Phase Transformation Of Boron Nitride Research Articles

Transformation of boron nitride under high pressure and shear deformation

Boron nitride cannot be found in nature and can be only synthesized. BN is isoelectronic to carbon and, like carbon, can exist as several polymorphic modifications. It is important material for many applications due to its prominent mechanical, chemical, and thermal properties. In our study, we consider certain phase transformations of boron nitride treated in a high-pressure diamond anvil cell with shear deformation. We studied wurtzitic BN (wBN) treated under pressure from 8 to 17 GPa and graphite-like BN (hBN) treated under pressure from 2.5 to 6 GPa with different shear deformation degrees. The samples were studied in a transmission electron microscope (TEM). It was found that wBN transforms into hBN under 8 GPa and 20°-shear angle, and forms onion-like curved structures. We have analyzed our results and characterized transformations of hBN under pressure up to 20 GPa and shear deformation.

Characterization of boron nitride phase transformations in the Li–B–N system under high pressure and high temperature

The possible phase transformations of boron nitride in the Li–B–N system have been discussed by the chemical reactions of Li3N, hBN and cBN at the conditions of 5.0GPa and 1300–1500°C. The results of the reaction between Li3N and hBN shows that certain Li–B–N eutectic compound(s) which were produced in the thermodynamical stable region of cBN have no catalytic effect for cBN growth. It indicates that a certain irreversible BN precipitation/dissolution process takes place in Li3N melt. However, the reaction between Li3N and hBN is preferential when Li3N, hBN and cBN coexist in Li–B–N system, and the regrowth of cBN is observed, namely the morphology of cBN changing from irregular to fine shape crystals with well-facetted (111) surface. Furthermore, the estimated sizes of regrown cBN are almost same as those of the raw cBN. This indicates a simultaneous cBN dissolution and precipitation process in Li3N+hBN/cBN system under high pressure and high temperature.

Phase transformation of boron nitride under hypothermal conditions

Phase transformation among different boron nitride (BN) phases in hydrothermal solution was investigated. It was found that hexagonal boron nitride (hBN) firstly formed in the solution at relatively low temperature (i.e., 220 °C). After that, a spot of hBN began to transform into wurtzite boron nitride (wBN) and cubic boron nitride (cBN) at 230 °C. More and more hBN converted into wBN and cBN with the increase in temperature, and this transformation process completed at 300 °C. In this paper, we have explained the mechanism of the above phase transformation by using a reported “puckering mechanism”.

Binary and ternary compounds in the Mg-Sb-B and Mg-Bi-B systems as catalysts for the synthesis of cubic BN

We have studied the phase relations in the Mg-Pn and Mg-B-Pn (Pn = Sb, Bi) systems, synthesized the magnesium pnictides Mg3Sb2 and Mg3Bi2 and the new magnesium boropnictides Mg3Pn2(B2), and determined their structure and unit-cell parameters. The synthesized compounds have been investigated at high pressures (4.0–6.5 GPa) and temperatures (700–1400°C). All of them have been found to promote the hexagonal-to-cubic phase transformation of boron nitride.

Crystallographic aspects related to the high pressure–high temperature phase transformation of boron nitride

Crystallographic relations between different forms of boron nitride (BN) appearing at the high pressure–high temperature structural phase transformation have been revealed by high-resolution transmission electron microscopy (HRTEM). As starting materials, crystalline hexagonal BN (hBN) with different degrees of crystallinity, or with defects intentionally introduced, were used. Cubic BN (cBN) is formed only as a minor component, the rest consisting of different forms of sp 2 bonded BN: hBN, compressed, monoclinic deformed hBN, or turbostratic BN (tBN). The small cBN crystallites (300–400 nm) contain many defects such as twins, stacking faults and nanoinclusions of other BN forms: tBN, rhombohedral BN (rBN) and wurtzite BN (wBN). The cBN phase grows epitaxially on the basal plane of hBN. The nucleation sites for cBN are revealed by HRTEM. They consist of nanoarches (sp 3 hybridized, highly curved nanostructures), frequently observed at the edges of the hBN crystallites in the starting materials. Based on HRTEM observations of specimens not fully transformed, a nucleation and growth model for cBN is proposed which is consistent with existing theoretical and experimental models.

Phase transformation of BN in the solid state during electrostatic deposition

Experiments on BN deposition were carried out using a new electrostatic accelerator with a cascade electrode arrangement. The most interesting result of the experiment is the solid state phase transformation of boron nitride. Particles built of the hexagonal BN phase, which under the present experimental conditions is thermodynamically stable, formed a coating composed of the E-BN phase, which is metastable.

Allotropie phase transformation induced by UV power laser irradiation of boron nitride

The optically induced allotropic phase transformations of boron nitride were studied. Under irradiation with laser beam of 337.1 nm wavelength, at energy density of 1.9mJ per 0.1 mm2 a hexagonal boron nitride transformed into the cubic form. The suggestion is made that the results obtained could be explained by the multiphoton excitation and recombination of electrons.

Phase transformations in boron nitride

>Crystal strncture of various boron nitride modifications is considered. Experimental data are presented on phase transformations of boron nitride at high pressures and temperatures under conditions of both static pressures and shock waves. Pressure and temperature diagram for BN is analyzed and the mechanism of direct phase conversion of graphite-like structures to diamond-like ones is discussed. The authors' papers on the investigation of fine crystal structure of wortzite-lixe phase resulting from graphite-like BN under shock pressure are considered in detail. (41 references) (auth)