Modifications Of Boron Nitride Research Articles

Thermal conductivity epoxy resin composites filled with boron nitride

Boron nitride (BN) micro particles modified by silane coupling agent, γ‐aminopropyl triethoxy silane (KH550), are employed to prepare BN/epoxy resin (EP) thermal conductivity composites. The thermal conductivity coefficient of the composites with 60% mass fraction of modified BN is 1.052 W/mK, five times higher than that of native EP (0.202 W/mK). The mechanical properties of the composites are optimal with 10 wt% BN. The thermal decomposition temperature, dielectric constant, and dielectric loss increase with the addition of BN. For a given BN loading, the surface modification of BN by KH550 exhibits a positive effect on the thermal conductivity and mechanical properties of the BN/EP composites. Copyright © 2011 John Wiley & Sons, Ltd.

Structural types of boron nitride particles produced by carbothermal synthesis

Transmission electron microscopy and microdiffraction are used to examine boron nitride produced by carbothermal synthesis (using saccharose as a carbon-containing component) in the temperature range 1000–1450°C in nitrogen. It is established that onion-like particles form during structural ordering of turbostratic boron nitride in the range 900 − 1000°C. The structural types of BN particles are classified according to: 1) crystal morphological features (onion-like particles, cylindrical and faceted tubes, and lamellas), 2) formation mechanisms (structures of growth formed through nucleation and structures of breakdown formed through structural transitions), and 3) synthesis temperatures. The tubes grow from the gas phase and their crystal morphological forms differ in phase composition (cylindrical tubes consist of the hexagonal BN phase and faceted tubes of the rhombohedral one). At T ≥ 1350°C, rhombohedral–hexagonal phase transition and transformation into multilayered polytypes occur in faceted tubes due to a highly anisotropic thermal expansion coefficient of graphite-like modifications of boron nitride.

Reversible phase transformation in boron nitride under pulsed mechanical action

It is established that hexagonal boron nitride (h-BN) exhibits transformation to high-pressure diamondlike phases (w-, c-BN) under conditions of treatment in a planetary ball mill. After a 12-h processing, the yield of these phases (≃20%) no longer grows with further increasing the process duration, which is evidence for a reversible character of the transformation. This conclusion is confirmed by the w-BN → h-BN transformation observed under the same process conditions. In addition to h-, w-, and c-BN, we have also found several new modifications of boron nitride.

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.

Sputtering of boron nitride single crystals of different structure

Results obtained in studying the sputtering of single crystals of hexagonal, rhombohedral, and cubic boron nitride modifications by computer simulations are reported. Data on sputtering the (0001) face of BN in two graphite-like modifications and the (111) face of BN crystals with a cubic lattice are presented. The energy and angular dependences of the sputtering yields and spatial and energy distributions of sputtered particles are considered for the cases of normal and oblique ion incidence. Specific features of the anisotropy of spatial distributions of sputtered particles and mechanisms of their formations are analyzed.

Analytical optimization of the lattice parameter using the binding energy calculated in the quasi-classical approximation

The dependence of the static binding energy of a crystal on the crystal structure as calculated in the initial quasi-classical approximation is shown to allow the equilibrium lattice parameter to be found analytically. The application of this method to boron nitride modifications leads to lattice parameters that coincide with experimental values to within several percent. This method gives lattice parameters of 2.66, 3.49, and 2.44 A for the BN hexagonal layer and the cubic c-BN and “ideal” wurtzite-like w-BN crystals, respectively. The corresponding binding energies are estimated to be 23.2, 14.1, and 13.6 eV/mol, respectively.

Laser Cutting of Diamond-Based Materials and Dense Modifications of Boron Nitride

The possibility of applying pulse-periodic laser irradiation for cleaving superhard materials for use as tools is studied. Experimental investigations are confirmed by calculations and it is shown that through the use of laser irradiation (wavelength 1.06 μm, pulse length 130 μsec, radiant energy 0.8 J, pulse repetition frequency 25-50 Hz) it is possible to attain cutting depths in superhard materials of up to 2.0 mm with width of 200-300 μm. The speed of laser cutting superhard plates is 3.5 times higher than with the use of the electrospark cutting technology. It is shown that phase transformation of diamond into graphite is either absent (Hexanite-R) or not significant (SKM-R) and do not affect the subsequent technological operations involved in fabrication of the tool.

Effect of hydrogen on the electronic structure and properties of boron nitrides

The electronic structure of three polymorphic modifications of boron nitride is calculated using the full-potential linear augmented plane wave method with exchange-correlation potential within the framework of the local density and generalized gradient approximation. The calculated values of the bulk modulus obtained from the three equations of state are in reasonable agreement with experimental data. The effect of hydrogen on the electronic structure and physicochemical properties of hexagonal and cubic boron nitride is studied.

