Study On The Thermoelectric Properties Of ?-?-?₂ Family Compounds

As a new kind of environmental-friendly energy material,thermoelectric conversion materials can realize the mutual conversion of and electricity.Its research and development are of great significance to alleviating the increasingly serious environmental pollution and energy crisis.Ternary chalcogenides ?-?-?₂compounds have attracted extensive attention for thermoelectric applications due to their intrinsically low lattice thermal conductivity.This type of material usually has a relatively flat and multi-valley valence band structure.Thus,it also shows the potential for good electrical properties.In addition,?-?-?₂materials,as derivatives of traditional IV-VI materials,are often been formed solid solutions with IV-VI compounds,such as GeTe-Ag Sb Te₂(TAGS),Ag Pb_mSb Te_2+m(LAST)material systems,which show extremely excellent thermoelectric properties in the medium temperature range.This paper mainly focuses on ?-?-?₂group materials as the research object.First,the typical Ag Sb Te₂materials are selected,which have been frequently reported to show excellent thermoelectric properties.However,the thermodynamic stability of Ag Sb Te₂has always been a controversial issue.Studies have shown that the nominal Ag Sb Te₂is unstable at low temperatures and is partially decomposed into second phases such as Ag₂Te and Sb₂Te₃during thermal cycling.Therefore,the preparation of stable and single-phase materials is the key to the practical application of Ag Sb Te₂.Ag_0.366Sb_0.558Te is the composition for stabilizing the single-phase according to the Ag₂Te-Sb₂Te₃phase diagram,while the thermoelectric transport properties have rarely been reported.To evaluate the possibility of replacing Ag Sb Te₂by Ag_0.366Sb_0.558Te,this paper systematically studied the thermoelectric properties of non-stoichiometric Ag_0.366Sb_0.558Te.Secondly,AgBiSe₂,as one of a few n-type semiconductors among these compounds,shows the potential to be a promising thermoelectric material.According to the Ag₂Se-Bi₂Se₃phase diagram,single-phase AgBiSe₂has a wide range of composition in(Ag₂Se)_1-x(Bi₂Se₃)_x.This paper systematically studied the thermoelectric properties of(Ag₂Se)_1-x(Bi₂Se₃)_xby adjusting the content of x.Finally,combining the relationship between the crystal structure of AgBiSe₂and GeTe,the effect of the crystal structure manipulation of(Ag_0.5Bi_0.5Se)x(GeTe)_1-xsolid solution on the thermoelectric properties of AgBiSe₂and GeTe was studied.The main content and results of this paper are as follows:(1)The non-stoichiometric Ag_0.366Sb_0.558Te prepared by the method of"melting,quenching,and long-term annealing"is a single-phase cubic crystal structure material with good thermal stability.Sn/Sb substitution is found to effectively increase the carrier concentration from?5×10¹⁹cm^-3to?4×10²¹cm^-3.A single parabolic band model with acoustic phonon scattering enables a good understanding of the charge transport.Sn doping improves the density-of-states effective mass.The increased carrier concentration effectively suppresses the bipolar effect at high temperatures.The obtained Ag_0.366(Sb_1-xSn_x)_0.558Te intrinsically has lower lattice thermal conductivity,and the electrical properties are improved by Sn doping.The peak z T is about 1.3 at 650 K,and the average is about 0.9 in the range of 300-700 K,indicating that Ag_0.366(Sb_1-xSn_x)_0.558Te is potential thermoelectric material.(2)AgBiSe₂materials with a broad composition range were prepared according to(Ag₂Se)_1-x(Bi₂Se₃)_x,and the composition of(Ag₂Se)_1-x(Bi₂Se₃)_xwas determined to be a single phase within the range of x from 0.5 to 0.56.The carrier concentration increases from 1.0×10¹⁹cm^-3to 5.7×10¹⁹cm^-3through the composition manipulation in the single-phase range,which enables a comprehensive assessment of electronic transport properties based on a single parabolic band model with acoustic scattering.The experimental results are in good agreement with the model predictions,and the highest carrier concentration obtained is close to the theoretical optimal one.AgBiSe₂has intrinsically low lattice thermal conductivity,which can be attributed to the strong anharmonicity and low sound velocity.with the increase of x,the lattice thermal conductivity is decreased,the lowest value obtained at high temperatures is?0.35Wm^-1K^-1.(Ag₂Se)_1-x(Bi₂Se₃)_xachieves the highest z T value of about 0.5 at 700 K.This work offers a reference and direction for the study of thermoelectric performance optimization of AgBiSe₂.(3)Through the solid solution of AgBiSe₂and GeTe,the continuous transformation of crystal structure symmetry from low to high is achieved on both the AgBiSe₂matrix and the GeTe matrix.For Ag_0.5Bi_0.5Se-rich(Ag_0.5Bi_0.5Se)_x(GeTe)_1-x,the samples with 18%or higher GeTe content exhibited a cubic crystal structure at room temperature.Compared with the intrinsic AgBiSe₂,the carrier concentration and density of state effective mass of the GeTe alloyed samples at room temperature are improved.At high temperature,the Seebeck coefficient and resistivity of the samples gradually decrease with the increase of temperature,thereby reducing the power factor.The lattice thermal conductivity decreases with the increasing content of GeTe.The maximum z T obtained for Ag_0.5Bi_0.5Se-rich(Ag_0.5Bi_0.5Se)_x(GeTe)_1-xat about 600 K is about 0.3.In the GeTe-rich(Ag_0.5Bi_0.5Se)_x(GeTe)_1-xsamples,the influence of the solid solution content of AgBiSe₂on the crystal structure,and the thermoelectric transport performance of rhombohedral GeTe was studied.we show the effect of AgBiSe₂-alloying on the crystal structure as well as thermoelectric transport properties of rhombohedral GeTe.The results show that AgBiSe₂-alloying is found to not only finely manipulate the crystal structure for band convergence and thereby an increased band degeneracy,but also flatten the valence band for an increased band effective mass.Both of them result in an increased density of state effective mass and therefore an enhanced Seebeck coefficient along with a decreased mobility.Moreover,a remarkably reduced lattice thermal conductivity of?0.4 Wm^-1K^-1is obtained due to the introduced additional point defect phonon scattering and bond softening by the alloying.With the help of Bi-doping at Ge site for further optimizing the carrier concentration,thermoelectric figure of merit,z T,of?1.7 at 625 K and average z T of?0.9 among 300?640 K are achieved in 5%AgBiSe₂-alloyed rhombohedral GeTe,which demonstrates this material as a promising candidate for low-temperature thermoelectric applications.