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cBN-based materials are well known for their high chemical inertness and high performance. The variation of binder phase type and amount enable improvement of performance even more. Comprehensive investigation of the influence of type of niobium compound (NbC and NbN) as a binder phase for cBN-based materials were done. Samples were prepared by high pressure high temperature (HPHT) sintering under pressure of 7.7 GPa. Sintered samples were investigated in terms of phase composition (XRD), microstructure (SEM, EDX) and mechanical properties. Performance of investigated tools were tested in conditions of high speed machining of Inconel 718 alloy and Caldie tool steel.

Polycrystalline cubic boron nitride (PCBN) tool materials with NbN binder without additives and PCBN with Al2O3 or Si3N4 micropowder and whisker additives were manufactured and compared. PCBN materials with Si3N4 whisker reinforcement have the best mechanical properties of all the evaluated materials. Composites reinforced with Al2O3 whiskers have the lowest fracture toughness. However, Al2O3 whisker-reinforced tools outperform both commercial and Si3N4 reinforced tools when machining hardened steel. Thus Al2O3 whisker-reinforced PCBN materials are promising for industrial applications, likely due to their higher resistance to oxidation and diffusional wear mechanisms during cutting operations. (Less)

Автори висловлюють подяку канд. фіз.-мат. наук, ст. наук. співробінику фізичного факультету КНУ ім. Т.Г. Шевченка Н.М. Білявиній за інтерпретацію результатів рентгенівського фазового аналізу та участь в обговоренні результатів. Дослідження виконано в рамках European Union’s Horizon 2020 Research and Innovation Programme проект Flintstone2020 (грант № 689279).

The sintering of cubic boron nitride (cBN) based ceramic matrix materials with zirconium carbide and aluminum has been carried out under high pressures and temperatures. The performed study has revealed that the interaction between mixture components with the formation of a new phase, namely, zirconium diboride (ZrB2) occurs at sintering temperatures above 1750°C and an applied pressure of 7.7 GPa. A further increase in the sintering temperature leads to a growth in the volume ZrB2 content and reduces the microhardness and crack toughness of the studied materials. The flank wear of specimens under the conditions used for the processing of AISI316L stainless steel (vc = 300 m/min, t = 300 s) grows with an increase in the sintering temperature to attain the critical value VB = 325 µm, when the specimen sintered at temperature of 2300°C is used, and VB = 200 µm for the specimen sintered at 1750°C.

Ceramic matrix composites based on cubic boron nitride with a TaC binder phase are sintered under conditions of high pressure and high temperature. The sintering is carried out in a high-pressure apparatus of the toroid type in the temperature range of 1450–2450°C. It is shown that a chemical reaction between components of the mixture with the formation of TaB2 takes place at a sintering temperature above 2150°C. The microhardness of the composites increases with an increase in the sintering temperature and reaches a maximum value (34 GPa) at 2150°C. The composites show high resistance to mechanochemical wear when used for high-speed machining of nickel-based alloy Inconel 718.

Three types of cBN-based composites with TaN binder—without whiskers, with Al2O3 whiskers, and with SiC whiskers—have been produced by the high pressure-high temperature (HPHT) sintering. Density, hardness, Poisson ratio, and fracture toughness of the composites have been studied. The whisker-reinforced cBN-based composites exhibit a higher level of mechanical properties (hardness, fracture toughness) in comparison with the non-reinforced materials.

The structure, phase composition, and temperature dependence of electrical resistivity of the thermobarically sintered cBN–NbN composite are presented. The semiconductor character of the change in resistivity from 2.5 to 1.8 Ohm cm in the range of 300–700 K is revealed.

