Kinetics and mechanism of cubic boron nitride formation in the AlN–BN system at 6 GPa

The kinetics of hBN-into-cBN transformation at 6 GPa and 1770, 1880 and 1990 K in the presence of AlN have been studied. The transformation proceeds without liquid phase. It has been established that a limiting stage of the transformation is diffusion of boron and nitrogen atoms in wurtzitic aluminum nitride. Activation energy of the transformation is 170±40 kJ/mol. On the basis of our results we conclude that hexagonal BN dissolves in aluminum nitride solid solution and supersaturates it with respect to cBN, and then cubic boron nitride precipitates from the supersaturated solution of BN in AlN. An AlN–BN system phase diagram is proposed.     

Morphology of cubic boron nitride crystals synthesized using (Fe,Co, Ni)–(Cr,Mo)–Al alloy solvents under pressure

The growth region of the cubic boron nitride (cBN) using (Fe, Ni)–Cr–Al and Co–(Cr, Mo)–Al solvents were presented in the pressure range of about 4–6 GPa and the temperature range between 1200 and 1700 °C. The minimum pressure for cBN formation was confirmed at about 4–4.1 GPa for both (Fe, Ni)–Cr–Al and Co–(Cr, Mo)–Al solvents. Based upon this pressure–temperature condition of the cBN growth region, the morphology of cubic boron nitride crystals was examined under various compositions of the solvents. The morphology of cBN crystals was affected by not only the reaction pressure and but also the composition of the solvents. It was found that the variation of alloy composition provides various morphologies and grain sizes of cBN crystals.     

Oxidation of the zirconium boride-silicon nitride composite in the temperature range 1100–1300°C in air

Composite materials and coatings based on zirconium boride and silicon nitride have been prepared by heat treatment of mixtures of the initial components in air. The chemical reactions result in the formation of the glass melt encapsulating initial particles, which provides an increased heat resistance of the material. The kinetics of high-temperature oxidation of the composite based on the ZrB₂-Si₃N₄ system has been studied. The interaction between solid components of the ZrB₂-Si₃N₄ system, atmospheric oxygen, and the glass melt has been described in terms of the formal kinetic equations for heterogeneous processes. Thermogravimetric, thermal, and X-ray powder diffraction analyses have been carried out, and the electrical resistivity of the compact samples has been determined.     

Effect of pressure and treatment temperature on the structural evolution of mechanically alloyed Ti(W)(C,N)–Al admixes in boron nitride

The effect of pressure and temperature on the structural changes of admixtures of cBN, Al and Ti(C_0.5N_0.05) or Ti(C_0.5N_0.5)_0.6 mechanically alloyed powders with 40 mass% W were investigated by means of the X-ray diffraction technique. It emerged that pressure and temperature affected the crystal structures and compositions of the binder phases as well as the behaviour of the contaminating Fe. High pressure–high temperature (HPHT) sintering favoured the formation of Ti(W,Al)(C,N) solid-solutions, whereas vacuum annealing favoured the formation of W(Ti,Al) solid-solutions. Products of Ti(C,N)-based crystal lattices remained stable under high pressure (5 GPa), whereas W based crystal lattices were more stable under vacuum (0.001 Pa). Inert single phase binders were formed in HPHT sintered PcBN compacts. Formation of Ti(W,Al)(C,N) by reactions between mechanical alloyed Ti(W)(C,N) powder particles and liquid Al prevented the formation of AlN, AlB₂, α-AlB₁₂, TiN and TiB₂ particles in PcBN compacts. Sintering of PcBN occurred by dissolution of B and N atoms in Ti(W,Al)(C,N) and re-precipitation on cBN particles.     

Synthesis and nanostructure of [Mo(C6H4O2)3]·2(C4H8N2O) and the synthesis from 1,2-PDA to (C4H8N2O)

Continuing our work on the synthesis of MoO₂L₂ and MoO₃L_AL_B that show excellent anti-cancer activities in vitro, the MoL₃ have been synthesized by the solvothermal reaction of Na₂MoO₄ with catechols and 1,2-DPA in the mixed solvent of MeCN/MeOH. X-ray diffraction revealed that Mo in chiral octahedral geometry coordinate with three catechol ligands formed three five-membered rings, and the [Mo(C₆H₄O₂)₃] are linked by hydrogen bonds Mo(OC₆H₄)O…H–N(C₄H₈O)N–H…O(C₆H₄O)Mo through the by-product (C₄H₈N₂O) that are formed by one 1,2-DPA with one CO₂ on the catalysis of Mo-complex. Also, we have disassembled bulk crystal into nano-aggregates, and under TEM mono-lamella morphology of nanostructures was observed, which agrees well with the previous conclusion that the morphologies of the nano-aggregates are associated with the quantum motifs in their crystal lattices. [Mo(C₆H₄O₂)₃] have also been characterized by UV–vis spectra, cyclic voltammogram and thermogravimetric analysis.     

