Thermoelectric Transport Of Sodium-chloride Type Complex ?-?-?₂ Solid Solutions

With the global energy crisis and negative influence of fossil fuels on environment,a compelling demand occurs for thermoelectric materials,which can directly convert heat into electricity.In the past few years,thermoelectric conversion technology has got rapid progress as a result of the newly proposed strategies and development of the transport theory.Specifically,IV-VI family compounds with sodium chloride structure have been intensively studied owing to the high symmetry of crystal structure,which leads to good electrical transport performance;as an derivative to IV-VI family compounds,I-V-VI₂family semiconductors has also been intensively studied.Nevertheless,most I-V-VI₂semiconductors reveal p-type conduction,which cannot readily be tuned into the n-type via doping while the n-type counterparts with high thermoelectric performance and similar crystal structure in I-V-VI₂compounds are missing and some of them suffer from structural phase transitions.Such issues largely impede the practical application of these compounds as TE modules.This work mainly focused on the thermoelectric transport properties of solid solutions formed between I-V-VI₂semiconductor AgBiSe₂and IV-VI semiconductor Pb Se and Pb Te,which are synthesized via traditional solid state method.Our study was carried out according to the present research status and potential problems,aiming to boost thermoelectric performance and elucidate the underlying physical mechanism,which may provide solid foundation for practical application of relevant compounds.The contents and results of this study are as follows:1.The influence of the variation of configurational entropy to the crystal structure and themoelectric performance of AgBiSe₂has been studied.Solid solutions were formed by alloying AgBiSe₂with Pb Se,and the high-temperature cubic phase was stabilized to ambient conditions.Based on the model analysis,the optimal increment of entropy was determined,which simultaneously ensure the phase stability and high thermoelectric performance.After carrier concentration optimization,a maximum power factor of 0.44 m W m^-1K^-2was attained for(AgBiSe₂)_0.7(Pb Se)_0.3.Meanwhile,the cubic AgBiSe₂reveals an ultralow lattice thermal conductivity,which originates from the high Grüneisen parameter?(>2.0)ascribing to the high-symmetry average structure but distorted local structure.Finally,a decent average z T value of 0.42 was achieved for cubic AgBiSe₂.These results pave the way for design new TE materials via entropy engineering.2.The effect of dual site solid solution on thermoelectric performance was investigated for AgBiSe₂based compounds.Pb Te was selected to alloy to the pristine AgBiSe₂.Substitution of Pb for the cation sites ensures the phase stability and Te alloying for the anion sites further optimize the electronic structure,which leads to the enhanced intrinsic weighted mobility.A maximum power factor of 0.69 m W m^-1K^-2and peak z T value of 0.92 was achieved.This work opens a new avenue for further improving the thermoelectric performance of AgBiSe₂based compounds by multiple site substitution,providing stepping stone for searching for more complex solid solutions.3.The origin of ultra-low lattice thermal conductivity of Pb Te based solid solution with glass-like behavior was investigated by utilizing the sound speed and low-temperature heat capacity measurement,combining with the theoretical calculation and transmission electron microscope.The forming solid solution with nominal composition of(Pb Te)_0.95(Ag Bi Te₂)_0.05exhibits a glass-like lattice thermal conductivity,which stems from the low-lying optical phonons and low Debye temperature.Further reduction in lattice thermal conductivity of(Pb Te)_0.85(Ag Bi Te₂)_0.15can be ascribed to the nano-precipitates.These results shed light on the origin of glass-like ultra-low lattice thermal conductivity of some specific compounds.4.As a supplement to previously performed study,the thermoelectric transport properties of a novel Te based III-VI family compound of In Te was investigated.It is found that,the high-temperature carrier moblity suffers from the exponentially decline with respect to temperature due to inter-valley scattering.By introducing minute amount of antimony(1-2 mol%),which successfully converts the predominant scattering mechanism from inter-valley scattering to acoustic phonon scattering at high temperature.Finally,with the noticeable improvement in both carrier mobility and power factor,a maximum and average z T of 0.8 and 0.51 is achieved in In Te-Sb_0.01sample.This study provides some useful guidance for suppressing inter-valley scattering in certain semiconductors.