Band Structures And Thermoelectric Transport Properties Of Tellurides And Antimonides

The demand for exploiting of new energy and promoting energy efficiency is becoming more imperative due to the upgraded energy crisis and deteriorating environment issue. Thermoelectric (TE) materials, which can convert temperature gradient into electrical gradient and vise versa, are versatile in heat recycling, improving the energy efficiency and substituting Chloro Flouro Carbon compounds in refrigeration. Moreover, theories predicted no obvious upper limit for thermoelectric performance, indicating the promising application of thermoelectric materials, and TE devices are environmental benign, maintenance-free, small and lightweight etc., having wide perspective and practicability.However, the in depth investigations of these good TE candidates for use, which are beneficial and critical for developing new thermoelectric materials, are still far from enough. In this paper, great efforts are made in measuring galvomaganetic and thermomagnetic properties and clarifying the origin of high power factor and low thermal conductivity for peudo-binary compounds of AgSbTe2 andⅣ-Ⅵsemiconductors and VA element doped PbTe1-xSex solid solutions. At the same time, InSb semiconductors are exploited as potential thermoelectric materials, with thermoelectric performances presented and transport parameters calculated.The main conclusions are listed as follows:1. The low temperature galvamagnetic and thermomagnetic properties of (AgSbTe2)15(GeTe)85 (TAGS85) with different Ag/Sb ratio are reported and a maximum ZT reaching～1.5 at about 700 K when the Ag/Sb ratio is equal to 0.6 is presented. From four transport properties (Seebeck coefficient, resistivity, Hall and Nernst coefficients) it is revealed that TAGS85 materials are dominated by acoustic phonon scattering and have a very heavy density of states effective mass, as a result of the high degeneracy of the Fermi surface reached when the Fermi level moves deep enough into valence band. Furthermore, the carrier mean free path of TAGS85 materials is comparable to the lattice parameter, illustrating the materials have reached the minimum possible mobilities compatible with band conduction and near loffe-Regal limit. Through our investigation in TAGS85 materials, we have shown that multiply-degenerate Fermi surface pockets (band convergence) provide a route to substantially increase power factor of thermoelectric materials. Based on the conclusion of minimum possible carrier mobilities in TAGS85 materials, melt spinning is used in material preparation for smaller grain size. The carrier mobilities of melt spun samples and mean free path are comparable with that in slow cooled samples while the lattice thermal conductivities are depressed effectively as a result of the increasing defects and boundaries, which provides experimental validation that materials near Ioffe-Regal limit could be further optimized by rapid solidification.2. Single phased (Ag0.366Sb0.558Te)1-x(SnTe)x (0<x≤1) and (Ag0.366Sb0.558Te)1-x(GeTe)x (0<x≤8%) solid solutions are prepared and their electrical and thermal transport properties are presented. For (Ag0.366Sb0.558Te)1-x(SnTe)x solid solutions, Seebeck coefficient, resistivity and thermal conductivities are measured from 80 K to 630 K and analyzed systematically, with two compositions showing a ZT maximum identified.The unusual composition dependence of hall coefficient and Seebeck coefficient at x～20% are explained in terms of band crossing betweenΔband (degeneracy 6) and L band (degeneracy 12), where ZT reached a maximum value of 0.8. For (Ag0.366Sb0.558Te)1-x(GeTe)x (0<x≤8%) solid solutions, lattice thermal conductivities are calculated by using Lorenz number under consideration of two carrier conduction mechanism. The calculated lattice thermal conductivity increased monotonically with GeTe content, suggesting that unharmonic Umklapp scattering dominates thermal conduction in this material system. By considering intrinsic thermal conductivity conducted only by acoustic phonons and in which there are only interactions among the phonons themselves by anharmonic Umklapp processes, the increasing lattice thermal conductivity after alloying is identified as a result of the increase in Debye temperature and the decrease in Gruneisen parameter.3. InSb samples with different carrier concentrations are prepared and the best carrier concentration of n-InSb is identified as 5×1017cm-3 with its maximum power factor～65 Wcm-1K-1. Seebeck coefficient saturates under magnetic field at 80 K and this is used to calculate the Fermi energy, mobility and effective mass. Efforts are made in searching for resonant impurities and promoting electrical performance by double doping. With the help of water quenching and melt spinning as rapid solidification methods, InSb matrix with NiSb nanostructures between 100-200 nm evenly distributed are prepared and characterized by EPMA and SEM images. The thermoelectric performances of two phase materials are presented. The low temperature ZT is enhanced while the ZT at working temperature range is lowered due to the depressed mobility after rapid solidification, while the thermal conductivities stay comparable to that for single phase InSb materials. On the other hand, the experimental exploration of resonant conduction demonstrate that Te, Y, Sn, Ni are not resonant impurities in InSb materials.4. The VA-element doped Pb9.6M0.2Te10-xSex(M=Sb, Bi, x=6,7,8) alloys have been synthesized and their thermoelectric properties were systematically investigated. VA element doping does not lead to a significant change in band structure, and the maximum ZT for Sb and Bi doped samples are 1.05 and 0.75 respectively. The experimental results indicate VA elements are typical dopants in PbTe1-xSex solid solutions. The calculated lattice conductivities are 0.55Wm-1K-1 and 0.6 Wm-1K-1 respectively by using corrected Lorenz number, both of which are near the theoretical limit. The HRTEM images show nanodots～5-10 nm evenly distributed in the matrix, which may act as scattering center for phonons and account for the reduction of lattice thermal conductivity.