubic boron nitride in the Li3N-BN system has been shown to change in the range 4 to 10 wt % lithium nitride. We have determined the conversion, size composition, lattice parameter, abrasive ability, and paramagnetic structure of cubic boron nitride powders containing 4 to 10 wt % Li3N.

Synthesis and characterization of boron nitride sponges as a novel support for metal nanoparticles

This paper describes a simple synthetic route for the synthesis of hexagonal boron nitride (h-BN) powders with high specific surface area, in which BBr3, NH4Cl and Al powders are used as starting materials. The structure and composition of the powders were characterized by electron diffraction, Fourier transformation infrared spectroscopy and X-ray photoelectron spectroscopy in the selected area. X-ray diffraction shows wide peaks of crystalline h-BN with the particle size on the nanometer scale, and transmission electron microscopy reveals that the products have a novel spongy morphology. Silver nanoparticles loaded h-BN sponges were prepared via a one-step synthesis method. Different reaction conditions for the formation of h-BN sponges were also investigated.

Phase composition and surface properties of cubic boron nitride micron powders

X-ray diffraction analysis and IR spectroscopy have been used to study the phase composition of KM grades of cubic boron nitride powders of grain sizes 0.5/0, 1/0, 2/1, 3/1, and 3/2. It has been shown that in parallel with the cubic modification there is the hexagonal modification of boron nitride (from 0.8 to 6.0 wt %). The kinetics of capillary absorption in the interaction of the surface of KM powders with different systems has been analyzed and the effect of the addition of surface-active substances and of the powder metallization on the constant of the working media passing rate has been defined. The dynamics of the powder activation with palladium have been studied and the optimal conditions of active center recovering and growing on the powder surface have been defined.

Synthesis of zirconium diboride–zirconium nitride composite powders by self-propagating high-temperature synthesis

Formation of zirconium diboride–zirconium nitride composite powders by self propagating high temperature reaction of zirconium, boron and hexagonal boron nitride powders was investigated. Zirconium diboride–zirconium nitride powder mixtures with varying proportions were produced by changing the amount of boron nitride in the reactants. Products were subjected to powder X-ray diffraction analysis; and grain size and morphology was examined by scanning electron microscope. It was seen that the velocity of the wave front, adiabatic temperature of the reaction and grain size of the products decreased by increasing the boron nitride amount in the reactants. It was deduced from the scanning electron microscope examinations and crystal size calculations performed from the broadening of the peaks on the X-ray diffractions patterns that zirconium nitride grains were much finer than zirconium diboride grains.

Microstructure and electrochemical properties of the Co–BN composites

The Co–BN composites are synthesized by ball-milling metallic Co and boron nitride (BN) powder with a different Co/BN weight ratio. The microstructure, morphology and chemical state of the obtained Co–BN composites are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). After ball milling, Co nanoparticles of 10–30 nm in size are distributed on the amorphous BN matrix. The electrochemical measurements, including galvanostatic method and cyclic voltammetry (CV), show that the Co–BN composite with the Co/BN weight ratio of 5/1 has a good cycle performance and a high reversible electrochemical capacity in 6 M KOH solution. Moreover, the Co–BN composite can be discharged directly without the first charging process. The higher initial discharge capacity, observed in the first cycle of the pre-charged sample, can be explained by an initial reduction of the oxidized species on the surface of the Co–BN composite and subsequent oxidation. The partial dissolution of Co(OH) 2 in alkaline solution would further increase the active surface area of the material for the considerable decrease in overpotential after the first cycle. Based on a structure analysis and electrochemical measurement, the reversible faradic reaction of the highly dispersed active Co nanoparticles in the Co–BN composite is dominant.

A Study of hBN-Y<sub>2</sub>O<sub>3</sub> Composite Investment for Casting Ti-6Al-4V Alloy

An hBN-Y2O3 composite investment moulding material was developed for investment casting Ti-6Al-4V alloy. Hexagonal boron nitride (hBN) powders plus a few Y2O3 powders were used as the investment mould material to prepare the shell mould with colloidal yttria as binder. The results show that the hBN-Y2O3 composite investment mould can be used to cast Ti-6Al-4V alloy, and has lower cost compared with the traditional yttria based investment mould, because of chemically inert of hexagonal boron nitride and its lower density. Moreover the interfacial reaction between the Ti and hBN-Y2O3 composite shell mould was investigated with measurement of microhardness and scanning electron microscopy (SEM) with energy dispersive (EDS).

