Influences of potential barrier scattering on the thermoelectric, properties of sintered n-type PbTe with a small grain size

Sintered degenerate n-type PbTe samples with a grain size down to about 0.2 mum were prepared and their thermoelectric properties subsequently measured. Accordingly, the effects of grain size on thermoelectric properties were investigated. For samples with a grain size above similar to0.5 mum, the Seebeck coefficient and thermoelectric figure-of-merit increased with decreasing grain size. In particular, the room-temperature figure-of-merit reached a maximum value of similar to4 x 10(-4) K-1, which is about twice as large as the value of a large-grain-size sample with the same carrier concentration reported in the literature. For samples with a grain size below similar to0.5 mum, with decreasing grain size, the Seebeck coefficient increased but the figure-of-merit did not increase because of a substantial decrease in electrical conductivity. The increase in the Seebeck coefficient with decreasing grain size was probably caused by the potential barrier scattering at grain boundaries. On the other hand, the decrease in electrical conductivity is explained mainly by the porosity effect. Thus, we consider that if a sintered PbTe sample with a smaller grain size and a lower porosity is prepared, a larger, improvement in the figure-of-merit due to the potential barrier scattering can be obtained.