Athorn Vora-ud

Electronic Structure of Ge-Sb-Te System Calculated by DV-Xα Method

Germanium-antimony-tellurium (Ge-Sb-Te) system comprised the GeSb2Te4, GeSb4Te7, and Ge2Sb2Te5 compounds which are the most widely used materials with electronic structure of narrow band gap energy. In this work, we studied electronic structure of Ge-Sb-Te system by using the discrete variational (DV)-Xα molecular orbital method based on the self-consistent Hartree-Fock-Slater model in order to investigate thermoelectric materials. The calculation was designed based on Ge13Sb20Te52, Ge7Sb12Te40, and Ge14Sb6Te26 model cluster atoms which performed to GeSb2Te4, Ge2Sb2Te5, and GeSb4Te7 compounds with the symmetry (9Cv) in order to convert radial function for molecular orbitals of model cluster atoms. The findings were the electronic structure comprising the model cluster atoms, energy level, density of state, bond overlap population and electron density at wave function number of HOMO and LUMO showed narrow energy gap of 0.022 eV, 0.016 eV and 0.037 eV of Ge13Sb26Te52, Ge7Sb12Te40, and Ge14Sb6Te26, respectively. The narrow energy gap was relevant to improving the thermoelectric performance of Ge-Sb-Te system.

Predication of Thermal Conductivity of Mg2X (X = Ge and Sn) by Molecular Dynamics

Molecular dynamics (MD) simulations of elastic and thermal properties of Mg2X (X = Ge and Sn) based on anti-fluorite structure (CaF2) at temperature range 300−700 K were presented. The MD simulation in this study involving the Morse−type potential functions, and the Busing–Ida potential to determine the interatomic interaction among cluster atoms size 4×4×4 unit cells of 768 atoms {512−Mg1.2+, 256−(Ge, Sn)2.4−}. The potential parameter functions of the cluster atoms were indicated by random numerical method and fit lattice parameter from the experimental data obtained at room temperature. The calculation of lattice parameter, pressure, temperature and energy contributes to evaluation of the elastic properties. The results showed that Mg2Ge had better elasticity than Mg2Sn. On the other hand, Mg2Sn had less thermal conductivity than Mg2Ge. Since thermal conductivity decreases with increasing temperature, the interesting feature of thermal conductivity is particulary useful to enhance thermoelectric performance of materials.

Power Factor of Germanium Antimony Tellurium Thin Film on Al2O3 Ceramic Substrate Deposited by Pulsed–DC Magnetron Sputtering

Germanium–Antimony–Telluride (Ge–Sb–Te) has low electrical resistivity and thermal conductivity for good thermoelectric properties. The Ge–Sb–Te thin films were deposited on Al2O3 ceramic substrate by pulsed–dc magnetron sputtering system using a 99.99 % Ge:Sb:Te of 1:1:1 composite target and annealed at 573, 623, 673, and 723 K for 1 hour in vacuum. The phase identification, atomic composition, morphology and film thickness (d), carrier concentration (n), mobility (µ), Seebeck coefficient (S) and electrical resistivity (ρ) of the as–deposited and the annealed samples were investigated by X–ray diffraction (XRD), energy dispersive X–ray spectroscopy (EDX), field–emission scanning electron microscopy (FE–SEM), Hall–effect measurement, steady state method and calculation of from n and µ, respectively. The results demonstrated that the as–deposited Ge–Sb–Te film showed amorphous phase and annealing changed the phase crystalline. Morphologies of annealed Ge–Sb–Te films showed very large grain size and porosity to obtaining good n and µ. The approximately maximum power factor (P) was 4.22×10−4 W m−1 K−2 at annealing temperature of 723 K.