Thermoelectric Performance Of PbTe-based Compounds And Cu₂Se Synthesized Under High Pressure

With the development of society,the problems of energy and environment have become brook no delay.Therefore,it is urgent to look for a new clean energy.Thermoelectric material is a kind of functional semiconductor material which can directly realize the conversion between electricity and heat energy.Thermal power generation and cooling devices made of thermoelectric materials have been extensively applied in industrial waste heat harvesting,aviation,microelectronics,refrigeration and other fields.In recent years,with the intensively study of thermoelectric materials,the conversion efficiency of thermoelectric materials has been greatly improved.As mid-temperature thermoelectric materials,ZT of PbTe and Cu₂Se systems have exceeded 2.0 at about 900K,and have broad prospects for development.In this paper,PbTe and Cu₂Se thermoelectric materials were prepared by high pressure synthesis.The electron and phonon transport properties were controlled by solid solution and doping to improve properties of thermoelectric materials.The main results are as follows:Single elemental doped n-type PbTe compounds were prepared with high pressure synthesis?HPS?followed by spark plasma sintering?SPS?.Thermoelectric properties of these samples are closely related to carrier concentration.In general,peak ZT increases and occurs at higher temperature with increasing carrier concentration for each doping element,meanwhile the average ZT decreases due to substantially smaller ZT at low temperature.which is agiant the improvement of the conversion efficiency.Through analysis of the data obtained,it is found that the optimum carrier concentration for n-type PbTe materials is around 1x10¹⁹ cm^-3,and within this optimum carrier concentration range the average ZT of most doping elements are about 0.7.The average ZT of Cr-doped samples is as high as 0.75.Because average ZT determines the performance of thermoelectric devices,our study provided a solid foundation for further improving the performance of n-type PbTe thermoelectric devices.Al doped n-type PbTe-PbSe compounds were prepared with high pressure synthesis followed by spark plasma sintering,and explored an effective way to further improve the thermoelectric properties of PbTe materials.The incorporation of selenium has dual effects on the system.Firstly,the carrier concentration can be further modified with varying Se content,on the basis of Al doping.Secondly,the lattice thermal conductivity can be greatly suppressed due to the alloying effect,and the lowest value is 0.46Wm^-1K^-1.Meanwhile,the power factor is kept at an acceptable level by adjusting the carrier concentration.The thermoelectric properties of Al_xPbTe_1-ySe_y samples have been significantly improved through carrier concentration optimization and alloying effect.ZT_avg reached 0.82 in Al_0.02PbTe_0.75Se_0.25 sample,which was 60%higher than Al_0.02PbTe sample,and provide clues for further improving the thermoelectric properties of n-type PbTe materials.A series of n-type InSb doped and In,Sb co-doped PbTe compounds were prepared with high pressure synthesis followed by spark plasma sintering.The results of thermoelectric properties illustrate that compared with InSb doping,In,Sb co-doping is more effective in improving PbTe system thermoelectric properties.For In,Sb co-doping samples,at Sb content fixed circumstance,through changing Indium doping level,carrier concentration can be controlled at low temperature without affecting high temperature carrier concentration,which finally improves room temperature thermoelectric properties and ZT_avg.0.3%and 0.5%Indium doped samples can obtain peak ZT=1.1 and improve room temperature ZT to 0.3,which make ZT_avg reached 0.74 and much higher than Sb_0.02PbTe sample with ZT_avgvg of 0.56.Considering the cost of practical application and the impact on environment,we studied Cu₂Se thermoelectric materials.A series of non-stoichiometric Cu₂Se compounds were synthesized by cold pressing and studied Cu₂Se crystal structure and thermoelectric properties.Compared with traditional synthesis methods,cold pressing method has many advantages,such as high efficiency,energy conservation,eco-friendly,easy regulation of components,high repeatability and so on.Cu_2-xSe samples synthesized by cold pressing reacted completely and had good crystallinity.The density can reach 98%of the theoretical density.With the decrease of copper content,Cu_2-xSe samples gradually changed from a?monoclinic?phase to b?cubic?phase,and carrier concentration increased.The maximum power factor of Cu_1.9Se sample is 1600?Wm^-1K^-2 at 900K.The maximum ZT of Cu_1.975Se sample reacheed 1.4 at 900 K,which is superior to other synthesis methods.In addition,we found that there were intermediate products in the process of cold-pressing synthesis of Cu₂Se.The sequence of products appear was Cu₃Se₂,Cu_2-xSe and Cu₂Se.