Effects Of Doping On The Thermoelectric Properties Of Cu₃SbSe₄

Thermoelectric materials,which can realize the direct conversion between heat energy and electricity,has been attracted much attention in the past few decades.Generally,its advantages can be seen as follows,such as high environmental protection,convenience and less pollution.However,most of thermoelectric materials with high performance have some shortcomings,such as precious or toxic elements,high preparation requirements,which limit the large-scale commercial application.The copper-based thermoelectric materials,Cu₃SbSe₄,has been attracted more attention over the past several years for their potential thermoelectric properties in the moderate temperature range.In this dissertation,the bulk Cu₃SbSe₄ materials were prepared via vacuum melting-quenching-annealing,then combined with ball milling?BM?and hot pressing sintering?HP?process.Donor doping?Ni/Co/Ag on Cu site and In on Sb site?was studied to improve the electricity conductivity and depress the thermal conductivity.In order to further optimize the performance of the doped-Cu₃SbSe₄,the size of powder particles was adjusted by ball milling to improve the thermoelectric transport properties.The main research contents are as follows:The grain refinement of Cu₃SbSe₄ enhanced the phonon scattering,which decreased the lattice thermal conductivity and increased the electrical conductivity.The maximum ZT of0.42 was obtained at 650K after milling of 72 hours.In order to further improve the thermoelectric performance of Cu₃SbSe₄,the bulk Cu_3-xNi_xSbSe₄ samples doped with Ni at Cu sites were fabricated.The electrical conductivity and carrier concentration increased with the increasing of the holes in the system,while the Seebeck coefficient decreased.The thermoelectric properties of Cu_2.97Ni_0.03SbSe₄ compounds were significantly increased due to the depression of thermal conductivity.The lattice thermal conductivity of Cu_2.97Ni_0.03SbSe₄decreased from 3.0 Wm^-1K^-1 to 1.31 Wm^-1K^-11 at room temperature.After ball milling of 72hours,the ZT value of Cu_2.97Ni_0.03SbSe₄ reached 0.71 at 650K.Both the carrier concentration and the electrical conductivity increased with the the Co-doping increasing.Lattice distortion caused by the substitution of heterogeneous atom depressed the lattice thermal conductivity,which improved the ZT value significantly.In order to further optimize the lattice thermal conductivity and the electrical conductivity at room temperature,the effects of the size of Cu_2.98Co_0.02SbSe₄ powder particle on the thermoelectric properties were studied.The lattice thermal conductivity of Cu_2.98Co_0.02SbSe₄compound was about 1.7Wm^-1K^-1 after ball milling of 3 hours and decreased to 0.98Wm^-1K^-1after ball milling of 72 hours at room temperature.The highest ZT value of 0.70 was achieved for Cu_2.98Co_0.02SbSe₄ sample at 650K.Ag doping could effectively enhance the carrier concentration and electrical conductivity of Cu₃SbSe₄.The second phase of AgSbSe₂ was precipitated when the nominal content of Ag-doping was more than 0.03.It can be concluded that the addition of Ag and the reduction size of powder particles both can increase the ZT value of the Cu₃SbSe₄ compound.Finally,the ZT value of 0.70was obtained for the Cu_2.98Ag_0.02SbSe₄ sample at 650K.In order to further optimize the carrier concentration,donor doping?In-doping on Sb site?for the Cu₃SbSe₄ compounds was studied.As the In-doping increasd,the carrier concentration increased from 1.09×10¹⁸cm^-3 to 12.62×10¹⁸cm^-3,which also resulted in the increase of the electrical conductivity.The second phase In₂Se₃ formed when the nominal In-doping was more than 0.02.With the the size of Cu₃Sb_0.98In_0.02Se₄ particles decreasing,the lattice thermal conductivity decreased from 2.75 Wm^-1K^-11 to 1.70 Wm^-1K^-1,which can be explained by the double effect of lattice distortion and the point defect scattering.After ball milling of 36 hours,the highest ZT value of Cu₃Sb_0.98In_0.02Se₄ reached 0.75 at 650K.