Masato Shimono

Molecular dynamics study on formation and crystallization of Ti-Al amorphous alloys

Abstract Formation of the amorphous phase and its relaxation and crystallization processes in the Ti–Al binary system are investigated by using molecular dynamics techniques. For various Al concentrations ranging from 0.0 to 1.0, amorphous alloys are prepared by quenching from liquid state. By varying the cooling rates, we can obtain the critical cooling rate to get the amorphous state in each composition. The geometrical analysis shows that a structural inhomogeneity on nanometer scale exists in the amorphous state, which mainly consists of two regions. One is dominated by the icosahedral clusters and the other is filled with approximate crystalline packing. The excess of low frequency vibration modes called the boson peak is observed in the amorphous states, which is getting weaker during the annealing process and completely vanishes after crystallization.

Icosahedral Order in Supercooled Liquids and Glassy Alloys

Formation of the icosahedral order in supercooled liquids and glassy phases is investigated for a model alloy system by using molecular dynamics simulations. The simulation results show that the short-range icosahedral order grows in the supercooled liquids as well as in the glassy phases. Structural analyses reveal that the icosahedral clusters form a network in which the clusters are connected via the pentagonal-bicap sharing. Geometrical property of the network formed by the icosahedral clusters is an origin of medium-range order in the glassy phases

On the mechanism for martensitic transformation from fcc to bcc

Abstract The mechanisms for martensitic transformation from fcc to bcc, those by Bain, Kurdjumow and Sachs (K–S), Frank or Bogers and Burgers, are reexamined on the basis of the comparison of the cohesive energy. The cohesive energy of the transient lattices formed by those mechanisms is estimated by use of the embedded atom method potential for Fe. In the process of the calculation, it has been realized that the shear deformation involved in all of the latter three mechanisms must be carried out in a reinterpreted sense slightly different from the strict sense of the word. In an ideal situation, where the formation of the martensite without any constraint from the surrounding austenite is allowed, the saddle point energy for the latter three mechanisms is equal to the maximum energy for the Bain deformation process between fcc and bcc. However, if the constraint that the distance between the close-packed planes is kept constant during the martensitic transformation is imposed, only the mechanism by K–S can bring fcc lattice to bcc lattice among the latter three.

First-principles calculations of Seebeck coefficients in a magnetic semiconductor CuFeS2

We analyze the Seebeck coefficients of a magnetic semiconductor CuFeS2 using first-principles calculation methods based on density functional theory. The calculated temperature dependence of the Seebeck coefficient in the antiferromagnetic phase reproduces a distinctive behavior in a bulk CuFeS2, such as a peak structure at a low temperature and weak temperature dependence around room temperature. In doped systems, almost linear temperature dependence appears. Despite not including any effect beyond the conventional spin density functional theory in our calculations, the calculated results agree qualitatively with the experimental results. These agreements indicate that the behavior of the Seebeck coefficients in CuFeS2 is mainly determined by its electronic structure.

First-principles calculations of thermoelectric properties of TiN/MgO superlattices: The route for an enhancement of thermoelectric effects in artificial nanostructures

We present the thermoelectric properties of TiN/MgO superlattices employing first-principles calculation techniques. The Seebeck coefficients, the electrical conductances, the thermal conductances, and the figure of merit are investigated employing electrical and thermal transport calculations based on density functional theory combined with the nonequilibrium Greens function and nonequilibrium molecular dynamics simulation methods. The TiN/MgO superlattices with a small lattice mismatch at the interfaces are ideal systems to study the way for an enhancement of thermoelectric properties in artificial nanostructures. We find that the interfacial scattering between the two materials in the metal/insulator superlattices causes the electrical conductance to change rapidly, which enhances the Seebeck coefficient significantly. We show that the figure of merit for the artificial superlattice nanostructures has a much larger value compared with that of the bulk material and changes drastically with the superlat...

MOLECULAR DYNAMICS STUDY ON STRUCTURAL RELAXATION OF METALLIC GLASSES

Structural relaxation process in the Zr-Cu metallic glasses is investigated by using molecular dynamics simulations. The enthalpy change in isothermal annealing of the glassy state cannot be fitted by a simple exponential function but obeys a stretched exponential function, which indicates that the relaxation in glassy phase is not a single Debye type process. A close examination of individual atomic motion reveals that the enthalpy relaxation is related to a string-like cooperative motion of atoms. The analysis of the local symmetry around each atom shows that a network of the icosahedral clusters grows in the glassy phases during annealing and it closely relates to the free-volume annihilation in the structural relaxation.

Molecular dynamics study of isothermal and adiabatic elastic moduli prior to martensitic transformation

Abstract A difference between the isothermal and adiabatic elastic moduli of a crystal signals damping at finite frequencies of vibration. Isothermal and adiabatic elastic moduli of B2 alloys with complete and incomplete long range order are calculated by use of the fluctuation formula of molecular dynamics. The results of the calculation show a decrease of the shear elastic modulus ( c 11 − c 12 )/2 upon approaching M S . This temperature seems to decrease as the long range order decreases. An increase of damping as the temperature is lowered towards M S is not obtained from the present calculation.

Electronic band structure of TiN/MgO nanostructures

Various nanostructured TiN(001)/MgO(001) superlattices based on a repeated slab model with a vacuum region have been investigated by the total energy pseudopotential method. They are rectangular and rectangular parallelepiped TiN(001) dot structures on MgO(001)-2×2 and 3×3 substrates. A rectangular TiN(001) structure on a MgO(001)-2×1 substrate has also been calculated. Their detailed electronic and internal lattice properties were investigated systematically. The internal atomic coordinates in a unit cell were fully relaxed. The rectangular TiN(001) structure on the MgO(001)-2×1 superlattice, which is not a dot owing to its periodicity, corresponds to metallicity. The electronic states of relaxed rectangular TiN(001) dot/MgO(001)-2×2 and MgO(001)-3×3 superlattices are semiconducting. All relaxed rectangular parallelepiped TiN(001) dot/MgO(001)-2×2 and MgO(001)-3×3 superlattices correspond to metallicity. The electronic properties depend on the shape of the TiN dot and the size of the MgO substrate.

Gibbs energy functions with the vacancy complexes in the Al-Cu binary system

The Gibbs energy functions of the phases in the Al-Cu binary system are taken from the CALPHAD-type thermodynamic assessment (Witusiewicz et al., 2004; Ansara et al., 1998) [1], [2], where the effect of the monovacancy (Va), divacancy (VaVa) and Va-solute atom pair are taken into account based on the formulation (Abe et al., In press). The divacancy is modeled as an associate, VaVa, in the FCC solid solution. The contributions from the Va-solute pair are included through the ternary excess Gibbs energy term. Using the Gibbs energy functions provided in this data article, the fractions of the monovacancies and divacancies, even in various metastable conditions, can be calculated. Since the Gibbs energy functions and phase descriptions are written in the TDB (Thermodynamic DataBase) format, one can use this file with various thermodynamic software packages, such as OpenCalphad [3] etc.

Enhancement of Thermoelectric Properties in Surface Nanostructures

The thermoelectric properties of TiN/MgO surface nanostructures have been determined using first-principles calculations based on the nonequilibrium Green’s function (NEGF) method. Through structural modification of the surfaces at the atomistic level, we find that the metallic TiN thin-film layer becomes semiconducting with a small bandgap, which enhances the Seebeck coefficient, while the electrical conductivity remains high at room temperature. Hence, a much larger thermoelectric figure of merit is obtained compared with bulk. These findings indicate the possibility of designing thermoelectric devices with surface nanostructures.