Si is a very promising thermoelectric conversion material because it is inexpensive, non-toxic, and exists in large quantities on the earth. However, the ZT values are low due to the very high thermal conductivity. Previously, Y. Ashida et al. achieved a maximum ZT of 0.43 at 973 K in an excessively P-doped Si system with reduced thermal conductivity due to localized Si–P nanoprecipitates and defects. Alloying Si and Ge is known to cause a ignificant decrease in thermal conductivity due to alloy scattering. Therefore, in this study, we aimed to further improve the performance by adding Ge to this Si–P system. Excessively P-doped Si-Ge samples were prepared by arc melting, ball milling, and SPS procedures to evaluate thermoelectric properties. In the system with 5% Ge, ZT increased significantly to 0.67 at 973 K, but there was little change after 10%. The fact that ZT did not increase significantly with the addition of more than 10% Ge is thought to be due to the fact that electron transport is hindered by alloy cattering and the lower solid solution limit of P in Si-Ge.

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In this study, we developed a method for reproducibly fabricating high-performance nano-grained bulk Si- Ge thermoelectric materials free from severe oxidization. In our previous work, the oxidization of Si-Ge during mechanical alloying and sintering processes had led to poor reproducibility of the value of electrical resistivity. We found that co-sintering with Ti, which is more easily oxidized than Si and Ge near the sintering temperature, effectively reduces the oxygen concentration in the nano-grained bulk Si-Ge samples. The oxygen oncentration in the sample co-sintered with Ti was found to be less than 2.4 at.%, and electrical resistivity was found to be less than 3.9 mΩcm at 922 K with good reproducibility. High Seebeck coefficient (more than 400 μV K^－1) and low thermal conductivity (less than 1 Wm^－1K^－1) were simultaneously achieved by constructive electronic structure modification via iron doping and nano-crystallization, respectively. As a consequence, we succeeded in obtaining a surprisingly large value of dimensionless figure of merit, ZT＝4 at 922 K, and the temperature range of ZT exceeding 1 extended at high temperatures above 700 K.

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We have developed a system design platform for low-grade waste heat recovery thermoelectric generator with Model-Based Design (MBD). An MBD model was developed to predict the amount of electricity generated by a thermoelectric conversion unit, which utilizes the temperature difference between steam and cooling water to recover thermal energy, including latent heat. A mathematical model was proposed to predict the performance of a single thermoelectric module. Next, a model was developed to estimate the electricity generated by a thermoelectric unit comprising multiple modules, incorporating heat transfer coefficient estimation. Finally, an expanded MBD model was developed to predict the total power generation of a subsystem module (SSM) that integrates multiple thermoelectric units. The power generation was successfully predicted under various conditions. Moreover, the differences in power generation for each unit, depending on the combination of cooling circuits, were accurately predicted, demonstrating the potential applicability of this method to system optimization design.

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