Thermoelectric Performance Improvements Of Zintl Compound Mg₃Sb₂

Zintl phase Mg₃Sb₂ has attracted more and more attentions recently,because its element abundance,low cost,environmental friendliness,especially the unique property of"electronic crystal-phonon glass",which is expected to be applied in thermoelectric(TE)field.In this thesis,Mg₃Sb₂-based TE materials were prepared by high-energy ball milling combined with rapid hot-pressing.The effects of alloying with different elements on the thermoelectric properties were studied,the carrier transport and phonon scattering mechanisms of Mg₃Sb₂-based materials were analyzed.First,the TE properties of p-type Mg₃Sb_1.5Bi_0.5-based thermoelectric materials by Ge doping were investigated.The experimental results show that the mobility of Mg₃Sb_1.5Bi_0.5significantly increass after Ge doping on Sb site.The room temperature mobility changes from 1.82 cm²v^-1s^-1to 25.63 cm²v^-1s^-1,however it does not change too much when we keep increasing the content of Ge.And Ge doping can greatly increase the carrier concentration of Mg₃Sb_1.5Bi_0.5,the room temperature carrier concentration changes from 1.02×10¹⁹cm^-3to5.71×10¹⁹cm^-3.Since the second phase may be formed during the preparation process,the change trend of the carrier concentration is relatively complicated.Besides,the lattice thermal conductivity was reduced,it may owe to the scattering of phonons by the interface between the matrix phase and the second phase.Finally,p-type sample Mg₃Sb_1.5Bi_0.47Ge_0.03shows a peak ZT of 0.499 at 723 K.Secondly,The thermoelectric performances of n-type Mg_3.2Sb_1.5Bi_0.5-based materials by Zn doping at Mg site and Se doping at Sb site simultaneously.In this way,the mobility greatly increases while the carrier concentration almost unchanged,so that the conductivity was significantly improved.In addition,using some Zn atoms to replace Mg atoms will introduce point defects in the material,which further enhances acoustic phonon scattering and reduces the lattice thermal conductivity.In the end,Mg_3.18Zn_0.02Sb_1.5Bi_0.49Se_0.01sample achieved the maximum ZT about 1.77 at 623 K.Morover,the thermoelectric properties of n-type Mg_3.2Sb_1.5Bi_0.5-based materials had been improved significantly by co-doping of In and Se.It is benefited from the increase of effective mass and carrier concentration.The carrier scattering mechanism has been slightly changed,in which the ionization scattering has been weakened.Ultimately,the power factor and the thermoelectric figure of merit have been significantly improved,achieving maximum values at 573 K and 723 K,which are 15.1?W cm^-1K^-2and 1.64,respectively.