Robert Pcionek

Resonant states in the electronic structure of the high performance thermoelectrics AgPbmSbTe2+m: The role of Ag-Sb microstructures

Ab initio electronic structure calculations based on gradient corrected density-functional theory were performed on a class of novel quaternary compounds AgPb(m)SbTe(2+m), which were found to be excellent high temperature thermoelctrics with a large figure of merit ZT approximately 2.2 at 800 K. We find that resonant states appear near the top of the valence and bottom of the conduction bands of bulk PbTe when Ag and Sb replace Pb. These states can be understood in terms of modified Te-Ag(Sb) bonds. The electronic structure near the gap depends sensitively on the microstructural arrangements of Ag-Sb atoms, suggesting that large ZT values may originate from the nature of these ordering arrangements.

Nanostructuring and its Influence on the Thermoelectric Properties of the AgSbTe 2 -SnTe Quaternary System

The structural and thermoelectric properties of the AgSbTe2-SnTe quaternary system were studied. Powder averaged x-ray diffraction of Ag0.85SnSb1.15Te3 indicates a cubic NaCltype structure in contrast with the single crystal refinements, which point towards tetragonal symmetry. Furthermore, high-resolution electron microscopy imaging revealed the system to be a nano-composite formed by thermodynamically driven compositional fluctuations rather than a solid solution as it was viewed in the past. The lattice thermal conductivity attains very low values, which is in accord with recent theories on thermal transport in heterogeneous systems. The charge transport properties of the system exhibit a rich physical behavior highlighted in the coexistence of an almost metallic carrier concentration (~5×10 21 cm -3 ) with a large thermoelectric power response of ~160 μV/K at 650 K. This is attributed to a heavy hole effective mass that is almost six times that of the electron rest mass.

Phase Segregation and Thermoelectric Properties of AgPb m SbTe m +2 m =2,4,6, and 8

The preparation and characterization of the AgPbmSbTem+2 family of compounds with m=2, 4, 6, and 8 is reported. Phase segregation was observed in all of these materials. The lattice thermal conductivity of these samples is low (<1.1 W/m-K). Powder x-ray diffraction, thermal analysis, and electron microscope investigations of these systems show that ideal solid solutions are not formed. The transport properties of these composite materials are presented and suggest that they could have promising thermoelectric properties when optimized.

Thermoelectric properties of the nanostructured NaPb18-xSn xMTe20 (M=Sb, Bi) materials

The thermoelectric properties of materials with compositions NaPb 18-x Sn x MTe 20 (M=Sb, Bi, x=0, 3, 5, 9, 13, 16 and 18) were investigated in the temperature range 300-670K. All compositions exhibited p-type behavior over the measured temperature range. Electronic properties and transport were tuned through the manipulation of the Pb/Sn ratio. Increasing the Sn fraction results in an increase in electrical conductivity and a decrease in thermopower. The compositions NaPb 13 Sn 5 SbTe 20 and NaPb 9 Sn 9 SbTe 20 show a lattice thermal conductivity of ∼1 W/m/K at room temperature.

Effects of Antimony on the Thermoelectric Properties of the Cubic Pb 9.6 Sb y Te 10−x Se x Materials

The thermoelectric properties of Pb9.6SbyTe10-xSex were investigated in the intermediate temperature range of 300 – 700 K. The effect of the variation of Sb content (y) on the electronic properties of the materials is remarkable. Samples with compositions Pb9.6Sb0.2Te10xSex (y = 0.2) show the best combination of low thermal conductivity with moderate electrical conductivity and thermopower. For Pb9.6Sb0.2Te8Se2 (x = 2) a maximum figure of merit of ZT~ 1.1 was obtained around 700 K. This value is nearly 1.4 times higher than that of PbTe at 700 K. This enhancement of the figure of merit of Pb9.6Sb0.2Te8Se2 derives from its extremely low thermal conductivity (~0.7 at W/m.K at 700 K). High resolution transmission electron microscopy of Pb9.6Sb0.2Te10-xSex samples shows broadly distributed Sb-rich nanocrystals, which may be the key feature responsible for the suppression of the thermal conductivity.