Preparation Of Sn-Mn-Doped GeTe By Mechanical Alloying And Study On Thermoelectric Properties

Germanium telluride（GeTe）has received extensive attention as a thermoelectric material.However,due to the inherent Ge vacancies in GeTe,which often lead to high carrier concentrations beyond the optimal range,further improvement of its thermoelectric performance is limited,and its phase transition problems also brings great challenges to regulating the thermoelectric transport mechanism.Studies have shown that Mn Te-SnTe-GeTe can form solid solutions in a wide range of compositions,and there are reports that Sn/Mn elements are used for doping of Te-based thermoelectric materials,which can optimize the thermoelectric properties of Te-based materials by doping.In view of the above problems and phenomena,in this paper,a Sn/Mn co-doped GeTe thermoelectric material was synthesized by mechanical alloying（MA）combined with spark plasma sintering（SPS）technology.Here,the crystal structure,microstructure,energy band structure and thermoelectric transport mechanism are systematically studied by different control methods,which lays a foundation for the further application of GeTe matrix materials.The specific research contents and conclusions are as follows:1.Since both GeTe and SnTe have excellent thermoelectric properties,and both tin and germanium are adjacent elements of group IV with similar atomic radii,homomorphic element doping is a commonly used thermoelectric material control method.In this paper,Sn-doped GeTe thermoelectric materials,namely solid solution alloy Ge_1-x Sn_xTe（x=0,0.1,0.15,0.2）,were synthesized by mechanical alloy and SPS sintering technology again.For layered GeTe materials,Sn atom doping increases the atomic disorder,strengthens the lattice anharmonic property of the material,and enhances the mass/stress field fluctuation of the sample.The resulting strong phonon scattering results in a sharp decline in thermal conductivity over the whole temperature range.In particular,the thermal conductivity at room temperature decreases from 7.1 Wm^-1K^-1 of pure GeTe to 2.2 Wm^-1K^-1 of doped Ge₀.8Sn₀.2Te.In addition,according to hall measurement results,Sn doping can increase the effective mass of the material and significantly improve its seebeck coefficient.In addition,the seebeck coefficient at room temperature increases from 23.08 (1 without GeTe to 70.25 (1（Ge0.9Sn₀.1Te）.The final sample Ge₀.8Sn₀.2Te obtained the highest ZT（¹.49）at 730 K,which was nearly half higher than the maximum ZT of pure GeTe sample.Meanwhile,the average ZT value of Ge_1-x Sn_x Te（x=0,0.1,0.15,0.2）increases with the increase of x,especially the average ZT value of Ge₀.8Sn₀.2Te between 300 K and 800 K is about 0.98.Meanwhile,the co-doped sample Ge_0.8-y Sn₀.2Mn_y Te was prepared by mechanical alloy method to explore the optimization of the thermoelectric properties of the sample Ge₀.8Sn₀.2Te by Mn doping.In the measurement of infrared absorption wavelength of sample Ge_0.8-y Sn₀.2Mn_y Te,the results show that Mn doping significantly reduces the energy gap of sample Ge_0.8-y Sn₀.2Mn_y Te,thus increasing the Seebeck coefficient of sample Ge_0.8-y Sn₀.2Mn_y Te.This results in a significant increase in the power factor over the 300-767 K temperature range.In addition,alloying can increase disorder and expand phonon dispersion in codoped samples,which further reduces the thermal conductivity.For example,the thermal conductivity of Ge0.76Sn₀.2Mn0.04 Te decreases to about 1.90 Wm^-1K^-1 at 300 K.Finally,the peak ZT of Ge₀.64Sn₀.2Mn₀.16 Te reaches 1.81 at 727 K due to the excellent electric transport performance and extremely low thermal conductivity of the co-doped sample.In this study,the lower limit and upper limit of temperature are 300 K and 800 K respectively,and the maximum ZT_ave value of Ge₀.64Sn₀.2Mn₀.16 Te is 1.19,which makes the manufacture of thermoelectric devices have broad application prospects.2.A high quality co-alloy sample Ge₀.8Mn₀.1Sn₀.1Te_1-x（x=0,0.02,0.04,0.06）was synthesized by mechanical alloy combined with SPS sintering process.The results show that Mn/Sn co-doping and Te defects have significant effects on the microstructure and thermoelectric transport properties of the sample.Due to the synergistic effect of preparation technology and its doping properties,although the semiconductor conductive behavior cannot be changed,Mn/Sn co-alloy doping and introduction defects can effectively reduce the hole concentration in GeTe and increase the effective mass of carriers,so that the doped sample can obtain a higher Seebeck coefficient.For example,the Seebeck coefficient of Ge₀.8Mn₀.1Sn₀.1Te0.94 is about 4 times that of pure GeTe at 500 K.For example,the PF value of the defective sample Ge₀.8Mn₀.1Sn₀.1Te0.94 at 625 K is twice that of the pure sample.In addition,the thermal conductivity of the doped samples decreased significantly,which was caused by the multi-scale full frequency phonon scattering composed of mass and stress field fluctuations,point defects,dislocations,nanoparticles and noncoherent interfaces.For example,the lattice conductivity of the defective Ge₀.8Mn₀.1Sn₀.1Te0.94 sample decreased by⁷0% compared with the pure GeTe sample at room temperature.Due to the improvement of the conductivity and decrease of the thermal conductivity of the doped sample,the maximum ZT value of the final defective sample Ge₀.8Mn₀.1Sn₀.1Te0.94 is about 1.68 at 737 K.Compared with other conventional solid-phase synthesis methods,the maximum and average ZT values reported here are improved,which opens a new way to optimize the thermoelectric properties of GeTe-based semiconductors and lays the foundation for commercial applications.