Collaborative effects of Zn and Sb Co-doping in magnesium silicide for thermoelectric applications

The effects of Zn and Sb co-doping in Mg2Si are investigated for thermoelectric applications, particularly focusing on the bonding with an Ni electrode and the grain boundaries. In the case of single Sb doping in Mg2Si, Sb is segregated at the interface with the Ni electrode, often accompanied by void formation due to the Kirkendall mechanism. On the other hand, in the case of Zn and Sb co-doping in Mg2Si, fast diffusion of Zn and the formation of a stable Zn-Sb alloy result in a smooth interface and void-free Zn-Sb-rich region embedded by the intermetallic ω-phase on the Mg2Si side of the interface with the Ni electrode. The thermoelectric properties and microanalyses at the grain boundaries with and without post-annealing reveal significant atomistic redistributions of Zn to form Zn-O segregation at the grain boundaries while keeping the Sb distribution uniform. Such Zn-O segregation possibly reduces the barrier height at the grain boundaries and stabilizes the grains compared to the case without post-annealing, where reactive oxygen remains. The long-term annealing test up to 1300 h under 773 K in air shows good stability of the materials with the Ni electrode. These collaborative effects will be particularly important for achieving reliable thermoelectric modules using Mg2Si. •Sb doping in Mg2Si often causes fatal cracks at the interface with Ni electrode.•Zn and Sb co-doping in Mg2S yields a void-free smooth interface at the Ni electrode.•Segregation of Zn-O at grain boundaries possibly reduces barrier height for electrons.