A Study On The Synthesis And Thermoelectric Performance Of Sn-S Based Compounds

Thermoelectric effects enable direct conversion between thermal and electrical energy,which can be used for both power generation and refrigeration.The search for high-performance thermoelectrics with low cost and earth abundant elements is attracting significant attention because of energy efficiency,conservation and management concerns.The low costs,low toxictity,abundance and promising thermoelectric properties of sulfides have attracted growing interest in recent years.Nonetheless,sulfides with high ZT values are very rare.Thus,investigations on thermoelectric performance of new sulfides and on how to improve thermoelectric properties of existing sulfides are very meaningful.This work focused on the binary sulfides,SnS,whose thermoelectric properties were enhanced by the method of optimizing synthesis process,microstructure designing based on its layed structure and seeking for the proper dopant.In addition,thermoelectric properties of ternary sulfides Cu₂SnS₃ were investigated and improved.The phase transition caused by doping was also investigated.At first,pristine SnS was synthesized by mechanical alloying combining spark plasma sintering.It revealed that SnS had potential as a good thermoelectric material benefiting from its intrinsically low thermal conductivity below 1.0 W/m/K above 400 K and its high Seebeck coefficient over 500 ?V/K.Changing the stoichiometric ratio and sintering temperature had less apparent influence on the microstructure and thermoelectric properties of SnS because of the redundant S in the powders were decomposed during the sintering process.The anisotropy of thermoelectric properties was investigated and the textured SnS was prepared and characterized.It revealed that texture is not an effective method to enhance thermoelectric properites in Sn S system.Then SnS nanorods were synthesized by a facile solvothermal method,which were compacted by cold isostatic pressing and the rapid annealing process.The bulk sample consolidated from the nanorods synthesized by solvothermal showed much higher carrier mobility than that from the powders synthesized by mechanic alloying.On the other hand,lower thermal conductivity was obtained due to the porous nanograined structure.As a result,the maximum ZT value was enhanced from 0.10 to 0.25 at 873 K.For the porpose of increasing carrier concentration,Na,K and Ag elements were doped to SnS.Na or K dopants could bring a lot of defects into matrix and the bipolar thermal conductivity increased,resulting in that the ZT value only reached 0.30.Ag doping could not only increase the carrier concentration to 10¹⁸cm^-3,but also make thermal conductivity lower than the undoped one.Thus the ZT value reach 0.6 at 873 K,which is the highest ZT value of reported SnS system so far.In addition,monoclinic Cu₂SnS₃ bulk materials were synthesized by the mechanical alloying combined with spark plasma sintering,and their crystal structures,electronic transport properties and enhanced thermoelectric performance were investigated.In doping caused the phase transition of Cu₂SnS₃ and both the monoclinic phase and tetragonal phase coexist in the samples.In doping significantly improved the electrical properties meanwhile it reduces the lattice thermal conductivity,and the ZT values were boosted as a consequence.The ZT value of Cu2Sn0.9In0.1S3 reach to 0.56 at the relatively low temperature?773 K?.