We report on the synthesis of Sb 2 Te 3 nanoparticles with record-high figure of merit values of up to 1.5. The central thermoelectric parameters, electrical conductivity, thermal conductivity and Seebeck coefficient, were independently optimized. The critical influence of porosity for the fabrication of highly efficient thermoelectric materials is firstly demonstrated, giving a strong guidance for the optimization of other thermoelectric materials.Thermoelectric materials directly convert heat fluxes into useable electricity and are therefore discussed as a key-enabler in waste heat recovery. For this vision, the main challenge is to develop thermoelectric materials with sufficiently high conversion efficiencies, expressed by the material's figure of merit zT = a 2 sT/k, where a, s, k and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature, respectively. a 2 s is called power factor. It is assumed that zT D 1.5 is necessary for most technical applications. 1 Nanostructuring of thermoelectric materials has been demonstrated experimentally and theoretically to greatly improve the figure of merit by reducing the lattice contribution to the thermal conductivity. 2,3 Different types of scattering centers for the heat carrying phonons were implemented as design concepts for thermoelectric materials, such as nanoscale precipitates or interfaces. 4,5 To effectively scatter the broad spectrum of phonon wavelengths, a hierarchical design of the nano-and microstructure was developed which led to record-high zT values. 6 On the other hand, an increase of disorder in the nano-and microstructure of a crystal simultaneously increases the charge carrier scattering, which limits the nano-and microstructuring approach since the charge carrier mobility gets affected adversely. A high electrical conductivity further requires a high charge carrier concentration. The latter compromises the Seebeck coefficient which is best when the charge carrier concentration is low. Thus, to optimize a thermoelectric material, a careful decoupling of the competing and interrelated thermoelectric transport coefficients is mandatory.We herein report on a strategy that allows to individually address and optimize each of the three thermoelectric transport coefficients in the nanobulk fabrication process as demonstrated for a standard thermoelectric material, Sb 2 Te 3 . Synergistic effects result in a dramatic enhancement of zT up to 1.5 around 300 1C. This specific versatile approach opens up a new synthetic strategy to a general route to highly efficient thermoelectric materials.Control of the charge carrier concentration is the first important aspect in optimizing the performance of a thermoelectric material. Metal chalcogenides such as Sb 2 Te 3 have a phase width (are not line compounds) and are intrinsically doped by anti-site defects. As a consequence, the Seebeck coefficient of Sb 2 Te 3 obtained by traditional synthetic approaches tends to be lower than required for thermoelectric application. Thus, the an...
We review the Raman shift method as a non-destructive optical tool to investigate the thermal conductivity and demonstrate the possibility to map this quantity with a micrometer resolution by studying thin film and bulk materials for thermoelectric applications. In this method, a focused laser beam both thermally excites a sample and undergoes Raman scattering at the excitation spot. The temperature dependence of the phonon energies measured is used as a local thermometer. We discuss that the temperature measured is an effective one and describe how the thermal conductivity is deduced from single temperature measurements to full temperature maps, with the help of analytical or numerical treatments of heat diffusion. We validate the method and its analysis on 3-and 2-dimensional single crystalline samples before applying it to more complex Si-based materials. A suspended thin mesoporous film of phosphorus-doped laser-sintered Si 78 Ge 22 nanoparticles is investigated to extract the in-plane thermal conductivity from the effective temperatures, measured as a function of the distance to the heat sink. Using an iterative multigrid Gauss-Seidel algorithm the experimental data can be modelled yielding a thermal conductivity of 0.1 W/m K after normalizing by the porosity. As a second application we map the surface of a phosphorus-doped 3-dimensional bulknanocrystalline Si sample which exhibits anisotropic and oxygen-rich precipitates. Thermal conductivities as low as 11 W/m K are found in the regions of the precipitates, significantly lower than the 17 W/m K in the surrounding matrix. The present work serves as a basis to more routinely use the Raman shift method as a versatile tool for thermal conductivity investigations, both for samples with high and low thermal conductivity and in a variety of geometries.
Seeding zone melting is applied to produce bulk Bi 1.625 In 0.375 Te 3 with 7.5 atom % In in solid solution. The concentration distribution is markedly homogeneous and exhibits pronounced anisotropic electrical and thermal conductivity. Subsequent precipitation from the solid solution leads to the formation of a highly anisotropic composite thermoelectric material consisting of aligned microscaled Bi 2 Te 3 and extended micro-to nanoscaled In 2 Te 3 plates. By the precipitation, an increase of zT by a factor of 6 compared with the parent supersaturated solid solution crystal is achieved. This is attributed to the combination of a decrease of In concentration from 7.5 to 3 atom % in the Bi 2 Te 3 layer and an increasing interface density due to the precipitation of In 2 Te 3 . The Bi 2 Te 3 /In 2 Te 3 interface is determined as coherent, and the crystallographic orientation between the two phases is determined as ⟨2̅ 11⟩ In2Te3 //⟨11̅ 00⟩ Bi2Te3 , {111} In2Te3 //{0001} Bi2Te3 .
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