Thermoelectric Studies of P-Type Narrow-Bandgap Bi-Sb and Modeling of Transport Properties

Thermoelectric research has been performed on p-type Bi-Sb system. We systematically study the doping effects of Ge, Sn, and Pb in Bi-Sb alloys. The samples are made using the melt spinning technique and the spark plasma sintering method. A “high pressure low temperature” method is developed to resolve the low solubility issue of Ge and Pb in Bi-Sb. We observe an increased doping level for Ge and an increased doping efficiency for Pb in Bi-Sb. Several strategies have been applied to enhance the thermoelectric performance of p-type Bi-Sb samples including the Ge + Pb/Sn co-doping method, transition metal + Sn/Pb co-doping method, Te + Sn/Pb co-doping method and the post high-pressure method. Enhanced power factors are obtained in p-type Bi-Sb samples and most of them can be attributed to the decreased ratio of the electron mobility to the hole mobility. The thermal conductivity is measured using the hot disk method. The Bi88Sb12Ni1Pb0.5 sample shows a peak of figure of merit zT ~ 0.13 around 200 K which is comparable to the performance of the best single crystal p-type Bi-Sb by far.

We apply a two-band effective mass model to fit the data of undoped and pure Ge, Sn, and Pb doped Bi-Sb samples and the band gap is set as a changing parameter. We find that the band gap needs to shrink as temperature rises in order to fit the data well. This helps explain why it is difficult to achieve a good thermoelectric performance in p-type Bi-Sb system. If the band gap shrinks as temperature rises, the electron channel starts to contribute to the transport at relatively low temperature which is detrimental to the p-type Seebeck coefficient. Besides, the bipolar thermal conductivity becomes significant with both the hole channel and the electron channel participating the transport. Those factors lead to a low figure of merit zT for p-type Bi-Sb.

Moreover, we have studied p-type Si-Ge alloys. The ball milling technique and the spark plasma sintering method are applied to synthesize Si-Ge samples. We measure the resistivity and the Seebeck coefficient of Fe/Ni/Mo codoped Si-Ge, Si-Ge embedded with aerogel particles, and Si-Mo-B samples. The carrier concentration and the carrier mobility are affected by the Fe/Ni/Mo and aerogel particles which change the electrical conductivity and the Seebeck coefficient of Si-Ge alloys. Overall, we get about 10 % increase of the power factor in the transition metal codoped Si-Ge compared with the referenced p-type Si-Ge result.