The available data indicate that solubility of nitrogen in the liquid transition metals at high temperatures attains very interesting, high values even for relatively low pressure of N2 gas. For instance, at pN2 = 10 MPa (app. 100 bar) and 1600 °C, the N solubility in Fe-35%Cr solution is as high as 10 at%, what is significantly above N-solubility in liquid gallium at the typical conditions of high pressure solution growth of GaN (pN2 = 1.0 GPa, T = 1500 °C). Therefore, for pressures required for GaN stability at T > 1300 °C (much higher than 100 bar) one can expect N solubility in Fe and its mixtures with Co and Cr on the level of 10–20 wt% which is extremely promising for growth of GaN from liquid solution. The results of some “proof – of – concept” experiments exploring Fe-Ga-N system in both high pressure gas systems of high volume (1.0 GPa, 1600 °C, 1–5 dcm3) used previously for crystal growth of GaN at the Institute of High Pressure Physics PAS and in the toroid high pressure (8.0 GPa, 2500 °C) systems for bulk growth of diamond at the Institute of Superhard Materials in Kyiv are shown and discussed. The results of studies of: melting of Fe and Fe-Ga alloys in contact with nitrogen by DTA, dissolution of nitrogen from gas and solid (GaN) sources as well as physical properties of GaN crystals grown in Fe-Ga-N system are reported.

The processes of formation of ceramicmatrix materials in the cBN(Al)-SiB4 WC system under high pressure conditions (7.7 GPa) in the temperature range of 1600-2300 °C are studied. It is shown that, for the composition chosen by us (BL group), 60 % vol. cBN, 5 % vol. Al, 25 % vol. SiB4 and 10 % vol. WC, nonporous superhard materials are virtually formed in the entire temperature range with hardness of no less than 33 GPa and Young’s modulus of 613 GPa, which is attributable to the formation of a highstrength ceramic matrix both as a result of the liquidphase sintering using aluminium and of an active chemical interaction of silicon tetraboride with WC. It is demonstrated by experiments that the micropowder consolidation process should be carried out at temperatures of no less than 1800 °C. The heating of the system above 2200 °C leads to the excessive cBN graphitization and accumulative recrystallization in general. According to the XRD analysis, it was established that, as a result of the thermal decomposition of silicon tetraboride and further chemical reaction with WC, new compounds are formed: W2B5 and WSi2, and the original aluminum is oxidized to α-Al2O3, thereby relieving the system of excess oxygen. All new compounds formed in the course of producing the ceramics are represented by microcrystalline forms, not exceeding 1—3 μm in size, that are arranged in the intergranular space of the core matrix, which contributes to an additional increase in hardness and fracture resistance. The obtained superhard ceramic plates can be used for turning tempered (up to 60 HRC) and highalloyed (including inconel) steel at rising temperatures in the cutting area.

In this paper we initiate further discussion on phase diagrams and structural transitions in tetrahedrally bonded semiconductors with particular emphasis on GaN. Comprehensive experimental data and the most important theoretical predictions are comprised in an effort to bring up a generalized universal scheme that explain melting that may or may not be accompanied by the change of the coordination number. The scheme considers the “anomalous” melting temperature dependence on pressure for GaN. Principal possibility of GaN melting into low density semiconducting liquid is demonstrated and challenges on the way to experimental observation of such liquid phase are disputed. Prospects of future experiments that can provide reliable information on the nature of atomic bonding for different GaN phases are laid out.

Influence of sintering temperature on both phase composition and structure of ceramic matrix composites based on cubic boron nitride with binders in a form of Al and the Ti, Cr, V carbides are investigated by methods of physical materials science. As the object of study, three cBN–TiC–Al, cBN–Cr3C2–Al, cBN–VC–Al compositions containing 60% vol. cBN are selected. The composites are obtained by high-pressure high-temperature (HPHT) sintering in high-pressure apparatus AVTT-30 in the temperature range of 1600– 2450°C under pressure of 7.7 GPa. In the process of HPHT sintering of composites in the cBN–TiC–Al and cBN–Cr3C2–Al systems, the formation of titanium and chromium diborides is found at the temperatures of sintering Тsint ≥ 1850°С and Тsint ≥ 2150°С, respectively, simultaneously with the formation of alu-minium nitride. (Less)

A method of the reaction hot pressing was used to produce composites from the B4C–(5–15%)(TiH2–VC) system, established the peculiarities of their structures and measured physico-mechanical parameters. The dense dispersion-hardened materials having a high bending strength (Rbm = 420–580 MPa) and increased fracture toughness (KIc = 4.1–4.8 MPa·m1/2) on retention of the HKN = 19–20 GPa are promising for the use in the conditions of the severe abrasive wear.

