Preparation And Properties Of Polycrystalline SnSe Based Nanocomposite Thermoelectric Materials

Thermoelectric material is a new type of green functional material that can directly change heat energy and electric energy to each other without producing pollutants.However,due to low conversion efficiency and high production cost,the large-scale commercial application of thermoelectric materials is limited.Therefore,it is of great significance to further improve the performance of thermoelectric materials and search for new low-cost thermoelectric materials.In recent years,SnSe-based compound materials have become a hot topic due to their excellent thermoelectric properties.Although single crystal SnSe material has excellent thermoelectric performance ZT value,but long preparation period and high equipment requirements,and single crystal material is prone to cleavage due to poor mechanical properties,which is not conducive to the practical application of its corresponding thermoelectric devices.Compared with single crystal SnSe,polycrystalline SnSe has become a research hotspot due to its better mechanical properties and simple processing.However,polycrystalline SnSe is inferior to single crystalline SnSe due to its poor electrical properties.Through a variety of strategies to optimize the thermoelectric properties,some breakthroughs have been made in the thermoelectric properties of polycrystalline SnSe in a relatively short period of time,and several recent works have prepared polycrystalline SnSe with ZT values above1.5.However,one problem exposed in these studies is that the preparation cycle of SnSe is generally long,and the solid-state synthesis method is dominant and requires high production cost.Therefore,it is urgent for researchers to develop a synthesis method of polycrystalline SnSe with short synthesis period,low cost and mass production.The intrinsic low conductivity of polycrystalline SnSe materials hinders the improvement of thermoelectric properties.Therefore,it is an important breakthrough to improve the thermoelectric properties of polycrystalline SnSe by improving its conductivity.An excellent thermoelectric material needs to maintain a high ZT value over a wide temperature range.However,most of the reported p-type polycrystalline SnSe have excellent thermoelectric performance only in the middle and high temperature region,and few reports have focused on the performance of material in the middle and low temperature region.Recent studies have shown that composite magnetic nanomaterials can improve their thermoelectric properties.However,no relevant work has reported the effect of polycrystalline SnSe composite magnetic nanomaterials on their thermoelectric properties.In view of the above problems existing in SnSe materials,in this paper,the hydrothermal method can be used to quickly and low-cost synthesis of polycrystalline SnSe,and then study the introduction of high conductivity Ag nanoparticles,Sb₂Te₃nanosheets,conductive polymer PEDOT:PSS and Fe nanoparticles on the microstructure and thermoelectric properties of polycrystalline SnSe based materials.The main research contents and results of this paper are as follows:（1）Polycrystalline Sn_0.95Cu_0.05Se was prepared by hydrothermal method with high speed and low cost synthesis.Due to the low conductivity of SnSe-based materials,Ag nanoparticles were introduced into SnSe-based materials,and the effects of Ag nanoparticles on the microstructure and thermoelectric properties of SnSe-based materials were studied.The results of SEM showed that Ag nanoparticles were uniformly dispersed on the surface of Sn_0.95Cu_0.05Se nanosheets,and the morphology of the sample was almost unaffected by the addition of Ag nanoparticles.XRD and XPS tests show that Ag Sn^-point defects occur in the complex bulk materials,which can increase the carrier concentration.The interface formed between Ag and Sn_0.95Cu_0.05Se can form an energy barrier,which can filter low energy hole carriers.The Seebeck coefficient S and conductivityσcan be optimized simultaneously,that is,the power factor PF can be increased.For heat transport,the point defects caused by Ag’s position in place of Sn and the increased two-phase interface provide phonon scattering centers,which can effectively reduce the lattice thermal conductivity.When 2.25 mol%Ag was added at 773 K,the maximum ZT value was 0.7,and the thermoelectric parameters were decoupled.This study indicates that the introduction of Ag nanoparticles is one of the effective methods to improve the thermoelectric performance of Sn_0.95Cu_0.05Se.（2）Sb₂Te₃ nanosheets were synthesized by reflow method,and the composite of Sb₂Te₃ nanosheets and Sn_0.98Se nanosheets was formed by simple mechanical stirring.The effect of composite Sb₂Te₃ nanosheets on the microstructure and thermoelectric properties of Sb₂Te₃ was studied.Although the interface between the two phases becomes the phonon scattering center,it reduces the thermal conductivity,but ultimately deteriorates the thermoelectric properties of the sample.（3）Few reports focus on the thermoelectric properties of SnSe-based materials in the middle and low temperature region.Many reports have reported that adding organic thermoelectric materials to inorganic thermoelectric materials can optimize the properties of inorganic thermoelectric materials.It is found that the PEDOT:PSS interface and SnSe interface are in close contact,forming conductive interface.The conduction interface can well control the transmission of carriers and phonons.On the one hand,the mobility of carriers is improved and the conductivity is increased;on the other hand,the increased two-phase interface can scatter phonons and let the thermal conductivity reduce,thus realizing the decoupling of thermoelectric parameters to a certain extent.When 1 m L PEDOT:PSS was added,the maximum ZT value was 0.16,which was 114%higher than that of pure SnSe.This study indicates that the addition of organic thermoelectric material PEDOT:PSS to SnSe can significantly improve the thermoelectric properties of SnSe.（4）Recently,the addition of magnetic nanoparticles into thermoelectric materials to achieve performance improvement has attracted widespread attention.In this paper,Fe nanoparticles were selected as the composite object to study the effect of Fe nanoparticles on the microstructure and thermoelectric properties of SnSe.The study shows that the composite samples with Fe nanoparticles have ferromagnetic properties.Compared with the pure SnSe material,the conductivity is improved by a large increase in the carrier concentration,although the Seebeck coefficient is reduced to a small extent,which eventually leads to an increase in the power factor PF.The increased two-phase interface helps to reduce the thermal conductivity,resulting in an optimization of the thermoelectric properties of the composite samples.The results show that Fe nanoparticles can improve the properties of SnSe materials.