Study On Thermoelectric Transport Properties Of Tin-based Layered Compounds

With the increasing global demand for green energy,people are committed to search for advanced energy materials that enable harvesting electric energy directly from waste heat.Thermoelectric conversion technology is a green energy conversion technology that directly converts waste heat into electrical energy,which exhibits broad application prospects in waste heat recovery.In recent years,layered thermoelectric materials,by virtue of a unique electronic structure and strong lattice anharmonicity,have raised a worldwide range of research interests among scientistic community.In this work,we take layered Sn-based thermoelectric materials with intrinsically low lattice thermal conductivity as the research objects.Based on the promoted quality factor,the thermoelectric performance of these compounds is greatly improved by optimizing the carrier concentration.Meanwhile,with the integration of the theoretical calculations and experimental results,the physical mechanism behind the electrical and thermal transport of these compounds is studied systematically.The details are briefly summarized as follows:(1)Firstly,the electronic structure of SnS was obtained by density functional theory(DFT)calculation.The results show SnS is an indirect band gap semiconductor with band gap of 0.89 e V and the valence band edge has multi-valley characteristics.In addition,the calculated valence band edge morphology is in good agreement with the angular resolved photoelectron spectroscopy(ARPES)results.Based on the analysis of the calculated Fermi surface and effective mass of each energy valley near band edges,the effective mass of valence band edges shows strong anisotropy,while that of conduction band edges has no obvious anisotropy.Meanwhile,for p-type SnS-based materials,the effective mass along the a-axis is larger than that of the other directions(b-axis and c-axis),which indicates the carrier mobility along the a-axis is lower.In contrast,for n-type SnS-based materials,the effective mass along the a-axis is small so that the high mobility can be obtained.The difference between valence band edges and conduction band edges results from the overlaps of charge density near the van der Waals layers along the a-axis.Through the analysis of lattice dynamics and charge density,it is found that the intrinsically low lattice thermal conductivity of SnS is due to the strong lattice anharmonicity caused by the Sn?5s²lone pair electrons near the van der Waals layers.By analyzing the phonon transport,it is found that all thermally activated optical phonons above room temperature contribute to 67%of the lattice thermal conductivity,as a result of some optical phonons having large phonon group velocity and phonon lifetimes.In addition,it is found that three-phonon scattering processes dominate the thermal transport for SnS in the measured temperature range,as suggested by the temperature dependence of Raman mode frequency and linewidth,which is further corroborated by the good consistency between experimental and calculated thermal conductivity.Moreover,a peak z T of the SnS single crystal without optimization along the crystal b-axis at 870 K reaches 0.4.(2)Based on the calculation results,an in-depth experimental study of the SnS-based materials was carried out.Large-size and high-quality Sn_1-xNa_xS(x=0,0.001,0.01,0.02,0.03,0.04)single crystals were successfully grown via a modified Bridgman method.Similar to other single crystal,the thermoelectric transport properties of all Sn_1-xNa_xS samples along each axis disclose strong anisotropy.In addition,compared with the SnS-based polycrystalline samples,the single crystal samples have a larger carrier mobility due to the lack of the grain boundary for electron scattering,and then obtain improved material quality factor.Furthermore,the high power factor(2.0m W m^-1 K^-2)was achieved at room temperature,which benefits from the optimized hole concentration by sodium(Na)doping on the tin(Sn)sites.Based on the single parabolic(SPB)model and first-principles calculation,it is found that Na doping not only optimizes the carrier concentration,but also moves the Fermi level to the position that allows for multi-valley carrier transport,which is pivotal for improving the electrical properties of SnS-based samples.As a result,the highest z T value of the 2 at.%Na-doped SnS single crystal reaches 1.1 at 870 K along the b-axis direction.Moreover,through the analysis of the electrical transport model,it is found that the maximum z T value of 1.4 can be achieved,when the Seebeck coefficient reaches 250?V K^-1 by further optimizing hole concentration.(3)To further improve the thermoelectric potential of the SnS-based samples,we synthesized a series samples of Na_ySn_1-yS_1-xSe_x(0?y?0.03,0?x?0.5)using the traditional solid-phase method,and systematically studied their thermoelectric properties.By analyzing the calculated and experimental results,the inherent characteristic of the markedly reduced lattice thermal conductivity with increasing Se alloying contents is ascribed to low sound velocity caused by weakened chemical bonds and increased masses,as well as the enlarged Grüneisen parameter.Moreover,in contrast to SnS,the more localized Sn?5s² lone pair electrons in SnS_0.5Se_0.5are beneficial to reinforce the anisotropy of Fermi surface and lattice anharmonicity,resulting in a higher quality factor.Finally,the carrier concentration was optimized by Na doping,a peak z T value of Na_0.02Sn_0.98S_0.5Se_0.5sample at 820 K reaches 1.1.(4)In order to extend the research of thermoelectric performance of Sn-based layered system to the ternary systems,we successfully synthesized a series of SnSb₂(Te_1-xSe_x)₄(0?x?0.25)compounds and investigated systematically their thermoelectric properties in the temperature range of 320 to 720 K.Combining lattice dynamics and local chemical bonding analysis,it was found that the inherent mechanism of the intrinsically low lattice thermal conductivity of the pristine SnSb₂Te₄can be attributed to the relatively weak chemical bonding as well as the asymmetric local crystal structure.In addition,with increasing amount of Se content,the Hall carrier concentration is found to decrease significantly from 9.5×10²⁰ cm^-3(x=0)to4.5×10²⁰ cm^-3(x=0.25)at 320 K,which results from the increased formation energy of intrinsic defects after alloying Se on Te sites.Meanwhile,multiple energy valleys convergence is achieved by Se substitution,which is beneficial for improving the density of states(DOS)effective mass.Based on the synergistic effect of increased effective mass and optimized carrier concentration,a significant enhancement in the Seebeck coefficient for the Se-doped samples is obtained.Finally,a peak z T of 0.5 and an average z T of 0.3 in the range of 320 to 720 K are obtained in SnSb₂(Te_0.75Se_0.25)₄samples,with corresponding z T values enhanced by 100%and 200%,respectively,compared to the pristine compound.