Optimizing Thermoelectric Performance Of SnTe-based Materials By Doping And Multi-scale Microstructure Manipulation

Heat and electricity are two forms of energy.Heat is ubiquitous,such as emanating from the Sun,industrial sectors,automobiles,or even the human body,but most of this heat is lost in the form of waste heat.Electricity is versatile,but its production is demanding.Thermoelectric technology provides a simple and environmentally friendly solution for the direct conversion of waste heat to electric energy.Meanwhile,thermoelectric generators are solid-state devices without moving parts,no noise,zero emission,stable operation,long life,and can be miniaturized,which is the only choice to realize waste heat recycling at present.The field of thermoelectric materials has stagnated several times in its recent 60-year history,but each time it was rejuvenated by new paradigms.Especially in the last twenty years,thermoelectric conversion technology has steadily attracted the attention of researchers.Improving the conversion efficiency of thermoelectric devices is the focus of thermoelectric research,and the conversion efficiency mainly depends on the dimensionless figure-of-merit?ZT?.Therefore,the central issue in thermoelectrics materials research is to increase ZT.High-performance SnTe is a promising lead-free thermoelectric material.However,compared with its Pb analogue,SnTe has limited thermoelectric potentials because of the low Seebeck coefficients and the relatively large thermal conductivity.The dramatic enhancements in the figure of merit achieved in SnTe thermoelectric materials have come from the reduction in lattice thermal conductivity and improvement in power factors.This thesis focuses on the fabrication of SnTe-based thermoelectric materials and the optimization of thermoelectric properties.The main contents are as follows:Se-doped SnTe_1-xse_x nanothermoelectric materials were synthesized by microwave-stimulated wet-chemical approach.The feasibility of using microwave method with low energy and high efficiency to prepare thermoelectric nanomaterials was proved,and we obtained the regular octahedron SnTe_1-xse_x particles with uniform morphology.Se-doping was beneficial to control the growth of SnTe with nanoscare size.Se substituted the Te site in SnTe,increase the band gap of SnTe and optimize the electrical properties of SnTe based materials.Se/Cd co-doped SnTe composite materials were fabricated by microwave-stimulated solvothermal method combined with spark plasma sintering?SPS?.Se/Cd dopants can control the growth shapes and induced lattice distortion.Dense SnTe_1-ySe_y and Sn_1-xCd_xTe_0.90Se_0.10 bulk samples can be obtained by improving the SPS process.Se/Cd doping can optimize the electrical properties of SnTe materials and increase their seebeck coefficient.Meanwhile,point defects,nanoprecipitated phase and a large number of grain boundaries are introduced.The multiphonon scattering mechanism reduces the thermal conductivity of SnTe,and finally improves the ZT value.The maximum ZT value of Sn_0.98Cd_0.02Te_0.90Se_0.10 samples reached 0.78 at 773 K.Zn-doped and Zn/In co-doped SnTe compounds were synthesized by microwave method.It was found that the solid solubility of Zn in SnTe was very low,and only a small amount of Zn could be doped into the SnTe matrix,while excessive Zn would form Zn Te nano precipitates to adhere to/embed into the SnTe matrix.Zn doping failed to optimize the electrical properties of SnTe,but the point defects and Zn Te nano precipitates can enhanced phonon scattering,reduced the thermal conductivity of SnTe,and finally improved ZT value.The maximum ZT value of Sn_0.97Zn_0.03Te reached 0.69at 823 K.Meanwhile,In doping can introduce resonance level and increase the power factor of SnTe.Finally,the thermoelectric property of Zn/In co-doping SnTe system increases significantly.A crowding-out effect strategy using Ag Cl as extruder was designed to fabricate SnTe thermoelectric materials with all scale hierarchical structure.Low melting point Ag Cl mixed with SnTe powder.Ag Cl was crowded out during spark plasma sintering and some of nano pores were created,the porosity enhanced phonon scattering.The crowding-out effect of Ag Cl induced all-scale hierarchical structure,including point defects,dislocation,nanopores and mesoscale grain boundary,etc.,inhibited phonon transmission and effectively reduced the lattice thermal conductivity of the material.(Sn_0.985In0.₀₁₅Te)1-x?Ag Cl?x?x=0,0.05,0.10,0.15?particles were synthesized by microwave method.It was found that In doping could change the growth morphology of SnTe,inducing SnTe grain with regular tetrahedral structure.Add a small amount of Ag Cl,some of nano pores were created on the surface of SnTe particles,while excessive Ag Cl would form a large number of Ag₂Te nanoparticles.In doping introduces resonance levels inside the valence bands of SnTe,leading to a significant improvement in the Seebeck coefficient.Ag Cl can induce the preparation of SnTe with full scale microstructure to enhance phonon scattering and reduce the thermal conductivity of SnTe.Synergistic effect of resonance level and full-scale structure can significantly increase the thermoelectric performance of SnTe,and the ZT value of?Sn0.985In0.015Te?0.90?Ag Cl?0.10 at 823 K is 0.86.