Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent on identifying materials with higher thermoelectric efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelectric materials is approaching. We review recent advances in the field, highlighting the strategies used to improve the thermopower and reduce the thermal conductivity.

Materials and Methods 1. Sample preparation method Tl-doped PbTe was made by direct reaction of appropriate amounts of Pb, Te, and Tl 2 Te in a fused-silica tube sealed under a vacuum. Each sample was briefly melted at 1273 K for 24 h and lightly shaken to ensure homogeneity of the liquid, then furnace cooled to 800 K and annealed for 1 week. The obtained ingot was crushed into fine powder and hot-pressed at 803 K for 2 hours under a flowing 4% H 2-Ar atmosphere. The final form of each polycrystalline sample was a 2mm thick disk about 10 mm in diameter. Phase purity was checked by powder X-ray diffraction. No impurity phases were found in the XRD patterns, indicating that all added Tl was dissolved in PbTe. The purities of all starting materials were at least 99.99%. The samples were stable in air at room temperature.

Mitigation of the global energy crisis requires tailoring the thermal conductivity of materials. Low thermal conductivity is critical in a broad range of energy conversion technologies, including thermoelectrics and thermal barrier coatings. Here, we review the chemical trends and explore the origins of low thermal conductivity in crystalline materials. A unifying feature in the latest materials is the incorporation of structural complexity to decrease the phonon velocity and increase scattering. With this understanding, strategies for combining these mechanisms can be formulated for designing new materials with exceptionally low thermal conductivity.

Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent upon identifying materials with higher thermoelectric efficiency, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly regarding nanoscale materials, a new era of complex thermoelectric materials is approaching. Several new classes of compounds have been discovered which show enticingly high efficiencies. By reviewing recent advances in the field, we discuss the most promising strategies that can help guide the development of revolutionary thermoelectric materials: (a) quantum confinement of electrons to enhance thermopower (Seebeck coefficient), (b) low lattice thermal conductivity through structural complexity on various length scales, and (c) substructure approaches which separates the 'electron-crystal' from the 'phonon-glass'. Finally, we briefly discuss the integration of new thermoelectric materials into devices and the challenges of thermoelectric measurements.

Zintl phases and related compounds are promising thermoelectric materials; for instance, high zT has been found in Yb14MnSb11, clathrates, and the filled skutterudites. The rich solid-state chemistry of Zintl phases enables numerous possibilities for chemical substitutions and structural modifications that allow the fundamental transport parameters (carrier concentration, mobility, effective mass, and lattice thermal conductivity) to be modified for improved thermoelectric performance. For example, free carrier concentration is determined by the valence imbalance using Zintl chemistry, thereby enabling the rational optimization of zT. The low thermal conductivity values obtained in Zintl thermoelectrics arise from a diverse range of sources, including point defect scattering and the low velocity of optical phonon modes. Despite their complex structures and chemistry, the transport properties of many modern thermoelectrics can be understood using traditional models for heavily doped semiconductors.

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...

No abstract

No abstract