Silicide based compounds are widely used for coatings due to their high melting temperature, oxidation resistance and moderate density. Employment of binders based on silicides of transitional metals can provide cBN-based cutting tools with higher chemical stability and better performance. The relationship between phase composition, microstructure and mechanical properties of novel polycrystalline cubic boron nitride (PcBN) materials were investigated. Three series of PcBN samples were made by high pressure high temperature (HPHT) sintering. Silicides of chromium – CrSi2, vanadium – VSi2 and molybdenum MoSi2 were used as a binder phase in each case, while aluminum was introduced to the mixture as an oxygen getter. During HPHT sintering at temperatures above 1850 ∘C the formation of borides of binder phase were observed in cases with VSi2 and MoSi2. For system with CrSi2 binder, temperature of boride formation was found to be lower – 1600 ∘C. Materials with MoSi2 binder phase demonstrated the highest level of microhardness. Performance of materials were investigated in conditions of machining of stainless steel AISI 316L and Inconel 718.

The corrosion resistance, structure and physico-mechanical properties of B4C–VC hot-pressed composites are studied. Materials perspective for the use as the end seals at chemical productions.

(Y0.99Eu0.01)2O3 nanoceramics have been produced by sintering of stable cubic nanopowders under 8 GPa at temperature in the range of 25–500 °C with the use of Low Temperature High Pressure (LTHP) technique. During consolidation step irreversible phase transition from cubic to monoclinic yttria occurs resulting in two-phase nanoceramics with a grain size in the 10–40 nm range. It has been demonstrated that composite nanoceramics possess a high transmittance in the visible and mid IR ranges due to small light scattering on the nanoscale pores and low birefringence due to extremely small grain size. It has been shown that Eu3+ ions act as a luminescent probe in composite (Y0.99Eu0.01)2O3 nanoceramics since their 4f–4f luminescence strongly depends on the crystallographic environment. The luminescence spectra excited in the charge transfer band (CTB) are presented by superposition of emission from europium ions in cubic and monoclinic yttria. A new wide emission band of (Y0.99Eu0.01)2O3 ceramics in the λ = 500–650 nm wavelengths range (λex. = 307 nm) were attributed to luminescence of Eu3+ ions located in perturbed sites at grain boundaries or interfaces.

Superhard cBN-based materials with Cr3C2, Cr2N and CrB2 binders were sintered in a high-pressure high-temperature (HPHT) toroidal apparatus under a pressure of 7.7 GPa and in the temperature range of 1600-2450°С. Initial mixtures of three compositions were chosen with 60 vol.% of cBN, 35 vol.% of binder phase and 5 vol.% of Al. Phase composition and microstructure of sintered samples were investigated by X-ray analysis and scanning electron microscopy, respectively. Elastic properties were measured using the ultrasonic pulse-echo technique. Composites with Cr3C2 and CrB2 binders sintered at 2000°C have the highest values of hardness.

The structure and electro-physical properties of islet gold films on the surface of aluminum nitride are reported.

The structure of island gold films on the surface of cubic boron nitride is reported. The average size of islands, the nature of their distribution on the surface, and the average number per surface unit are given.

The phase diagram of the Al–B–C system has been calculated at pressure 7.7 GPa using models of the phenomenological thermodynamics with the interaction parameters derived from the experimental data on the phase equilibria at high pressures and temperatures.

During a metal cutting process, chemical wear can become the dominant mechanism of tool degradation under the high temperatures and contact pressures that arise between the tool and the metal workpiece. This study focuses on the chemical and diffusional interactions between superalloy Inconel 718 and cubic boron nitride (cBN) tool material with and without TiC binder. It covers thermodynamic modeling and experimental tests in the pressure range of 0.1 Pa to 2.5 GPa at temperatures up to 1600 °C. The methods used include diffusion couples under both vacuum and high pressure, transmission electron microscopy (TEM) analysis and in-situ synchrotron observations. It is shown that cBN is prone to diffusional dissolution in the metal and to reactions with niobium, molybdenum, and chromium from Inconel 718. Adding TiC binder changes the overall degradation process because it is less susceptible to these interaction mechanisms.

