The conversion of heat to electricity by thermoelectric devices may play a key role in the future for energy production and utilization. However, in order to meet that role, more efficient thermoelectric materials are needed that are suitable for high-temperature applications. We show that the material system AgPb(m)SbTe(2+m) may be suitable for this purpose. With m = 10 and 18 and doped appropriately, n-type semiconductors can be produced that exhibit a high thermoelectric figure of merit material ZTmax of approximately 2.2 at 800 kelvin. In the temperature range 600 to 900 kelvin, the AgPb(m)SbTe(2+m) material is expected to outperform all reported bulk thermoelectrics, thereby earmarking it as a material system for potential use in efficient thermoelectric power generation from heat sources.

Energy conversion Energy conversion V 1650Cubic AgPb m SbTe 2+m : Bulk Thermoelectric Materials with High Figure of Merit.-The title materials with m = 10 and 18 are n-type semiconductors exhibiting a high thermoelectric figure of merit of about 2.2 at 800 K. They are expected to outperform all reported bulk thermoelectrics in the temperature range 600-900 K and to find applications in thermoelectric power generation from heat sources, such as vehicle exhaust, coal-burning installations, or electric power utilities. With a hot source of 900 K and a temperature difference of about 500 K, a conversion efficiency of more than 18% may be achieved. -(HSU, K. F.; LOO, S.; GUO, F.; CHEN, W.; DYCK, J. S.; UHER, C.; HOGAN, T.; POLYCHRONIADIS, E. K.; KANATZIDIS*, M. G.;

Thermoelectric (Peltier) heat pumps are capable of refrigerating solid or fluid objects, and unlike conventional vapor compressor systems, they can be miniaturized without loss of efficiency. More efficient thermoelectric materials need to be identified, especially for low-temperature applications in electronics and devices. The material CsBi(4)Te(6) has been synthesized and its properties have been studied. When doped appropriately, it exhibits a high thermoelectric figure of merit below room temperature (ZT(max) approximately 0.8 at 225 kelvin). At cryogenic temperatures, the thermoelectric properties of CsBi(4)Te(6) appear to match or exceed those of Bi(2-x)Sb(x)Te(3-y)Se(y) alloys.

Most materials expand upon heating. Although rare, some materials expand on cooling, and are said to exhibit negative thermal expansion (NTE); but the property is exhibited in only one crystallographic direction. Such materials include silicon and germanium at very low temperature (<100 K) and, at room temperature, glasses in the titania-silica family, Kevlar, carbon fibres, anisotropic Invar Fe-Ni alloys, ZrW2O3 (ref. 4) and certain molecular networks. NTE materials can be combined with materials demonstrating a positive thermal expansion coefficient to fabricate composites exhibiting an overall zero thermal expansion (ZTE). ZTE materials are useful because they do not undergo thermal shock on rapid heating or cooling. The need for such composites could be avoided if ZTE materials were available in a pure form. Here we show that an electrically conductive intermetallic compound, YbGaGe, can exhibit nearly ZTE--that is, negligible volume change between 100 and 400 K. We suggest that this response is due to a temperature-induced valence transition in the Yb atoms. ZTE materials are desirable to prevent or reduce resulting strain or internal stresses in systems subject to large temperature fluctuations, such as in space applications and thermomechanical actuators.