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.

Improvement of the Production Yield of Spherical Si by Optimization of the Seeding Technique in the Dropping Method

Spherical Si with a diameter distribution of approximately 1.0 mm, which is applicable to spherical solar cells, was formed by the dropping method. To control the solidification of the molten Si droplet to occur at shallow undercooling, a seeding technique was developed. In this paper, we describe the optimization of the seeding technique with emphases on high production yield and high minority carrier lifetime. An effective seeding supply system was first introduced. It was found that the seeding with Si powder causes impurities, such as Fe, into Si spheres, and the crystallinity becomes poor with increasing yield. SiO, SiO2, Si3N4, and boron nitride powders were shown to have the same seeding effect as Si powder. Among these, SiO2 was the best candidate. The seeding with SiO2 powder did not increase the concentration of Fe and O impurities and did not decrease the crystallinity. The dependences of the yield on the size and the density of SiO2 powder were systemically investigated. The best yield of nearly 70% was realized at a high SiO2 powder density.

Wear/Friction Behaviour of FeB and FeB/h-BN Coatings Sprayed by Atmospheric Plasma Spraying

Abstract The crushed and rounded ferroboron (FeB) powders of Fe-18.8B-0.2C-0.5Si-0.8Al (wt.%) were deposited onto an aluminum substrate by atmospheric DC plasma spraying to improve its tribological properties. Hexagonal boron nitride (h-BN) powders which have excellent lubricating properties similar to graphite were incorporated into the boride powders as a solid lubricant by a sintering process to obtain protective coatings with a low friction coefficient. As-sprayed coatings are composed of mainly FeB, iron matrix supersaturated with boron owing to the rapid solidification of molten droplets flattened on a substrate. The friction and wear behaviour of each coating were evaluated using a ring-on-disk type wear tester in paraffin based oil with an air atmosphere. It was found that sintering of iron boride powder with h-BN powder (5 wt.%) is an acceptable method for producing a protective composite coating against friction and wear. The evaluation of friction and wear results shows that iron boride coatings exhibit high seizure load and good anti-wear performance.

Pressureless Sintering of Hexagonal Boron Nitride Powders

Pressureless sintering of hexagonal boron nitride (BN) with and without additives was performed using a powder which was previously nitrided at 900oC and then heat treated at 1400oC. Sintering additives were B2O3 (2, 4, 6 wt %) and Y2O3 (2, 4, 6 wt %) and sintering temperatures was 1900oC. Sintered samples were characterized by XRD, SEM, micro hardness, density and mercury porosimeter measurements. Maximum density values of samples were 1.56 g/cm3 without additives and 1.63 g/cm3 with additives. Hardness values of samples were found to improve with the presence of additives.

Synthesis of highly concentrated TiO2 nanocolloids and coating on boron nitride powders

Highly concentrated TiO2 nanocolloids were synthesized by hydrolysis method of TiCl4. Reactions were performed in a TiCl4/HCl/H2O aqueous solution and particle characteristics such as shape, size and aggregation were controlled by polymerization and adsorption of acrylamide on the surface of TiO2 nanoparticles. Moreover, aminopropyltriethoxysilane was added instead of acrylamide, and their effects were compared with each other. The prepared colloids dispersed well and showed strong absorption peaks at 350–370nm, which blue-shifted to 20–30nm, as compared with macroparticles. The obtained techniques from TiO2 nanocolloids synthesis were utilized in coating boron nitride powders, which are nonpolar and isoelectronic materials of carbon. Their surface morphology, structure, thermal stability and ultraviolet (UV) absorption characteristics were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG/DTA), ultraviolet–vis spectroscopy (UV–vis).

Microstructural characterization of amorphous and nanocrystalline boron nitride prepared by high-energy ball milling

