Hexagonal Boron Nitride Powder Research Articles

Preparation of Nanostructured Hexagonal Boron Nitride Powder

In this communication, we describe an inexpensive and feasible method for the preparation of hexagonal boron nitride (h–BN) nanorods in the absence of metal catalyst. Tertiary calcium phosphate (Ca3(PO4)2) and ammonium biborate hydrate (NH4HB4O7·3H2O) were selected as starting materials where calcium phosphate was used as a diluting agent to prevent the formation of bulk B2O3 during the thermolysis of ammonium biborate hydrate. The mixture was nitrided at 900°C in the flowing ammonia and was transformed into h–BN nanorods after subsequent crystallization. After crystallization at 1650°C for 2 h, the unique microstructure of h–BN nanorods was observed.

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.

Oxidation of Boron Nitride Powder in Wet Oxygen.

Oxidation kinetics of hexagonal boron nitride powder in 8% O2-20% H2O/balance argon was investigated at 900-1100°C. Paralinear weight change curves were observed. The rate constant of BN oxidation in 8% O2-20% H2O/balance argon was almost the same as that in 20% H2O/balance argon at the temperature below 1000°C, while at higher temperatures it was almost the same as that in 10% O2/balance argon.

Characterization of Hexagonal Boron Nitride Powders

In this work, the characterization of two hexagonal boron nitride (h-BN) samples has been carried out using several techniques, mainly XRD, SEM, TEM and Electron Diffraction. XPS was used for the analysis of powder surfaces. Despite the same synthesis procedure being used for both samples, there are differences between them. The two samples are highly crystalline according to the XRD results. The results suggest that there are minor impurities. Thus, XPS data have shown differences between the surface and bulk composition with presence of oxygen, associated with boric oxide. There are also differences in particle size between the samples, as deduced from electron microscopy and particle size measurements by laser scattering. This was also confirmed by pore volume variations after nitrogen adsorption measurements. The characterization of these materials is of primary importance in order to better understand the processing behaviour using these powders.

Preparation of Boron Nitride Fibers Using Hydrated Cellulose. Part 3. Nitriding Hydrated Cellulose Fibers Impregnated with Boric Acid

The effect of preliminary heat treatment (PHT) for hydrated cellulose fibers impregnated with boric acid on the composition and structure of nitrided products is studied. Subsequent nitriding after PHT in helium leads to formation of carbon fibers coated with boron nitride, but after PHT in air there is formation of hexagonal and rhombohedral boron nitride powder. Data obtained are compared with results reported previously.

Magnetic relaxation properties of helium-3 adsorbed on hexagonal boron nitride

We present pulse NMR measurements on ${}^{3}\mathrm{He}$ adsorbed on hexagonal boron nitride powder in the registered phase. We observe strong coupled magnetic relaxation with substrate ${}^{11}\mathrm{B}$ spins, mediated by ${}^{14}\mathrm{N}$ surface-layer spins via the level-crossing or quadrupole-dip (QD) effect at 4.5 MHz. Ab initio electronic structure calculations of the BN surface-layer electric field gradients, dramatically modified by the ${}^{3}\mathrm{He}$ were essential in justifying our QD explanation. BN crystallite geometry plays a key role in producing the QD. Stretched exponential fits to the QD relaxation data may provide a new tool for studying specifically the commensurate phases on BN and related materials.

Formation, characterization, and properties of a new boron-carbon-nitrogen crystal

Amorphous boron-carbon-nitrogen (BCN) powders were produced by mechanically milling a mixture of hexagonal boron nitride (h-BN) and graphite powders for 120 h in an argon atmosphere. By annealing the amorphous BCN powders isothermally under 4 GPa at temperatures above 880 K, a new B–C–N crystal with a tetragonal structure was obtained. Its lattice constants decrease with the increasing annealing temperature and are a=1.685 nm and c=0.537 nm, respectively at an annealing temperature of 1170 K. The conductivity measurement suggests the new B–C–N phase is a semiconductor with a very small band gap and the band gap changes depending on the temperature in the temperatures between room temperature and 840 K.

Investigation of the Physical and Mechanical Properties of Hot‐Pressed Boron Nitride/Oxide Ceramic Composites

Billets of hexagonal boron nitride powders (h‐BN) were hot‐pressed, varying the alignment of the platelike particles and the amount of oxide additives. Increasing either alignment of individual grains or the amount of additives was shown to increase flexural strength, to approximately 120 MPa at ambient temperatures. h‐BN was shown to deflect cracks initially propagating normal to its basal planes.

