Preparation And Thermoelectric Properties Of Ag(Pb_1-ySny )_m SbTe_2+m-based Materials

Thermoelectric (TE) materials, which can realize the direct conversion between electrical and thermal energy, have significant application prospects in many areas. Ag-Pb-Sb-Te (LAST) material has the highest ZT value that had ever been reported for bulk thermoelectric materials in the middle temperature. As reported by many researchers, there is a very close relationship between the excellent thermoelectric performance of AgPb₁₈SbTe₂₀ and the nanoscaled second phase rich in AgSb. Nanostructure can decrease the thermal conductivity and greatly improve the thermoelectric performance of thermoelectric materials. As the thermoelectric properties of AgPbmSbTe_2+m are sensitive to preparation conditions and chemical composition, it is meaningful to conduct research on the influence of the composition, fabrication process on their thermoelectric performance. Currently, LAST materials are mainly fabricated by the method of melting at high termperature, which is quite time-taking and energy-consuming. Furthermore, high Pb content in LAST is very negative to the environment. LASTT materials, which were obtained by substituting Pb fully or partially with Sn in LAST materials, were proved to possess excellent performance and be more environmental benign. In this work, we fabricated the powders of LAST materials and LASTT materials by mechanical alloying. Subsequently, the as-MAed powders were consolidated by plasma activated sintering and hot pressing sintering, respectively. We explored the influence of Pb, Sn,Ag and Sb content on the thermoelectric properties of LASTT materials, and the effect of fabrication technologies on the microstructure and thermoelectric properties of the compounds was also investigated. The following results have been obtained:Starting from elemental powder mixtures of Sn50Te50 and AgSn₁₈SbTe₂₀, SnTe compound and SnTe-based solid solution were synthesized by mechanical alloying (MA) respectively. The MAed AgSn₁₈SbTe₂₀ has a SnTe-based solid solution structure. With the milling time prolonging, more and more Ag and Sb atoms enter into the SnTe-based structure of AgSn₁₈SbTe₂₀ and substitute for Sn, so the lattice spacing of SnTe-based solid solution decreases. Starting from elemental powder mixtures of AgPb₁₈SbTe₂₀ and Ag_0.85Pb₂₂SbTe₂₀, the PbTe-based solid solution of AgPb₁₈SbTe₂₀ and Ag_0.85Pb₂₂SbTe₂₀ compound were synthesized by mechanical alloying for 3 hours.Ag(Pb1–ySny)mSbTe_2+m compounds were also synthesized by mechanical alloying for 3 hours. As shown by the XRD patterns, with the increase of Sn content in these compounds, the peaks of SnTe became increasingly intensified and shifted to high angle direction, indicating that the lattice constants decreased. Evidently the content of SnTe phase increases with the increase of Sn.The as-MAed Ag(Pb1–ySny)mSbTe_2+m powders were consolidated by plasma activated sintering. With increase of Sn content, the content of SnTe compound also increased, and the electrical resistivity decreased apparently. The power factor of AgSn₁₈SbTe₂₀ attained 1.98×10-3 Wm-1K-2 at 723 K, which was the highest value among the Ag(Pb1–ySny)mSbTe_2+m compounds of all tested compositions, and it was higher than those of NaSn₁₈SbTe₂₀ and SnTe reported before. It also shows that the codoping of Ag and Sb to SnTe compounds had better electrical properties than the undoped-SnTe compounds within some temperature range.Due to its high electrical resistivity, P type AgPb₁₈SbTe₂₀ thermoelectric material had small power factor. Adjusting the content of Ag and Pb in this material, n type Ag_0.85Pb₂₂SbTe₂₀ thermoelectric material was obtained. Its thermoelectric properties were much better than those of AgPb₁₈SbTe₂₀, and its largest power factor of 1.54×10-3 Wm-1K-2 was obtained at 725 K and it highest ZT value attained 0.91 at 675 K. The as-MAed Ag(Pb1–ySny)mSbTe_2+m powders were consolidated by hot pressing subsequently. The maximal Seebeck coefficient of the as-HPed Ag_0.85Pb₂₂SbTe₂₀ was -287μV/K at 525 K, and its power factor was 1203μW.m-1.K-2 at 725 K. Although the composition dependence of the thermoelectric properties of the as-HPed samples were similar to that of the as-PASed samples, the as-PASed samples had better thermoelectric performance than those of the as-HPed samples for the same composition. Compared with the as-PASed LASTT materials, the as-HPed LASTT materials had higher electrical resistivity and smaller power factor. The ZT value of the HPed Ag_0.85Pb₂₂SbTe₂₀ and PASed Ag_0.85Pb₂₂SbTe₂₀ were 0.78 and 0.91 respectively. However, the HPed samples had better mechanical properties than the PASed samples.