Thermoelectric performance optimizations of chalcogenides via electrical transport properties enhancement

Abstract

Metal Chalcogenides have a long history as thermoelectric materials with a decent performance, and several new types of chalcogenide thermoelectric materials have received extensive attention in the past few decades. The intrinsic vacancy of metal cations or chalcogen anions will lead to different intrinsic carrier concentrations of the materials, and an optimized carrier concentration level is required for their best thermoelectric performance. In this dissertation, the thermoelectric performance of multiple types of chalcogenide thermoelectric materials has been optimized through their electrical transport properties control. First, the carrier concentration of ternary I-V-VI2 chalcogenide AgBiSe2 has been enhanced via Se vacancy control and aliovalent cation doping. The low carrier concentration limits the thermoelectric performance of AgBiSe2 by reducing its electrical conductivity, and the increase of Se vacancy can increase the electrons in the material and n-type carrier concentration, while the aliovalent cation doping further provides electron donors and increases the carrier concentration. By reducing 0.25% of Se and doping 3% of Cd on the Ag site, the dimensionless figure of merit reaches ~0.65 at 773 K for this material, which has a significant increase from the pristine material and shows the effect of carrier concentration optimization. Second, the IV-VI thermoelectric material SnTe has been optimized via carrier concentration control and heavy alloying. The pristine SnTe has a high intrinsic carrier concentration due to the intrinsic Sn vacancy, and excess Sn is added for the suppression of the Sn vacancy and reduces the carrier concentration. The doping of AgBiSe2 into SnTe further reduces the carrier concentration and increases the Seebeck coefficient by increasing the effective mass. Meanwhile, the Mn alloying helps with the band convergence and increase in Seebeck coefficient, and Ge is introduced to further increase the solubility of Mn, and the effective mass drastically increases for Mn alloying with the existence of Ge. The maximum zT of Sn0.73Ge0.1Mn0.2Te+3% AgBiSe2 reaches ~1.44 at ~823 K, which is attributed to the optimization of carrier concentration, increase in Seebeck coefficient, and suppression of thermal conductivity. Finally, a new ternary copper and titanium chalcogenide Cu4TiTe4 has been successfully synthesized, and its thermoelectric performance is also measured and optimized. The formation of the material requires both the ball milling and heat treatment processes, and the pristine material has a relatively high electrical resistivity and low carrier concentration. By introducing Cu vacancies, not only is the impurity phase reduced in the material, but the electrical properties and thermoelectric performance of the material are also enhanced. After doping Se on Te site, the carrier concentration of the material is further enhanced, and the maximum zT at ~723 K is enhanced from ~0.4 of pristine material to ~0.8 in the composition with ~2% of Cu deficiency and ~5% of Se doping. The high maximum zT value and low intrinsic lattice thermal conductivity show the high thermoelectric potential of the material. In this dissertation, different electrical optimization strategies are attempted on the materials, and the optimizations are successful. The results provide the different optimization methods on the electrical properties of thermoelectric materials and also the synthesis methods of thermoelectric materials.