Investigation On The Preparation And Properties Of MnTe Based Thermoelectric Materials

Due to the advantages of low thermal conductivity,low cost and low environmental burden,MnTe is promising p-type thermoelectric?TE?material for mid-temperature applications.However,its low ZT is yet mismatching with the well-established n-type TE materials because of the relative high electrical resistivity.In this scenario,the chemical composition was modified by doping,and point defects were introduced correspondingly to regulate the electrical and thermal transport properties of MnTe,and a significant enhancement of the TE performance of MnTe has been achieved.In this work,MnTe-based bulk TE materials were prepared by a combined method of melt-quenching and hot pressing.The phase structure,microstructure and composition were characterized by XRD,FESEM and TEM.Hall coefficient test system was employed to measure the carrier concentration and mobility,and the mechanical properties have been evaluated by means of Vickers hardness measurement.By means of the laser flash method,the thermal conductivity has been measured,and the Seebeck coefficient and electrical resistivity were measured simultaneously by a home-designed thermoelectric measurement system.Based on the above measurent and investigations,the main results are summarized as follows:Cu was doped into MnTe to substitute Mn cations and the effect of Cu doping on the electrical and thermal transport properties of the Mn_1-xCu_xTe samples was investigated.The band gap of Mn_1-xCu_xTe samples decreased correspondingly,and the electrical conductivity and power factor of Cu-doped samples increased with increasing the Cu doping concentration due to the increase of carrier concentration.In addition,the thermal conductivity was reduced due to the intensive scattering of phonons by the extra point defects introduced by Cu doping.As a consequence,a maximum ZT value of⁰.55 was obtained in the Mn_0.925Cu_0.075Te sample at 773 K,which increase by 35%in compared with the pristine MnTe sample.Na₂S is adopted as the dopant to regulate the TE performance of MnTe,and the effect of anion and cation co-doping by Na₂S on the thermoelectric properties of MnTe-based material was systematically studied.Owing to the increased hole concentration by Na doping,the power factor has been significantly improved.On the other hand,the point defects introduce extra phonon scattering in the doped samples and the lattice thermal conductivities were greatly reduced.As a result,an unprecedented maximum ZT value of¹.1 at 873 K has been achieved in the 0.5 at%Na₂S doped sample,which improves about57%in comparison with the pristine MnTe sample.It is noteworthy that the average ZT of0.5 at%Na₂S-doped sample is⁰.52 between 300 and 873 K,it increases about 190%in comparison with that of the pristine MnTe sample?0.18?.The mechanical performance and thermal stability of the samples?MnTe+0.5 at%Na₂S?,which presents a maximum ZT as stated above,have also been studied.The sample was annealed at 873 K for 100 h,and the power factor of this sample was very close to that of the un-annealed sample in the whole measured temperature range.Meanwhile,its Vickers hardness value?122?is higher than that of pristine MnTe?100?and some other TE materials.This significant improvement in TE performance coupled with good mechanical and thermal stability underlines that Na₂S-doped polycrystalline MnTe is a very promising middle temperature TE material for TE power generation.