Investigation On The Ag Based Near Room Temperature Thermoelectric Materials

Thermoelectric(TE)materials are used to make the energy conversion between electric and thermal energy through the transportion of internal carriers.They are more safe,reliable and clean than fossil fuel,which have a broad prospect in the field of thermal power generation and thermoelectric refrigeration.Until now,Bi2Te3-based materials are still the only widely used TE materials at near room temperature.However,bismuth and tellurium are both rare and expensive elements in the crust,and the tellurium contained in Bi2Te3-based compounds is harmful to human body and environment.So these compounds are not suitable for large-scale preparation.Ag₈SiSe₆ and Mg Ag Sb,two kinds of Ag based compounds,are made up of environmental friendly,nontoxic and abundant elements.They are supposed to be the promising TE materials at near room temperature.To improving their TE performance,we optimized the preparation process and got control of the composition and microstructure of Ag₈SiSe₆ and Mg Ag Sb.The second phase,dislocation,stacking fault and energy barrier are introduced into them to improve the TE transport performance.Finally,high-performance Ag₈SiSe₆ and Mg Ag Sb based TE materials are obtained.The main experimental results of this paper are as follows:1.Ag₈SiSe₆ based TE materials prepared by high-temperature melting and hot pressing sintering has the second phase of nano-sized Ag2 Se and Si.And the content of the second phase in the matrix changes with the hot pressing sintering temperature.When the material is sintered at 525°C,the content of Ag2 Se and Si is the lowest with the lowest resistivity(11.11 ??m).And Because of the energy barrier between Ag₈SiSe₆,Ag2 Se and Si,low-energy carriers are effectively filtered.The Seebeck coefficient of Ag₈SiSe₆ based TE material is increased,and the power factor is optimized.In calculation,the maximum power factor of the sample sintered at 525 °C is 1772 ?W/m K2 at 125 °C.2.The existence of Ag2 Se and Si nano-sized second phase enhanced scattering of phonons by Ag2 Se and Si nano-sized second phase,and reduced the lattice thermal conductivity of Ag₈SiSe₆ based thermoelectric materials.The ZT value of samples sintered at 525 °C is close to 0.9 at 125 °C,and its TE performance is similar to n-type Bi2Te3 based thermoelectric materials.Since Ag₈SiSe₆ based TE material is more environmental friendly,cheap and has excellent mechanical properties,it can be widely used in TE application.3.The MgAgSb based TE material was fabricated by microwave-assisted process and subsequent spark plasma sintering,which has a certain amount of Ag3 Sb.The micro morphology and distribution of Ag3 Sb were determined by EPMA.Comparing with the traditional melting method,the existence of Ag3 Sb improves the carrier concentration of Mg Ag Sb based TE materials and greatly reduces the resistivity.Although the Seebeck coefficient has deteriorated,the power factor of Mg Ag Sb was nearly doubled,reaching to 1963 ?W/m K2.4.The microstructure like flake sub-grain,dislocation and stacking fault caused by fast microwave melting reduce the lattice thermal conductivity and optimize the thermal properties of Mg Ag Sb.Finally,the ZT value of Mg Ag Sb based TE material prepared by microwave-assisted process is close to 0.8 at 270 °C,which is 20% higher than the sample prepared traditional melting method.Moreover,because of the advantages of easy operation and energy saving,this method is more useful in large-scale preparation.