| The emergence of increasingly severe energy depletion problems encourages the researchers to develop new methods to harvest energy.Thermoelectric materials,which can convert waste heat to electrical energy,have been considered a novel method to alleviate the energy crisis.Thermoelectric power generation and electric refrigeration based on thermoelectric materials have a wide application prospect in waste heat utilization and device refrigeration.In recent years,the thermoelectric properties of simple alloy compounds have been significantly improved by means of resonance energy level and energy band degeneracy,such as Bi3Te3,PbTe materials.To further improve the thermoelectric properties of simple alloy compounds or to explore new thermoelectric material systems is the development trend in the thermoelectric materials.In addition to optimization of the thermoelectric properties of existing simple binary alloy compounds the multi-alloy material system with special crystal structure and intrinsic low thermal conductivity also has the potential to be the excellent thermoelectric material.Quaternary chalcogenides have common composition of A2B?C?X4 where A=Ag,Cu;B=Zn,Cd,Mn,Hg,Fe,Co;C=Si,Ge,Sn and X=Se,S,Te.These quaternary compounds have the tetragonal lattice structure which is also called the low-symmetry non-cubic structure.This structure can be described as a double-periodic cubic zinc-blende supercell in the z-direction.All atoms have a tetrahedral surrounding and the tetrahedron is formed by one selenium surrounded by four metal atoms.Cationic elements form a tetrahedral void with anionic Se atoms residing inside it.The difference of electronegativity between cations and variant interatomic distances results in natural superlattice structure with localized lattice distortions.At the same time,most of quaternary chalcogenides have a wide band gap,which can effectively avoid the conduction of high temperature polaron,so the quaternary alloys have the potential to be the excellent medium and high temperature thermoelectric material and has been widely concerned by researchers.Among these quaternary compounds,Cu2ZnSnSe4 alloy has the advantages of high element enrichment,less toxic nature and simple synthesis process.However,the electrical properties of these materials need to be optimized to further improve their thermoelectric properties.Single/dual doping and vacancy introduction are commonly used to improve the electrical properties of materials.Through these methods,the carrier concentration of the material is increased,and then the power factor is significantly improved.In summary,the solid-phase melting method is adopted in this paper to optimize the synthesis conditions and study the relationship between the synthesis conditions and the material preparation.Effect of unit doping and dual doping on the thermoelectric properties of Cu2ZnSnSe4.The research results of this paper are as follows:1.Cu2.1Zno.9SnSe4 samples annealed at 600?,650?,700? and 725? are synthesized by solid phase melting method.The main phase of all samples is kesterite structure and all samples have high relative density.The impurity peak strength of CuSe in the sample decreased gradually with the increase of annealing temperature,and the electrical properties of the sample are improved.As a result,the sample annealed at 725? has the highest power factor of 398.9 ?W/K2m at 673 K attributing the lowest resistivity and moderate Seebeck coefficient.The grain size of the sample increases with the increase of annealing temperature,and the number of grain boundaries decreases,which reduces the phonon scattering by grain boundaries and increases the lattice thermal conductivity of the material.The lowest lattice thermal conductivity of 1.9 Wi-1K-1 is obtained at 673 K for the sample annealed at 600?.The highest zT value of 0.1 is obtained at 673 K for the sample annealed at 725?,which attribute to the highest power factor and relatively moderate thermal conductivity.2.The Cu2ZnSnSn4 and Cu2ZnSn0.98Pb0.02Se4-xTex(x=0.005,0.010,0.015,0.020,0.025,0.030)samples are prepared by solid phase melting method.All the samples have the main phase of kesterite structure.In addition,all samples have high relative density and uniform element distribution.The x=0.025 sample has low resistivity and moderate Seebeck coefficient resulting in highest power factor value of 678.3 ?W/K2m among all the samples.That the high symmetry of lattice structure and the point imperfection scattering synergistically affect the lattice thermal conductivity of materials.Therefore,the lowest lattice thermal conductivity of 1.5 Wm-1K-1 is obtained by the x=0.025 sample at 673 K.And the highest zT value 0.24 is obtained at 673 K for the x=0.025 sample,which is 1.4 times of the value of pristine Cu2ZnSnSe4.3.High density Cu2ZnSnSe4 and Cu2.1Zn1-xCoxSnSe4(x=0.00,0.01,0.02)samples are successfully synthesized by solid phase melting method.All samples have the single phase of kesterite structure.The resistivity of Cu2.1Zn1-xCoxSnSe4(x=0.00,0.01,0.02)is much higher than that of Cu2ZnSnSe4,due to which Cu2ZnSnSe4 has the highest power factor 436 ?W/K2m at 673 K.Cu and Co co-doping introduced point defects into the material and successfully reduced the lattice thermal conductivity.Therefore,the lowest lattice thermal conductivity of 1.2 Wm-1K-1 is obtained by Cu2.1Zn0.98Co0.02SnSe4 sample at 673 K.Cu2ZnSnSe4 sample has the highest power factor and relatively low thermal conductivity.Therefore,the highest zT value of 0.2 is obtained by Cu2ZnSnSe4 sample at 673 KIn this thesis,the influence of synthesis conditions on crystal structure and thermoelectric properties of quaternary alloy Cu2ZnSnSe4 is systematically studied.On this basis,the single/dual doping method is used to optimize the carrier concentration of the sample,and then the electrical properties of the sample are adjusted.The changes of thermal conductivity are further analyzed by lattice distortion factor and point defect.Finally,the thermoelectric properties of the sample are optimized.Through the above specific research work,this thesis summarized the relationship between the preparation conditions and the sample microstructure and thermoelectric properties,to deepen the understanding of element doping regulation of the thermoelectric properties of the quaternary alloy. |
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