High Pressure Synthesis And Thermoelectric Performance Of Mid-Temperature Thermoelectric Material

Thermoelectric materials have garnered significant attention as green energy materials capable of converting thermal energy into electrical energy.The principle of thermoelectric effect positions these materials as crucial components in renewable energy and energy conversion fields.With the increasing global energy demand and escalating environmental issues,the research and development of thermoelectric materials hold immense importance in addressing current energy and environmental challenges.Therefore,this paper aims to investigate the performance optimization of mid-temperature thermoelectric materials by enhancing their thermoelectric properties through the optimization of carrier concentration,alloy scattering,and microstructure control.Specifically,four mid-temperature thermoelectric materials were selected for this study:Mg₂Si_0.4Sn_0.6 solid solution,Cu₂Se_0.5S_0.5 solid solution,Pb Te,and Pb Se.Among these materials,the Mg₂Si_0.4Sn_0.6 solid solution exhibits promising potential for applications in waste heat recovery in aerospace,automotive,and other fields.This is attributed to its lightweight nature,low cost,abundant reserves,and environmentally friendly constituent elements.Another material of interest is the Cu₂Se_0.5S_0.5 solid solution,which is based on Cu₂Se and possesses liquid-like characteristics along with an extremely low lattice thermal conductivity.This material has attracted extensive attention and undergone in-depth research.Additionally,Pb Te-based thermoelectric materials are widely recognized as one of the most outstanding mid-temperature thermoelectric materials.They are known for their high valence band degeneracy,strong anharmonicity,simple crystal structure,and tunable microstructure,making them a focal point in thermoelectric research.Furthermore,Pb Se-based thermoelectric materials share similar crystal and band structures with Pb Te,and their abundant raw material reserves make them an ideal substitute for Pb Te.As a result,they have garnered significant research interest.To conduct this study,the aforementioned materials were synthesized using a high-pressure synthesis method.Following synthesis,their microstructure and thermoelectric properties were characterized and analyzed.The n-type Mg₂(Si_0.4Sn_0.6)_1-xSb_x samples were prepared by a high-pressure synthesis method combined with spark plasma sintering technique,and their structure and thermoelectric properties were characterized and analyzed.The energy band convergence was achieved by manipulating the Si/Sn ratio,which enhanced the Seebeck coefficient and optimized the electrical transport properties;In addition,the large atomic mass difference between Si and Sn introduced point defects and lattice distortion,which reduced the thermal conductivity.Under the combined effect of both thermal and electrical aspects,the Mg₂Si_0.385Sn_0.6Sb_0.015 sample achieves a peak ZT of～1.6 at 803 K.The average ZT value over the entire test temperature region is～0.96,corresponding to a single-leg conversion efficiency of～14%.The optimal carrier concentration of thermoelectric materials increases with increasing temperature.However,conventional aliovalent doping usually provides an approximately constant carrier concentration over the whole temperature range,which can only match the optimal carrier concentration in a narrow temperature range.In this work,n-type indium and aluminum codoped Pb Te were prepared with high-pressure synthesis,followed by spark plasma sintering.While Al doping can provide a roughly constant carrier concentration with varying temperatures,In doping can trap electrons at low temperatures and release them at high temperatures,thus optimizing the carrier concentration over a broad temperature range.As a result,both electrical transport properties and thermal conductivity are optimized,and a significantly enhanced thermoelectric performance is achieved in In_xAl_0.02Pb_0.98Te.The optimal In_0.008Al_0.02Pb_0.98Te shows a peak ZT of 1.3 and an average ZT of 1,with a decent conversion efficiency of 14%.The n-type In_xAl_0.02Pb_0.98Te samples were prepared by high-pressure synthesis method combined with spark plasma sintering technique,and their structure and thermoelectric properties were characterized and analyzed.Inclusion of In into Pb Te creates a deep defect level which can optimize the carrier concentration over the whole temperature range.In addition,the thermal conductivity is substantially reduced due to the decrease in electronic thermal conductivity.The simultaneous optimizations of electrical and thermal transport properties contribute to the thermoelectric properties enhancement.Specifically,the peak ZT of 1.3 at 763 K and average ZT of 1 are achieved in In_0.008Al_0.02Pb_0.98Te,with a decent conversion efficient of 14%.The n-type In_yAl_0.02Pb Se_0.75Te_0.25samples were prepared by high-pressure synthesis method combined with spark plasma sintering technique,and their structure and thermoelectric properties were characterized and analyzed.The decrease in lattice thermal conductivity is due to atomic mass fluctuations and lattice distortion caused by Se alloying.The lattice thermal conductivity of the Al_0.02Pb Se_0.25Te_0.75 sample was reduced to～0.89Wm^-1K^-1 at 683 K.The lattice thermal conductivity of the sample was optimized by additional In doping in the broad temperature region.Finally,a peak ZT of～1.2 was achieved in In_0.008Al_0.02Pb Se_0.75Te_0.25,with an average ZT value of～0.91 over the entire test temperature region,corresponding to a single-leg conversion efficiency of～13.7%.The p-type Cu_2-xS_0.5Se_0.5 samples were prepared by high-pressure synthesis method combined with spark plasma sintering technique.The transition from monoclinicα-phase to cubicβ-phase and the hole concentration of the samples were modified by tuning the Cu deficiency;the carrier concentration and thermal conductivity were reduced by solid solution of Cu₂Se in S-element,and the high-temperature lattice thermal conductivity of all Cu_2-xS_0.5Se_0.5 samples was around 0.4 Wm^-1K^-1,this weak temperature dependence of such lattice thermal conductivity is the typical characteristics of liquid-like materials.Finally,a peak ZT of～1.9 at 903 K was obtained for the Cu_1.85Se_0.5S_0.5 sample.