High Thermoelectric Performance in PbTe Due to Large Nanoscale Ag2Te Precipitates and La Doping

Abstract

wileyonlinelibrary.comAdv. Funct. Mater. 2011, 21, 241-249 the infl uence of nano-particle content, density and size, as well as the infl uence from alloy-scattering and electronic doping effects. In this article, we describe the controlled synthesis and thermoelectric properties of fi ne and uniformly dispersed Ag 2 Te precipitates embedded in PbTe. In contrast to many previously studied PbTe-based systems, [ 5 , 10 , 11 , 13-16 ] the Ag 2 Te precipitates are much larger (50-200 nm), are not isostructural to PbTe and do not introduce considerable electronic doping effect to PbTe. We show that these precipitates scatter the phonons effectively, leading to a low lattice thermal conductivity which approaches the minimum expected of PbTe above 650 K. Moreover, doping with La independently optimizes the carrier concentration and results in a thermoelectric fi gure of merit of 1.6 in La-doped PbTe-Ag 2 Te composites at 775 K. This value is about twice that of the state-of-the-art n-type PbTe [ 1 , 26 ] and arises from the low lattice thermal conductivity at this temperature. (PbTe)1 MicrostructureThe pseudo-binary phase diagram of PbTe-Ag 2 Te (see Figure 1 ) [27][28][29] shows signifi cant and strongly temperature dependent solubility of Ag 2 Te in PbTe. Similar behavior in the PbTe-Sb 2 Te 3 phase diagram has been harnessed to yield Widmanstätten precipitates of Sb 2 Te 3 in a matrix of PbTe. [ 25 ] Here, we utilize the variance in maximum solubility of Ag 2 Te in PbTe, which is about 7-11 mol.% [ 27 , 29 ] at the eutectic temperature of ∼ 970 K and quickly drops to about 1 mol.% at ∼ 770 K. [ 27 ] From these features, one can expect that after melting (step 1 in Figure 1 ) and homogenizing the solid solution at ∼ 970 K (step 2 in Figure 1 ), Ag 2 Te precipitates will be obtained during a lower temperature anneal at ∼ 770 K (step 3 in Figure 1 ).Four compositions of (PbTe) 1 − x (Ag 2 Te) x are considered here ( x = 1.3, 2.7, 4.1, 5.5), all of which have compositions ( Table 1 ) greater than the solubility limit for Ag 2 Te at the annealing temperature (770 K). Following this thermal treatment, Ag-rich precipitates are observed to be homogeneously distributed in the PbTe matrix, as shown by fi eld emission scanning electron microscopy images ( Figure 2 a ). As the Ag 2 Te content increases, the Ag 2 Te is incorporated as a solid solution in the PbTe matrix [27][28][29] ). The open circle at 773 K shows the experimental Ag solubility in PbTe, [ 27 ] consistent with the current study. Starting with a homogeneous melt at a composition of Ag5.5 (point 1), the sample is quenched and then annealed within the single phase region (point 2) for homogenization. Phase separation is then achieved by annealing at 773 K (point 3). up to the solubility limit ( ∼ 1%) at the annealing temperature. [ 27 ] Beyond the solubility limit, the volume fraction of Ag 2 Te particles increases with increasing Ag 2 Te content in the mixture (Figure 2 ), but the Ag content in the PbTe matrix remains constant. The solubility limit is dir...

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