The Investigation On Thermoelectric Modification Of Ag And Cu Doped In SnSe Polycrystalline

Thermoelectric material is a kind of functional material for direct conversion between thermal and electrical energy.It can provide a device for thermoelectric power generation and refrigeration,which has broad potential application in the field of the production of clean energy and refrigeration.The orthorhorhombic layer structure SnSe single crystal shows excellent thermoelectric properties,low toxicity and abundant.Much attention has recently payed to the investigation of SnSe polycrystalline due to the complication fabrication and unsuitable for production of SnSe single crystal in large scale.SnSe polycrystalline shows poor thermoelectric properties due to its high electrical resisitivity.In this paper,we investigated the SnSe polycrystalline doped with Ag and Cu.Four series of alloys SnSe_1+x,（SnSe）_1-x（AgI）_x,（SnSe）_1-x（Ag₂Se）_x and（SnSe）_1-x（Cu₂Se）_x were prepared by melting,quenching and SPS sintering techniques.The effects of the doping on phases,electrical resistivity,Seeebeck coefficient,thermal conductivity and figure of merit of the SnSe polycrystalline in the perpendicular and parallel to the pressure direction were investigated.（1）The investigation of SnSe_1+x（x=-0.13,-0.1,0,0.1,0.15）series alloys shows that all alloys form orthorhombic SnSe single solid solutions with large scale texture without any other phase.The alloy shows N-type conduction as Se is insufficient and P-type conduction as Se is excessive,with the transformation of N to P-type conduction as x=0.The electron carrier concentration decreases with the increase of Se content in the Se insufficient alloy,but the hole carrier concentration is almost unchanged in the Se excessive alloy,With the increase of Se content,the electrical resistivity and Seebeck coefficient increase,while the thermal conductivity decreases,in both Se in sufficient and excessive alloys,and in the perpendicular or parallel to the pressure direction.The thermoelectric properties of SnSe polycrystalline can be improved with adjusting the Se content.The maximum ZT value of 0.454 at 873 K was achieved in the sample with x=0.15 in the perpendicular to the pressure direction,which is46%higher than that of stoichiometric SnSe polycrystalline.（2）The investigation of Ag I double-doped（SnSe）_1-x（AgI）_x（x=0,0.01,0.02,0.03,0.04）series alloys shows that all the alloys keep the Pnma structureand P-type conductivity of SnSe.It is benefit for doping elements to enter SnSe lattice with AgI as doping raw material.AgI doping in SnSe increases the carrier concentration and thus reduces the electrical resistivity.As a results,the power factor increases.Although the thermal conductivity of the alloy increases with AgI doping,the thermoelectric properties can be totally improved.The effects of AgI doping in SnSe polycrystalline on the electrical resistivity,Seebeck coefficient and thermal conductivity show different in some degree between the perpendicular and parallel to the pressure direction,but the figure of merit ZT is is not much obvious different in both directions.The maximum ZT value of 0.57 at 873 K was achieved in the sample with x=0.03in the direction perpendicular to pressure direction,which is 124%than that of undoped sample.（3）The investigation of Ag doped（SnSe）_1-x（Ag₂Se）_x（x=0,0.001,0.003,0.005,0.01 and0.02）series alloys show that the alloy formed SnSe solid solution as the amount of Ag doped in SnSe was small（x less than 0.005）.A minor secondary phase Ag₂Se appeared in the alloy with x=0.005,which indicates that the solid solubility of Ag in SnSe is less than 0.5 at%.A minor secondary phase AgSnSe₂ appeared in the alloy with x=0.01.The Ag⁺¹ ion replaces the Sn⁺² in Ag-doped Sn Se,which increases the hole carrier concentration of the compound and thus decreases its electrical resistivity and Seebeck coefficient,but increases the thermal conductivity.As a results,the thermoelectric properties can be improved with Ag doping.The figure of merit ZT is not much obvious different in both the perpendicular and parallel to the pressure direction.The maximum ZT value of 0.882 at 873 K was achieved in the sample with x=0.005 in the direction perpendicular to pressure direction.which is 195%improvement than that of the undopedsample.（4）The investigation of Cu doped（SnSe）_1-x（Cu₂Se）_x（x=0,0.002,0.005,0.01,0.02,and0.03）series alloys show that all the alloys keep the Pnma structureand P-type conductivity of SnSe.The secondary phase Cu₂Se₄Sn increases with the increase of Cu content.Cu-doped SnSe,which increases the carrier concentration of the compound and thus decreases its electrical resistivity,as a results,the power factor increases.Although the thermal conductivity of the alloy increases with Cu doping,the thermoelectric properties can be totally improved.The maximum ZT value of 0.812 at 873 K was achieved in the sample with x=0.005 in the perpendicular to the pressure direction,which is 55%higher than that of pure sample.