Thermoelectric (TE) power generation has come to be appreciated as an attractive means of low-cost conversion of waste heat to useful electrical energy with a small environmental impact. For a compound to qualify as an efficient thermoelectric material it should exhibit the highest TE figure of merit, ZT, possible at the temperature of operation, T. ZT is defined asand it involves the simultaneous manipulation of the TE power (absolute Seebeck coefficient) S, the electrical conductivity r, and the thermal conductivity j. The search for efficient TE materials mainly focuses on degenerate semiconductors since the underlying physics of these systems allow the coexistence of high thermopower values with high electrical conductivity to achieve high power factors: PF = S 2 r. The Seebeck coefficient is inversely related to the electrical conductivity according to the Boltzmann transport equation, and, as a result, maximization of one cannot be achieved without minimization of the other. An interesting alternative that has been recently suggested to achieve high power factors is the quantum-confinement effect; however, definite experimental verification of this is still lacking.[1]Another route to achieving high-performance TEs is through the minimization of the thermal conductivity. To this end, many suggestions have been made to increase ZT. These include the phonon-glass electron-crystal approach [2] (where loosely bound atoms rattle in cage structures [3] ) as in clathrates, [4] and the thin-film multilayer approach where the introduction of interfaces significantly reduces phonon propagation.[ [10] where compositional fluctuations at the nanoscopic level, resulting in a distinct type of nanostructuring, seem to play a key role in the previously reported very low thermal conductivity. [11] In contrast to the thin-film multilayers, bulk nanocomposite systems offer the advantages of large-scale industrial production and the sustenance of large thermal gradients for extended time. The challenge, therefore, lies in identifying equally efficient p-type materials so that they can be employed in the fabrication of TE modules.Here we report on the Ag(Pb 1 -y Sn y ) m SbTe 2 + m series and show that certain compositions exhibit high performance p-type TE properties (e.g., ZT ∼ 1.45 at 630 K) as a result of their very low thermal conductivity. We show as well that the Ag(Pb 1 -y Sn y ) m SbTe 2 + m systems are in fact bulk nanocomposites. We demonstrate that varying the m and y values, as well as the Ag and Sb concentrations, allows for control over a wide range of properties such as carrier concentration, TE power, and thermal conductivity. These exceptional properties, derived from specific compositions, outperform the standard state-of-the-art p-type systems like TAGS ((AgSbTe 2 ) 0.15 (GeTe) 0.85 , ZT ∼ 1.2 at 720 K [12] ), PbTe (ZT ∼ 0.7 at 740 K [13] ), and Zn 4 Sb 3 (ZT ∼ 1.3 at 670 K [14] ).The electronic-transport properties of the Ag(Pb 1 -y Sn y ) mSbTe 2 + m system can be tuned primarily through carefully control...

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The highly anisotropic material CsBi(4)Te(6) was prepared by the reaction of Cs/Bi(2)Te(3) around 600 degrees C. The compound crystallizes in the monoclinic space group C2/m with a = 51.9205(8) A, b = 4.4025(1) A, c = 14.5118(3) A, beta = 101.480(1) degrees, V = 3250.75(11) A(3), and Z = 8. The final R values are R(1) = 0.0585 and wR(2) = 0.1127 for all data. The compound has a 2-D structure composed of NaCl-type [Bi(4)Te(6)] anionic layers and Cs(+) ions residing between the layers. The [Bi(4)Te(6)] layers are interconnected by Bi-Bi bonds at a distance of 3.2383(10) A. This material is a narrow gap semiconductor. Optimization studies on the thermoelectric properties with a variety of doping agents show that the electrical properties of CsBi(4)Te(6) can be tuned to yield an optimized thermoelectric material which is promising for low-temperature applications. SbI(3) doping resulted in p-type behavior and a maximum power factor of 51.5 microW/cm.K(2) at 184 K and the corresponding ZT of 0.82 at 225 K. The highest power factor of 59.8 microW/cm.K(2) at 151 K was obtained from 0.06% Sb-doped material. We report here the synthesis, physicochemical properties, doping characteristics, charge-transport properties, and thermal conductivity. Also presented are studies on n-type CsBi(4)Te(6) and comparisons to those of p-type.

A New Thermoelectric Material: CsBi 4 Te 6 . -The title compound is obtained in quantitative yield by reaction of Cs 2 Te and Bi 2 Te 3 at 700°C (60 h). CsBi 4 Te 6 crystallizes in the monoclinic space group C2/m with Z = 8 (single crystal XRD). The structure consists of NaCl-type [Bi4 ] layers and Cs + ions residing between the layers. The anionic layers are interconnected by Bi-Bi bonds with a distance of 3.2383 Å. CsBi 4 Te 6 is a narrow gap semiconductor. The electrical properties can be tuned by doping to yield an optimized thermoelectric material which is promising for low-temperature applications. SbI3 doping results in p-type behavior and a maximum power factor of 51.5 µW/cm·K 2 at 184 K and the corresponding thermoelectric figure of merit ZT of 0.82 at 225 K. The highest power factor of 59.8 µW/cm·K 2 at 151 K is observed for the 0.06% Sb-doped material. -(CHUNG, D.-Y.; HOGAN, T. P.; ROCCI-LANE, M.; BRAZIS, P.; IRELAND, J. R.; KANNEWURF, C. R.; BASTEA, M.; UHER, C.; KANATZIDIS*, M. G.; J. Am. Chem. Soc. 126 (2004) 20, 6414-6428; Dep. Chem., Mich. State Univ., East Lansing, MI 48824, USA; Eng.) -Schramke 33-018 Te -6