Enhanced Thermoelectric Performance of Ba₈Ga₁₆Ge₃₀ Clathrate by Modulation Doping and Improved Carrier Mobility

Type-I inorganic clathrates are promising high temperature thermoelectric materials. They are known for their intrinsic low thermal conductivity, but a moderate power factor leaves room for further improvement. In this paper, a new route for improving the power factor by enhanced carrier mobility achieved via modulation doping is reported. A series of clathrates with composition Ba-8(AlxGa1-x)(16)Ge-30 are synthesized through ball milling and spark plasma sintering of mixtures of Ba8Al16Ge30 and Ba8Ga16Ge30. Among the materials with x = 0.20, 0.23, and 0.25, it is found that the electrical conductivity is significantly enhanced with increasing x, while the Seebeck coefficient decreases slightly. It is further revealed that the carrier mobility of the sintered sample x = 0.25 is greatly increased, reaching a value that exceeds that for a single crystal. Electron micro-scopy analysis reveals that the material consists of a heterostructure and is composed of a Ga-rich clathrate matrix phase and Al particles, suggesting that the power factor enhancement is due to modulation doping. As a result, the highest power factor is achieved for Ba-8(Al0.25Ga0.75)(16)Ge-30, with a value of 1.89 mW m(-1) K-2 at 800 degrees C. Consequently, the maximum zT of sample x = 0.25 reaches 0.93 at 800 degrees C.