Tosawat Seetawan

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.

La/Sm/Er Cation Doping Induced Thermal Properties of SrTiO3 Perovskite.

The La/Sm/Er cations with different radii doping SrTiO3 (STO) as model Sr0.9R0.1TiO3 (R = La, Sm, Er) were designed to investigate structural characteristics and thermal properties by the molecular dynamics simulation with the Green-Kubo relation at 300-2000 K. The structural characteristics were composed of lattice constant, atoms excursion, and pair correlation function (PCF). The thermal properties consisted of heat capacity and thermal conductivity. The lattice constant of R-doped exhibited less than the STO at 300-1100 K and more than STO at 1500-2000 K, which was encouraged by atom excursion and PCF. The thermal properties was compared with literature data at 300-1100 K. In addition, the thermal properties at 1100-2000 K were predicted. It highlights that thermal conductivity tends to decrease at high temperature, due to perturbation of La, Sm, and Er, respectively.

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.

Theoretical Enhancement of Thermoelectric Properties of Sr1–xLaxTiO3

The electronic structure and density of state (DOS) of Sr1–xLaxTiO3 (x = 0, 0.06, 0.125, and 0.25) have been investigated on the first principle molecular orbital calculation. The La was substituted Sr site in SrTiO3 and caused an increase of electrical conductivity. The electronic structure, DOS and band structure of Sr1–xLaxTiO3 (x = 0, 0.06, 0.13, and 0.25) were calculated by discrete variational (DV)-Xα method and Material studio to estimated the thermoelectric properties. The Seebeck coefficient and electrical conductivity of Sr1–xLaxTiO3 were estimated form DOS by Boltzmann theory: Mott and Jones equation. It was found that, the substitution of La made high electrical conductivity, made high power factor and enhancement thermoelectric properties. The Sr0.87La0.13TiO3 cluster shows maximum power factor about 2.55×10−3 W/m·K2 at 1200 K. For maximum enhancement thermoelectric properties of SrTiO3 x = 0.13 should be used in the synthesis of Sr1-xLaxTiO3.

Synthesis and Characterization of Mg2Si Thermoelectric Material

The Mg2Si compound was synthesized by the solid state reaction method. The powder precursors of Mg and Si were thoroughly mixed in ballmilling for 24 hr in an argon atmosphere. Mixed powder was pressed at 170MPa and sintered at 800 °C for 6 hr in an argon atmosphere. The sinter powder was crushed in mortar for 1 hr. The crystal structure and microstructure were measured and observed by using XRD and SEM. The microstructure and the crystal structure were analyzed. TheMg2Si shows single phase, cubic structure and particle size about 1-10 mm.

Thermoelectric Module of P-Type Ca-Co-O/N-Type ZnO Thin Films

Thin films thermoelectric module fabricated by pulsed-dc magnetron sputtering system using Ca3Co4O9 (p-type) and ZnO (n-type) targets of 60 mm diameter and 2.5 mm thickness, which were made from powder precursor, and obtained by solid state reaction. Thin films of p-Ca-Co-O (Seebeck coefficient = 143.85 µV/K, electrical resistivity = 4.80 mΩm, power factor = 4.31 µW/m K2) and n-ZnO (Seebeck coefficient =229.24 µV/K, electrical resistivity = 5.93 mΩm, power factor = 8.86 µW/m K2) were used to make a thermoelectric module, which consist of four pairs of legs connected by copper electrodes (0.5 mm thickness, 3.0 mm width, and 3.0-8.0 mm length). Each leg is 3.0 mm width, 20.0 mm length, and 0.44 µm thickness on a glass substrate of 1.0 mm thickness in dimension 25.0x50.0 mm2. For preliminary test, a module was used to thermoelectric power generation. It was found that the open circuit voltage increased with increasing temperature difference from 3 mV at 5 K up to 20 mV at 78 K. The internal resistance of a module reached a value of 14.52 MΩ. This test indicated that a module can be generated the electrical power. Therefore, it can be used as an important platform for further thin films thermoelectric module research.

Designing and Fabricating of Low Cost Thermoelectric Power Generators

Fossil fuel is the main energy resources of the world. About 80-90% of its primary energy need to supply by oil, coal, natural gas, and oil shale [1]. These energy resources will also be of importance in the future but non-renewable and cause problems to the environment as a result of their relatively high amount of carbon dioxide (CO2), carbon monoxide (CO), and other environmentally harmful emissions. We are investigating to look for alternative energy resources which are clean, safe, and long-term reliable. Thermoelectricity is one of the renewable energy resources that has been widely investigated and is expected to be feasible in the near future. Moreover, it is a clean energy generation, since it can directly convert heat to electrical energy by using non-polluting thermoelectric devices. These are reasons for the growing interest in further research and development of the thermoelectric technology. The search for new thermoelectric materials is important that the transition metal oxides were interested such as p-type Ca3Co4O9 [2-7] and n-type CaMnO3 [8-12]. There have been synthesized using different techniques in the form of powder and bulk. However, the doped metals have been expected to be one of the candidates for good thermoelectric materials, including thermoelectric module consists of two or more materials of p-type and n-type [13-15]. Recently, the thermoelectric module is also being used as the thermoelectric generators, thermoelectric coolers, etc. [16-17].

NANOSIZE POWDERS OF SILVER-DOPED SODIUM COBALT OXIDE SYNTHESIZED BY POLYMERIZED COMPLEX METHOD

Nanosized powders used for the preparation of bulk Na1.5Co2O4 and Ag-doped Na1.5Co2O4 nanosized crystalline grains were synthesized by the polymerized complex (PC) method. X-ray diffraction analysis revealed that the nondoped and Ag-doped PC products were composed of Co3O4 and Na2CO3 phases. After a subsequent calcination at 800°C, the nondoped PC product was converted to powder of single phase γ-NaxCo2O4, whereas the Ag-doped PC products remained as mixed phases of Co3O4 and Na2CO3 and Ag2O with a small trace of γ-NaxCo2O4. SEM and TEM investigations showed that all the calcined products were powders of about 200–500 nm particle size.

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.

Investigating Power Factor of CaMnO3 Added Carbon Nanotubes

CaMnO3 (CMO) thermoelectric material is large Seebeck coefficient but high electrical resistivity. To reduce electrical resistivity by adding carbon nanotubes (CNTs) in CMO material and may be decreased Seebeck coefficient. In this work, we simulated electronic structure of CMO and CNTs-added CMO by DV-Xα method to investigation of power factor and enhance the thermoelectric performance. The Seebeck coefficient and electrical resistivity were calculated by Maxwell-Boltzmann distribution and Mott’s law to investigate power factor. The DV-Xa calculated show the energy level and density of state (DOS) of CMO and CNTs-added CMO demonstrated that the energy gap reduces from 3.33 eV to 0.19 eV affect to enhance the power factor of CMO with Seebeck coefficient and electrical resistivity are decreases. The power factor of CNTs-added CMO was increased with increasing CNTs content.