Experimental studies of diffusion of oxygen in bulk wurtzite-type GaN crystals grown by Halide Vapor Phase Epitaxy (HVPE) are reported. Oxygen concentration profiles were studied in as-grown GaN crystals and also after annealing of crystals at temperatures up to 3400 K and pressures up to 9 GPa. Investigated crystals contained large conical defects i.e. pinholes of significantly higher oxygen concentration ( N O =(2–4)×1019 cm −3 ) than that in the bulk matrix ( N O −3 ). The pinholes were revealed by a photo-etching method in as-grown and annealed GaN samples. Confocal micro-Raman spectroscopy was applied to measure the profiles of free electron concentration, which directly corresponds to the concentration of oxygen impurity. Lateral scanning across the interfaces between pinholes and matrix in the as-grown HVPE GaN crystals showed sharp step-like carrier concentration profiles. Annealing at high temperature and high pressure resulted in the diffusion blurring of the profiles. Analysis of obtained data allowed for the first time for estimation of oxygen diffusion coefficients D O ( T , P ). The obtained values of D O ( T , P ) are anomalously small similarly to the values obtained by Harafuji et al. by molecular dynamic calculations for self-diffusion of nitrogen. Whereas oxygen and nitrogen are on the same sublattice it could explain the similarity of their diffusion coefficients.

In order to investigate new materials for metal cutting applications, cubic boron nitride, cutting tools with different amounts of binder phase were sintered in HPHT toroid type apparatus under 7.7 GPa and in temperature range of 1450-2450°C. Initial mixtures of three composition were chosen with 50, 60, 65 vol. % of cBN, 5 vol. % of Al was added to mixture to prevent oxidation. Phase composition, microstructure, elastic properties, hardness, fracture toughness and cutting performance were investigated. The highest value of the mechanical properties and tool life demonstrated samples sintered in temperature range 1850-2150 °C.

The journal of physical chemistry / C 122(15), 8505 - 8509 (2018). doi:10.1021/acs.jpcc.8b00102, Phase relations in the B–BN system have been studied ex situ and in situ at pressures 2–20 GPa and temperatures up to 2800 K. The evolution of topology of the B–BN phase diagram has been investigated up to 24 GPa using models of phenomenological thermodynamics with interaction parameters derived from our experimental data on phase equilibria at high pressures and high temperatures. There are two thermodynamically stable boron subnitrides in the system i.e. B$_{13}$N$_2$ and B$_{50}$N$_2$. Above 16.5 GPa, the $B_{50}N_2 ⇄ L + B_{13}N_2$ peritectic reaction transforms to the solid-phase reaction of $B_{50}N_2$ decomposition into tetragonal boron $(t′-B_{52})$ and B$_{13}$N$_2$, while the incongruent type of B$_{13}$N$_2$ melting changes to the congruent type only above 23.5 GPa. The constructed phase diagram provides fundamentals for directed high-pressure synthesis of superhard phases in the B–BN system., Published by Soc., Washington, DC

Gallium nitride (GaN) is considered to be one of the most important semiconductors nowadays. In this report a solution of the long standing puzzle regarding GaN decomposition and melting under high pressure and high temperature is presented. This includes the discussion of results obtained so far. The possibility of a consistent parameterisation of pressure (P) evolution of the melting temperature (Tm) in basic semiconductors (GaN, germanium, silicon…), independently from signs of d T m / d P is also presented.

The article presents a study of the potential for the use of B6O superhard boron suboxide as a binder in composite materials with a low cBN content. Superhard B6O is characterized by higher mechanical properties than TiC widely used in commercial materials today. Composites containing 60 vol % cBN and different binder compositions that included B6O and TiC have been sintered in a toroid-type high-pressure apparatus at a pressure of 7.7 GPa in a temperature range from 1500 to 2000 °C. The microstructure, phase and elemental composition of the sintered materials have been examined via electron microscopy and X-ray diffraction analysis. Mechanical properties have been analyzed via indentation techniques. The performance of the cutting tools produced from the sintered composites has been evaluated in turning hardened cold work tool steel.

The evolution of topology of the B-B2O3 phase diagram has been studied at pressures up to 24 GPa using models of phenomenological thermodynamics with interaction parameters derived from experimental data on phase equilibria at high pressures and high temperatures.

