Optimization of Thermoelectric Properties of SnTe via Multi-element Doping

Thermoelectric materials can realize the direct conversion of heat and electric energy, and have broad application prospects in the fields of thermoelectric power generation and semiconductor refrigeration. Both SnTe and PbTe thermoelectric materials belong to the IV-VI group, and have the same NaCl-type crystal structure, but SnTe possesses poor thermoelectric properties. In this work, SnTe-based thermoelectric materials were prepared by a fast method, known as self-propagating high-temperature synthesis under high-gravity field (HG-CS) combined with spark plasma sintering (SPS). The effect and mechanism of multi-element doping on the thermoelectric properties of SnTe compounds were also studied. Multi-element doping, equivalent ions Ge2+ 2+ and Pb2+ 2+ in cation of SnTe and anionic S 2- and Se2-, 2- , causes a large number of lattice distortion point defects. At the same time, rapid solidification under the supergravity field brings about plastic deformation and introduces a stress field and a large number of dislocations, which results in the formation of multilevel microstructural defects and strong scattering of medium- and high-frequency phonons. As a result, the room-temperature thermal conductivity decreases dramatically from 7.28 W<middle dot>m-1<middle dot>K-1 -1 <middle dot>K -1 (undoped SnTe) to 2.74 W<middle dot>m-1<middle dot>K-1 -1 <middle dot>K -1 (Sn 0.70 Ge 0.15 Pb 0.15 Te 0.80 Se 0.10 S 0.10 ), with a minimum thermal conductivity of only 1.38 W<middle dot>m-1<middle dot>K-1 -1 <middle dot>K -1 at 873 K. These microstructural defects scatter phonons and carriers, leading to a decrease in carrier mobility and conductivity. It is worth mentioning that doping decreases the bandgap of SnTe and increases the Seebeck coefficient, so that the power factor PF of the doped material remains at a high value. Finally, the peak thermoelectric figure of merit ZT of Sn 0.70 Ge 0.15 Pb 0.15 Te 0.80 Se 0.10 S 0.10 sample is greatly improved to 1.02 (873 K).