【摘要】 化石燃料产生的热能一部分以废热的形式流失到环境中。常常，废热的温度较低且低温废热占比很高，有效利用中低温废热将大大提高能源的利用率。热电材料器件能够实现热能与电能相互转换。Zintl相化合物因拥有“电子晶体-声子玻璃”的复杂晶体结构特征，具有优异的热电性能。Mg₃X₂（X=Sb,Bi）作为一种Mg基Zintl相化合物，具有成本低、组成元素储量丰富且热电性能优异等特点，被认为是一种极具应用潜力的中低温热电材料。近年来，关于如何提升Mg₃X₂的热电性能成为研究的热点。首先，综述了Mg₃X₂晶体结构的特点和其热电性能具有各向异性的特征，又分别综述了掺杂的Mg₃Sb₂和Mg₃Bi₂化合物的热电性能和最新研究进展。在相同温度下，n型Mg₃X₂比p型Mg₃X₂的热电性能更高。通过不等价n型或p型元素掺杂，大部分Mg₃X₂基材料虽然Seebeck系数值有所降低，但其电导率、功率因子和优值都增加，可有效改善Mg₃X₂基材料的热电性能。通过调整Mg₃（Sb,Bi）₂固溶体中Mg₃Bi₂的含量，可调整能带电子结构和带隙，在70%含量下(Mg₃Bi_1.4Sb_0.6合金)可获得最大的优值ZT。基于n型Mg₃Bi_1.4Sb_0.6合金在低温废热有效利用率已经超过现有的n型Bi₂Te₃合金，Mg₃X₂基很有可能在未来取代n型Bi₂Te₃合金，成为新的低温热电材料。更多还原

【Abstract】 A portion of the thermal energy generated by fossil fuels is released toward the environment in the form of waste heat.In general,low temperature waste heat is more.Effective use of medium and low temperature waste heat will greatly improve energy utilization rate.Thermoelectric material devices can directly carry out the mutual conversion of thermal energy and electrical energy,which possess these characteristics of simple devices,noiseless and environment friendly.Zintl phase compounds have excellent thermoelectric properties due to the complex crystal structure characteristics of"electronic crystal-phonon glass".Namely,they have the characteristics of high conductivity of crystal and low thermal conductivity of glass.Mg-based Zintl phase compound,such as Mg₃X₂（X=Sb,Bi）has the characteristics of low cost,abundant constituent elements,and excellent thermoelectric properties.It is widely studied as a potential medium and low temperature thermoelectric material.In recent years,it has been paid more attentions how to improve the thermoelectric performance of Mg₃X₂-based materials.In this paper,the characteristics of the crystal structure of Mg₃X₂and its anisotropic thermoelectric properties were first summarized.For single crystals,the electrical transport performance of Mg₃X₂material was anisotropic,and for polycrystalline Mg₃X₂material,the thermoelectric performance was isotropic.The thermoelectric performance greatly improved along texture direction for polycrystalline Mg₃X₂material after the texturing treatment in the optimum crystal orientation.Then the thermoelectric properties and the latest advances in the doped Mg₃Sb₂and Mg₃Bi₂compounds were introduced respectively.The references showed that n-type Mg₃X₂had higher thermoelectric figure of merit compared with p-type Mg₃X₂at the same temperature.It was related to band degeneracy of N_v=6 for n-type Mg₃X₂and N_v=1 for p-type Mg₃X₂.The thermoelectric properties were poor for undoped p-type Mg₃Sb₂.At 750 K,the maximum power factor and figure of merit（zT）values were only 1.92μW·cm^-1·K^-2and 0.26.By doping with unequivalent n-type impurity elements such as La,Te and Nb et al.or p-type impurity elements such as Li,Na and Pb et al.,Seebeck coefficient values decreased for most Mg₃X₂-based materials.But their electrical conductivity,power factor and zT increased,which could effectively improve the thermoelectric properties of Mg₃X₂-based materials.By doping with chalcogen element Te,n-type Mg_3.2Sb_1.99Te_0.01was obtained.At 300～750 K,the average zT value was 0.41,which greatly exceeded the thermoelectric performance of p-type Mg₃Sb₂.Doping with rare earth element La had a great influence on improving the electron carrier concentration.At this time,the electron carrier concentration of n-type Mg_3.19La_0.01Sb_1.5Bi_0.5could reach 9×10¹⁹cm^-3.The maximum zT value at 693 K was～1.6 and the average zT value at 300～700 K zT～0.9.By doping with Mn element,the carrier mobility and power factor had been significantly improved.The highest zT value reached～1.85 at 723 K.The average zT value was～1.25 in a range of temperature from 300 to723 K,which was higher than other n-type Mg₃Sb₂based thermoelectric materials.The atomic mass fluctuation in lattice produced by equivalent doping of Zn,Cd or other elements in p-type Mg₃Sb₂increased the collision process of phonons between lattices and reduced thermal conductivity,optimizing the thermoelectric properties of the material.At 773 K,zT value of p-type Mg_2.69Li_0.01Cd_0.5Sb₂was～0.68,and zT value of Mg_2.4875Na_0.0125Zn_0.5Sb₂could reach 0.8.Mg₃X₂materials prepared according to the stoichiometric element content generally exhibited p-type characteristics due to the formation of Mg vacancy defects in the lattice during the preparation.To compensate for the Mg vacancy defects in lattice,the n-type characteristics of Mg₃X₂-based thermoelectric materials could be obtained by enriching Mg in Mg₃X₂lattice.However,if the rich Mg amount was more,a magnesium-alloy second phase would form,which would increase the thermal conductivity of Mg₃X₂-based materials and deteriorate their thermoelectric properties.Thus,for preparing ntype Mg₃X₂,it was necessary to add an appropriate amount of Mg element according to the actual preparation situation to optimize the thermoelectric performance.Like the Mg₃Sb₂,undoped Mg₃Bi₂had excessively poor thermoelectric properties,which was related to the semi-metal nature of Mg₃Bi₂lattice.In a range of temperature of 300～500 K,the average Seebeck coefficient value of p-type Mg₃Bi₂was only～50μV·K^-1,and the thermal conductivity was very high(3.5 W·m^-1·K^-1),the average value of zT was only 0.06.By doping Sb,the obtained Mg₃（Sb,Bi）₂solid solution could effectively improve the thermoelectric properties of Mg₃Bi₂.By adjusting the amount of Mg₃Bi₂in the Mg₃（Sb,Bi）₂solid solution,the energy band electronic structure and band gap could be adjusted,and the maximum value of zT could be obtained at 70%content of Mg₃Bi₂(Mg₃Bi_1.4Sb_0.6solid solution),the average zT value of Mg₃Bi_1.4Sb_0.6alloy could reach 1.0 at 300～500 K.The low-temperature waste heat would widely be utilized by replacing the existing n-type Bi₂Te₃material based on effective utilization rate of n-type Mg₃Bi_1.4Sb_0.6alloy exceeding Bi₂Te₃material.In the future,Mg₃X₂alloys would become a new-typed low-temperature thermoelectric material.更多还原