Study On Preparation And Thermoelectric Properties Of N-Type Mg₃Sb₂ Based Materials

Thermoelectric?TE?material is a kind of functional materials that can realize direct convertion between thermal energy and electrical energy,and can be used for waste heat recovery or for cooling applications.Thermoelectric devices are scalable,stable,reliable,and have no mechanical moving parts,which can be used in the areas where conventional heat engines are not capable of.N-type Mg₃Sb₂-based materials attract much attention because of the best TE properties,and the advantages of rich reserves of constituent elements,low price,and no pollution.According to recent research,n-type Mg₃Sb₂-based alloy is also a kind of promising room temperature TE materials,which is comparable with the commercialized n-type Bi₂Te₃-based materials.Moreover,since element Te is rare on the earth,the development of Mg₃Sb₂-based alloy instead of Bi₂Te₃ has great significance for the commercial application of TE materials.In this paper,n-type Mg₃Sb₂-based TE materials were prepared by directly ball milling and hot pressing.By doping different elements at different positions,the TE figure-of-merit of Mg₃Sb₂ was optimized.Then the effects of different doping on the electron and phonon transport process of Mg₃Sb₂-based materials is investigated.The main research results obtained are as follows:By codoping with Se and transition metals,room temperature and peak ZT values are simultaneously improved,achieving the highest average ZT between 323 K and 523 K so far.Based on earlier report,the carrier concentration of the Se-doped Mg₃Sb₂ is optimized,reaching ?1.9�10¹⁹ cm^-3,which is close to that of generally used Te-doped Mg₃Sb₂.Meanwhile the thermal conductivity keeps unchanged.At last,the ZT value is greatly improved.A highest ZT ?1.4 at 723 K is obtained for Mg_3.2Sb_1.5Bi_0.49Se_0.01.Further optimization was performed for the above doped samples.By codoping the transition metals?Mn,Co,Fe,Hf,and Ta?at the Mg site,the room temperature carrier scattering mechanism is changed?i.e.the ionized impurity scattering mechanism is changed to a complex mixed scattering mechanism?,which greatly improves the room temperature carrier mobility.Thus the electrical properties of Mg₃Sb₂-based materials near room temperature is greatly improved.In the sample doped with the transition metal Co or Mn,the lattice thermal conductivity is greatly decreased.Because Mn prefers to occupy interstitial site?2/3,1/3,0.182 or 1/3,2/3,0.581?while Co prefers to occupy Mg1 site?0,0,0?,Mg_3.15Mn_0.05Sb_1.5Bi_0.49Se_0.01 has stronger phonon scattering,contributing to a high room temperature ZT value ?0.6,and a high peak ZT value ?1.70 at 673 K.We also explored the doping effects of different rear earth elements on the My site of Mg₃Sb₂-based TE materials.It is found that the doping efficiency and doping limit of the rear earth elements on the Mg site is higher than that of the chalcogen elements.This is possibly because that the formation energy of rare earth elements doping at Mg site is lower.So an extremely high carrier concentration of ?9�10¹⁹ cm^-3 is obtained in Mg_3.18La_0.02Sb_1.5Bi_0.5 and Mg_3.185Ce_0.015Sb_1.5Bi_0.5 which is close to the theoretical doping limit.The obtained higher carrier concentration activates much more electronic energy bands,leading to the enhancement of Seebeck coefficient together with the increased electrical conductivity.A highest ZT value ?1.56 at 693 K was achieved in Mg_3.19La_0.01Sb_1.5Bi_0.5,as well as ?1.50 at 693 K for Mg_3.195Pr_0.005Sb_1.5Bi_0.5 and Mg_3.19Ce_0.01Sb_1.5Bi_0.5,ZT which is comparable with that of Te-doped samples.The Se-doped n-type Mg₃Sb₂-based TE material is also prepared by high-pressure method.Similar to the transition metal-doped sample,the carrier scattering mechanism around the low-temperature range was changed from ionized impurity scattering mechanism to complex scattering mechanism,thereby increasing the carrier mobility and electrical conductivity near room temperature.This part of work shows that it is feasible to optimize TE properties of materials by changing preparation method.