Thermodynamic Modeling of Phase Equilibria in the U–Zr–N System

Inorganic Materials - A thermodynamic model is proposed for condensed phases in the ternary system U–Zr–N in the range 298–2800 K. The model is based on previously reported models...     

Thermodynamic Modeling of Mechanical Alloying in the Fe–Sn System

X-ray diffraction analysis and thermodynamic calculations in the Miedema model are used to study the sequence of solid-state reactions during mechanical alloying in the Fe–Sn system. The results indicate that the formation of FeSn₂ in the initial stages of the process is a quasi-equilibrium process reducing the volume contribution to the free energy. A simple model, considering a random distribution of particles, is used to obtain an expression for the total free energy, comprising the volume and interface contributions, as a function of particle size. It is shown that the formation of a supersaturated solid solution is thermodynamically possible over a wide composition range if the particle size is reduced to a few nanometers and surface Sn segregation occurs. The limit of the supersaturated solid solution obtained by mechanical alloying is determined by the relationship between the limit of size reduction and the particle size which ensures stabilization of the solid solution. In the Fe–Sn system, the limit of the supersaturated fcc solid solution is found to be 20 at. % Sn.     

Formation and Transport of Atomic Hydrogen in Hot-Filament Chemical Vapor Deposition Reactors

In this paper we focus on diamond film hot-filament chemical vapor deposition reactors where the only reactant is hydrogen so as to study the formation and transport of hydrogen atoms. Analysis of dimensionless numbers for heat and mass transfer reveals that thermal conduction and diffusion are the dominant mechanisms for gas-phase heat and mass transfer, respectively. A simplified model has been established to simulate gas-phase temperature and H concentration distributions between the filament and the substrate. Examination of the relative importance of homogeneous and heterogeneous production of H atoms indicates that filament-surface decomposition of molecular hydrogen is the dominant source of H and gas-phase reaction plays a negligible role. The filament-surface dissociation rates of H2 for various filament temperatures were calculated to match H-atom concentrations observed in the literature or derived from power consumption by filaments. Arrhenius plots of the filament-surface hydrogen dissociation rates suggest that dissociation of H2 at refractory filament surface is a catalytic process, which has a rather lower effective activation energy than homogeneous thermal dissociation. Atomic hydrogen, acting as an important heat transfer medium to heat the substrate, can freely diffuse from the filament to the substrate without recombination.     

Diamond Crystallization in the System B4C–C

Superhard polycrystalline diamond material consisting of crystallites less than 20 m in size and containing less than 5 wt % B₄C is synthesized in the graphite–B₄C system at 2600–2800 K and 8–9 GPa. In the Raman spectrum of this material, the main band (1332 cm^–1) is shifted to lower frequencies by 40 cm^–1, typical of heavily boron-doped diamond films. Based on experimental data, a mechanism is proposed for the transformation of graphite into polycrystalline diamond at temperatures between the melting points of the B₄C–diamond and B₄C–graphite eutectics.     

Equilibrium Vapor Composition in the Pb–I2 System

The equilibrium vapor composition in the Pb–I₂ system at temperatures from 400 to 2000 K and total pressures from 10² to 10⁵ Pa was assessed by thermodynamic analysis. The results show that the dominant vapor species is PbI₂. PbI₂ dissociation is significant starting at 1000–1300 K, depending on the total pressure in the system.     

Optimization of PVD Parameters for the Deposition of Ultrahard Ti–Si–B–N Coatings

Multicomponent Ti–Si–B–N coatings were deposited on high-speed steel (HSS) substrates by reactive magnetron sputtering using a SHS TiB + 20 wt% Si target. The influences of the substrate temperature, bias voltage, and nitrogen partial pressure on the structure and the elemental compositions of the films were studied. The films were characterized by high-resolution transmission electron microscopy (HRTEM), Auger spectroscopy (AES), and X-ray diffraction (XRD). The results of HRTEM analysis indicated the formation of an ordered–disordered structure with fine crystalline grains of hexagonal Ti(B,N) _x phase and amorphous integrain layers. The stoichiometry of the Ti(B,N) _x phase was strongly affected by PVD process parameters. The films were characterized in terms of their microhardness and wear resistance. The reasons for the high value of microhardness appear to be the result of stoichiometric phase composition, compressive residual stress, and dense and fine microstructure of the Ti–Si–B–N coatings. The tribological wear test results indicated the superior wear-resistant properties of Ti–Si–B–N coatings compared to TiN and Ti(C,N) coatings.     

Modeling the Process of Liquid Flow in the System “Formation–Pipeline”

Journal of Engineering Physics and Thermophysics - A model of the process of nonstationary motion of a liquid in the conjugate system “formation–pipeline” has been constructed,...     

Thermodynamic Properties of Melts and Phase Equilibria in the Copper–Zirconium System

The vapor composition over and thermodynamic properties of Cu–Zr melts (5.1–85.0 at. % Zr) are studied by Knudsen cell mass spectrometry between 1191 and 1823 K. The data set obtained comprises more than 1100 activity values for various compositions and temperatures. The thermodynamic behavior of Cu–Zr melts is described in terms of the associated-solution approach with an accuracy no worse than the experimental accuracy. The melts are shown to contain two molecular species: CuZr and Cu₂Zr. The contributions of different types of chemical bonding to the Gibbs energy and enthalpy of formation of Cu–Zr melts are asymmetrical and shifted from the equiatomic composition in opposite directions: the extremum of covalent bonding is shifted to the Cu-rich side, while metallic bonding is more significant in Zr-rich alloys. The rapid temperature variation of covalent bonding leads to a large excess heat capacity C _p ^E of the melts and a negative excess entropy _f S ^E, which rapidly drops with decreasing temperature. It is shown that not only C _p ^E and _f S ^E but also their temperature variations are governed by the parameters of association reactions and depend more strongly on the entropy than on the enthalpy of complex formation. This indicates that, in the general case, the glass-forming capabilities of melts are independent of the interparticle interaction and accounts for the pronounced tendency of Cu–Zr melts toward amorphization.     

