Solvothermal Synthesis And Electrical Transport Properties Of PbTe Based Thermoelectric Materials

Thermoelectric (TE) materials are a kind of semiconductor functional materials, which can be used to convert directly heat energy to electricity or reversely. They are of interest for applications in TE cooling devices and power generators. PbTe and its alloys, which have the highest dimensionless figure of merit about 2.2 as reported, are known as one of the best TE materials currently available at medium-temperature. The figure of merit of TE materials could be significantly improved if the materials were nanostructured and dimensionally reduced.In the present work, PbTe powders and Ag/Sb doped PbTe based powders have been synthesized using Pb(CH₃COO)₂·3H₂O and Te powder as the precursors via solvothermal route, hydrothermal route and low temperature aqueous chemical route. The structures and morphologies of the nanopowders were investigated by XRD, SEM and TEM. Chemical reactions during the synthesis of pure PbTe and doping mechanism of Ag/Sb doped PbTe based powders have been experimentally investigated and detailedly discussed. The bulk materials of some PbTe based powders have been prepared by vacuum hot pressing and spark plasma sintering. The electrical transport properties of sintered samples have been measured. Some important results of the present work are listed as fellows:1. Pure PbTe powders have been successfully prepared by solvothermal (hydrothermal) route using Pb(CH₃COO)₂·3H₂O and Te powder as the precursors, NaBH₄ as the reductant, NaOH as the alkaline agent, ethanol, acetone, glycol, ethylenediamine, DMF and distilled water as solvent. The effect of various reaction conditions on the product phase and microstructure has been investigated. We have found that, it was more difficult to synthesize pure PbTe powders via hydrothermal route than solvothermal route. Higher temperature is needed in hydrothermal route to synthesize pure PbTe powders. In solvothermal route, cubic PbTe grains were synthesized using ethylenediamine and DMF as solvent, and the size of grains in DMF was more homogeneous and smaller. When the reactant concentration increased, higher temperature and longer reaction time were needed to synthesize pure PbTe.2. PbTe nanopowders were synthesized by low temperature aqueous chemical route in open system. A two-step charge method, instead of the one-step charge method as we used in solvothermal and hydrothermal route, was used in the temperature aqueous chemical route. In this route, Te was reduced to Te_a+1^2- firstly, and then Pb²⁺ was added to the system tocombine with Te^2- to form PbTe nuclei. PbTe phase could be synthesized in 70℃ which was the lowest temprature reported, though Te impurity was involved in product. When the reaction temperature reached 100 ℃ , pure PbTe nano-sized powders about 20nm were prepared.3. In the synthesis process of PbTe powders, we believe that atom reaction mechanism and ion reaction mechanism existed in solvothermal and hydrothermal route, but only ion reaction mechanism existed in low temperature aqueous chemical route. To synthesize pure PbTe phase, higher temperature was needed in atom reaction, but comparatively lowertemperature was needed in ion reaction. So the higher temperature which could make PbTe grains grow greatly was needed in solvothermal and hydrothermal route, and the lower temperature in aqueous chemical route could suppress the growth of PbTe grains to form nano-sized powders. The alkaline and reducing atmosphere in which NaBKU as the reductant and NaOH as the alkaline agent was necessary in solvothermal, hydrothermal and aqueous chemical routes. NaOH could not only make Te be dissolved easily, but also suppress the reaction between NaBrtj and H+ to ensure the reducing atmosphere in the whole reaction process.4. On the base of solvothermally synthesized PbTe powders, Ag doped PbTe based powders, Sb doped PbTe based powders and Ag and Sb co-doped PbTe based powders were synthesized in solvothermal route. It was found that element Ag and Sb enter into PbTe crystal lattice, and there were two doping mechanism, substitution doping and gap doping. Further analysis indicated that the molecular formula of our doping products were not the formula we had designed, and there were some silver telluride and antimony telluride in the final product. So the synthesis technic was still need to be further optimized.5. Some powders were sintered to bulk materials using modern rapid sintering technic, and the electrical transport properties were measured. It was found that grains grew greatly during hot pressing process, but SPS could suppress the growth of grains. The power factors of bulk sintered materials made from solvothermally synthesized Ag/Sb doped PbTe powders were smaller than solvothermally synthesized and hot-pressed PbTe bulk material because Ag2Te and Sb2Te3 impurities existed in the final products. In order to improve the electrical transport properties, the solvothermal synthesis technics were still need to be further optimized to make more Ag and Sb elements enter into PbTe crystal lattice and avoid the A&Te and Sb2Te3 impurities existing in final products.