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Si1-xGex alloys show the high power generating efficiency as a thermoelectric- conversion material. We evaluate the thermoelectric power of the n-type SiGe system on the basis of the first principles calculations. The electronic-band-structure calculation is performed using all-electron full-potential linearized augmented-plane-wave (FLAPW) method within the local density approximation (LDA). The Seebeck coefficient is analyzed by the Bloch-Boltzmann equation. We find that the ordered rhombohedral SiGe has high Seebeck coefficient in comparison with zincblende SiGe. The efficiency of the thermoelectric power in Si1-xGex is gained by the local atomic configuration rather than the Ge concentration.
We investigated the Srn+1TinO3n+1 Ruddlesden-Popper homologous series. This material exhibits a wide range of electric behaviors, from a high-dielectric-constant tunable-paraelectric in its undoped form to a metallic superconductor when it is doped with a variety of elements.
The Srn+1TinO3n+1 and the yttrium-doped Srn+1TinO3n+1 Ruddlesden-Popper homologous series were prepared through the thermal decomposition of a metal citric acid complex. The starting solution consisted of the raw materials strontium acetate and titanium alkoxide. Citric acid was used as the chelating agent, and ethanol and distilled water were mixed and used as a solvent. Single phase Sr2TiO4 and Sr3Ti2O7 were produced without creation of SrCO3 by heat-treatment at 1073 and 1473 K, respectively, for three hours. We were unable to produce single phase Sr4Ti3O10. The samples consisted of Sr4Ti3O10 and SrTiO3 phases. Scanning electron microscopy showed that the grain shape of the Sr2TiO4 was plate-like, while those of the Sr3Ti2O7 and Sr4Ti3O10 were not and the grains had large numbers of pores. High-density Sr2-xYxTiO4 (x= 0 to 0.06) ceramic samples were produced under hot-press conditions (1823 K for one hour at a uniaxial pressure of 31.2 MPa). Although the undoped samples were insulators, the yttrium-doped samples showed high electrical conductivity (i.e. that of Sr1.98Y0.02TiO4 was ρ= 8.5×10−5 Ωcm). The Seebeck coefficient of Sr1.98Y0.02TiO4 was –160.7 μV/K, and the thermal conductivity was 3.38 W/mK.
Thermoelectric properties of Sr1-xYxTiO3 (x = 0-0.08) were calculated using a virtual crystal method. Using a Pechini's method, Sr1-xYxTiO3 (x = 0-0.06) precursor powder was prepared. Using this method, we prepared pure SrTiO3 at a lower temperature than would be using the conventional solid-state reaction method. The precursor solution was heated at 823 K for 5 h after drying at 353 K for 8 h to produce the precursor powder. The powder was sintered using a hot pressing technique. The relative densities of ceramics were more than 98%. The Seebeck coefficient and electrical conductivity of the samples were measured using the standard four-probe method in a flowing He gas atmosphere in a temperature range of 323 to 923 K. The conductivities of SrTiO3, Sr0.97Y0.03TiO3, and Sr0.94Y0.06TiO3 at room temperature were 6.61 × 102, 5.61 × 103, and 1.58 × 104 S/m, respectively. The Seebeck coefficients of SrTiO3, Sr0.97Y0.03TiO3, and Sr0.94Y0.06TiO3 at room temperature were -548, -264, and -196 μV/K, respectively.
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