Synthesis and Thermal Decomposition Studies of Silicon (IV) Compounds with N,N’-Bis(Salicylidene)Ethylenediimine

The silicon (IV) compounds possessing SiN₄O₂ (6), SiN₂O₂C₂ (7, 8), SiN₂O₂CCl (9) and SiN₄O₄ (10) coordinating frameworks have been synthesized in high yield by the reaction of the O,N,N,O- donor salen-type ligand N,N’-bis(salicylidene)ethylenediimine L with different silanes i.e. diethoxydiisocyanatosilane-(C₂H₅O)₂Si(NCO)₂ 1, dichlorodiphenylsilane-Ph₂SiCl₂ 2, dichloromethyl- phenylsilane-MePhSiCl₂ 3, trichlorophenylsilane-PhSiCl₃ 4 and silicon tetrachloride-SiCl₄ 5. The compounds 6–10 have been characterized by elemental analysis, IR, ¹H, ¹³C and ²⁹Si NMR and mass spectrometry. The decomposition reactivity of these compounds has been studied using a Thermal Gravimetric Analyzer (TGA) at a heating rate of 10°C/minute in an inert atmosphere.     

Studying the mechanism of the interaction between silicon nitride, oxygen, and nitrogen in heating and the processes of defect formation in Si3N4 — gas systems

The method of precision thermal massometry is used to study the interaction between silicon nitride, oxygen, and nitrogen in nonisothermal heating at 300 -1300 K and a partial gas pressure of 19,998.3 Pa (150 Torr). The behavior of Si₃N₄ heated in forevacuum is studied. A special computer analysis of the experimentally obtained dependencies has provided kinetic equations for analyzing and determining the mechanisms of these processes and computing their activation energy. The experimental data are used for creating a hypothesis that the nitrogen sublattice of Si₃N₄ is dispersed before the beginning of its interaction with oxygen and nitrogen.     

Transport of Thorium(IV) Across a Supported Liquid Membrane Containing N,N,N′,N′-Tetraoctyl-3-oxapentanediamide (TODGA) as the Extractant

The transport behavior of Th⁴⁺ was investigated from a feed containing 3.0 M HNO₃ into a receiver phase containing 0.1 M oxalic acid across a PTFE flat sheet supported liquid membrane (SLM) which contained TODGA (N,N,N′,N′-Tetraoctyl-3-oxapentanediamide) in n-dodecane as the extractant. Effects of the nature of the strippant, extractant concentration, Th concentration in the feed, and feed acidity on the transport rates were investigated. The transport behavior apparently depended on the rate of extraction of the metal ion at the feed-membrane interface and was not diffusion controlled. Influence of Th concentration on flux was also investigated. Transport mechanism was elucidated and the diffusion coefficient was calculated to be 2.13 × 10^?7 cm²/s. Solvent extraction studies at varying feed acidity and TODGA concentration were also carried out.     

Spontaneous nucleation of diamond in the system MgCO3–CaCO3–C at 7.7 GPa

High-pressure and high-temperature experiments were carried out to determine the minimum temperature required for spontaneous nucleation of diamond in the system comprising a carbonate mixture (60 mol% MgCO₃ and 40 mol% CaCO₃) and graphite at 7.7 GPa, for 1 h and longer reaction times up to 12.5 h, and also in the systems MgCO₃–graphite and CaCO₃–graphite for comparison. The above carbonate mixture melted at a temperature between 1600°C and 1700°C. The minimum temperature for spontaneous nucleation of diamond was 1900°C for a reaction time of 1 h; it was lowered to 1700°C for 11 h, but no diamond was formed at 1600°C for 12.5 h. These results indicate that the molten state of the solvent-catalyst and enough reaction time are necessary for diamond nucleation in the systems examined at 7.7 GPa.     

Application of zirconium (IV) acetylacetonate to the copolymerization of glycolide with ɛ-caprolactone and lactide

Summary A study on the copolymerization of glycolide with lactide and glycolide with ɛ-caprolactone was performed in the presence of zirconium (IV) acetylacetonate at moderate temperatures (100° and 150°C). Zirconium acetylacetonate appeared to be an efficient initiator of copolymerization. The obtained polymers were characterized by high molecular weights. Considerable influence of transesterification on the polymer chain microstructure was found. Received: 13 October 1998/Revised version: 4 January 1999/Accepted: 4 January 1999     

High pressure synthesis of tungsten carbide–cubic boron nitride (WC–cBN) composites: Effect of thermodynamic condition and cBN volume fraction on their microstructure and properties

