Thermoelectric properties of type-I clathrate Na8Al8Ge38

Abstract

To investigate the use of Na-containing clathrates as thermoelectric materials, we prepared several sintered samples of the title clathrate and studied their thermoelectric properties and electronic structure. The Na8Al8Ge38 samples had low lattice thermal conductivities, 15 mW cm−1K−1 at 300 K, because of alloy disorder scattering for phonons due to the Al–Ge framework, compared to a previously reported Na8Ga8Ge38 (21 mW cm−1K−1). Due to the s-orbital of the Na guest atoms, the Na8Al8Ge38 clathrate had a larger effective mass, 3.6me (me: free electron mass), than Ba8Ga16Ge30 (1.5me), although it was a little lower than that 4.6me of Na8Ga8Ge38. However, the Na8Al8Ge38 samples had comparable carrier mobility, 8 cm2V−1s−1 at 300 K, to that of Na8Ga8Ge38, probably because of alloy disorder scattering for electrons. The dimensionless figure-of-merit ZT of Na8Al8Ge38 was almost the same as that of Na8Ga8Ge38, and the former was higher than the latter at high temperatures. The maximum value of ZT was 0.76 at 870 K.