Investigation of the boron nitride/polybenzoxazine interphase

The influence of a hexagonal boron nitride powder surface on the polymer- ization of a benzoxazine monomer is examined by differential scanning calorimetry (DSC). By varying the thickness of the benzoxazine coatings on boron nitride particles, a distinct influence of the surface substrate on the polymerization reaction could be observed. At a coating thickness calculated to be on the order of a monolayer, the heat of reaction is reduced, while the exothermic peak temperature and peak width at half height are increased relative to that of the neat resin values. The reduced heat of reaction and increased exothermic peak temperature indicate that the boron nitride surface is inhibiting the benzoxazine polymerization reaction. In the intermediate coating thickness region, the heat of reaction increases and the exothermic peak temperature and peak width at half height decrease with increasing coating thickness. With even thicker coatings, the heat of reaction and exothermic peak temperature and peak width at half height reach values corresponding to that of the neat resin and no longer change with thickness. In addition, the influence of surface treated boron nitride particles on the thermal polymerization behavior, as well as the effect of the surface treatment on the viscosity, dynamic mechanical properties, and flexural properties of the filled composites are also investigated. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2360 -2372, 1999

A solid-state process for formation of boron nitride nanotubes

The formation of boron nitride (BN) nanotubes via a solid-state process is demonstrated. The nanotubes are produced by first ball-milling hexagonal BN powder to generate highly disordered or amorphous nanostructures, followed by annealing at temperatures up to 1300 °C. The annealing leads to the nucleation and growth of hexagonal BN nanotubes of both cylindrical and bamboo-like morphology. Unlike previous mechanisms for nanotube formation, the reordering and solid-state growth process of our nanotubes does not involve deposition from the vapor phase nor chemical reactions.

パルスレーザー堆積法によるＢ‐Ｃ‐Ｎ系薄膜の合成

B-C-N thin films were prepared onto silicon substrates by pulsed-laser deposition (PLD) method. The targets were prepared by hot-pressing of mixtures of hexagonal boron nitride (h-BN) and graphite powders with various weight ratios. The deposited films were evaluated by the x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and x-ray diffraction (XRD). XRD patterns of the deposited films show that intensity of peak corresponding to h-BN (002) tends to increase with increasing content of h-BN in the target. Raman spectra show that carbon in the deposited films composes carbon network like diamond-like carbon (DLC) or amorphous carbon. FT-IR spectra and XPS spectra indicate existence of B-C and C-N bond as well as that of B-N and C-C bond within the deposited films.

Suspension properties of hexagonal BN powders: effect of pH and oxygen content

Suspension characteristics for two types (HCV and HCP) of hexagonal boron nitride (h-BN) powder have been examined and correlated to the amount of boric oxide present (B2O3) on the BN surface. The effects of B2O3 on the suspension behavior of HCV-BN and HCP-BN were determined by measuring the zeta (ζ) potential and viscosity as a function of pH. Results indicate the poor suspension quality of HCV-BN powders is linked to the increased presence of B2O3 on the particle surface. Because of its weak ionization characteristics, B2O3 acts to prevent stable suspension formation by decreasing the double layer length, thereby lowering the barrier to agglomeration. Hence, as the amount of B2O3 increases on the HCV-BN powder surfaces, the stability decreases and the particles agglomerate, rapidly settling out of higher viscosity suspensions. In contrast, HCP-BN having less B2O3 on the powder surface, exhibits higher ζ-potentials and increased stability against agglomeration. Therefore, HCP-BN forms suspensions of single, unagglomerated particles which are less viscous, settle at a slower rate and are more suitable for slip casting operations.

Discrete element modeling of shock compression of hexagonal boron nitride powder with and without copper addition

A multi-physics, discrete element computer code called DM2 was used to model shock compression of hexagonal boron nitride (hBN) powder at the particle level. Included in this analysis are an anisotropic property of powder grains and heat conduction between hBN and copper particles. Results confirm the earlier observations that the effects of copper are to increase shock pressure and to act as a heat sink to cool BN grains. But, the simulation also reveals that the shock processing at the particle level is far more complex than those suggested in the literature and that the interpretation of process parameters based on macroscopic conditions requires caution.