Tungsten carbide (WC) with different amounts of Cubic boron nitride (cBN) were synthesized by High Pressure-High Temperature (HPHT) method. The mapping correlation between thermodynamic condition, cBN addition, and microstructure, mechanical properties of WC–cBN composites was established and analyzed by response surface methodology. The main factors affecting the properties of composites were identified by ANOVA. The optimum thermodynamic condition was calculated. It was found that a minor phase transformation of cBN into the low-hardness hBN occurred at a temperature of 1300 °C and intensified at 1500 °C. The homogeneously dispersed cBN particles in the WC matrix promoted an improvement of hardness and fracture toughness, but the phase transition of cBN and its truss effect can dramatically reduce the mechanical properties. The Vickers hardness and fracture toughness of the well-sintered WC-cBN bulks reached a high value of 34 GPa and 13.6 MPa·m^1/2, which are improved by 17% and 52% respectively compared with the pure WC samples sintered under similar high-pressure level.     

Synthesis of AlN powder from Al(OH)3 by reduction–nitridation in a mixture of NH3–C3H8 gas

Aluminum nitride (AlN) powders were synthesized by gas-reduction–nitridation of aluminum hydroxide (Al(OH)₃) powders using a mixture of NH₃ and C₃H₈ gases. A high conversion to AlN (over 70% of naitridation ratio) and single-phase AlN were achieved over 1200 °C. The specific surface area of the products decreased with increasing of reaction temperature and soaking time, and products obtained contain much lesser oxygen than those of the previous researches. The reason why such nanosized AlN powders were easily obtained from Al(OH)₃ was considered to be the higher surface areas of transition alumina formed by dehydration of Al(OH)₃ during firing.     

Understanding the adsorption behavior of oxygen on the 3C–SiC(1 1 0) surface: A first-principles study

The adsorption behavior of atomic oxygen and molecular O₂ on the 3C–SiC(1 1 0) surface is investigated by first-principles calculations. The atomic O prefers to be adsorbed at the C top site (C–O) with adsorption energy of −1.95 eV after zero-point energy correction, followed by the C–O–Si bridge site, Si–O–Si bridge site, and the Si top site (Si–O) with adsorption energies of −1.46, −1.36, and −1.13 eV, respectively. The molecular O₂ separately trapped by the second nearest neighboring C and Si atoms (C–O–O–Si, M4 type) is the most stable configuration with the adsorption energy of −2.46 eV, which is followed by the Si–O–O–Si (M5 type), C–O–O–Si (M3 type), O–Si–O (M2 type), and Si–O=O (M1 type) configurations with the adsorption energies of −2.24, −1.87, −1.07, and −0.75 eV, respectively. All these molecular O₂ adsorption configurations exhibit high tendency to dissociate with the dissociation barriers range of 0.09–0.19 eV. The adsorbed atomic O seems to be easily trapped at the C–O site due to the extremely low diffusion barrier. In addition, the infrared spectra of all the atomic O and molecular O₂ adsorption configurations are predicted and compared with available experimental observations.     

Synthesis and spark plasma sintering of the α-Si3N4 nanopowder

A two-step process (milling and then heat treatment) was used for the preparation of α-Si₃N₄ nanopowder. The influence of the milling time and heat treatment temperature as processing parameters were investigated on the formation of α-Si₃N₄. Silicon nitride ceramic was produced by spark plasma sintering at 1700 °C for 15 min, using MgSiN₂ additive. The optimum sample was produced in a 30 h milling time, heat treatment at 1300 °C, and a 22 °C/min heating rate conditions. X-ray fluorescence analysis showed that the purity of the final product is above 98%. Nanoindentation hardness and Young’s modulus of the SPS-ed sample were measured as 17±2.0 GPa and 290±11.0 GPa, respectively.     

Extraction of Lanthanides(III) with N,N′-Bis(Diphenylphosphinyl-Methylcarbonyl)Diaza-18-Crown-6 in the Presence of Ionic Liquids

The extraction of microquantities of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y from nitric acid solutions into an organic phase containing N,N′-bis(diphenylphosphinyl-methylcarbonyl)diaza-18-crown-6 and ionic liquid (IL) 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (BMImTf₂N) has been studied. The effect of HNO₃ concentration in the aqueous phase and that of the extractant and IL concentration in the organic phase on the extraction of metal ions is considered. The stoichiometry of the extracted complexes has been determined. A considerable synergistic effect was observed in the presence of IL in the organic phase containing a neutral organophosphorus ligand. This effect is connected with the hydrophobic nature of the IL anion. The partition of IL between the equilibrium organic and aqueous phases is the dominant factor governing the extractability of lanthanide (III) ions in the extraction system. The potentialities of polymeric resin impregnated with compound I and BMImTf₂N for the preconcentration of lanthanides(III) from nitric acid solutions are demonstrated.     

