[en] The phase equilibria and thermodynamic properties of the Mg-Ca-Zn ternary system were analyzed and a complete thermodynamic description of the system was obtained using a computerized optimization procedure. Based on the experimental data, one ternary intermetallic compound was considered for the model, while another ternary intermetallic compound was speculated. The calculated results were compared with the experimental data in the literature

[en] The phase relations in the BaO-TiO_2-δ system within the concentration range of 14-65 mol% BaO under reducing conditions were investigated using high temperature DTA and X-ray powder diffraction. The stable compounds under these conditions were determined, together with the temperatures of the invariant reactions and the liquidus boundaries. The phase relations in this BaO-TiO₂-Ti₂O₃ system under elevated temperatures and reducing atmospheres are represented in the form of quasi-binary phase diagrams

[en] The thermodynamic properties of Al, Ni, NiAl, and Ni₃Al were studied using the first-principles approach. The 0-K total energies are calculated using the ab initio plane wave pseudopotential method within the generalized gradient approximation. The contribution to the free energy from the lattice vibration was calculated using the phonon densities of states derived by means of the ab initio linear-response theory. The thermal electronic contribution to the free energy was obtained from the one-dimensional numerical integration over the electronic density of states. With the deduced Helmholtz free-energy, the thermal expansion and enthalpy as a function of temperature were calculated and compared with the experimental data. Our calculations show that the enthalpies of formation are slightly temperature dependent with a slope of -1.6 J/mol/K for NiAl and -1.2 J/mol/K for Ni₃Al. For Ni, the inclusion of thermal electronic excitation results in a 10% increase in thermal expansion and 15% increase in enthalpy at 1600 K

[en] The MgB₂ compound has a superconducting temperature of 39 K and its film has been successfully deposited by means of the hybrid physical chemical vapor deposition (HPCVD). In this paper, the CALPHAD technique was applied to study the reactivity of magnesium with several substrate materials during the HPCVD process. The calculated results showed that MgO, SiC, AlN and TaN are stable in the current HPCVD process. The Si, ZrO₂, TiN, SiO₂, Al₂O₃ and GaN substrates were also investigated. It was found that Mg reacts with these substrates under current HPCVD conditions. However, if the Mg vapor is low enough but still in the stability range of the MgB₂ compound, the Si, ZrO₂ and TiN substrates can be stable in the HPCVD process. The methodology used in the present work can be extended to other systems, and the current findings may be useful to guide other MgB₂ thin film deposition processes

[en] A phase-field model for predicting the coherent microstructure evolution in constrained thin films is developed. It employs an analytical elastic solution derived for a constrained film with arbitrary eigenstrain distributions. The domain structure evolution during a cubic->tetragonal proper ferroelectric phase transition is studied. It is shown that the model is able to simultaneously predict the effects of substrate constraint and temperature on the volume fractions of domain variants, domain-wall orientations, domain shapes, and their temporal evolution. [copyright] 2001 American Institute of Physics

[en] The phase equilibria and thermodynamic properties of the Re-Ta and Re-W systems were analyzed and complete thermodynamic descriptions of the two binary systems were obtained with the CALPHAD technique using a computerized optimization procedure. The thermodynamic descriptions of pure Re, Ta and W elements were taken from the SGTE database. Two binary intermetallic compounds were considered in both binary systems, namely σ phase and χ phase based on available experimental data. Good agreement was obtained between calculated results and experimental data in the two binary systems. (orig.)

[en] We propose a mixed-space approach using the accurate force constants calculated by the direct approach in real space and the dipole-dipole interactions calculated by linear response theory in reciprocal space, making the accurate prediction of phonon frequencies for polar materials possible using the direct approach as well as linear response theory. As examples, by using the present approach, we predict the first-principles phonon properties of the polar materials α-Al₂O₃, MgO, c-SiC, and h-BN, which are in excellent agreement with available experimental data. (fast track communication)

[en] A first-principles approach to calculating the elastic stiffness coefficients at finite temperatures was proposed. It is based on the assumption that the temperature dependence of elastic stiffness coefficients mainly results from volume change as a function of temperature; it combines the first-principles calculations of elastic constants at 0 K and the first-principles phonon theory of thermal expansion. Its applications to elastic constants of Al, Cu, Ni, Mo, Ta, NiAl, and Ni₃Al from 0 K up to their respective melting points show excellent agreement between the predicted values and existing experimental measurements.

[en] Research highlights: → Implemented the eight frequency model for impurity diffusion in hexagonal metals. → Model inputs were energetics/vibrational properties from first princples. → Predicted diffusion coefficients for Al, Ca, Zn and Sn impurity diffusion in Mg. → Successful prediction of partial correlation factors and jump frequencies. → Good agreement between calculated and experimental results. - Abstract: Diffusion in dilute Mg-X alloys, where X denotes Al, Zn, Sn and Ca impurities, was investigated with first-principles density functional theory in the local density approximation. Impurity diffusion coefficients were computed as a function of temperature using the 8-frequency model which provided the relevant impurity and solvent (Mg) jump frequencies and correlation factors. Minimum energy pathways for impurity diffusion and associated saddle point structures were computed with the climbing image nudged elastic band method. Vibrational properties were obtained with the supercell (direct) method for lattice dynamics. Calculated diffusion coefficients were compared with available experimental data. For diffusion between basal planes, we find D_Mg-Ca > D_Mg-Zn > D_Mg-Sn > D_Mg-Al, where D is the diffusion coefficient. For diffusion within a basal plane, the same trend holds except that D_Mg-Zn overlaps with D_Mg-Al at high temperatures and D_Mg-Sn at low temperatures. These trends were explored with charge density contours in selected planes of each Mg-X alloy, the variation of the activation energy for diffusion with the atomic radius of each impurity and the electronic density of states. The theoretical methodology developed herein can be applied to impurity diffusion in other hexagonal materials.

[en] Thermodynamic fluctuation among many electronic states in systems with 'itinerant-electron' magnetism is addressed with a first-principles formulation of the Helmholtz energy. The Ce γ-α phase transition is used as an illustrative case. The k_Bln(2J+1) form for the magnetic entropy that is commonly found in the literature is replaced with an expression that is derived from configurational fluctuations among nonmagnetic, ferromagnetic, and antiferromagnetic electronic states. Predicted first- and second-order magnetic phase transitions are in close accord with experiment. The mixture of states leads to a Schottky anomaly in the Ce specific heat.