The Study Of P-Type Mg₂X?X=Si,Ge,Sn? Thermoelectric Performance

Thermoelectric materials have receipted worldwide concerns since fossil energy crisis.The eco-friendly Mg2X?X=Si,Ge,Sn?thermoelectric materials have advanced characters of abundance row material,low price,no pollution and low density for mid-temperature?400⁸00 K?power generation.The thermoelectric figure of merit ZT of n type Mg2X thermoelectric materials has achieved¹.3,but ZT of p type Mg2X thermoelectric materials is less than 1.So,it is important to improve the thermoelectric performance of p type Mg2X thermoelectric materials.Firstly,Mg₂Si_1-xSn_x?x=0.4,0.5,0.6,0.7,0.8?and Mg₂Ge_1-ySn_y?y=0.4,0.5,0.6,0.7,0.8?solid solutions were synthesized by two-step solid-state reaction,ball-milling and hot-pressing method.The phase analysis for Mg₂Si_1-xSn_x and Mg₂Ge_1-ySn_y indicates:Mg₂Si_1-xSn_x solid solutions are single phase materials for x?0.7 and Mg₂Ge_1-ySn_y solid solutions are single phase materials for 0.4?y?0.8.The results of thermoelectric properties from room temperature to 773 K show:Mg₂Si_1-xSn_x and Mg₂Ge_1-ySn_y solid solutions are n type semiconductor;there is the lowest thermal conductivity for Mg₂Si_0.4Sn_0.6 solid solution and Mg₂Ge_0.3Sn_0.7 solid solution.Secondly,with comparison of Mg_2?1-x?Ag_2xSi_0.3Sn_0.7?x=0,0.01,0.02,0.03,0.04,0.05?and Mg_2?1-y?Li_2ySi_0.3Sn_0.7?y=0,0.02,0.04,0.06,0.08?thermoelectric materials,we have measured the room-temperature physical parameters and thermoelectric properties from room temperature to 773 K,and investigated the influence of different dopant on solid solubility,microstructure,carrier concentration,electrical properties and thermal transport.The phase composition shows that the solid solubility of Ag and Li for Mg₂Si_0.3Sn_0.7 is respectively x=0.03 and y=0.06.The comparison results demonstrate:the Mg_2?1-y?Li_2ySi_0.3Sn_0.7 samples have higher carrier concentration without obvious bipolar effect;the maximum ZT for Mg_1.92Li_0.08Si_0.3Sn_0.7 achieves 0.54 at 773 K;the maximum ZT for Mg_1.9Ag_0.1Si_0.3Sn_0.7achieves 0.34 at 623 K.The lattice thermal conductivity of Li or Ag doped samples is decreased obviously due to the stronger mass and strain field fluctuations in phonon transport.Finally,we have researched the microstructure,carrier concentration,scatter factor,bandgap and thermoelectric performance for Mg_1.92Li_0.08Ge_0.4Sn_0.6-xSi_x?x=0,0.025,0.05,0.075?.The results demonstrate that the grain boundaries can form the wide bandgap nanoscale Si-rich phase,which can create the interface barrier to filter the low-energy hole in the valence band and block the thermal excited electron in conduction band.Therefore,the nanoscale Si-rich phase could observably improve power factor of samples and effectively decrease lattice thermal conductivity and electrical thermal conductivity.Finally,the maximum figure of merit ZT of Mg_1.92Li_0.08Ge_0.4Sn_0.525Si_0.075 achieves 0.75 at 723 K.