Enhanced thermoelectric performance of Bi_0.5Sb_1.5Te₃-expanded graphene composites by simultaneous modulation of electronic and thermal carrier transport

Solution-based synthesis of thermoelectric nanoplates, which provides a low thermal conductivity due to the grain boundary scattering, has received considerable attention as a scalable method. However, the scattering also decreased electrical conductivity leading to a low thermoelectric figure of merit (ZT). Here we employed expanded graphene to enhance thermoelectric performance of p-type Bi_0.5Sb_1.5Te₃ composites by simultaneous improvement in electrical conduction and phonon scattering. The addition of expanded graphene (0.1vol%) improved both carrier concentration and electrical conductivity of composites due to the high intrinsic p-type carrier concentration of graphene. Besides, it significantly decreased lattice thermal conductivity due to the phase boundary phonon scattering in spite of the high intrinsic thermal conductivity of graphene. The increased carrier concentration also suppressed the bipolar conduction resulting in a moderate increase in power factor and a slow increase in bipolar thermal conductivity at elevated temperatures. Overall, the maximum ZT increased by 45% (1.13 at 360K) by the addition of expanded graphene. A similar trend with a greater maximum ZT (1.24 at 360K) was observed when ball-milled Bi_0.5Sb_1.5Te₃ ingot powders were employed providing reliability of the suggested mechanism.