Thermodynamic calculation of the phase diagram for the Al–B–C system at pressure 7.7 GPa

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.     

p–T–x Phase Diagram of the Sb–O System

Vaporization processes in the Sb–O system are studied by Knudsen cell mass spectrometric measurements. The results indicate that Sb₂O₄ sublimes congruently and is the most stable oxide in the system. Experimental data are used to evaluate the enthalpies of formation of Sb₂O₃, Sb₂O₄, and Sb₆O₁₃ and to construct the p–T–x phase diagram of the Sb–O system.     

Deterministic Model of Homophase and Heterophase Fluctuations in a Liquid–Vapor System

Within the framework of the assembly-type catastrophe model, a nonlinear dynamic equation (DE) homogeneous in the parameter t with an aftereffect is constructed, in which t characterizes the deviation of the reduced density of a thin surface layer on the liquid-vapor interface from the mean density of the vapor-liquid system. This equation is used to treat a second-order nonlinear DE with a variable damping coefficient for a vapor-liquid system excited by periodic impacts (acts of evaporation and condensation of molecules). This DE is integrated over a finite time interval to find a two-dimensional mapping whose numerical solution describes the chaotic dynamics of the density in time, including homophase and heterophase fluctuations. For this system, the bifurcation diagrams are constructed and the Lyapunov exponents are found.     

Thermodynamics of the System InCl3–HCl–H2O at 25° C

A comprehensive equation for the thermodynamic properties of the system at 25°C in the ion-interaction (Pitzer) equation form is generated on the basis of a very recent and comprehensive array of electrochemical-cell measurements of the HCl activity, together with older published measurements of the activity of InCl₃ in mixtures with 0.02 molal HCl. Alternate equations with and without explicit consideration of the ion pair InCl²⁺ as a separate species are tested. Excellent agreement is obtained on either formulation between calculated and measured activities, although considerable uncertainty remains concerning the standard potential for the in electrode.     

p–T–x Phase Equilibria in the Cd–Zn–Te System

The p–T–x–y phase equilibria in the Cd–Zn–Te system are analyzed. The p–T and T–x–yprojections of the p–T–x–y phase diagram and a T–x–y isobar (for pressures at which Cd_1–x Zn _x Te_{1 ±} solid solutions sublime congruently in terms of Te) are mapped out. The key features of the sublimation behavior of the solid solution are examined. The p–T projection is studied by static vapor pressure measurements at temperatures from 700 to 1300 K and pressures of up to 101.3 kPa. The p–T sections of the phase diagram are constructed for x = 0.05, 0.10, 0.15, 0.25, 0.50, 0.75, 0.90, and 1. The solid solution containing 35 mol % ZnTe is found to phase-separate at 473 K.     

Special features of the diamond crystallization in the Mg–Zn–B–C system

The diamond crystallization in the Mg–Zn–B–C system occurring in the diamond thermal stability region have been considered. The phase transformations, which take place during the preparation of the alloy–solvent for carbon and its structure, the diamond crystallization and properties of the resultant diamond crystals have been studied. The formation of the acceptor centers and inclusions in diamond crystals caused by the addition of boron into the growth system have been considered. It has been found that the use of the diamond powder produced in this system for abrasive machining surfaces of sapphire parts makes it possible to increase the machining efficiency and quality as compared with that of the powder produced in the Ni–Mn–C system.     

Synthesis of Intercalation Compounds in the Graphite–HNO3–H3PO4System

The highly oriented pyrolytic graphite–HNO₃–H₃PO₄system was studied by x-ray diffraction and potentiometry at different acid concentrations. The results demonstrate that chemical and electrochemical intercalation in the graphite–98% HNO₃–85% H₃PO₄system yields graphite nitrate, a binary graphite intercalation compound (GIC). H₃PO₄is shown to have an ambiguous effect on the concentration ranges of different stages of graphite nitrate, shifting them to lower HNO₃concentrations as compared to the graphite–HNO₃–H₂O system. In the graphite–98% HNO₃–100% H₃PO₄system, a stage II ternary GIC is obtained, with an intercalate layer thickness d _i 4.7 Å. Stages II–VI of this GIC were prepared via exchange reaction between graphite nitrate and 100% H₃PO₄. A mechanism for the formation of the ternary GIC is proposed. The synthesis of the cointercalated GIC is likely to involve two steps: in the first step, graphite nitrate is formed; subsequent reaction in the intercalate layer leads to partial replacement of solvated HNO₃by H₃PO₄molecules.     

Properties of Phases and Alloys of the Mo–Ni–B System in the Ni–MoNi–Mo2NiB2–Ni2B Region

Materials Science - For alloys of the Mo–Ni–B system annealed at subsolidus temperatures (and some as-cast alloys) in the Ni–MoNi–Mo2NiB2–Ni2B region, we study the...