Self-propagating High-temperature Synthesis And Thermoelectric Performances Of Cu₂SnSe₃ Based Compounds

With the increasing energy crisis and environmental pollution,there has been a serious threat to human survival and social stability.Therefore,new energy materials and new energy conversion technologies have caught worldwide attention of the international community.Among them,the thermoelectric conversion technology which can directly convert thermal energy and electric energy with each other has attracted wide attention of researchers because of its great application prospect in waste heat recovery and utilization.At present,many thermoelectric materials such as Bi₂Te₃,PbTe,GeTe and skutterudite are reported to have excellent thermoelectric properties,but most of them contain toxic or precious elements,and the preparation process is complicated and energy-consuming.This greatly limits the scale of application of these materials.Therefore,the development of a kind of thermoelectric material with simple preparation process,low cost and excellent performance is of great significance for its scale application.In recent years,the ternary Cu-based diamond-like structure thermoelectric semiconductor material Cu₂SnSe₃ compound,for its excellent thermoelectric properties,conforms to the standard of"phonon glass electronic crystal"?PGEC?,and is free from toxic or expensive Pb,Te and Ag element,has received more and more attention.It's been considered to be a promising p-type medium-temperature thermoelectric material.The methods to optimize the thermoelectric properties of materials are mainly to improve the electrical transmission properties of materials by element doping or compound recombination,and to reduce the thermal conductivity of materials.Studies have shown that there is a three-dimensional Cu-Se conductive network for hole transport in Cu₂SnSe₃ compounds.The existence of Cu-Se bond network plays a main role in carrier conduction,while the Sn element contributes a little to electric transport.So doping in Sn position reduces the lattice thermal conductivity of the material can renduce the impact on that the material's electrical transport network while optimizing the thermoelectric performance.However,the mechanism of the influence of doping on the electrical transmission performance is still unclear,and the energy band structure of the material needs further study.In addition,the preparation process of the material needs to be improved,and it is currently required to perform long-time annealing to obtain a single-phase Cu₂SnSe₃compound,which wastes energy.Therefore,in order to solve these problems,it's essential to explore a new process with short cycle,low energy consumption,simple operation and pure product,to explore the change of the band structure of Cu₂SnSe₃compound in the deviation from the stoichiometric ratio of elements,and to further optimize its thermoelectric performance.The main research contents and research results of this thesis are as follows:The phenomenon of self-propagating combustion synthesis of Cu₂SnSe₃compound was first discovered,and a new technology for ultra-fast preparation of high-performance thermoelectric Cu₂SnSe₃ compound by SHS-PAS was developed.The phase transition mechanism of the ternary compound in the SHS reaction process was studied by quenching experiment and DSC simulation of the SHS reaction process for the first time.In the initial stage of the reaction,the Cu,Sn and Se elemental mixture first reacted between Cu and Se during the heating process.Solid phase reaction produced CuSe,then solid Cu reacted with liquid Se to form Cu₂Se,then Sn and Se reacted in liquid state to obtain SnSe,and Cu₂Se gradually reacts with Se until all of them were converted to CuSe,and finally after CuSe reacting with SnSe,a Cu₂SnSe₃ compound was obtained.Finally,the thermoelectric transport properties of the bulk materials were tested.The Cu₂SnSe₃ compound prepared by the SHS-PAS process achieved a maximum ZT value of 0.53 at 773 K,the thermoelectric performance of which was slightly higher than the products prepared by other methods.Cu_2-xSnSe₃?x=0.075⁰.175?and Cu₂Sn_1-y-y Se₃?y=0.06⁰.1?compounds were prepared by SHS-PAS process.The result of the X-ray diffraction?XRD?and electron probe?EPMA?test shows that the missing limit of Cu atoms is 0.15,and the doping limit of Sn atoms is 0.09.By using the density functional theory-based VASP software?Vienna ab-initio simulation package?to optimize the Cu₂SnSe₃crystal structure and energy band calculation,we found that with the increase of Cu or Sn loss,the Fermi level will go into valence bands to form multi-band transmissions and increase effective mass.The phase composition and microstructure of the two groups of compounds and the thermoelectric transport properties were studied:?1?As the amount of Cu missing increased,the conductivity of the material decreased.However,a significant increase in the effective mass results in an increase in the Seebeck coefficient and a decrease in the power factor of the Cu-dificient samples.At the same time,Cu deficiency can effectively enhance the scattering of phonons of materials,so the thermal conductivity?and lattice thermal conductivity?