Orientation and Growth of Grains in Copper‐Activated Hot‐Pressed Hexagonal Boron Nitride

Hexagonal boron nitride powder was hot pressed in an environment of metallic copper. When compared with a copper‐free system, the sintered body exposed to the copper was composed of substantially thicker grains having an unusual arrangement, consisting of preferred alignment of (0001) basal planes parallel to the pressure axis. The character of the orientation of the boron nitride grains substantially influenced the mechanical strength of the ceramics. The difference in the orientation of the grains is explained by the interaction between copper atoms and boron nitride crystals selectively occurring on the (0001) basal planes of the latter.

Sintering and crystallization of ground hexagonal boron nitride powders

Sintering and crystallization of ground hexagonal boron nitride powders

Oxidation kinetics of hexagonal boron nitride powder

The isothermal oxidation of hexagonal boron nitride powders was carried out at 900–1050°C in dry oxygen and air. The oxidation kinetics were found to obey linear rate law and were described by the surface chemical reaction-controlled shrinking cylindrical model. The apparent activation energies were found to be 298 and 330 kJ mol−1in dry oxygen and air, respectively. The oxygen partial pressure dependence of the oxidation rate constant at 1000 °C is well represented by a Langmuir-type equation. Microscopic examination of the oxidized sample after removal of the oxide scale with water suggested that the rate of attack by oxygen was determined by an anisotropy due to the crystallographic direction, similar to oxidation in graphite. The volatilization of B2O3 was observed only in dry oxygen and obeyed a linear rate law, and was found not to affect the oxidation reaction.

ChemInform Abstract: Synthesis of Boron Nitride Ceramics from 2,4,6‐Triaminoborazine.

AbstractPartially ordered hexagonal boron nitride powder can be readily obtained from the pyrolysis of a solid, insoluble poly(borazinylamine) precursor that is easily formed from the well‐known borazene monomer ((Me2N)B=NH)3 and NH3.

Fast luminescence from carbon-related defects of hexagonal boron nitride

Luminescence from carbon-related defects of hexagonal boron nitride powders is studied as to transient characteristics. Lifetimes of four luminescence groups classified previously range from 1.15 ns to 17 ns. The transitions determining the lifetimes are assumed to be totally or almost radiative. The transient characteristics provide some new aspects of the luminescence and support for the previous classification.

Laser clad lubricant layer with graphite/BN

A composite thin clad layer containing graphite and hexagonal boron nitride powders was tried to form on stainless steel using a continuous-wave C0~ gas laser. The layer formed only with boron nitride has no appreciable effect on friction reduction. Increase in graphite content in the coatings led to lower friction. A good lubricant layer with the lowest friction and the longest life was obtained from the powder mixture with higher graphite content about

Synthesis of Cubic Boron Nitride

This research was carried out on a part of study of synthesis of cubic boron nitride. I part alloy catalyst of Fe3Al, Ag18Cd82 or Ag58Cd42 and 4 parts hexagonal boron nitride were filled up in a platinum tube approximately 4mm in diameter and 8mm long. Three filled methods were used, that is, sandwich type, mixture type of alloy catalyst and hexagonal boron nitride, and bar type of catalyst. The assembly was placed for 30 minutes at maximum about 80kbars and about 2200°C, using a modified girdle type high pressure apparatus. The cubic boron nitride thus formed was isolated from the mixture and was identified by Debye-Scherrer pattern, hardness test and etc.In the case of Fe3Al alloy catalyst, cubic boron nitride with tetrahedron, octahedra and etc., having a zinc-blende struture, could be synthesized over about 1550°C and 45kbar. Its crystal size was about 80μ. The hexagonal boron nitride powder showed an increase in crystal size with increasing temperatures and pressures. The equilibrium line between the hexagonal and cubic forms was located at the extensions of Wentorf's experimental line and the line calculating from thermodynamic data.In the case of Ag18Cd82 and Ag58Cd42 alloys, cubic boron nitride could be obtained over about 1000°C and 32kbar and high yields of it did with increasing temperatures and pressures. The cubic-hexagonal boron nitride equilibrium line was in the same manner as Fe3Al alloy catalyst. Crystal size was about 80 and 150μ, using Ag58Cd42 and Ag18Cd82 alloys, respectively.Alloy of Ag-Cd system was diffused into hexagonal boron nitride at the rate of about 0.10mm/min.