Microstructural parameters like crystallite size, lattice strain, stacking faults and dislocation density were evaluated from the X-ray diffraction data of boron nitride (BN) powder milled in a high-energy vibrational ball mill for different length of time (2–120 h), using different model based approaches like Scherrer analysis, integral breadth method, Williamson–Hall technique and modified Rietveld technique. From diffraction line-broadening analysis of the successive patterns of BN with varying milling time, it was observed that overall line broadening was an operative cause for crystallite size reduction at lower milling time (∼5 h), whereas lattice strains were the prominent cause of line broadening at higher milling times (>19 h). For intermediate milling time (7–19 h), both crystallite size and lattice strain influence the profile broadening although their relative contribution vary with milling time. Microstructural information showed that after long time milling (>19 h) BN becomes mixture of nanocrystalline and amorphous BN. The accumulations of defects cause this crystalline to amorphous transition. It has been found that twin fault ( β′) and deformation fault ( α) significantly contributed to BN powder as synthesized by a high-energy ball-milling technique. Present study consider only three ball-milled (0, 2 and 3 h) BN powder for faults calculation because fault effected reflections (1 0 1, 1 0 2, 1 0 3) disappear with milling time (>3 h). The morphology and particle size of the BN powders before and after ball milling were also observed in a field emission scanning electron microscope (FESEM).

Preparation of Amorphous Si-B-C-N Powders and Nano-Sized Ceramics

Amorphous nano-sized silicoboron carbonitride (Si-B-C-N) powders with average grain size <50 nm were fabricated by high energy shaker mill using hexagonal boron nitride, graphite and amorphous silicon powders as starting materials. The powders were consolidated by spark plasma sintering at 1900° and 1950°C. Amorphous phase were partially retained in ceramic sintered at 1900°C. For ceramic sintered at 1950°C, amorphous Si-B-C-N ceramic transferred to hexagonal BN and cubic SiC.

Rheological Properties of Boron Nitride-Silica Composite Suspensions

In this paper, a novel colloidal forming process has been developed to prepare boron nitride-silica composites. The rheological properties of mixed boron nitride-silica powders in deionised water have been investigated by adjusting the concentration of dispersant, pH value of the solution and other process parameters. The isoelectric points for both boron nitride and silica powders are in the pH range of 2 to 3. Boron nitride doped with or without A-1 (an anionic dispersant) as a dispersant shows a similar electrophoretic behavior to silica powder, i.e. Zeta potential becomes more negative with the increase of pH value from 2 to 12. The sedimentation and rheological results of aqueous boron nitridesilica suspensions indicate that the optimized pH value is around 7.2. It depends upon the synthetic effects of dissociation of A-1 and sufficient positive adsorption sites on the particle surfaces in aqueous media. The optimum concentration of the dispersant at pH=6 is 0.6wt % (dry weight basis of composite powders). Boron nitride-silica composite suspensions with a solid content as high as 50vol% have also been achieved. A ball milling process can improve the fluidity of the suspensions.

Preparation and Properties of BN-SiO<sub>2</sub> Composite Ceramics

A series of BN-SiO2 composite ceramics with high relative densities have been fabricated by hot pressing of a mixture of hexagonal boron nitride powder and silica sol. X-ray diffraction (XRD) results show that silica are distributed in the BN matrix as noncrystalline state. Microstructure of as-sintered composite ceramics is observed by scanning electron microscopy (SEM). The influence of the secondary phase on the mechanical properties is studied by varying the silica content from 10 wt% up to 40 wt%. Dielectric properties of composite ceramics are also measured from 1 MHz to 2.0 GHz.

Optimization of rolling process parameters for wurtzite boron nitride powders

The paper presents some results of experimental investigations of the influence of geometrical and physical-mechanical parameters of the process of rolling wurtzite boron nitride powders on the formation of a powdered rolled material, its characteristics, and technological properties of the granules prepared from this material. The process variables have been optimized to produce granules of required density through multiple rolling in the initial state or with premoistening. It is demonstrated that high-density BNw granules are most efficiently produced by means of a rolling mill with 100-mm-diameter rolls propped up, with a zero roll clearance and a linear roll rotational speed varying from 0.8 to 1.5 m/min.

Electronic properties of c‐BN thick film layers obtained on silicon and ceramics substrates

AbstractA novel thick film has been produced using paste based on single crystal cubic boron nitride powders. The major paste component are particles of cubic boron nitride of different grain shape and size obtained by different methods. Another component is inorganic glass based on bismuth oxide with high wetting characteristics for binding together the particles of c‐BN and ensure good adhesion of the layer to the substrate. Structures were crated with c‐BN films acting as an insulator which enabled electrical characterization by means of current‐voltage measurements. Observed variation in specimen characteristics resulted primarily from different content of the secondary phase (glass) as well as different types of used c‐BN powders. The results of the experimental work has enabled a preliminary analysis of the room temperature conduction mechanisms in structures with c‐BN based thick film layers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)