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High Thermoelectric Power Factor of Si–Mg2Si Nanocomposite Ribbons Synthesized by Melt Spinning

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journal contribution
posted on 07.02.2020, 20:35 by Daiki Souda, Kazuto Shimizu, Yuji Ohishi, Hiroaki Muta, Takashi Yagi, Ken Kurosaki
Si has been considered as a promising thermoelectric material because of its many advantages, such as its nontoxic characteristics and abundance. In this study, the concept of modulation doping is adopted to enhance the power factor of Si. The structure that favors modulation doping consists of two phases: a heavily doped and an undoped phase. In modulation doping, it is desirable that the heavily doped phase has an optimum electronic band structure against the undoped phase and is uniformly dispersed at the nanoscale. Herein, we attempt to demonstrate modulation doping in the Si–Mg2Si system, whereby Si is considered as the undoped phase and Mg2Si as the heavily doped phase. The Si–Mg2Si nanocomposite ribbons are synthesized by a melt spinning method, and their thermoelectric properties are measured. A large power factor value of 2.7 mW m–1 K–2 is obtained at 773 K for the ribbons with optimized structure and chemical composition. It is revealed that the best composition is the eutectic composition between Si and Mg2Si, i.e., Si0.56(Mg2Si + 1.3 at. % Bi)0.44, where Bi is a dopant for Mg2Si. Compared to the theoretical calculation model which was based on the effective medium theory, this unique experimental result is evoked because of modulation doping at temperatures >500 K.