Optimization on the figure-of-merit of P-type Ba8Ga16Ge30 Type-I clathrate grown via the Bridgman method by fine tuning Ga/Ge ratio

The thermoelectric properties of polycrystalline p-type Ba8Ga16Ge30 type-I clathrates were explored by fine tuning Ga/Ge ratios of clathrates and compared with the carrier properties. The clathrates with five different Ga/Ge ratios were grown from melt by means of the vertical Bridgman method with the raw materials prepared by vacuum arc melting. The carrier concentration of the clathrates decreases with increasing the Ga/Ge ratio from 0.567 to 0.586. The temperature dependence of electrical conductivity shows that these clathrates are heavily doped semiconductors. The Seebeck coefficient increases with the Ga/Ge ratio at room temperature, while the sample with the Ga/Ge ratio of 0.579 has the highest Seebeck coefficient at 550 °C. These tendencies can be explained in dependence of carrier concentration. The clathrate with a Ga/Ge ratio of 0.572 has the optimal dimensionless figure-of-merit of 1.0 at 500 °C, while its power factor and thermal conductivity are 2.11 × 10−3 W/mK2 and 1.55 W/mK respectively. However, the clathrate of with Ga/Ge ratios between 0.567 and 0.579 possesses zT values close to 1.0. This indicates a Ga/Ge ratio range for steady zT values.