_L of the material decrease with increasing temperature.Finally,the Cu_1.875SnSe₃ sample has a ZT value of 0.95 at 800 K.?2?With the increase of Sn content,the sample mobility?decreases slightly,but the carrier concentration n increases significantly,so the material conductivity improved greatly.Under the effect of the conductivity,the power factor PF was much improved.Finally,the sample with a nominal composition of Cu₂Sn_0.93Se₃ had a ZT value of 0.87Ag-doped Cu_2-xAg_x SnSe₃?x=0.075⁰.175?samples,Sn-doped Cu₂Sn_1-y-y In_ySe₃?y=0.06⁰.1?samples,Ag and In co-doping Cu_1.85Ag_0.15Sn_1-yIn_ySe₃?y=0.06⁰.09?samples were prepared by SHS-PAS process.?1?In the Ag-doped Cu_2-xAg_xSnSe₃?x=0.075⁰.15?compounds,the doping limit of Ag atom is about x=0.15,and the Ag atom doped at the Cu site can improve the carrier effective mass of the Cu₂SnSe₃ material.In turn,the Seebeck coefficient of the material is greatly increased.However,because the conductivity of the material is lowered,finally the power factor is degraded.However,the phonon scattering is enhanced,which leads to a significant decrease in the lattice thermal conductivity of the material.Finally,the Cu_1.85Ag_0.15SnSe₃ sample achieved the maximum thermoelectric figure of merit ZT of 1.04 at 800 K,which is 100%higher than the intrinsic sample.?2?In the In-doped Cu₂Sn_1-yIn_y Se₃?y=0.06⁰.09?compounds,the doping limit of the In atom is about y=0.09.The In atom doped at the Sn site can effectively increase the carrier concentration of the compounds,thereby increasing the conductivity of the samples.Finally,the sample with y=0.09 achieved a maximum ZT value of 0.91 at 800 K,which was 75%higher than the intrinsic sample.?3?In the Ag,In co-doped Cu_1.85Ag_0.15Sn_1-yIn_ySe₃?y=0.06⁰.09?compounds,the conductivity is better than that of the intrinsic Cu₂SnSe₃ due to the interaction of two doping atoms.The increase is greatly improved,and the decrease of Seebeck coefficient and the increase of thermal conductivity are suppressed to some extent.The sample Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃ achieves the maximum ZT value of 1.12 at 800 K.,an increase of 115%compared to the intrinsic sample.The in-situ recombination experiments of powdered PAS sintering of Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃ compound were carried out by using Ag₂S,Ag₂Se and Ag.The reaction mechanism of three compounds with matrix was studied.The effects of Ag₂S,Ag₂Se and Ag on the phase composition,microstructure and electrothermal transport properties of Cu₂SnSe₃-based compounds were investigated.?1?Ag₂S decomposes into Ag element and S element under the high temperature condition of PAS sintering.S element is gasified and desorbed from the reaction system at high temperature,while high activity Ag element reacts with matrix.Under the process,the solid solution limit of Ag in the Cu₂SnSe₃ based compound increases,and the second phase of the compound nanometer phase of Cu_1.85-2xAg_0.15+2x.15+2x Sn_0.91In_0.09Se₃?x=3⁵%?is obtained,which is enriched in the grain boundary of the material.The nano-phase reduces the grain boundary barrier,improves the Seebeck coefficient of the material,and at the same time enhances phonon scattering and reduces the lattice thermal conductivity of the material.So that the ZT value of the material is improved,,and the maximum ZT value of the sample of x=4%is 1.58 at 800 K,which is 41%higher than that of the Ag and In co-doping samples.The intrinsic Cu₂SnSe₃ compound was increased by 190%.?2?The Ag₂Se compound is directly reacted with the matrix to obtain a second phase of the compound nanometer of Cu_1.85-2yAg_0.15+2ySn_0.91In_0.09Se₃?y=3⁵%?,which is also distributed in the grain boundary of the material.The y=5%sample achieved a maximum ZT value of 1.26at 750 K,an increase of 12.5%compared to the Ag,In co-doping sample,and an increase of 133%compared to the intrinsic Cu₂SnSe₃ compound.?3?Ag element does not increase the solid solubility of Ag in the Cu₂SnSe₃ based compounds.In addition,the process forms a second phase with a large scale in the matrix,and the distribution is disordered,and there is obvious agglomeration.Therefore,the direct combination of Ag will lead to the deterioration of the thermoelectric properties of the materials.