The isobaric sections of the phase diagram of the silicon–carbon system at 8 GPa has been calculated using the phenomenological thermodynamics models with parameters of the interaction that have been defined based on the experimental data on the phase equilibria at high pressures and temperatures.

Superhard cutting tool materials were sintered in cBN–(Ti3SiC2–TiC) system via high pressure–high temperature method. Sintering was performed under the pressure 8 GPa in the 1400–2400°C temperature range. The initial mixtures of three compositions were chosen with 90, 80 and 60 vol % cBN. The mixtures were prepared by mixing cBN (1–3 μm) and Ti3SiC2–TiC (< 2 μm). It was found, that upon sintering, the compositions of the obtained samples differed from the initial mixtures in all cases as a result of chemical reactions. Microstructure observations, phase composition estimation, and mechanical properties of the obtained tool materials were carried out. The results indicate that both the varying cBN content and the applied sintering conditions have a direct effect on the structure, properties, and kinetics of reactions.

A concept is proposed to produce Y2O3:Eu3+ optical nanograined ceramics by the transformation-assisted consolidation of nanospheres under 8 GPa pressure. The nanoceramics were prepared by high-pressure low-temperature sintering (~0.04–0.2Tm, where Tm is the melting temperature) accompanied by the cubic-to-monoclinic phase transition. The effects of sintering conditions upon phase composition, grain size evolution, density, morphology, optical and luminescent properties of sintered ceramics have been studied. It has been shown that Y2O3:Eu3+ nanograined ceramics consisting of individual (cubic or monoclinic) phases or their mixture can be obtained by variation of the sintering temperature. The use of transformation-assisted consolidation makes it possible to prepare Y2O3:Eu3+ nanoceramics with average grain size three times smaller (12 nm) than that of the starting nanopowders (37 nm), which corresponds to extremely low grain growth factor of 0.3. The grain size refinement is related to numerous nucleation events in the parent phase of cubic yttrium oxide. The preparation conditions of translucent (T=50%) composite Y2O3:Eu3+ nanograined ceramics with a relative density of 99±1 % have been determined. The obtained two-phase ceramics show high optical transparency due to negligible birefringence at extremely small (~12 nm) average grain size.

Thermogravimetry and differential thermal analysis have been used to study the resistance to the air oxidation of high-density samples of Ti3AlC2, Ti2AlC and Ti2Al(C1−xNx) solid solutions. It has been shown that the Ti3AlC2 samples are more stable than Ti2AlC and Ti2Al(C1−xNx) solid solutions and as the nitrogen content of the solid solution increases to x = 0.75, the oxidation resistance decreases. The following characteristics have been exhibited by the material containing 89 wt % Ti3AlC2 (the rest being Al2O3 and TiC) having density 99% of theoretical: bending strength Rbm = 500 MPa, compressive strength Rcm = 700 MPa, fracture toughness KIc = 10.2 MPa·m0.5, hardness HRA = 70 GPa, HV = 4.6 GPa, Young modulus = 149.4 ± 28.7 GPa. After sintering with diamonds or cBN (50 wt %) at 5.5–7.7 GPa and 1350–1960°C for 0.07–1.0 h the Ti3AlC2 MAX phase decomposes to form TiC and TiAl or TiB2 and a thin layer of Al4C3 forms at the interface with diamond. The Al4C3 decomposition in a composite material due to the interaction with the air moisture results in the crack initiation along the diamond perimeter, which brings about the material fracture in 1–2 weeks. It has been found that the Ti3AlC2 powder is efficient for polishing natural and synthetic jewelry crystals and competitive in polishing efficiency and quality with ACM 2/1 grade diamond.

Using models of phenomenological thermodynamics with interaction parameters derived from experimental data on phase equilibria at high pressures and high temperatures, the evolution of topology of the aluminum-boron phase diagram has been studied at pressures up to 8 GPa.

