| Environmental pollution and energy shortage are the two major problems of human society in this century,and due to the low efficiency of current fossil energy utilization,resulting in a large amount of energy spilling in the form of waste heat and serious emissions of carbon dioxide,it is crucial to establish a clean and efficient energy recovery mechanism.As a semiconductor material that can realize the conversion of electrical energy and thermal energy,thermoelectric materials have irreplaceable applications on the power sources in deep space exploration,and at the same time,because the thermoelectric refrigeration devices have the advantages of small sizes,noise free and high reliability,they also have important applications in the refrigeration of portable electronic devices and miniature components.At present,thermoelectric materials applied in the room temperature and middle temperature range are mainly based on Bi2Te3,PbTe and other sulfide semiconductors,due to the extremely low and abundant and high price of tellurium,which greatly limits the large-scale application of thermoelectric materials in the energy field,so it is of great significance to develop low-cost new thermoelectric materials.The thermoelectric materials of Zintl phase compounds have the characteristics of "electron crystal-phonon glass",which can make the material meet both high conductivity and low thermal conductivity requirements,so it has inherent advantages in thermoelectric consideration,which has attracted widespread attention from materials scientists.This paper focuses on the exploration and performance of thermoelectric materials around a new class of Li-based Zintl phases,and the specific content is summarized as follows:1.The 8-electron half-Heusler structure Li-based Zintl phases LiAlTt(Tt=Si,Ge)was synthesized,and the thermoelectric properties and the corresponding electron band structure,electron state density,phonon dispersion relationship,phonon state density,etc.were studied.Theoretical calculations have shown that phonon transport dominated by vibration of Tt atoms results in high lattice thermal conductivity,and p-type LiAlGe has a lower state density effective mass and single-band effective mass compared to the same type of compound.Experimental results confirmed that the Seebeck coefficient of n-type LiAlSi reached a peak value of-260 μV·K-1 at 423K,but its high thermal conductivity and resistivity make the overall thermoelectric performance poor;For p-type LiAlGe the Seebeck coefficient approached the maximum of 215μV·K-1 at 473K,which also has a high thermal conductivity,but its resistivity is relatively low,at 673K,LiAlGe exhibited a zT value of 0.24.2.Based on above results,the thermoelectric performance of the LiAlTt(Tt=Si,Ge)system is optimized through reducing the lattice thermal conductivity by constructing the LiAlSixGe1-x solid solutions.The results show that the power factor of the materials is first improved and the electrical performance is optimized after Si/Ge incorporated in the structure,and the effective mass of the density states is significantly improved.In addition,the Si/Ge-containing solid solutions can promote a significant decrease in the lattice thermal conductivity of the materials,and when x=0.15,the lattice thermal conductivity of the sample is reduced by about 50%over the entire test temperature range compared to the pristine samples.The above optimization makes the thermoelectric properties of the material effectively improved,of which LiAlSi0.15Geo.85 has a zT value of 0.43 at 673K,which is about 5 times that of the LiAlTt material reported in the literature.3.Li-based Zintl phases with the cubic structure such as Li3Bi have abnormally low lattice thermal conductivity.Due to the large difference in mass and sizes between Li and Bi atoms,significant nonharmonic vibrations can occur leading to a low thermal conductivity.However,due to the high Li-defect level in the compound high electrical thermal conductivity as well as low Seebeck coefficient were resulted.For these reason,Li3-yMgyBi solid solutions were constructed by incorporating Mg into the Li3Bi structure and the thermoelectric properties was tuned as well.The results indicated that by doping Mg into Li3Bi,the electron thermal conductivity and lattice thermal conductivity was simultaneously reduced,and the zT value of Li2.85Mg0.15Bi reaches 0.08 at 673K,which is about 4 times that of undoped Li3Bi. |
