The synthesis and thermoelectric properties of Yb/Sr double-atom-filled YbxSr8－xGa16Ge30 type-I clathrates

n-type Yb/Sr double-atom-filled YbxSr8-xGa16Ge30 (x=0, 05, 10, 15) clathrates have been synthesized by combining melting reaction with the spark plasma sintering (SPS) method. The effects of double-atom filling on thermoelectric properties have been investigated. The results show that the solubility limit of Yb in the Sr-Ga-Ge system is between 10 and 15 when it is expressed by the formula of YbxSr8-xGa16Ge30. With increasing Yb content x, the room-temperature carrier concentration of the YbxSr8-xGa16Ge30 samples increases, while the room-temperature carrier mobility decreases. For the double-atom-filled samples, the electrical conductivity raises with increasing x, while the Seebeck coefficient reduces, and in which the x=05 sample has a comparable electrical conductivity and a remarkably higher Seebeck coefficient compared with the single-atom-filled Sr8Ga16Ge30 sample in the temperature range of 300—800 K. The double-atom filling of Yb/Sr has significant influence on the lattice thermal conductivity of the YbxSr8-xGa16Ge30 samples and the lattice thermal conductivity decreases gradually with increasing x. Of all the YbxSr8-xGa16Ge30 samples, the maximum dimensionless figure of merit ZT of 081 is obtained at 800 K for the Yb10Sr70Ga16Ge30 sample. Compared with that of single-atom-filled Sr8Ga6Ge30 sample, it is 35% higher at the same temperature.