Comparative investigation of the luminescent properties of Y 3 Al 5 O 12 (YAG) nanoceramics with the properties of single crystals counterpart is performed under excitation by synchrotron radiation in the exciton range of YAG host. Analysis of the luminescent properties of such different crystalline forms of YAG allows us to conclude that the behavior of YAG nanoceramics is close to the properties of single crystal analogue with large content of Y Al antisite defects (ADs). We also have found that the relative intensity of F + –AD coupled centers is significantly higher in YAG nanoceramics than that for single crystal counterpart. This presupposes the strong coupling of the antisite defects and oxygen vacancy-related centers in YAG nanoceramics, mainly at the boundaries of grains.

The phase formation in the Al-B system under high pressures and temperatures as well as of crystal structures of aluminum borides forming in the system have been studied. The existence of AlB2, Al16, AlB25, AlxB12 have been detected by X-ray diffraction and scanning electron microscopy. It has been shown that the crystal structures of boron-rich high-pressure synthesized Al16, AlB25, AlxB12 phases are closely related to the crystal structures of known boron modifications.

Y2O3:Eu3+ (1 at.%) translucent nanostructured ceramics with total forward transmission achieving ∼70% of the theoretical limit has been obtained by the transformation-assisted consolidation of custom-made cubic Y2O3:Eu3+ nanopowders under high pressure (HP). Sintering under the pressure of 7.7 GPa and temperatures in the 100–500 °C range leads to the partial cubic-to-monoclinic phase transition that results in two-phase Y2O3:Eu3+ nanoceramics. The average grain size of ceramics d ≤ 50 nm for both Y2O3:Eu3+ polymorph is comparable with crystallite size of initial nanopowders (d ∼ 40 nm), indicating that the grain growth factor is near unity. The phase compositions, morphology, densities, preliminary optical and luminescent properties of synthesized nanostructured ceramics have been studied.

The formation of composites in the Al-B-C system at high p, T parameters using aluminum borides, diamonds, boron and aluminum carbides as the initial components has been studied. It has been shown by transmission and scanning electron microscopies that chemical interaction of diamond with aluminum borides in the Cdiam-AlB2 and Cdiam-AlB12 systems takes place starting at 1500°C and at T ≥1700°C becomes more pronounced. The composite produced at 1900–2100°C has been characterized only by intragranular fracture of diamond, which is indicative of the high-strength of phase boundaries. The Al-B4C system is characterized by the formation of high-strength composites that consist of substitutional solid solutions based on the boron carbide lattice.

Using micro- and nanoindentation techniques, we have studied the mechanical properties of Y3Al5O12 optical ceramics: coarse-grained, prepared by vacuum sintering, and nanostructured, prepared by high-pressure, low-temperature sintering. The effect of grain size on the strength of the ceramics has been analyzed.

Y3Al5O12 nanostructured ceramics with total transmittance of 63% at λ = 1064 nm has been obtained by low-temperature high-pressure sintering. According to high-resolution transmission electron microscopy ceramics is near pore-free and consists of close-packed grains of 20–40 nm in size. The transmittance spectrum in the visible and IR wavelength range, stationary X-ray excited luminescence and thermally stimulated luminescence were studied to characterize Y3Al5O12 nanostructured ceramics in comparison to single crystals of the same composition. The observed differences in the optical and luminescent properties were interpreted as a consequence of high defectivity level of nanoceramics arising from its non-equilibrium character, extremely large concentration of grain boundaries and surface states.

X-ray diffraction with synchrotron radiation has been used for the first time to study chemical interaction in the MnO-ZnO system at 4.8 GPa and temperatures up to 1600 K. Above 750 K, the chemical reaction between MnO and ZnO has been observed that resulted in the formation of rock salt (rs) Mn1-xZnxO solid solutions (0.3 \leq x \leq 0.7). The lattice parameters of these solid solutions have been in situ measured at high pressure as a function of temperature, and corresponding thermal expansion coefficients have been calculated., Comment: 9 pages, 2 figures, 2 tables

The evolution of topology of the B-C phase diagram has been studied at pressures up to 24 GPa using models of phenomenological thermodynamics with interaction parameters derived from experimental data on phase equilibria at high pressures and high temperatures.

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.

