V. Z. Turkevich

High pressure phase equilibria in the Li3N-BN system: in situ studies

Abstract Powder diffraction of synchrotron radiation at pressures up to 6.3 GPa and DTA at ambient pressure were used to study phase equilibria in the Li 3 N-BN system at temperatures up to 1820 K. The construction of the Li 3 N-BN quasibinary section of the Li-B-N ternary system has been refined. The melting of the high-pressure phase of Li 3 BN 2 has been shown to be of an incongruent nature. The existence of the Li 3 BN 2 (O) ⇄ L + BN peritectic equilibrium at 1620 K and 5.3 GPa has been established. It was shown that this peritectic reaction defines the lowest possible temperature of formation of cubic boron nitride in the Li 3 N-BN system (1670 ± 50 K in the 5.3–6.3 GPa range).

Phase Diagram of the B−BN System at 5 GPa

The chemical interaction and phase relations in the B-BN system have been in situ studied at 5 GPa and temperatures up to 2800 K using X-ray diffraction with synchrotron radiation. The thermodynamic analysis of the B-BN system based on experimental data allowed us to construct equilibrium and metastable phase diagrams of the system at 5 GPa. The only thermodynamically stable boron subnitride, B(13)N(2), melts incongruently at 2600 K and forms eutectic equilibrium with boron at 2300 K and 4 at. % of nitrogen.

Phase diagrams and synthesis of cubic boron nitride

On the basis of phase equilibria, the lowest temperatures, Tmin, above which at high pressures cubic boron nitride crystallization from melt solution is allowable in terms of thermodynamics have been found for a number of systems that include boron nitride.

Kinetics of diamond spontaneous crystallization from the melt of the Fe-Al-C system at 6.5 GPa

Abstract Kinetics of diamond spontaneous crystallization from the Fe–Al–C system melt in equilibrium with graphite has been studied in situ at 6.5 GPa in the range 1720–1920 K using diffraction of synchrotron radiation. The process under consideration is controlled by carbon diffusion in the melt at 1720 and 1820 K. Kinetics data might be best fitted by a model that assumes instantaneous nucleation. At 1920 K the mechanism of diamond crystallization changes; during growth, continuous nucleation is observed and the kinetics of the processes on the surface of growing crystals becomes the stage that controls the crystallization. The rate of spontaneous crystallization in the Fe–Al–C system decreases with increasing temperature as the graphite–diamond equilibrium line is approached.

THERMOANALYTICAL STUDY OF THE POLYMORPHIC TRANSFORMATION OF CUBIC INTO GRAPHITE-LIKE BORON NITRIDE

The polymorphic transformation of cubic into graphite-like boron nitride was studied by using DTA, high-temperature calorimetry and dilatometry over the temperature range 1000–2500 K. The transformation enthalpy was determined to be +22−11+5 kJ/mol.ZusammenfassungMittels DTA, Hochtemperaturkalorimetrie und Dilatometrie im Temperaturbereich 1000–2500 K wurde die polymorphe Phasenumwandlung von kubischem Bornitrid in graphitähnliches untersucht. Die Umwandlungsenthalpie wurde mit +22−11+5kJ/mol bestimmt.

Synchrotron radiation study of MgB2 formation under high pressure

X-ray diffraction with synchrotron radiation was used to study the MgB2 formation in the Mg–B–N system at pressures up to 5.5 GPa and temperatures up to 1900 K. The sequence of chemical reactions and phase transformations in the Mg–B, Mg–B–BN and Mg–BN mixtures was established from energy-dispersive diffraction patterns collected in situ at a constant pressure in the course of a linear heating (or cooling). Over the whole pressure and temperature ranges under study the appearance of the 101 line of MgB2 in diffraction patterns was observed prior to the melting of magnesium in all the cases except the Mg–BN mixture. No other new phases apart from MgB2 were detected in the experiments. Cooling of reaction mixtures leads to the crystallization of MgB2 as the main phase along with the occurrence of MgB4 as a minor impurity.

Kinetics of cBN crystallization in the Li3N-BN system at 6.6 GPa

Abstract Kinetics of cBN crystallization from the Li 3 N-BN melt being in equilibrium with hBN has been studied in situ at 6.6 GPa in the 1720–1820 K range using diffraction of synchrotron radiation. The process under consideration has been found to have the activation energy of 115 ± 5 kJ mol −1 and to be controlled by BN diffusion in the melt. Kinetics data might be best fitted by a model that assumes an instantaneous nucleation in the initial stage of crystallization and nucleation at constant rate when hBN-to-cBN conversion degree is higher than 0.2. Using the model or regular solutions, the maximum solubilities of hBN and cBN in the melt of the Li 3 N-BN system were calculated, which allowed the BN diffusion coefficient in the melt at 6.6 GPa and 1770 K to be evaluated at (3 ± 1) × 10 −7 cm 2 s −1 .

On the problem of the phase relations in the B-BN system at high pressures and temperatures

Phase relations and chemical interaction in the B-BN system have been studied in situ using X-ray diffraction with synchrotron radiation at pressures to 5.3 GPa and temperatures to 2700 K. The results obtained have been used for the thermodynamic analysis of this system and for the construction of its phase diagram at 5 GPa. The values of thermodynamic functions of the phases competing in the B-BN system at high pressures and temperatures have been calculated in the framework of phenomenological models, unknown parameters of which have been found from the experimental data. It has been shown that only one thermodynamically stable boron subnitride (rhombohedral B13N2) exists in the system. It melts incongruently and forms an eutectic equilibrium with boron.

Production of the Y3Al5O12 transparent nanostructured ceramics

The nanocrystalline Y3Al5O12 ceramics with different phase compositions, microstructures, and optical characteristics have been prepared by low-temperature consolidation of the Y3Al5O12 powder at pressures from 6 to 7.7 GPa in the temperature range from 250 to 550°C. The conditions have been defined for obtaining transparent nanostructured ceramics Y3Al5O12 having grains of size 20–40 nm and a transmission coefficient in the visible region of 40–45%. The criteria for the transparency of the nanostructured ceramics have been formulated and the ways of an improvement of its optical characteristics are discussed.

In situ studies of boron nitride crystallization from BN solutions in supercritical N–H fluid at high pressures and temperatures

The reaction of boron with products of the thermal decomposition of hydrazine and the crystallization of boron nitride from BN solutions in supercritical N–H fluid have been studies in situ at pressures up to 5.2 GPa and temperatures up to 1600 K using angle- and energy-dispersive X-ray diffraction with synchrotron radiation. Graphite-like boron nitride produced by the reaction between boron and N–H supercritical fluid dissolves in the latter to form associated solutions of different concentrations. On cooling, the disappearance of short-range order in this solution is observed, accompanied by the precipitation of solid phases (cBN or hBN and BN–NH3 intercalation compound depending on the pressure, temperature and concentration). Spontaneous crystallization of cubic boron nitride has been observed down to 1.9 ± 0.2 GPa, which is the lowest pressure of the cBN crystallization from a solution reported so far. The BN–NH3 hypothetical quasibinary section of the phase diagram of the B–N–H system at 4 GPa has been constructed.