Research On Improving The Performance Of Thermoelectric Materials PbTe And CU_2-χS In The Mid-temperature Region By Microstructure Design

As a green energy technology,thermoelectric conversion technology can realize the direct conversion between thermal energy and electric energy.Thermoelectric devices have the advantages of long service time,stable performance,simple structure and no moving parts,and play an important role in the field of special power supply and local refrigeration.The energy conversion efficiency of thermoelectric devices mainly depends on the thermoelectric properties of the materials used,so improving the thermoelectric properties of the materials has become an urgent task.In this paper,the middle temperature thermoelectric materials PbTe and Cu2-xS（x=0～0.25）system were studied to improve their thermoelectric properties.Due to the high symmetry crystal structure and high degeneracy of energy band,PbTe has become an excellent thermoelectric material in the middle temperature region.With the rapid development,the thermoelectric properties of p-type PbTe have been greatly improved,ZTmax～2.5.However,due to the difference of energy band structure,in order to improve the performance of n-type PbTe,the microstructure and thermoelectric properties of n-type PbTe are studied in this paper.Firstly,Sb2Te3 was introduced into PbTe,and the matrix（PbTe）81-Sb2Te3（PST81）was constructed.On this basis,Sb was doped to optimize the carrier concentration,and Cu2Te defects were introduced into the optimal carrier concentration PST81-0.6Sb to reduce the lattice thermal conductivity,the thermoelectric properties of ZTmax～1.6,and 323-823 K average ZTavg～1.0 have been realized in PST81-0.6Sb-2Cu2Te at 823 K.It is found that there is a coherent Sb/CuTe core-shell structure in the main phase lattice of PbTe,and its selective scattering of carriers and phonons can realize the decoupling of the electro-acoustic transport properties,is the material thermoelectric performance promotion main factor.In addition,three existing forms of Cu in PbTe,namely CuTe,substitution and interstitial Cu,have been proposed and characterized for the first time.In addition,the carrier mobility and lattice thermal conductivity of n-type PbTe were optimized by using the modulation doping design.The Cu was doped in the PbTe/PbS composite phase（PbTe）0.9（PbS）0.1,based on the difference of solid solubility of Cu gap caused by the space size of PbTe-PbS two-phase tetrahedral gap,the PbTe with low Cu gap concentration and high mobility was used as carrier transport channel,and the PbTe with high Cu gap concentration was used as carrier storage source,combining the scattering effect of the two-phase interface on the medium-and long-wave phonons,the increase of carrier mobility and the decrease of lattice thermal conductivity are realized simultaneously,finally,high carrier mobility（1.25 × 1019 cm-3）and extremely low lattice thermal conductivity（0.26 W/mK at 673 K）of 1001 cm2V-1s-1 were achieved in（PbTe）0.9（PbS）0.1-0.3%Cu,and the average thermoelectric merit of 323-873 K was ZTavg～1.06,it is one of the highest properties of the n-type PbTe based materials.Cu2-xS system,a thermoelectric material in the middle temperature region,has the advantages of abundant element reserves and friendly environment,and its liquid-like behavior of random occupation of Cu after ordered-disordered phase transition leads to extremely low lattice thermal conductivity,therefore,it is a potential thermoelectric material.The effect of Cu vacancy concentration（x=0～0.25）on the phase composition and thermoelectric properties was studied,the thermal stability of each phase of Cu2-xS was evaluated in the temperature range of 300-773 K,and the possible problems in measuring thermal conductivity of this kind of system by laser flash heating method were discussed.It is found that Cu1.8S and Cu1.96S have high thermal stability,and the corresponding samples have obvious hysteresis behavior in the testing process of electrical properties.In this paper,the selective scattering of carriers and phonons in n-type PbTe is realized by using Sb/CuTe core-shell structure and modulation doping structure,and the electrical and thermal properties of n-type PbTe thermoelectric materials are optimized,the enhancement of thermoelectric ZT in the whole temperature range was realized,and the microstructure was analyzed by spherical aberration transmission electron microscope（Cs-STEM）,and the mechanism of performance improvement was explained.The formation and thermal stability of each phase in Cu2-xS（x=0～0.25）system were studied by controlling the concentration of Cu vacancies,this work has a positive reference value for clarifying the complex and inconsistent thermoelectric transport properties of Cu2-xS system.