Grigorii Valentinovich Samsonov: Novosibirsk Period in the Study of Borides

This paper, dedicated to the memory of Grigorii Valentinovich Samsonov (15 February 1918 – 22 December 1975) [1, 2], corresponding member of the Ukrainian Academy of Sciences, gives a brief survey of the fruitful cooperation of this prominent researcher in materials science with the scientific community of the city of Novosibirsk. Results that have come from cooperation with G. V. Samsonov are unique indeed. They are related to the study of phase equilibria in boron-containing systems, mechanisms of interactions between boron and metals, and synthesis of high-melting borides (in particular, in the ultradisperse state). A vast amount of research has been focused on the development of coatings based on various modifications of boron nitride and related materials.

Thermodynamic modelling of the chemical vapour deposition of boron nitride in the B-N-H-He-O system

Thermodynamic analysis of the chemical vapour deposition (CVD) of the different modifications of boron nitride (hexagonal h-BN, cubic c-BN and wurzite w-BN) was performed for the B-N-H-He-O system using the method of the minimisation of Gibbs free energy. The calculations were carried out for total pressure P tol = 1.33 Pa in the temperature region of 300 - 2100 K. The composition of the initial gas mixture of borazine (B 3 N 3 H 6 ) and helium (with the oxygen impurity) was varied in a wide range. The results are presented in the from of CVD phase diagrams. The influence of the oxygen traces on the temperature boundaries of the c-BN deposition field was shown.

Surface modification of boron nitride in hydrogen plasma

The state of the surface of poorly crystallized or amorphous (a-BN) and highly oriented hexagonal (h-BN) boron nitride before and after treatment in a hydrogen plasma has been studied by high-resolution Auger electron spectroscopy. Treatment of both modifications of BN in hydrogen plasma is found to lead to a partial transformation of sp 2 bonds into sp 3 bonds on the surface. This is thought to be a result of terminating dangling bonds of sp 2-hybridized BN by hydrogen atoms. The formation of chemical NH bonds after treatment of BN in hydrogen plasma is found on the surface of both a-BN and h-BN, whereas in practice, BH bonds appear to form only on the treated surface of h-BN.

The influence of excited hydrogen species on the surface state of sp2-hybridized boron nitride

The surface of sp 2 -hybridized boron nitride, namely well-crystallized hexagonal and poorly-crystallized turbostatic boron nitride, after its treatment in a hydrogen plasma was studied with the aid of high-resolution Auger spectroscopy. The surface state of both modifications of boron nitride is found to change due to interaction with excited hydrogen species. This interaction is thought to lead to termination of π (dangling) bonds of sp 2 -hybridized boron nitride by hydrogen atoms, which results in transformation of the sp 2 -hybridized state into the sp 3 -hybridized state on the surface of boron nitride.

Phase transformations of the rhombohedral modification of boron nitride with shock compression

Features of crystal-orientated transformations of the rhombohedral modification of boron nitride (rBN) into dense modifications of the wurtzite (wBN) and sphalerite (spBN) types with shock compression in the range from 15 to 50 GPa are studied. Three compression schemes are used: in annular ampules (∼15 GPa); in the ampules of K. M. Rabinin (25–35 GPa); in a plane shock wave (∼50 GPa). With an increase in pressure from 15 to 50 GPa and a corresponding change in the loading scheme the degree of rBN transformation into dense modifications increases from 0 to 40%. There is a change in the rBN transformation mechanism: from longitudinal bending of layers (as a result of which wBN forms, and spBN is formed from it) to corrugation leading to spBN formation directly from rBN.

Normal lattice vibrations and the crystal structure of anisotropic modifications of boron nitride

A multicomponent infrared study of boron nitride (BN) pyrocrystals was performed in polarized light. A series of textured polycrystals, which included samples with different dergees of ordering, with various ratios of the concentrations of the hexagonal and rhombohedral phase, was prepared by chemical vapor deposition. This study revealed the essential influence of the microstructure of the pyrocrystal on the reflection and transmission spectra. It is shown that to identify the normal oscillations of the hexagonal phase, crystal-optical effects and effects associated with phonon scattering must be taken into account. The normal oscillations of the rhombohedral phase of BN were identified directly from experimental spectra of pyrocrystals which are similar in their properties to a single crystal. On the basis of the results obtained by us it is shown that ionic-covalent bonds exist between some of the atoms of neighboring layers in anisotropic modifications of BN. A refined model of the lattice structure eliminates an entire host of contradictions between the experimental data and shows that in addition to sp2 hybridized electrons, sp hybridized electrons participate in the formation of interatomic bonds in anisotropic BN.

