Preparation and Thermoelectric Properties of Yb-doped Si Clathrates

Abstract

Solubility of Yb to the Si clathrates Ba₈Ni_3.8Si_42.2 and Ba₈Cu_4.9Si_41.1 with the type-I clathrate structure has been studied with polycrystalline samples by the electron micro probe analysis. It is revealed that most of the doped Yb is precipitated as secondary phases and the Yb solubility limit is about 1% of the Ba site or less. Thermoelectric properties of those composite samples are also investigated. Yb-doped samples show lower electrical resistivity and Seebeck coefficient, compared to the pristine compounds. Thermal conductivity increases with the addition of Yb, which is attributed to the increase of the electronic thermal conductivity. Overall thermoelectric figure of merit is not improved for the Ni-system, while a slight enhancement is observed for the Cu-system above 800 K. The best performance is zT = 0.21 for the nominal composition Ba_7.8Yb_0.2Cu_4.9Si_41.1 at T = 1000 K.