The Thermoelectric Performance Of Novel Group Chalcogenides Materials

Developing eco-friendly thermoelectric materials is becoming a focus all over the world.In order to widely employ in applications of thermoelectric materials,lots of investigations are mostly focused on the following two aspects.one is to tailor the coupling of these parameters to improve the thermoelectric properties of the materials,in some ways,via the strategies such as doping,forming solid solution,electronic band engineering approaches?such as band convergence,etc.?and designing multi-scale microstructure.High thermoelectric properties are realized through the above mentioned methods.Another hot topic has been focused on exploring new thermoelectric materials with intrinsically low thermal conductivity.More and more attention has been paid to novel heavy chalcogenides due to their ultralow lattice thermal conductivity and tunable electrical properties.This work focuses on novel semiconductor compounds Ba₂SnTe₅-and BaAg₂SnSe₄-based compounds.Ba₂SnTe₅ and BaAg₂SnSe₄ compounds were successfully prepared by melting and annealing treatment combined with spark plasma sintering?SPS?.The results show that Ba₂SnTe₅ and BaAg₂SnSe₄ show a step-like cleavage and behave as transgranular fracture without detection of grain boundary.Both Ba₂SnTe₅ and BaAg₂SnSe₄ samples have high Seebeck coefficient and ultralow lattice thermal conductivity.However,due to the extremely low electrical conductivity of these materials,their thermoelectric properties are not high.Herein BaAg₂SnSe₄ compound achieved the maximum ZT=0.07 at 673 K,and this value is significantly higher than the ZT_Max=0.01of Ba₂SnTe₅ compound at 573 K.From the comprehensive consideration of structure and performance,compared with Ba₂SnTe₅,BaAg₂SnSe₄ samples have a better performance and prospects.For BaAg₂SnSe₄ with poor electrical performance mentioned above,we improve the electrical properties by adjusting the stoichiometric ratio?Ag deletion?of BaAg₂SnSe₄.And their high-temperature thermoelectric properties are systematically investigated.In addition,phonon spectral calculation and low temperature heat capacity reveal that the rattling behavior of Ag and Se atoms in the structure contributes to the low frequency optical phonon,coupling with acoustic phonons and suppressing the maximum of Debye frequency in the first Brillouin zone.Moreover,the weak chemical bonding feature in the structure results in the low elastic properties which soften phonon modes and slow down phonon propagation.Herein,the thermal conductivity of BaAg₂SnSe₄ as low as 0.26 Wm^-1K^-1 at 723 K is reported.A combination of the intrinsically ultralow thermal conductivity and the substantially enhanced power factor enables a peak ZT=0.25 at 673 K for BaAg_1.985SnSe₄,which is 2.6 times larger than that of pristine BaAg₂SnSe₄.On the other hand,the carrier concentration is also optimized by doping A elements?Ga,In?substituting for Sn.Theoretical calculation shows that the conduction band edge is mainly contributed by Se 4p orbits and Sn 5s orbits,while the valence band edge is mainly contributed by Ag 4d orbits and Se 4p orbits.With rising temperature,the electronic transport mechanism for BaAg₂M_x Sn_1-xSe₄?M=Ga,In?samples transits from a variable range hopping conduction at low temperature to the thermal activated conduction and further into band conduction at high temperature,and the transition temperature decreases with the increasing dopant content.A combination of the intrinsically ultralow thermal conductivity and the substantially enhanced power factor enables a peak ZT=0.25 at 673 K for BaAg₂Ga_0.002Sn_0.998Se₄,which is 2.6times larger than that of pristine BaAg₂SnSe₄.This work demonstrates that compounds with complex structure are interesting candidates for thermoelectric materials.