Graphite-like Hexagonal Boron Nitride Research Articles

Structure, properties and applications of graphite-like hexagonal boron nitride

A review of the literature is presented, including a description of the crystal structure and morphological features of graphite-like hexagonal boron nitride, as well as the preparation of materials based on it. The mechanical and dielectric properties of pyrolytic, hot-pressed (hexagonal and turbostratic modification) and reactive sintered boron nitride are presented. The positive effect of impregnation of porous samples based on boron nitride with organoboron and organosilicon compounds, followed by pyrolysis, on the level of physicomechanical characteristics is shown. Ill. 3. Ref. 46.

Structure, Properties, and Applications of Graphite-Like Hexagonal Boron Nitride

A review of the literature is presented, including a description of the crystal structure and morphological features of graphite-like hexagonal boron nitride, as well as the preparation of materials based on it. The mechanical and dielectric properties of pyrolytic, hot-pressed (hexagonal and turbostratic modification) and reactive sintered boron nitride are presented. The positive effect of impregnation of porous samples based on boron nitride with organoboron and organosilicon compounds, followed by pyrolysis, on the level of physicomechanical characteristics is shown.

Role of the particle size of graphite-like boron nitride in nucleation of cubic boron nitride

The influence of the reduced radius of grains of the graphite-like hexagonal boron nitride (h-BN) on the nucleation of the cubic boron nitride (c-BN) during synthesis from an initiator solution at a high pressure is analyzed. The colloidal mechanism of nucleation is confirmed experimentally. It is shown that there is a correlation between the nucleus sizes of the hexagonal boron nitride and the pressure of the onset of nucleation of the cubic boron nitride. The effect of these sizes of the hexagonal boron nitride on the concentration of crystal nuclei of the cubic boron nitride is studied. The kinetic nucleation curves are obtained. It is demonstrated that the concentration of crystallization centers depends on the thermodynamic and kinetic parameters, as well as on the particle size of the graphite-like hexagonal boron nitride.

Strain-induced disorder, phase transformations, and transformation-induced plasticity in hexagonal boron nitride under compression and shear in a rotational diamond anvil cell: In situ x-ray diffraction study and modeling

Plastic shear significantly reduces the phase transformation (PT) pressure when compared to hydrostatic conditions. Here, a paradoxical result was obtained: PT of graphitelike hexagonal boron nitride (hBN) to superhard wurtzitic boron nitride under pressure and shear started at about the same pressure ( approximately 10 GPa) as under hydrostatic conditions. In situ x-ray diffraction measurement and modeling of the turbostratic stacking fault concentration (degree of disorder) and PT in hBN were performed. Under hydrostatic pressure, changes in the disorder were negligible. Under a complex compression and shear loading program, a strain-induced disorder was observed and quantitatively characterized. It is found that the strain-induced disorder suppresses PT which compensates the promotion effect of plastic shear. The existence of transformation-induced plasticity (TRIP) was also proved during strain-induced PT. The degree of disorder is proposed to be used as a physical measure of plastic straining. This allows us to quantitatively separate the conventional plasticity and transformation-induced plasticity. Surprisingly, it is found that TRIP exceeds the conventional plasticity by a factor of 20. The cascade structural changes were revealed, defined as the reoccurrence of interacting processes including PTs, disordering, conventional plasticity, and TRIP. In comparison with hydrostatic loading, for the same degree of disorder, plastic shear indeed reduces the PT pressure (by a factor of 3-4) while causing a complete irreversible PT. The analytical results based on coupled strain-controlled kinetic equations for disorder and PT confirm our conclusions.

Femtosecond and ultraviolet laser irradiation of graphitelike hexagonal boron nitride

The effect of the femtosecond and nanosecond UV laser irradiation (below the ablation threshold) on graphitelike hexagonal boron nitride (hBN) has been studied. Experiments were carried out with the compacted powder under high vacuum at room temperature using the excimer KrF laser (248nm). In the nanosecond operation mode, the laser-induced fluorescence spectra are found strongly modified depending on the integrated doze, which is attributed to a progressive enrichment of the surface layer by an elemental boron. A slow sample recovery after the laser irradiation has been observed. On the other hand, in the femtosecond mode, the fluorescence spectra depend on the laser fluence, and the changes are reversible: low-energy fluorescence spectra are restored immediately when the laser energy decreases. This effect can be explained by a material bleaching, which favors a bulk centers emission. The ablation threshold has been determined as 78mJ∕cm2 in the femtosecond laser operational mode.

Optical absorption of hexagonal boron nitride involving nitrogen vacancies and their complexes

The pseudopotential and supercell methods are used to study the optical absorption of graphite-like hexagonal boron nitride involving deep levels of nitrogen vacancies and their clusters. The impurity-induced absorption is shown to be mainly related to electron transitions between the states that are antisymmetric with respect to the horizontal plane. Therefore, this absorption is highly anisotropic and is maximum for light waves polarized normally to the hexagonal axis. The optical absorption and photoconductivity spectra before and after neutron irradiation and thermal treatment are interpreted, and the activation energies for the thermoluminescence and conductivity of nitrogen-depleted boron nitride before and after fast-neutron irradiation and vacuum annealing are found.

Bandgap energy of graphite-like hexagonal boron nitride

We have carried out laser-induced fluorescence measurements of hBN powder at room temperature under different environmental conditions (dry powder and suspensions in water or ethanol). From fluorescence excitation spectra, the absorption onset, related to the bandgap energy of this indirect gap semiconductor, has been precisely determined to be E g=4.02±0.01 eV. This result is of importance in view of large discrepancies in previously published data.

The parameters of equilibrium between oxygen solid solutions in cubic and graphite-like hexagonal boron nitrides

Abstract A decrease in the onset temperature of the cBN-to-hBN solid-phase transformation with increase in the oxygen content of cBN samples has been found experimentally. This dependence might be attributable to the existence of oxygen solid solutions in cBN and hBN. From the experimental data and taking into account the kinetics of solid-phase transformations, the equilibrium onset temperature of the cBN-to-hBN transition has been derived for samples with different oxygen contents. In the framework of phenomenological thermodynamics, the model parameters have been determined and the lines of equilibrium between oxygen solid solutions in cubic and graphite-like hexagonal boron nitride have been calculated. It has been shown that a decrease in the oxygen content of the boron nitride–solvent system usually used for cBN synthesis should induce an increase in the thermodynamic stimulus to the hBN-to-cBN transformation at high pressures.