Critical current density (jc) of high-pressure (2 GPa) manufactured MgB2-based superconductors depends on the amount and distribution of higher borides (MgB12) in MgB2 matrix, which in turn are determined by the nature of the initial components first of all B or MgB2 and the temperature of sintering or synthesis. Ti and Ta additions can improve jc by promoting the higher boride formation via impurity hydrogen absorption, thus preventing MgH2 detrimental for jc being formed, which possibly increases the MgB12 nucleation barrier. SiC (0.2-0.8 mum) addition increases jc of MgB2, allowing us to get jc = 106 A/cm2 at 20 K in the 1 T field: pinning is increased by SiC and higher boride grains and there is no notable interaction between SiC and MgB2 . As the synthesis temperature increases from 800 to 1050degC, Ti and SiC additions may affect the oxygen segregation and formation of Mg-B-O inclusions enriched with oxygen as compared to the amount of oxygen in the MgB2 matrix, which can also promote an increase in pinning. Materials high-pressure synthesized from Mg and B taken in 1:4, 1:6, 1:7, 1:8, 1:10, 1:12, 1:20 ratios were superconductive with Tc of about 37 K. High jc (7middot104 - 2middot104 A/cm2 in zero field at 10-30 K, respectively) showed materials with the matrix composition near MgB12 stoichiometry, they have doubled microhardness of MgB2.

Phase equilibria with the participation of a liquid phase in the Mg-MgO-B ternary system at 2 GPa have been calculated in the framework of model approximations of the phenomenological thermodynamics. The melting diagram of the system is characterized by five nonvariant four-phase equilibria, three of which are peritectic and two eutectic ones. The region of the primary crystallization of magnesium diboride MgB2 in the diagram is near the Mg-B binary system region and as the oxygen concentration increases, disappears and is replaced by the crystallization regions of MgO and MgB4 at the point of peritectic equilibrium L + MgB2 ⇄ MgO + MgB4 (1345 K, 68 at % B, 10 at % O).

The quenching from high pressures, X-ray analysis, and scanning electron microscopy have been used to experimentally study phase equilibria in the Mg–B binary system at a pressure of 2–4 GPa. The experimental results have been used to define unknown parameters in phenomenological models of the phases competing at high pressures. Thermodynamic calculations have been made and the phase diagram of the Mg–B system at a pressure of 2 GPa has been constructed. Structure and properties of MgB2-based block materials manufactured at high pressure (2 GPa) have been considered. The blocks have been synthesized from Mg and B or sintered from MgB2 powder at 970–1370 K. Fine dispersion inclusions of higher borides (with stoichiometry close to MgB12) increase critical current density. Samples synthesized from the mixture of Mg and B with a ratio, which corresponds to MgB12 stoichiometry, contained MgB12, MgO, MgB2 phases and demonstrated superconducting behavior.

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.

Thermal phase stability of graphitelike BC{sub 4}N (g-BC{sub 4}N) has been studied up to 7 GPa and 2,100 K using in-situ powder diffraction of synchrotron radiation. It has been shown that the process of g-BC{sub 4}N decomposition at ambient pressure starts at 2,050 K and occurs to form highly ordered hexagonal graphitelike boron nitride (hBN) and disordered graphite. As the pressure increases up to 6.6 GPa, the temperature of the decomposition onset decreases to 1,070 K. At this pressure in the 1,070--1,400 K temperature range (decomposition degree of < 0.1), the decomposition products remain the same, whereas at higher temperatures, the decomposition of g-BC{sub 4}N is accompanied by the formation of cubic boron nitride (cBN) and disordered graphite. Measurement of 298 K equation of state of g-BC{sub 4}N results in a zero-pressure bulk modulus of 18.1 {+-} 0.2 GPa and its pressure derivative of 6.6 {+-} 0.1. Thermodynamic analysis has shown that the g-BC{sub 4}N decomposition is a nonequilibrium process and the phase itself is metastable.

The graphite-diamond transformation was investigated by in situ time-resolved X-ray diffraction experiments using aqueous fluid containing dissolved MgO as the diamond-forming catalyst under conditions of 6.6–8.9 GPa and 1400–1835 °C. The transformed volume fractions of diamond as a function of time under various pressure-temperature conditions were obtained and analyzed using the JMAK rate equation. Variations in the nucleation and growth processes during diamond formation as a function of pressure and temperature were clarified.