Research On The Preparation And Performance Optimization Of Te-based Thermoelectric Material

Thermoelectric materials enable direct conversion between heat and electricity,and which has a significance economic value and social meaning in the fields of waste heat recycling,alleviating energy crises,and reducing environmental pollution.With the in-depth research,the field of thermoelectric materials has become increasingly extensive.Among them,Te-based semiconductor materials are the most promising mid-temperature thermoelectric materials,but there is still significant room for improvement in their thermoelectric performance.This article systematically investigates the preparation and properties optimization of bulk thermoelectric materials for SnTe and Te using hot pressing and high temperature and high pressure(HTHP)methods,respectively.Based on the principle of doping modification,the thermoelectric properties of SnTe and Te have been optimized by using nonequivalent element doping and introducing the second phase to regulate carriers,combining with high-energy ball milling for nano-scaling(introducing grain boundaries)and high pressure inducing lattice defects(high-density dislocations,lattice distortions,etc.)and coupled with Te-Se solid solution(introduce amounts point defects)to reduce phonon thermal conductivity,the thermoelectric performance of SnTe and Te materials has been optimized.The structure and morphology of the synthesized bulk materials were analyzed by XRD,Raman,XPS,SEM and TEM.The thermoelectric properties of the synthesized samples were studied by resistivity,Seebeck coefficient and thermal conductivity.The mechanism and mechanisms underlying the performance optimization of the synthesized samples were discussed based on the analysis of experimental data.The conclusions are as follows:(1)Nanostructured In-doped SnTe(InxSn1-xTe)were prepared by high energy ball milling combining with vacuum hot pressing sintering.The structure,thermal stability and thermoelectric properties were measured and analyzed.The results demonstrate that In doping can effectively regulate the thermoelectric properties of SnTe.With the increase of In doping,the carrier concentration decreases from 2.83×1020 cm-3 of SnTe to 3.19×1019 cm-3 for In0.0125Sn0.9875Te at 300 K,and the Seebeck coefficient also increased from 80 μVK-1 to 119 μVK-1.The thermal conductivity was reduced to～2.7 Wm-1K-1 at 710 K derive from the reduced carrier concentration and decreased grain size.The power factor is higher than 20 μWcm-1K-2 in the whole test temperature range,which also makes the synthesized sample obtains the higher average figure of merit zTave～0.34 in the working temperature range of 300-610 K.(2)Based on the advantages of high pressure in improving doping efficiency and regulating microstructure,the thermoelectric properties of elemental semiconductor Te doped with Ag,Sb and Ag-Sb co-doping were studied.The electrical properties of Te can be improved by controlling the carrier concentration through Sb doping and enhancing the carrier mobility by constructing fast conducting channels through Ag doping.With Ag doping,the mobility of Te increased from 197.6 cm2V-1s-1 of Te to 499.4 cm2V-1s-1 of Ag0.0075Te0.9925 at 300 K.The carrier concentration increased from 7.2×1016 cm-3 of Te to 3.7×1019 cm-3 of Sb0.0025Te0.9975,leading to a decrease in resistivity from 5751.7 μΩm to 21.7 μΩm at 300 K.Furthermore,it’s worth to noting that the carrier concentration and mobility of element Te are simultaneously optimized through Ag-Sb co-doping,resulting in further improvements in the resistivity and Seebeck coefficient compared to single-doped Ag and Sb samples.When the doping concentration of Ag-Sb is 1 at.%,the sample of(AgSb)0.0iTe0.98 achieves the highest zT value of～1.17 at 600 K.In addition to optimizing the performance through doping,the thermal stability of bulk materials synthesized by HTHP methods is investigated,revealing improved service performance after thermal treatment.(3)In this study,elemental Te is used as the base material,and under the conditions of 4.5 GPa and 1000 K,the second phase of black phosphorus(BP)is incorporated into Te to overcome the poor stability and degradation issues of BP by constructing Te-BP heterostructures.A series of bulk materials,Te-BP,(Ge)Te-BP,(Ge,Se)Te-BP and(Ge,Sb)Te-BP are prepared.By introducing a small amount of(Ge)BP,the carrier concentration increases from 2.32×1018 cm-3 for pure Te to 3.18×1019 cm-3 for Te+1 GeP5 wt.%,and the resistivity decreases from 490 μΩm to 139 μΩm.Additionally,the bipolar effect is suppressed,resulting in an increase in the Seebeck coefficient and a decrease in thermal conductivity in the high-temperature region,leading to significant improvements in the thermoelectric figure of merit(zT)and average zT(zTave)values.Furthermore,Sb doping is employed to further enhance the electrical transport properties of Te,while Se is used to further reduce the thermal conductivity.The power factor and thermal conductivity of the Sb0.0025Se0.03Te0.9675+0.25 GePs wt.%are optimized to 9.4 μWcm-1K-2 and 0.53 Wm-1K-1,respectively,at 610 K,and the zT value is improved from the maximum value of 0.24 for pure Te to 1.1 for Sb0.0025Se0.03Te0.9675+0.25 GeP5 wt.%.(4)In previous experiments,it was confirmed that high-pressure environments can enhance the doping efficiency of dopants.Therefore,we investigated the influence of pressure on the thermoelectric properties of Te-based materials.Based on the optimized stoichiometric ratio of Sb0.0025Se0.03Te0.9675+0.25GeP5 wt.%,we explored the effects of pressure on the thermoelectric performance of the matrix.The results indicate that compared to traditional preparation methods,high-pressure environments can improve the doping efficiency of the dopant elements.Additionally,samples prepared under high pressure exhibit a more complex microstructure,scattering phonons at multi-scales,which contributes to a decrease in thermal conductivity.Under the same synthesis temperature,with increasing synthesis pressure,the carrier concentration of the Sb0.0025Se0.03Te0.9675+0.25 GePs wt.%increased from 1.86×1019 cm-3 at 1.5 GPa to 6.54×1019 cm-3 at 5.0 GPa.The minimum phonon thermal conductivity also decreased from a value of 0.81 Wm-1K-1 at 1.5 GPa to 0.40 Wm-1K-1 at 4.5 GPa.Both the electrical and thermal transport properties improved simultaneously,with the highest zT value increasing from 0.5 at 1.5 GPa to 1.1 at 4.5 GPa.It was concluded that synthesis pressure has distinct advantages in modulating the thermoelectric properties of Te-based materials.(5)In order to accurately analyze the presence of BP in Te-based materials and the effect on the thermoelectric properties of different matrix materials,BP was incorporated into elemental Te,Sb-doped Te,and Se-alloyed Te,and the electrical and phonon transport properties were analyzed.By high temperature in-situ Raman measurement,it was determined that constructing heterogeneous structures can effectively improve the stability of BP,allowing it to remain stable above 600 K.By analyzing the thermoelectric properties of BP introduced sample and non-BP,it was determined that the introduction of BP can simultaneously improve the electrical and thermal transport properties of Te matrix materials,resulting in a significant increase in the highest(average)zT value.The improvement percentages reached 79%(157%),158%(193%),353%(390%),30%(20%)and 49%(30%)respectively,providing a direction for the search for high-performance thermoelectric materials based on Te elements.