Thermoelectric Performance Optimization And Mechanism Study Of Tellurium-based SnTe And GeTe Materials

Thermoelectric conversion technology can realize the direct conversion between electrical and thermal energy,and is thus considered to be one of the most promising green technologies to solve the energy and environmental problems.However,the relatively low conversion efficiency limits its large-scale development and application.The electric-heat conversion efficiency of a thermoelectric device depends on the intrinsic figure of merit ZT of according materials,which is determined by compromise of the electro-thermal transport performance.However,the electrical and thermal transport parameters of aa specific thermoelectric material are often reversely coupled to each other,and it is difficult to achieve collaborative optimization.to the aim of this work is to at least partly realize decoupling between these entangled parameters in two representative green thermoelectric systems.SnTe and GeTe have received widespread attention in recent years as low-cost,green and environmental friendly medium temperature thermoelectric materials in recent years.However,the intrinsic thermoelectric properties of SnTe and GeTe materials are not excellent.For SnTe,it is mainly due to the following reasons:1.A large number of Sn vacancies cause the high carrier concentration to deteriorate the Seebeck coefficient;2.The energy offset between light and heavy valence band is large,and the heavy valence band does not participate in the conduction,resulting in a low valence band degeneracy and Seebeck coefficient;3.SnTe intrinsic lattice thermal conductivity is high.For GeTe,it is mainly due to:1.The low formation energy of Ge vacancies leads to many Ge vacancies and high carrier concentration;2.The edge of the valence band,the low symmetry and degeneracy limit the Seebeck coefficient further improve.In response to the above problems,the measures taken for SnTe are:adjustment of carrier concentration through heterovalent atom doping,improvement of Seebeck coefficient through energy band engineering,introduction of second phase and alloying to form nano-engineering and defect engineering to reduce lattice thermal conductivity.The measures taken for GeTe are:on the basis of(GeTe)17(Sb2Te3)(GST),the formation of the energy of the Ge vacancy is reduced by the substitution of the same-valent atoms,the content of Sb2Te3 is adjusted to improve the symmetry of GeTe,and the microstructure is controlled by annealing to reduce the crystal grid thermal conductivity.The main research results of this paper are as follows:1.On the basis of resonant levels created by the In-doped SnTe,the influence of InSb on the Sn0.995In0.005Te thermoelectric properties was studied.Studies have shown that InSb as nano-precipitated second phase exists in the SnTe,and the effect of InSb nano-precipitated for electrical properties is very small,but the lattice thermal conductivity is significantly reduced by nano-engineering.When the InSb content is 2%,there is ultra-low lattice thermal conductivity?0.32Wm-1K-1@873 K,which is 27%lower than that of Sn0.995In0.005Te sample,and the ZT is increased 37%;2.The effect of Cd doping SnTe for the SnTe valence band structure,the Cu2Te alloying can reduce the lattice thermal conductivity,and the effect of I doping on the carrier concentration were studied.Reserching have shown that Cd doping can modify the SnTe valence band structure,the light and heavy valence bands converge through energy band engineering,the energy offset between two valence band gap is reduced from 0.35 eV to 0.1 eV,which effectively improves the valence band degeneracy and Seebeck coefficient.When the Cd content is 5%,the Seebeck coefficient can reach ?180 ?VK-1@823 K;On the basis of Cd doping,Cu2Te alloyed can form interstitial Cu atoms and CdTe/Cu2Te eutectoid nanoprecipitation,which can significantly enhance phonon scattering to reduced the lattice thermal conductivity through defect engineering and nano-engineering.When the Cu2Te content is 5%,have ultra-low lattice thermal conductivity,which can be as low as? 0.42 Wm-1K-1@873 K;on this basis,through Iodine heterovalent atom doping further regulates the carrier concentration to optimize the electrical transport performance.Finally,the ZT can reach 1.42 at 823K for(Sn0.95Cd0.05Te)0.93(Cu2Te)0.07-1%I sample and the ZTave can reach 0.65 in the temperature range of 323-823K for(Sn0.95Cd0.05Te)0.93(Cu2Te)0.07-2%sample;3.The effects of Pb replacing Ge,Sb2Te3 content and annealing time on(GeTe)17(Sb2Te3)(GST)thermoelectric performance were studied.The results show that effective PbGe substitution can reduce Ge vacancy and carrier concentration,which significantly increase the Seebeck coefficient;By adjusting the content of Sb2Te3,the lattice symmetry and energy band degeneracy of GeTe are improved to further optimize the electrical properties;On this basis,the van der Waals gap was adjustment by annealing(Ge0.91Pb0.09Te)17.5(Sb2Te3)0.5 sample,that significantly reduces the lattice thermal conductivity through defect engineering.When the annealing time is at 4 days,we got an ultra-low lattice thermal conductivity?0.34 Wm-1K-1@773 K.