Xiaoma Tao

Physical properties of thermoelectric zinc antimonide using first-principles calculations

We report first-principles calculations of the structural, electronic, elastic, and vibrational properties of the semiconducting orthorhombic ZnSb compound. We study also the intrinsic point defects in order to eventually improve the thermoelectric properties of this already very promising thermoelectric material. Concerning the electronic properties, in addition to the band structure, we show that the Zn (Sb) crystallographically equivalent atoms are not exactly equivalent from the electronic point of view. Lattice dynamics, elastic, and thermodynamic properties are found to be in good agreement with the experiments and they confirm the nonequivalency of the zinc and antimony atoms from the vibrational point of view. The calculated elastic properties show a relatively weak anisotropy and the hardest direction is the y direction. We observe the presence of low energy modes involving both Zn and Sb atoms at about 5-6 meV, similar to what has been found in Zn4Sb3, and we suggest that the interactions of these modes with acoustic phonons could explain the relatively low thermal conductivity of ZnSb. Zinc vacancies are the most stable defects, and this explains the intrinsic p-type conductivity of ZnSb.

Physical properties of thallium-tellurium based thermoelectric compounds using first-principles simulations.

We present a study of the thermodynamic and physical properties of Tl(5)Te(3), BiTl(9)Te(6), and SbTl(9)Te(6) compounds by means of density functional theory based calculations. The optimized lattice constants of the compounds are in good agreement with the experimental data. The electronic density of states and band structures are calculated to understand the bonding mechanism in the three compounds. The indirect band gaps of BiTl(9)Te(6) and SbTl(9)Te(6) compounds are found to be equal to 0.256 and 0.374 eV, respectively. The spin-orbit coupling has important effects on the electronic structure of the two semiconducting compounds and should therefore be included for a good numerical description of these materials. The elastic constants of the three compounds have been calculated, and the bulk modulus, shear modulus, and Youngs modulus have been determined. The change from ductile to brittle behavior after Sb or Bi alloying is related to the change of the electronic properties. Finally, the Debye temperature and longitudinal, transverse, and average sound velocities have been obtained.

Prediction of formation of intermetallic compounds in diffusion couples

Formation of intermetallic compounds (IMCs) at the interface between two metals during soldering processing exerts much influence on the electrical and mechanical performance of integrate circuits (ICs). Considering both of the thermodynamic and kinetic factors (including nucleation and growth) on phase formation, a new model capable of predicting phase formation sequence at the interface between two metals with different structures has been proposed in this work. Application of this new model on the interfacial reactions between pure elemental pairs of metals such as Ni/Sn, Cu/In, Cu/Sn, and Co/Sn at different temperatures shows good agreement between predictions by this model and experimental observations.

First-principles calculations of the thermodynamic and elastic properties of the L12-based Al3RE (RE = Sc, Y, La–Lu)

Abstract First-principles calculations of the total energy and elastic properties of the L12-based Al3RE (RE = Sc, Y, Lanthanide) at T = 0 K are performed by using the projector augmented-wave method within the generalized gradient approximation. The lattice constants, formation enthalpies, elastic constants and bulk modulus of the L12-based Al3RE are obtained. Youngs modulus, shear modulus and Poissons ratios are also estimated in the present work. By using the Debye – Grüneisen model, the Debye temperatures, Grüneisen constants and the sound velocity are obtained for the L12-based Al3RE. All the calculated results are in good agreement with experimental values and other theoretical calculations available.

Phase stability and physical properties of Ta5Si3 compounds from first-principles calculations

We present a study of the thermodynamic and physical properties of Ta5Si3 compounds by means of density functional theory based calculations. Among the three different structures (D8m, D8l, D88), the D8l structure (Cr5B3-prototype) is the low temperature phase with a high formation enthalpy of -449.20kJ/mol, the D8m structure (W5Si3-prototype) is the high temperature phase with a formation enthalpy of -419.36kJ/mol, and the D88 structure (Mn5Si3-prototype) is a metastable phase. The optimized lattice constants of the different Ta5Si3 compounds are also in good agreement with the experimental data. The electronic density of states (DOS) and the bonding charge density have also been calculated to elucidate the bonding mechanism in these compounds and the results indicate that bonding is mostly of covalent nature. The elastic constants of the D8m and D8l structures have been calculated together with the different moduli. Finally, by using a quasiharmonic Debye model, the Debye temperature, the heat capacity, the coefficient of thermal expansion and the Gruneisen parameter have also been obtained in the present work. The transformation temperature (2303.7K) between the D8m and the D8l structures has been predicted by means of the Gibbs energy, and this predicted temperature (2303.7K) is close to the experimental value (2433.5K).

