Preparation And Thermoelectric Performance Of Liquid-like Argyrodite Compounds

Solid-state thermoelectric（TE）technology can be extensively used in power generation and cooling systems because of inherent heat-carrying electrons or holes in the materials.The TE devices have many advantages,such as noise-free,small size,light weight,no mechanical moving parts,long service life,high reliability,no pollution and so on,hence the TE technology is a good solution to the increasing energy crisis,waste heat recycling and carbon emissions from traditional energy.However,the performance of applied TE materials does not meet the practical requirement,hence more investigations are still needed.Guided by the proposal of“Phonon-Liquid Electron-Crystal”recently,the liquid-like Argyrodite compounds attract much attention in thermoelectrics.These materials are usually composed of two sublattice structures:one is the rigid crystalline sublattice formed by complex anions responsible for the electrical conduction while the rest cations or guest atoms provide the“phonon-glass”structure for suppressing the phonon transport.So these kinds of materials are conducive to raise the TE figure of merit（ZT）and thereby improve the TE performance.In this dissertation,several Argyrodite compounds as TE candidates were explored,such as Ag₈Sn Se₆,Ag₈Ge Se₆ and Cu₈Ge Se₆.The strategies used in this work involves the composition engineering,band structure engineering and entropy engineering,as well as the transport mechanism regulation etc.The detailed research activities and achievements are described as follows:（1）Ag₈Sn Se₆,Ag₈Ge Se₆ and Cu₈Ge Se₆ compounds were successfully prepared and their crystal structures and phase transitions were fully analyzed.（2）Improvement of the TE performance of Ag₈Sn Se₆ via the composition engineering.Upon doping Cu in Ag₈Sn Se₆,the carrier concentration of Ag₈Sn Se₆ is optimized,and at the same time the point defects and stronger Cu-Se bond than Ag-Se bond are introduced,thus enhancing the phonon scattering and weakening the“liquid-like behavior”of Ag⁺.As a result,the lattice thermal conductivity reduces from 0.21Wm^-1K^-1 to 0.12 Wm^-1K^-1,and the highest ZT value reaches 0.85 at 645 K.（3）Improvement of the TE performance of Ag₈Sn Se₆ via the entropy engineering.Through dissolving the single element Ga into Ag₈Sn Se₆,the configuration entropy（ΔS）and band structures of Ag₈Sn_1-xGa_xSe₆（x=0～0.6）are engineered,which improves the electronic transport property,and at same time eliminates the phase transition.Moreover,further analysis reveals that the solubility of Ga increases in Ag₈Sn Se₆ as the temperature increases,demonstrating an increased configuration entropy（ΔS）.This strengthens the lattice disorder and anhormonicity,reducing the lattice thermal conductivity at high temperatures.In detail,the second phase Ag₉Ga Se₆ disappears above 620 K,and the element Ga is completely dissolved into the matrix of Ag₈Sn Se₆to form the solid solution Ag₈Sn_1-xGa_xSe₆.The disssolving of Ga creates another tetrahedral unit[Ga Se₄]^5-in the crystal structure,allowing the presence of mixed tetrahedral units([Sn Se₄]^4-and[Ga Se₄]^5-)with highly random distribution,reducing the lattice thermal conductivity.As a result,the ZT value of Ag₈Sn_0.5Ga_0.5Se₆ sample reaches 1.15 at 723 K.（4）Improvement of the TE performance of the compound Ag₈Ge Se₆ via band structure engineering.Through in-depth study of Ag₈Sn Se₆ compound,the strategy to dissolve Sn into Ag₈Ge Se₆ was proposed,aiming at introducing a more conductive Sn-Se bond than Ge-Se bond,and at the same time creating[Sn Se₄]^4-tetrahedral unit,in addition to[Ge Se₄]^4-.Such a structural alteration would engineer the band structure,which enhances the carrier concentration by more than one order of magnitude at 373K.In the mean time,the lattice thermal conductivity（κ_L）reduces from 0.26 W/m K to0.16 W/m K at 750 K.As a consequence,the ZT value of Ag₈Ge_0.9Sn_0.1Se₆ is boosted to1.05.（5）Improved thermoelectric performance of P-type Argyrodite Cu₈Ge Se₆ via the simultaneous engineering of the electronic and phonon transports.In this work,the strategy to engineer both the electronic and phonon transports in Cu₈Ge Se₆ by incorporating a species In₂Te₃ was proposed to tune the carrier concentration and at the same time increase the phonon scattering on the point defects(In _Ge,interstitial In atom,Te _Se)and randomly distributed tetrahedras([In Se₄]^5-,[Ge Te Se₃]^4-).As a result,the phase transformation at 329 K in Cu₈Ge Se₆ is eliminated.The lattice part（κ_L）reduces from 0.52 Wm^-1K^-1（Cu₈Ge Se₆）to 0.28 Wm^-1K^-1(（Cu₈Ge Se₆）_0.9（In₂Te₃）_0.1),reducing by about 46%.In the meantime,the power factor（PF）increases from 5.08μW/cm·K²to 7.19μW/cm·K²,increasing by about 40%.As a consequence,the peak ZT value is enhanced from 0.27 for Cu₈Ge Se₆ to～0.92 for（Cu₈Sn Se₆）_0.9（In₂Te₃）_0.1 at 774 K,enhancing by about 2.4 times.It thus proves that the simultaneous engineering of the electronic and phonon transports is an effective way in regulating the TE performance of P-type Cu₈Ge Se₆.This paper has 70 figures,12 tables and 249 references.