We herein report on the large-scale synthesis of ultrathin Bi(2)Te(3) nanoplates and subsequent spark plasma sintering to fabricate n-type nanostructured bulk thermoelectric materials. Bi(2)Te(3) nanoplates were synthesized by the reaction between bismuth thiolate and tri-n-octylphosphine telluride in oleylamine. The thickness of the nanoplates was ~1 nm, which corresponds to a single layer in Bi(2)Te(3) crystals. Bi(2)Te(3) nanostructured bulk materials were prepared by sintering of surfactant-removed Bi(2)Te(3) nanoplates using spark plasma sintering. We found that the grain size and density were strongly dependent on the sintering temperature, and we investigated the effect of the sintering temperature on the thermoelectric properties of the Bi(2)Te(3) nanostructured bulk materials. The electrical conductivities increased with an increase in the sintering temperature, owing to the decreased interface density arising from the grain growth and densification. The Seebeck coefficients roughly decreased with an increase in the sintering temperature. Interestingly, the electron concentrations and mobilities strongly depended on the sintering temperature, suggesting the potential barrier scattering at interfaces and the doping effect of defects and organic residues. The thermal conductivities also increased with an increase in the sintering temperature because of grain growth and densification. The maximum thermoelectric figure-of-merit, ZT, is 0.62 at 400 K, which is one of the highest among the reported values of n-type nanostructured materials based on chemically synthesized nanoparticles. This increase in ZT shows the possibility of the preparation of highly efficient thermoelectric materials by chemical synthesis.

We explored the effect of Cd substitution on the thermoelectric properties of PbTe in an effort to test a theoretical hypothesis that Cd atoms on Pb sites of the rock salt lattice can increase the Seebeck coefficient via the formation of a resonance level in the density of states near the Fermi energy. We find that the solubility of Cd is less than previously reported, and CdTe precipitation occurs to create nanostructuring, which strongly suppresses the lattice thermal conductivity. We present detailed characterization including structural and spectroscopic data, transmission electron microscopy, and thermoelectric transport properties of samples of PbTe-x% CdTe-0.055% PbI(2) (x = 1, 3, 5, 7, 10), PbTe-1% CdTe-y% PbI(2) (y = 0.03, 0.045, 0.055, 0.08, 0.1, 0.2), PbTe-5% CdTe-y% PbI(2) (y = 0.01, 0.03, 0.055, 0.08), and PbTe-1% CdTe-z% Sb (z = 0.3, 0.5, 1, 1.5, 2, 3, 4, 5, 6). All samples follow the Pisarenko relationship, and no enhancement of the Seebeck coefficient was observed that could be attributed to a resonance level or a distortion in the density of states. A maximum ZT of approximately 1.2 at approximately 720 K was achieved for the PbTe-1% CdTe-0.055% PbI(2) sample arising from a high power factor of approximately 17 microW/(cm K(2)) and a very low lattice thermal conductivity of approximately 0.5 W/(m K) at approximately 720 K.

We have performed a comparative investigation of the Ag 1-x Pb 18 MTe 20 (M ) Bi, Sb) (x ) 0, 0.14, 0.3) system to assess the roles of Sb and Bi on the thermoelectric properties. Detailed charge transport data including electrical conductivity, the Seebeck coefficient, the Hall coefficient, and thermal conductivity are presented. Optical reflectivity data support the conclusions of the transport studies. For comparable nominal compositions, the carrier concentrations are lower in the Sb analogs and the mobilities are higher. The Seebeck coefficient decreases dramatically in going from Sb to Bi. High resolution transmission electron microscopy (TEM) images of both samples reveal that all systems contain compositional fluctuations at the nanoscopic level and are nanostructured. Compared to PbTe, the lattice thermal conductivity of AgPb 18 BiTe 20 is substantially reduced. The lattice thermal conductivity of the Bi analog is, however, higher than the Sb analog, and this correlates with the decrease in the degree of mass fluctuation between the nanostructures and the matrix (for the Bi analog). As a result the dimensionless figure of merit ZT of Ag 1-x Pb 18 BiTe 20 is found to be substantially smaller than that of Ag 1-x Pb 18 SbTe 20 . Electronic structure calculations performed within the density functional theory and generalized gradient approximation show marked differences in the band structure near the Fermi level between the two analogs providing useful insights on the carrier transport in these systems.

Thermoelectric materials based on quaternary compounds Ag1−xPbmSbTe2+m exhibit high dimensionless figure‐of‐merit values, ranging from 1.5 to 1.7 at 700 K. The primary factor contributing to the high figure of merit is a low lattice thermal conductivity, achieved through nanostructuring during melt solidification. As a consequence of nucleation and growth of a second phase, coherent nanoscale inclusions form throughout the material, which are believed to result in scattering of acoustic phonons while causing only minimal scattering of charge carriers. Here, characterization of the nanosized inclusions in Ag0.53Pb18Sb1.2Te20 that shows a strong tendency for crystallographic orientation along the {001} planes, with a high degree of lattice strain at the interface, consistent with a coherent interfacial boundary is reported. The inclusions are enriched in Ag relative to the matrix, and seem to adopt a cubic, 96 atom per unit cell Ag2Te phase based on the Ti2Ni type structure. In‐situ high‐temperature synchrotron radiation diffraction studies indicated that the inclusions remain thermally stable to at least 800 K.

