Doping Modification And Mechanism Of YbMg₂Bi₂ Based Thermoelectric Materials

For the typical AM₂X₂compounds with the Ca Al₂Si₂-type structure,YbMg₂Bi₂compounds have received extensive attention which could be contributed to the unique structural and chemical characteristics.In the present work,for YbMg₂Bi₂ compounds,the optimization of thermoelectric properties is primarily focused on the substitution on Yb or Mg site,while the effect of anionic substitution is seldom explored.Although YbMg₂Bi₂ compounds possess excellent electrical properties,the lattice thermal conductivity is still relatively high which suppresses ZT values.In view of the present research situation,the effect of anionic substitution on the thermoelectric transport behavior of Bi-based Zintl phase compounds was investigated by partial substitution of Sb for Bi in Yb Mg₂Bi_1.98.Then,by doping Ba element at the Yb site,stress field fluctuations and point defects were introduced into the material,which effectively blocked the heat conduction process of phonons.The experiment confirmed that the partial substitution of Sb for Bi element enlarged the band gap and reduced the carrier concentration,which lead to the damage of the electrical conductivity and the improvement of Seebeck coefficient simultaneously.More importantly,the introduction of Sb element constructed a large number of point defects,and the lattice thermal conductivity was significantly inhibited.Finally,at 873 K,the thermoelectric value of Yb Mg₂Bi_1.58Sb_0.4 sample was about 1.05,about 40%higher than that of the matrix material.This experiment provides a new idea for the thermoelectric property optimization of Bi-based AM₂X₂ compound.It was found that difference in electronegativity between Yb and Ba increased the formation energy of cation vacancy of the doped material which resulted in the sacrifice of electrical conductivity.The Seebeck coefficient showed an opposite trend to the conductivity,but still could not compensate for the decreased conductivity,and the final power factor was lower than that of the undoped sample.Besides,due to the obvious difference in mass and size between host and guest atoms,the phonon scattering degree was significantly enhanced with the introduction of Ba element.At 300 K,the lattice thermal conductivity of Yb_0.75Ba_0.25Mg₂Bi_1.98 sample was as low as 1.21W m^-1 k^-1.Benefitting from the greatly reduced thermal conductivity,the engineering thermoelectric value of Yb_0.75Ba_0.25Mg₂Bi_1.98 sample reached a superior value of?0.64which was?46%greater than the initial one,when T_h is 873 K and T_c is maintained as300 K,exhibiting the promising application in the middle temperature fields.