Study On The Optimization And Mechanism Of Thermoelectric Properties For Liquid-like Ag₈SnSe₆-based Thermoelectric Materials

Thermoelectric materials can directly convert the thermal energy into electricity and vice versa.They have several advantages,such as no noise,easily miniaturization,no emission,and so on.They have the great potential to be widely applied in the fields of power generation and refrigeration.The key to promoting their large-scale application mainly lies on the improvement of thermoelectric performance.This study focuses on the improvement of thermoelectric performance as well as the transport mechanism of argyrodite compound Ag₈SnSe₆.The thermoelectric performance of Ag₈SnSe₆is optimized by regulating its self-defects and crystallinity,foreign atomic doping and introducing micro-nano structure as follows.?1?Ag₈SnSe₆nanocrystals were successfully synthesized by two different wet-chemistry methods?condensation refluxing method and hydrothermal reaction?.After the further heat-treating in a tubular furnace,the oxygen-containing groups and possible oxides on the nanocrystals were successfully removed.Bulk Ag₈SnSe₆polycrystalline was successfully prepared by the hot-pressing sintering process.According to the results of X-ray diffraction?XRD?and thermoelectric measurement,the samples obtained by the hydrothermal reaction have relatively higher thermoelectric performance due to their higher purity.?2?The thermoelectric performance of Ag₈SnSe₆is improved by introducing excessive Sn to regulate the self-defects.The measurement of Hall effect at room temperature suggests that the introduction of excessive Sn simultaneously increases the carrier concentration and mobility.Thus,the power factor is increased from 530?W m^-1K^-2?Ag₈SnSe₆?to 758?W m^-1K^-2(Ag₈Sn_1.005Se₆)at 715 K.In addition,the local structural distortion caused by the introduction of self-defects effectively hindered the phonon transport,especially at low temperature,leading to the significant reduction of lattice thermal conductivity.Finally,a maximum ZT value of1.22 for the sample Ag₈Sn_1.005Se₆is achieved at 715 K,which is 34%higher than that of the sample Ag₈SnSe₆.?3?The samples Ag₈SnSe_6-xM_x?M=Cl or Br?were prepared,and the influence of foreign atomic doping on the thermoelectric properties was further investigated.Because the ionic radius of Br^-and Se^2-is much close,the carrier concentration of Br-doped samples is increased significantly.Besides,significant grain boundary and dislocation can be observed in the Br-doped samples,which can act as scattering centers to strengthen phonon scattering.As a result,the lattice thermal conductivity is significantly reduced.The ZT value of 4%Ag Br-doped samples is 1.52 at 736 K,which is 39%higher than that of the undoped sample.It can be compared with many typical thermoelectric materials.Moreover,the 3%Sn Br₂-doped sample also owns a high ZT_avg?0.89?.This study shows that self-defects regulation and foreign atomic doping play a very important role in the optimization of thermoelectric performance of Ag₈SnSe₆-based thermoelectric materials,and may provide a guidance for improving the thermoelectric performance of other argyrodite compounds.