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Highly efficient thermoelectric materials have attracted tremendous attention because of various technological applications such as power generation from waste heat and environmentally friendly refrigeration.[1] The efficiency of thermoelectric materials is generally evaluated in terms of thermoelectric figure of merit ZT = (sS 2 /k)T, where s is the electrical conductivity, S is the Seebeck coefficient, k is the thermal conductivity, and T is the absolute temperature. Recently, various nanostructured thermoelectric materials have been reported to exhibit high ZT values. This increase in thermoelectric efficiency was attributed to the decrease of thermal conductivity caused by the increased interfaces to scatter phonons or the enhancement of power factor (sS 2 ) by quantum confinement effects.[2] However, most of the high-ZT nanostructured materials were prepared by costly and complicated processes, making it very difficult to inexpensively synthesize a large quantity of nanostructured materials. More recently, several kinds of nanostructured bulk materials with high ZT values were fabricated in large quantity by a ball-milling process and subsequent hot-press process. [3] Recently, colloidal chemical methods have been used to synthesize large quantities of uniform-sized nanocrystals. [4] These chemical methods can synthesize uniform-sized nanocrystals in a size-controlled manner, allowing the characterization of size-dependent properties, [5] which is very difficult to perform using top-down physical methods, such as the ballmilling process.Over the past few decades, intensive research has attempted to characterize the electrical properties of bulk bismuth (Bi), because it is semimetallic with a small band overlap and has high carrier mobility and extremely small carrier effective mass. Furthermore, thermoelectric properties of Bi nanocrystals were intensively studied, [6] because theoretical calculations predicted that Bi nanocrystals can exhibit a ZT value as high as 10 at 77 K. Moreover, Bi costs around one tenth of the price of bismuth telluride, which is one of the most popular thermoelectric materials.[7] However, a high ZT value has not yet been realized experimentally for Bi nanostructured materials. Although several chemical syntheses of Bi nanocrystals have been reported, [8] the thermoelectric properties of spherical Bi nanocrystals have rarely been studied. Herein, we report a simple and largescale synthetic method to produce uniform-sized Bi nanocrystals with controlled sizes and characterized their sizedependent thermoelectric properties. The size-dependant electrical and thermal properties were clearly demonstrated using uniform Bi nanocrystals with controlled particle sizes. Interestingly, the ratio of electrical to thermal conductivity increased with decreasing particle size, which leads to the enhancement of the ZT values.Bi nanocrystals were synthesized by reducing bismuth dodecanethiolate, which was generated by the reaction of dodecanethiol and bismuth neodecanoate in octadecene. Bismuth...

In thermoelectric energy conversions, thermal conductivity reduction is essential for enhancing thermoelectric performance while maintaining a high power factor. Herein, we propose an approach based on coated-grain structures to effectively reduce the thermal conductivity to a much greater degree when compared to that done by conventional nanodot nanocomposite. By incorporating CdTe coated layers on the surface of SnTe grains, the thermal conductivity is as low as 1.16 W/m-K at 929 K, resulting in a thermoelectric figure of merit, i.e., zT, of 1.90. According to our developed theory, phonons scatter coherently due to the phase lag between phonons passing through and around the coated grain. Such scattering is induced by the acoustic impedance mismatch between the coated layer and the grain, resulting in a gigantic phonon-scattering cross section. The phonon-scattering cross section of the coated grains is several orders of magnitude larger than that of the nanodots with the same impurity concentration. The power factor was also slightly increased by the energy filtering effect at the coated surface and additional minority carrier blocking by the heterointerfaces. This scheme can be utilized for various bulk crystals, meaning a broad range of materials can be considered for thermoelectric applications.

ABSTRACT(R)-4-(3,4-Dihydro-8,8-dimethyl)-2H,8H-benzo [1,2-b:3,4-bЈ]dipyran-3yl)-1,3-benzenediol (glabridin), a flavonoid present in licorice extract, is known to have antimicrobial, antiinflammatory, and cardiovascular protective activities. In the present study, we report the inhibitory effect of glabridin on nitric oxide (NO) production and inducible nitric oxide (iNOS) gene expression in murine macrophages. Glabridin attenuated lipopolysaccharide (LPS)-induced NO production in isolated mouse peritoneal macrophages and RAW 264.7 cells, a mouse macrophage-like cell line. Moreover, iNOS mRNA expression was also blocked by glabridin treatment in LPS-stimulated RAW 264.7 cells. Further study demonstrated that the LPSinduced nuclear factor (NF)-B/Rel DNA binding activity and NF-B/Rel-dependent reporter gene activity were significantly inhibited by glabridin in RAW 264.7 cells and that this effect was mediated through the inhibition of inhibitory factor-B degradation and p65 nuclear translocation. Moreover, reactive oxygen species generation was also suppressed by glabridin treatment in RAW 264.7 cells. In contrast, the activity of mitogen-activated protein kinases was unaffected by glabridin treatment. In animal model, in vivo administration of glabridin increased the rate of survival of LPS-treated mice and inhibited LPS-induced increase in plasma concentrations of nitrite/ nitrate and tumor necrosis factor-␣. Collectively, these data suggest that glabridin inhibits NO production and iNOS gene expression by blocking NF-B/Rel activation and that this effect was mediated, at least in part, by inhibiting reactive oxygen species generation. Furthermore, in vivo anti-inflammatory effect of glabridin suggests a possible therapeutic application of this agent in inflammatory diseases.