Low-cost preparation of Si3N4–SiC micro/nano composites by in-situ carbothermal reduction of silica in silicon nitride matrix

SiC/Si₃N₄ nano/micro composites were prepared from a mixture of α-Si₃N₄, amorphous carbon (carbon black), and Y₂O₃ by carbothermal reduction of SiO₂ present at the surface of Si₃N₄ matrix grains, or added deliberately to the starting mixture. A special heating regime allowed the outgassing of CO(g) (a product of carbothermal reduction) and reduced the residual porosity to less than 2%. The inter- and intragranular SiC inclusions containing the amorphous oxygen-rich layer at the interface between SiC and the Si₃N₄ grains were present, originating from the reaction of free carbon with the silica melt. The reaction consumes silica in the grain boundary phase. The change of the grain boundary chemistry influences both room and high temperature properties of the nanocomposite.     

Interphase Distribution of V(IV) in the Polyethylene Glycol 1500–Sodium Nitrate–Water System

Theoretical Foundations of Chemical Engineering - A study has been made of the interphase distribution of V(IV) ions in a nitrate system with polyethylene glycol 1500. The dependences of the...     

The influence of Ti: Si3N4 ratio on the microstructure evolution of Ti–Si3N4 reaction in Cu melts

In this work, through the study of the obtained Cu–Ti–Si–N intermediate products by controlling the reaction degree of Ti and Si₃N₄ powder in Cu melts at 1250～1300 °C, the effects of different Ti: Si₃N₄ mass ratios on the microstructure evolution of Ti–Si₃N₄ reaction in Cu melts were verified. When the mass ratio of Ti: Si₃N₄ is higher, such as 3.09:1, TiN will cooperate with Ti₅Si₃ and depend on each other to nucleate and grow to form TiN/Ti₅Si₃ composites. The formed TiN are spherical and wetted by Ti₅Si₃ to uniformly disperse in Cu melts. As a result, the TiN–Ti₅Si₃ hybrid reinforced Cu matrix composites will be formed. However, when the mass ratio of Ti: Si₃N₄ is lower, such as 1.37:1, Ti and Si₃N₄ will firstly react to form TiN and Ti–Si liquid. The formed TiN are irregularly polygonal and connect with each other. The Ti–Si liquid will combine with Cu melts to form Cu–Ti–Si liquid and finally form δCu₄Si, ηCu₃Si and τ₁-CuSiTi phases during the colling stage. In this case, TiN are difficult to be wetted, and the Ti–Si₃N₄ compact will keep its original shape and not spread in Cu melts.     

Radiolytic Stability of N,N,N′,N′-Tetraoctyl Diglycolamide (TODGA) in the Presence of Phase Modifiers Dissolved in n-Dodecane

The radiolytic stability of a promising extractant for actinide partitioning from high-level radioactive liquid waste, namely N,N,N',N'-tetraoctyl diglycolamide (TODGA) was investigated in the presence of several phase modifiers, viz. N,N-dihexyloctanamide (DHOA), tri-n-butyl phosphate (TBP), 1-decanol, and iso-decanol dissolved in n-dodecane. The distribution ratio of Am(III) decreased with increased radiation dose studied up to 1000 kGy. Nevertheless, all the compositions of extractants showed fairly high extraction of Am(III) up to 500 kGy (D_Am: ≥ 50), beyond which significant decrease was observed. However, the D_Am values were sufficiently high for process applications for the chosen compositions even at an absorbed dose of 1000 kGy. The stripping behavior of Am(III) with 0.2 M HNO₃ was found to be favorable with increased absorbed dose by the solvent up to 1000 kGy. With an increased absorbed dose, the loading of Nd(III) in the organic phase decreased due to depletion of ligand/extractant concentration (TODGA) in the organic phase. There was marginal variation in the hydrodynamic parameters such as density, viscosity, and interfacial tension (IFT) of the irradiated solvents vis-a-vis fresh/unirradiated solvent.     

Hydrothermal Synthesis of a Nano-sized Mercury(II) N–N–O Donor Coordination Compound

A novel nano-structures mercury(II) coordination compound, [Hg (HPCIH) I₂] (1), (“HPCIH” is the abbreviation of 2-pyridinecarbaldehyde isonicotinoyl hydrazone) has been synthesized by a hydrothermal method that produces the coordination compound at nano size. The new nanostructure was characterized by transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction elemental analysis and IR spectroscopy. Compound 1 was structurally characterized by single crystal X-ray diffraction and the single-crystal structure of this complex shows that each mercury(II) center is five coordinated with two N-donor and one O-donor atoms from “HPCIH” ligand and two iodo anions. Self-assembly of this complexes is realized by CH····I, I····I and π–π stacking interactions. The supramolecular features in these complexes are controlled by weak directional intermolecular interactions.