鉛-銀-銻-碲 四元熱電材料系統之相平衡、微結構與熱電性質

Thermoelectric material has been recognized as a promising candidate in the application of sustainable energy. The quaternary Pb-Ag-Sb-Te system has attracted much attention because it contains high-zT AgPb18SbTe20 (zT>2 at 800K), PbTe and AgSbTe2. The phase stability, microstructural evolution and resultant electrical/thermal transport properties of quaternary Pb-Ag-Sb-Te and the constituent systems (Ag-Sb-Te, Pb-Sb-Te and Ag-Pb-Te) are investigated. The liquidus projections of both ternary Ag-Pb-Te and Pb-Sb-Te systems are constructed. In particular, the Ag-Pb-Te ternary-eutectic alloy, with composition of Ag-4.3at%Pb-62.6at%Te, forms a partially aligned nano-sized lamellar microstructure, which comprises both the PbTe and Ag5Te3 phases, and an additional dotted PbTe of 200-600nm. This particular ternary-eutectic alloy is unidirectionally solidified using the Bridgman method, resulted in a nanostructured composite with an extremely low thermal conductivity(κ) of 0.3 W/mK and a zT peak of 0.41 at 400K. The phase diagrams of ternary Ag-Sb-Te system are constructed as well, including the 400℃ and 250℃ isothermal sections and the liquidus projection. The ternary AgSbTe2 is stabilized at 400℃ but not at 250℃, with homogeneity region of 49.0-53.0at%Te and 28.0-30.0at%Sb. The nano-scaled microstrucutre and crystal structures of the non-stoimeteric AgSbTe2 are analyzed by the transmission electron microscope (TEM). In particular, an ordered array of nano-wire microstructure, comprising a 200nm Ag2Te and a matrix of AgSbTe2+δ-Sb2Te, was resulted from a Class I reaction: L=AgSbTe2+Ag2Te+δ-Sb2Te with liquid composition of Ag-40at%Sb-36.0at%Te at 496.5℃. To understand and guide production of uniform bulk samples of this composite, the liquidus projection of quaternary Pb-Ag-Sb-Te system at 36.0at%Te isoplethal section is constructed experimentally using quenched samples. High-resolution transmission electron microscopy (TEM) confirms that these three phases are simultaneously present at the nanometer scale. Furthermore, unidirectional solidification experiments of the ternary eutectic alloy using the Bridgman method are carried out to examine the alloy's solidification behaviors. Pb-alloyed AgSbTe2 (PbxAg20Sb30-xTe50, x=3,4,5 and 6) are also unidirectionally solidified using the modified Bridgman method. The as-solidified 5at%Pb and 6at%Pb alloys, which exhibit high phase purity of AgSbTe2, contain grain-boundary inhomogeneity and nano-precipitates of δ-Sb2Te, leading to an extremely low thermal conductivity (κ) of 0.3-0.4 W/mK. A peak zT of 0.7-0.8 is found in as-solidified 5at% specimen at 425K. However, after annealing at 673K, the zT peak of 5at%Pb(annealed) decreases to 0.4, presumably due to increase in grain size and decrease in inhomogeneity.