Microstructure Control And Thermoelectric Performance Optimization Of SnTe Based Alloy

SnTe alloy is a kind of environmental-friendly thermoelectric material for its same crystal structure and similar energy band structure as Pb Te alloy.The Seebeck coefficient and power factor of SnTe alloy is relatively low due to the intrinsic high hole concentration and the large energy offset between light and heavy valence bands.Meanwhile,the lattice thermal conductivity is relatively high,resulting in the low figure-of-merit.One of the important ways to optimize the thermoelectric performance of SnTe is to reduce the lattice thermal conductivity and increase the power factor simultaneously.In this paper,based on the band convergence induced by Mn alloying,high-density dislocations are constructed by high-temperature quenching plus medium temperature annealing,and on the basis of resonant level induced by In alloying,nano-precipitates are constructed to reduce the lattice thermal conductivity of SnTe by Cu and Li doping and then improve its power factor and ZT value simultaneously.The effects of doping on the microstructure and thermoelectric transport properties of SnTe were systematically studied by using advanced characterization equipment such as X-ray diffraction,scanning electron microscope?SEM?,aberration-corrected scanning transmission electron microscope?ac-STEM?,Seebeck coefficient and electrical conductivity measurement system,Hall effect measurement system and laser flash diffusivity instrument.The results showed that the excess Sn self-compensating cation vacancy could effectively reduce the carrier concentration and improve the Seebeck coefficient,and then improve power factor and ZT value in the whole temperature range.With the increase of excess Sn content,_average power factor(PF_ave)and _average ZT value(ZT_ave)of Sn_1+?Te alloys increased first and then decreased.When ? was 0.03,PF_ave and ZT_ave between 300 K ? 873 K reached the maximum of 15.97 ?W cm^-1K^-2 and 0.358,respectively.On this basis,the thermoelectric performance of Sn_1.03-xMn_xTe alloy were further improved by combining the energy band convergence induced by Mn doping with the high density dislocation formed by high temperature quenching plus medium temperature annealing.With the Mn content increasing,the dislocation density and Seebeck coefficient increased,while electrical conductivity,lattice thermal conductivity and total thermal conductivity decreased,which resulted in that the power factor and figure of merit first increased and then decreased.When x = 0.09,the maximum value of PF_ave and ZT_ave were 18.21 ?W cm^-1 K^-2 and 0.78,respectively.When the temperature of cold and hot sides was 300 K and 873 K respectively,the calculated maximum conversion efficiency reached 11.5%.It was found that on the basis of resonant state induced by indium alloying at low temperature range,Cu doping reduces the carrier concentration and increases the Fermi level of?SnTe?_2.94?In₂Te₃?_0.02-?Cu₂Te?x alloys,and the energy band convergence effect appears in the high-temperature region.High Seebeck coefficient and high power factor are obtained then.At the same time,coherent Cu₂ Te precipitates and Cu interstitial atoms appeared in the alloy,which enhanced the phonon scattering and decreased the lattice thermal conductivity.When x = 0.18,the lattice thermal conductivity at 873 K decreased to 0.47 W m^-1 K^-1,and the corresponding ZT value reached 1.55.It was pointed out that Li doping caused the formation of dispersed and coherent precipitates,Li Te₃,with size less than 20 nm in the indium alloyed SnTe,which enhanced phonon scattering and reduced the lattice thermal conductivity.With the increase of Li doping,the lattice constant,Seebeck coefficient and lattice thermal conductivity of?SnTe?_2.94?In₂Te₃?_0.02-?Li₂Te?_x alloy decreased,and the carrier concentration and electrical conductivity increased.When x = 0.045,the PF_ave between 300 K and 873 K reaeched 28.01 ?W cm^-1 K^-2,which is 192% higher than that of the pure SnTe alloy.At the same time,the calculated maximum output power density and conversion efficiency are 5.63 W cm^-2 and 9.67% respectively when the length is 4mm and the temperature of the hot and cold side are 300 K and 873 K respectively,owing to the insensitive of self compatibility to temperature.