Preparation And Thermoelectric Properties Of Intermetallic Compound CoSbS

The modernization of the world economy has brought great convenience to people's lives,and with the rapid economic development,the energy crisis and environmental pollution problems become increasingly serious.Therefore,it is important to find environmental friendly clean energy and improve energy efficiency.Thermoelectric conversion technology can achieve direct conversion of thermal energy and electric energy.The key factor to improve the efficiency of thermoelectric conversion is to find thermoelectric materials with high conversion efficiency.High-performance thermoelectric materials should have as low thermal conductivity as glass,and have excellent electrical properties like crystals.Intermetallic compounds people have found high-performance thermoelectric materials such as skutterudite and other intermetallic compounds.The compounds of this type are mainly composed of elements with small difference in electronegativity.Recently,it has been found that the natural mineral based intermetallic compound CoSbS is a potential medium and high temperature thermoelectric material.It has a high power factor(1.5 mW m^-1 K^-2 @ 773 K)and a high thermal conductivity(3.9 W m^-1 K-1 @ 773 K).The electronic structure calculation shows that the high power factor of CoSbS is derived from its characteristics in the valence band and the conduction band with low energy band dispersion.Ni-doped Co-site can effectively improve the carrier mass concentration and density of state effective mass(power factor is increased to 2 mW m^-1 K^-2 @ 873 K);Te doped on Sb site,can simultaneously increase the material loading The carrier concentration and mobility of the current and the power factor can reach 2.7 mW m^-1 K^-2 @ 730 K.However,the lattice thermal conductivity of the material is still relatively high,which greatly limits the thermal power generation efficiency of CoSbS.In this work,CoSbS is chosen as the research object,using the traditional solid-phase synthesis and ball milling.The use of traditional solid phase synthesis in the Se solution S increased CoSbS in the mass fluctuations and stress fluctuations,effectively reducing the lattice thermal conductivity of the material.The Cu and Zn-doped CoSbS samples were prepared by high-energy ball milling.The thermal and electrical properties of the bulk materials were optimized,and the thermoelectric properties of the composites were improved.The main contents of this thesis are as follows:1.Pure CoSbS compound was synthesized by the traditional solid-state reaction method.The related thermoelectric properties were tested and compared with the CoSbS pristine reported by other groups.It is found that different synthesis methods have a significant effect on the thermal and electrical behavior of CoSbS,because different preparation methods will cause different degrees of S vacancy,which will affect the electrical behavior of the material.2.The reason for the high thermal conductivity of CoSbS relies on the smaller Gruneisen parameter,the high Debye temperature and the strong bond?ionic bond?.The method of alloying S with Se effectively reduces the lattice thermal conductivity?adding specific values?.The analysis of Debye-Callayway model shows that the lattice thermal conductivity of the system is reduced by solid solution,namely the mass fluctuation and the stress field fluctuation caused by the difference of S atom mass and atomic radius.On the other hand,because of the smaller electronegativity difference between Se and S,the power factor of the material is also improved while the thermal conductivity of the system is reduced,and finally the ZT of the CoSbS0.85Se0.15 sample is reached 0.35 at 923 K,59% enhancement compared to the pure CoSbS.This also proves that the solid solution is an effective means to improve the thermoelectric performance of CoSbS system.3.Based on the previous discussion the thermoelectric properties of the CoSbS1-xSex compound were optimized when the Se content was 0.15.In this chapter,the carrier concentration of CoSbS0.85Se0.15 is further control by Cu and Zn dopeing to optimizing the power factor.The results show that carieer concentration increasing via Cu doping.At room temperature,the P-N transition behavior occurs when the Cu doping concentration is 5%.The band structure shows that the Cu-d orbital impurity energy level is near the Fermi level,on the one hand activation of holes caused by impurity level reduced the electron concrntration in the compound.On the other hand it increased the carrier scattering probability.The Cu doping reduces the power factor and thus ZT value of the material.When the Zn doping concentration is 5%,the second phase appears in the material.The mobility of the electrons in the material exhibits a decrease-up-down behavior,and the carrier concentration appears to decrease first and then rise.In the end,the lattice thermal conductivity of the compound by introducing the impurity atoms is effectively suppressed,and the ZT value of the material increases from 0.17 to 0.34.