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Most primary energy sources, such as the fossil fuels of oil, coal, and natural gas, produce waste heat. Recycling of this unused thermal energy is necessary in order to increase the efficiency of usage. Thermoelectric (TE) conversion technologies, by which waste heat is directly converted into electricity, have been extensively studied, and the development of these technologies has continued. TE power-generation has attracted significant attention for use in self-powered wireless sensors, which are important for our increasingly sophisticated information society. For the middle-temperature range (i.e., 600–900 K), with applications such as automobiles, intensive studies of high-performance TE materials have been conducted. In this study, we review our recent experimental and theoretical studies on alkaline-earth silicide Mg2Si TE materials, which consist of nontoxic abundant earth elements. We demonstrate improvements in TE performance brought about by lightly doping Mg2Si with isoelectronic impurities. Furthermore, we examine the electrode formation and material coatings. Finally, we simulate the exhaust heat requirements for the practical application of TE generators.

To improve the thermoelectric (TE) performance of Mg2Si by optimizing the carrier concentration and reducing thermal conductivity, we focus on codoping Sb and Zn using theoretical and experimental methods. First-principles calculations show that Sb is a stable and controllable n-type dopant for Mg2Si, whereas Zn considerably shrinks the Mg2Si cell. We fabricate dense and high-purity polycrystalline Mg2MxSi (M = Sb, Zn; x = 0, 0.1, 0.3, and 0.5 at.%) via the all-melt process of the conventional vertical Bridgman (VB) method and examine the influence of dilute codoping of Sb and Zn on the TE properties of Mg2Si. VB-grown Mg2Si doped with 0.5 at.% Zn and Sb shows higher electrical conductivity than pure Mg2Si, achieving an increased power factor by 4.62–15.23% over that of the sintered specimen under the same doping rate at 323–873 K. Because the decreased lattice thermal conductivity of the codoped specimens nullifies the increased electronic thermal conductivity, the total thermal conductivity is similar to that of pure Mg2Si. Consequently, the dimensionless figure of merit of VB-grown Mg2Si doped with 0.5 at.% Zn and Sb reaches 0.82 at 873 K.

Direct growth of oxide film on silicon is usually prevented by extensive diffusion or chemical reaction between silicon (Si) and oxide materials. Thermodynamic stability of binary oxides is comprehensively investigated on Si substrates and shows possibility of chemical reaction of oxide materials on Si surface. However, the thermodynamic stability does not include any crystallographic factors, which is required for epitaxial growth. Adsorption energy evaluated by total energy estimated with the density functional theory predicted the orientation of epitaxial film growth on Si surface. For lower computing cost, the adsorption energy was estimated without any structural optimization (simple total of energy method). Although the adsorption energies were different on simple ToE method, the crystal orientation of epitaxial growth showed the same direction with/without the structural optimization. The results were agreed with previous simulations including structural optimization. Magnesium oxide (MgO), as example of epitaxial film, was experimentally deposited on Si substrates and compared with the results from the adsorption evaluation. X-ray diffraction showed cubic on cubic growth [MgO(100)//Si(100) and MgO(001)//Si(001)] which agreed with the results of the adsorption energy.

To investigate the possibility of p-type doping of [alpha]-SrSi.sub.2, a promising as an eco-friendly thermoelectric material, the energy changes of substitutions of the Si site of [alpha]-SrSi.sub.2 by group 13 elements were evaluated using first-principles calculations. It is found that Ga doping was the most energetically favorable dopant while In is the most unfavorable. We examined the synthesis of Ga- and In-doped [alpha]-SrSi.sub.2 using the vertical Bridgeman method and investigated their thermoelectric properties. The Ga atoms were doped to [alpha]-SrSi.sub.2 successfully up to 1.0 at. %, while In atoms could not be doped as suggested by calculations. For experimental prepared Ga-doped samples, the carrier density was observed to increase with Ga doping, from 3.58 x 10.sup.19 cm.sup.-3 for undoped [alpha]-SrSi.sub.2 to 4.49 x 10.sup.20 cm.sup.-3 for a 1.0 at. % Ga-doped sample at 300 K. The temperature dependence of carrier concentrations was observed to change from negative to positive with increasing Ga content. In addition, the temperature dependence of the Seebeck coefficient was also observed to change from negative to positive with increasing Ga content. The results indicate that [alpha]-SrSi.sub.2 undergoes a semiconductor-metal transition with Ga doping. The power factor for the undoped sample was quite high, at 2.5 mW/mK.sup.2, while the sample with 0.3 at. % Ga had a value of 1.1 mW/mK.sup.2 at room temperature.

To investigate the possibility of  p -type doping of α-SrSi 2 , a promising as an eco-friendly thermoelectric material, the energy changes of substitutions of the Si site of α-SrSi 2 by group 13 elements were evaluated using first-principles calculations. It is found that Ga doping was the most energetically favorable dopant while In is the most unfavorable. We examined the synthesis of Ga- and In-doped α-SrSi 2 using the vertical Bridgeman method and investigated their thermoelectric properties. The Ga atoms were doped to α-SrSi 2 successfully up to 1.0 at. %, while In atoms could not be doped as suggested by calculations. For experimental prepared Ga-doped samples, the carrier density was observed to increase with Ga doping, from 3.58 × 10 19  cm −3 for undoped α-SrSi 2 to 4.49 × 10 20  cm −3 for a 1.0 at. % Ga-doped sample at 300 K. The temperature dependence of carrier concentrations was observed to change from negative to positive with increasing Ga content. In addition, the temperature dependence of the Seebeck coefficient was also observed to change from negative to positive with increasing Ga content. The results indicate that α-SrSi 2 undergoes a semiconductor–metal transition with Ga doping. The power factor for the undoped sample was quite high, at 2.5 mW/mK 2 , while the sample with 0.3 at. % Ga had a value of 1.1 mW/mK 2 at room temperature.