First-principles calculations of elastic constants of DO3-Mg3 RE (RE = Sc, Y, La, Ce, Lu)

The elastic constants of DO3-Mg3 RE (RE = Sc, Y, La, Ce and Lu) have been calculated at T = 0?K by applying the projector augmented wave (PAW) method within the generalized gradient approximation (GGA). The polycrystalline shear modulus, Youngs modulus, Poissons ratio and the anisotropic ratio of DO3-Mg3 RE are also estimated. The density of states and bonding charge densities of DO3-Mg3 Ce and DO3-Mg3 Er are calculated to understand the bonding characteristics of Mg3 RE. The calculated results indicate that the rare earth (RE) additions can improve the elastic modulus of Mg alloys. The predicted elastic constants of DO3-Mg3 RE provide helpful guidance for the design of novel Mg alloys.

Molecular dynamics simulation of diffusion bonding of Al?Cu interface

The effects of temperature on diffusion bonding of Al–Cu interface have been investigated by using molecular dynamics (MD) technique with the embedded atomic method (EAM) potentials. The simulated results indicate that the Cu atoms predominantly diffuse into the Al side in the process of diffusion bonding, and the thickness of the interfacial region depends on temperature, with higher temperatures resulting in larger thickness. In the course of diffusion bonding, the interfacial region became disordered. In addition, the Cu atoms diffuse at low ratios but can deeply diffuse into the interior of Al, and the Al atoms diffuse at high ratios but hardly diffuse into the interior of Cu. The results show that the appropriate temperature range for diffusion bonding of Al–Cu interface is 750–800 K, and the diffusion activation energies of Al and Cu are 0.77 eV and 0.50 eV, respectively. Finally, in this work, three diffusion mechanisms of Cu atoms in Al lattice have been found and the main diffusion mechanism is the nearest neighbor hopping mechanism.

Experimental Investigation and Thermodynamic Modeling for the Mg-Nd-Sr System

Microstructure examination, phase analysis, and DSC measurement on the equilibrated ternary alloys not only enabled exploration of the phase equilibria relations of the Mg-Nd-Sr ternary but also thermodynamic assessment of the ternary system in the context of the CALPHAD approach. Thermodynamic modeling, further coupled with first principle calculations, was thereafter used to predict a panoramic phase diagram for the Mg-Nd-Sr ternary system at the Mg-rich corner.

Elastic constants and thermophysical properties of Al–Mg–Si alloys from first-principles calculations

Abstract The lattice constants and elastic constants for Al–Mg–Si alloys have been calculated by using first-principles total energy calculations within the generalized gradient approximation. The calculated results are in good agreement with available experimental and theoretical results. The polycrystalline shear modulus, Youngs modulus and Poissons ratio are also estimated from the calculated single crystalline elastic constants. The Youngs modulus and shear modulus increase following the precipitation sequence in Al–Mg–Si. The Debye sound velocity, Debye temperature, Grüneisen constant, heat capacity and linear coefficients of thermal expansion are predicted for the considered Al–Mg–Si alloys based on the Debye–Grüneisen model. The calculated values of Mg2Si agree well with the previous experimental and theoretical results.

Phase stability and physical properties ofTa5Si3compounds from first-principles calculations

We present a study of the thermodynamic and physical properties of Ta5Si3 compounds by means of density functional theory based calculations. Among the three different structures (D8m, D8l, D88), the D8l structure (Cr5B3-prototype) is the low temperature phase with a high formation enthalpy of -449.20kJ/mol, the D8m structure (W5Si3-prototype) is the high temperature phase with a formation enthalpy of -419.36kJ/mol, and the D88 structure (Mn5Si3-prototype) is a metastable phase. The optimized lattice constants of the different Ta5Si3 compounds are also in good agreement with the experimental data. The electronic density of states (DOS) and the bonding charge density have also been calculated to elucidate the bonding mechanism in these compounds and the results indicate that bonding is mostly of covalent nature. The elastic constants of the D8m and D8l structures have been calculated together with the different moduli. Finally, by using a quasiharmonic Debye model, the Debye temperature, the heat capacity, the coefficient of thermal expansion and the Gruneisen parameter have also been obtained in the present work. The transformation temperature (2303.7K) between the D8m and the D8l structures has been predicted by means of the Gibbs energy, and this predicted temperature (2303.7K) is close to the experimental value (2433.5K).