A thermodynamic approach to chemical vapor deposition of boron nitride thin films from borazine

Thermodynamic analysis of the chemical vapor deposition (CVD) of boron nitride has been performed for the BNH system using the method of the minimization of Gibbs' free energy. The calculations were carried out for two gas mixtures with borazine: (1) B 3N 3H 6 + N 2 and (2) B 3N 3H 6 + NH 3 for total pressures P tot of 1 and 10 −2 Torr, temperature range 800–2300 K and for a variety of initial gas mixture compositions. Theree different modifications of boron nitride (hexagonal h-BN, cubic c-BN and wurtzite w-BN) were taken into consideration. It was obtained that c-BN is formed at temperatures up to 1804 K, h-BN modification is most stable above this temperature, w-BN is a metastable phase at all possible variations of process conditions. Mixture borazine with nitrogen is more promising to deposit h-BN at low pressure.

Theoretical study of boron nitride modifications at hydrostatic pressures

This paper presents a detailed study of boron nitride modifications at hydrostatic pressures. Cohesive properties of zinc blende (c-BN), wurtzite (w-BN), hexagonal (h-BN), rhombohedral (r-BN), and rocksalt structure are calculated by systematic optimization of unit cell parameters and atomic positions using total-energy density-functional methods. With focus on the very rarely discussed layered modifications the p-V equations of states are derived. It is confirmed that the isothermal bulk modulus of the ${\mathrm{sp}}^{2}$-bonded phases is more than 10 times smaller in comparison to the dense phases. Additionally, the equilibrium line of c-BN and h-BN in phase p-T diagram is calculated. According to recent experimental reports c-BN is predicted as a stable modification at standard conditions.

On thermodynamic equilibria of solid BN and gas phases in the B−N−H−Cl−He system

Thermodynamic analysis of the chemical vapour deposition (CVD) of boron nitride was performed for the B−N−H−Cl−He system, with new thermodynamic data for different modifications of boron nitride (hexagonal h-BN, cubic c-BN and wurtzite w-BN). The phase equilibria were calculated for the temperature region 673–2273 K, at total pressures of 1.013 × 10 3 and 1.013 × 10 5 Pa and fora wide range of atomic ratios of elements in the system. The results arc presented in the form of CVD phase diagrams which are a good way of illustrating the influence of the system parameters on the formation of stable phases. c-BN is formed in a quasi-equilibrium process at temperatures below 1804 K, the h-BN modification is most stable above this temperature, and w-BN is a metastable phase for all possible variations of the process conditions.

Chemical bond and electronic structure anisotropies in the graphitic and rhombohedral modifications of boron nitride

The pseudopotential method is used to calculate the band spectra, valence charge densities, and electroneutrality levels in the graphitic (h-BN) and rhombohedral (r-BN) modifications of boron nitride. It is shown that these compounds are indirect-band insulators with the minimum energies of the indirect (4.65 and 4.8 eV) and direct (5.27 and 5.5 eV) forbidden zones in h- and r-BN, respectively. The band energies of h-BN correlate well with the experimental data. Both crystals are characterized by ionic-covalent chemical bonds. In the plane of hexagonal layers, the bonds are more covalent in h-BN. In h- and r-BN, the electroneutrality levels correspond to the energy of about 1.9 eV over the top of the valence band, which is responsible for the hole conductivity of the substances. In superlattices based on these compounds, the rhombohedral phase is a quantum well for holes and a barrier for electrons.

Characteristics of vacuum dissociation of shock-sintered BNsph polycrystals

We have used mass spectrometry to investigate the characteristics of dissociation of the sphalerite modification of boron nitride sintered in shock waves of various intensities as compared to the original BNsph powder and a specimen sintered under static conditions. We establish a correlation between the dissociation pressure and the crystallite sizes and internal distortions of the second kind. The increase in the thermal stability of the boron nitride is a result of the strengthening of chemical bonds via changes in the degree ofsp hybridization and the difficulty of nitrogen diffusion due to the decrease in the crystallite sizes and the increase in the level of lattice distortions. We calculate the energy parameters of the dissociation processes (enthalpy and Gibbs free energy).

Lattice Parameters of Boron Nitride Polymorphous Modifications as a Function of Their Crystal-Structure Perfection

The data available in the literature on the lattice parameters of boron nitride (BN) polymorphic modifications have been analysed. A precise measurement of interplanar spacing has been carried out for hexagonal BN of different degrees of crystalline perfection and rhombohedral BN. The possibilities for alteration of wurtzitic BN lattice parameters have been studied. The results obtained have been used to establish the fundamental reference values of lattice parameters for all crystalline modifications of BN.