| Thermoelectric (TE) materials have received renewed attentions for their potential applications in environmentally-friendly cooling and power generation based on the di-rect energy conversion between heat and electricity based on the Seebeck effect and the Peltier effect. Theoretically, an efficient TE material must exhibit a high thermo-electric dimensionless figure of merit, ZT=α2σT/κ, where α, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conduc-tivity, respectively. Thus, it is evident that TE material should possess high values of Seebeck coefficient and electrical conductivity as well as low thermal conductivity to obtain a high value. Due to the low thermal conductivity combined with reasonably good electronic properties, lead chalcogenides PbTe and PbSe are the most widely s-tudied TE material in the intermediate temperature range. However, most studies of lead chalcogenides have been focused on the doping and nanostructuring. Considering the reliability and stability, it is more important to improve the TE properties of bulk materials.Accordingly, this dissertation selected the lead chalcogenides as the matrices and introduced different interfaces into the bulk materials to improve the thermoelectric per-formance. As the state-of-art thermoelectric materials, PbTe-and PbSe-based compos-ites were fabricated by combinative means of facile hydrothermal/solvothermal synthe-sis, suspension induction melting and hot pressing. Four types of TE materials includ-ing AgPb18SbTe2o micro-nano composite, PbTe-Sb2Te3composite with self-assembled lamellar structure, PbSe composite containing C@PbTe core-shell spheres and dually-doped PbSe with at both cationic and anionic sites were obtained. The correlation be-tweenn these microstructure and thermoelectric properties were systematically investi-gated. The details were summarized as follows:(1) AgPb18SbTe2o (LAST-18) powders with a heterostructure of nanocubes and nanorods are synthesized by a facile one-pot solvothermal method using cheap chem-ical reagent as starting materials and cetyltrimethyl ammonium bromide (CTAB) as a surfactant at180℃for24h. The results show that CTAB is crucial for the formation of nanorods owing to the preferential growth on the{100} faces or along the<100> direc-tion with high surface energy. However, the powders are characterized by nanospheres in case mass equivalent SDBS are used under the same condition. The effects of grain size, remained porosity and nanophase additions on the thermoelectric properties of n- type LAST-18fabricated by suspension induction melting and hot pressing were inves-tigated. The sample hot-pressed at600℃exhibits the best thermoelectric performance with the maximum value of0.62at550K for the figure of merit ZT. The presence of sovelthermally syntheszied LAST-18nanospheres or nanorods in the bulk yields re-markably improved thermoelectric properties. A maximum ZT=0.92at673K for the nanorod-containing composite is achieved mainly due to the reduced thermal conduc-tivity. Furthermore, the temperature of ZT peaks shift to a higher range originated from the enlarged energy gap.(2) The eutectic PbTe-Sb2Te3composite with self-organized lamellar structure was synthesized using induction melting and the effects of annealing on the microstructure and thermoelectric properties were investigated. The PbTe and Sb2Te3lamellae are crystallographically oriented with coherent interface, which are found to insignificant-ly affect the electronic flow. After annealed, the spheroidization of the lamellae occurs, and the carrier concentration decreases. Despite the increased phase boundaries, the lattice thermal conductivity increases with the annealing time. Largely due to the en-hanced Seebeck coefficient, a maximum ZT,0.48at550K for the sample annealed for20h is achieved, showing a60%improvement over that of the as-melted one.(3) PbTe coated amorphous carbon sphere core-shell structure (C@PbTe) was em-bedded into PbSe nanopowders and subsequently assembled into macroscopic compos-ites by hot pressing. Thermoelectric properties of the samples with0,1, and5wt.%C@PbTe additions were examined between300-550K. Enhanced Seebeck coefficient and lower thermal conductivity are obtained in the PbSe/1wt.%C@PbTe sample. The maximum figure of merit is0.54at400K, showing a13%improvement over that of the PbSe matrix. The sample added with100nm spheres improves the Seebeck coefficient at low-middle temperature range, showing the highest ZT of0.58. These findings fa-cilitate the application of hydrothermally synthesized carbon spheres in thermoelectric materials and also provide a potential direction to enhance thermoelectric performance via constructing core-shell structures.(4) The lattice geometry and band structure of (Ag,Te),(Sn,Te) and (Sb,Te) dual-ly doped PbSe were investigated by first-principles calculations. The results indicat-ed that the cationic and anionic dopant pairs tend to cluster at the first nearest neigh-bors.(Ag,Te) and (Sb,Te) dual doping introduces additional bands near the Fermi levels, which can be anticipated to improve the electrical properties. However, the effect of (Sn,Te) dual doping on the band structure is not obvious. The experiment results show that the (Ag,Te) dual doped PbSe displays the best thermoelectric properties due to the highest electrical conductivity (596S/cm at room temperature). However, the Seebeck coefficient is significantly reduced to118μVK-1at room temperature. The (Sn,Te) dual doping slightly increases the electrical conductivity and decreases the Seebeck co-efficient. Due to the decreased carrier concentration, the lowest electrical conductivity and highest Seebeck coefficient of (Sb,Te) dual doping are obtained. The maximum figure of merit ZT is0.78at550K, showing63%improvement over that of the PbSe matrix. |
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