Study On The Thermoelectric Properties Of Flexible Silver Chalcogenide At Room Temperature And Medium Temperature Range

Thermoelectric?TE?technology can realize the direct conversion between thermal energy and electricity thus is widely used in numerous fields.Flexible TE devices can be used as the power source of wearable devices or the thermal-electric conversion systems in some“twisted”space.Meanwhile the main barrier of the development for flexible TE devices is the acquisition of high performance TE materials with ductility.So far,the research on flexible TE materials has been focused on the organic conducting polymers,organic-inorganic TE composite materials and inorganic ultrathin films.However,these materials cannot completely satisfy the demands of practical applications,and it becomes an urgent task to develop ductile inorganic TE materials.In this dissertation,based on the newly discovered ductile semiconductor material-silver sulfide?Ag₂S?,two series of inorganic ductile TE materials-Ag₂S_1-xSe_x and Ag₂S_1-xTe_x are synthesized.The effect of Se and Te substitution to sulfur on the structure and carrier behavior of Ag₂S has been systematically investigated.The electrical and thermal transport mechanism of Ag₂S_1-xSe_x and Ag₂S_1-xTe_x are studied.Furthermore,the performance of Ag₂S_0.5Se_0.5?Ag₄SSe?in medium temperature range is optimized by introducing Ag deficiency and doping Te.In this dissertation,the possibility of substituting S with some non-isoelectronic elements in Ag₂S is also investigated experimentally.The main achievements are listed as follows.1.The low carrier concentration is the major cause for the poor TE performance of Ag₂S at room temperature.Both the carrier concentration and carrier mobility are increased when doping Se into Ag₂S,thus the power factor of Ag₂S_1-xSe_x improves.The substitution of Se doesn`t cause the change of the structure of Ag₂S_1-xSe_x,when the amount of Se is less than 0.5,but makes the phase-transformation point shift to low temperature.Ag₂S intrinsically processes low thermal conductivity due to the strong acoustic phonon scattering by the optical branches.The lattice thermal conductivity is further reduced by the substitution of Se,because of the fluctuation of mass field and strain field in crystal.With the optimization of the electric properties,Ag₂S_1-xSe_x exhibits high performance at room temperature,and still maintain plasticity like Ag₂S.The zT of Ag₂S_0.5Se_0.5 can reach 0.26 at room temperature.2.The structure of Ag₂S_1-xTe_x is sensitive to the substitution of Te.When the amount of Te is less than 0.2,Ag₂S_1-xTe_x keeps monoclinic structure at room temperature,and the change of TE properties is similar to that of Se substitution.However,Ag₂S_1-xTe_x changes to BCC structure at room temperature when the Te substitution amount is larger than 0.2,and no longer undergoes monoclinic-BCC or BCC-FCC phase transitions.For these samples,the temperature dependence of electrical properties of the compounds exhibits the metal-like character,which is the feature of heavily doped semiconductor.3.Relatively high carrier concentration causes low zT of Ag₂S_0.5Se_0.5?Ag₄SSe?of medium temperature phase.Introducing Ag deficiency to Ag₄SSe decreases carrier concentration,thus improves TE performance.The maximum zT value of Ag_3.96SSe reaches 0.53 at 493 K.Carrier concentration is further optimized and carrier mobility is improved by partially substituting Se with Te.For Ag₄SSe_0.9Te_0.1,carrier concentration approaches to theoretically optimal value in SPB model,resulting maximum zT of 0.64 at 497 K.4.The substitution limit of non-isoelectronic elements in Ag₂S,such as Sb,P and Cu,is very low,and their substitution has tiny influence on TE performance at room temperature.However,the influence becomes evidential in cubic phase at medium temperature range.The substitution content of Cu greatly increase in the dual-substituted Ag_2-xCu_xS_0.9Se_0.1 samples and thus the influence of the Cu-substitution on the electrical transport becomes more evidential.