The Study On The Preparation And Thermoelectric Properties Of SnSe₂

Thermoelectric（TE）materials can achieve a direct conversion between heat and electricity and vice versa via the Seebeck and Peltier effects,with the advantages of emissions-free,no moving parts,high reliability and long life span.Therefore,thermoelectricity can play an important role in solving the energy and environmental crisis.In the past few decades,tremendous efforts have been devoted to improving thermoelectric properties and developing high performance TE materials through doping and electronic band structure manipulations,and forming solid solutions and introducing nanostructural features in the crystal lattice to enhance phonon scattering and thus lower the thermal conductivity.Despite much improved thermoelectric performance of the state-of-the-art thermoelectric materials,such as Bi₂Te₃,PbTe,GeTe,their large-scale commercial applications have not yet materialized because most of them contain expensive,low abundant and often toxic heavy metal elements.Consequently,it is essential to explore new,more environmentally friendly and cost effective high-performance TE materials in order to be competitive and make an impact on large scale energy conversion applications.Theoretical calculation shows that the maximum ZT value of n-type SnSe₂ single crystal can reach 2.95 at 773 K along the b axis,so SnSe₂ is also a potential high performance thermoelectric material.However,single-phase of SnSe2 are difficult to prepared,and few reports on the thermoelectric properties,so there is a lack of systematic research on their thermoelectric properties optimization.Therefore,this paper takes SnSe₂ thermoelectric materials as the research object,and a series of studies are carried out for the exploration of the SnSe₂ single-phase preparation and the optimization of the thermoelectric properties of single crystal and polycrystalline SnSe2.Its main purpose is to optimize the thermoelectric properties of materials comprehensively by controlling composition and microstructure.The research content and experimental results of this paper are as follows:（1）The influence of the preparation process on the single phase synthesis and microstructure of SnSe₂ is explored.The study found that through one step PAS sintering the SnSe₂ single-phase cannot be obtained,the reason is that Se element volatilized a lot during the PAS sintering thus leads to the major SnSe phase produced.The single phase SnSe₂ can not be obtained by melting-furnace cooling and melting-quenching,since the second phase of SnSe with different sizes is found in the products.The annealed ingot was annealed at different temperatures and time.It was found that the annealing process had no effect on the impurity SnSe.In order to get better single-phase,this study tried to prepare SnSe₂ single phase by zone melting method.It was found that choosing proper zone melting process can get SnSe₂ single phase and single crystal SnSe₂ material.（2）The single crystal SnSe₂ was prepared by the zone melting method.The study found that the intrinsic SnSe₂ was n-type semiconductor,and the conductivity was very low because of its low carrier concentration.Doping of Cl element in the Se site can provide an electron thus the carrier concentration of the material increased by two orders of magnitude after Cl doping,this makes the electrical conductivity greatly increase and the ZT value was enhanced from 0.1 to 0.4.However,the single crystal always possess poor mechanical properties since it is easy to cleave along the ab surface of SnSe₂ crystal,which seriously affects the preparation and characterization of the samples.In order to improve the mechanical properties of the material,the SnSe₂ polycrystalline material was prepared by melting and PAS method.（3）Cl-doped SnSe₂ composites with in-situ SnSe nanoprecipitates were prepared by melting-quenching method combined with the spark plasma sintering process.The remarkable influence of Cl doping on the thermoelectric properties and the structure of the heterojunction were systematically investigated.Doping with Cl in the system not only increased the carrier concentration by an order of magnitude but it also modified the heterojunction from that of the Schottky junction type（p-n junction）in undoped samples to junctions having an Ohmic contact（n-n junction）when the samples were doped with Cl,increasing their carrier mobility in the process.On account of the simultaneously boosted carrier concentration and carrier mobility upon Cl doping,the electrical conductivity were greatly increased.Moreover,the enhanced point defect phonon scattering following Cl doping coupled with the interface phonon scattering resulted in a suppression of the thermal conductivity.As a consequence,the maximum ZT value of 0.56 at 773 K was achieved in the 6%Cl-doped SnSe₂/SnSe nanocomposite measured in the direction parallel to the pressing direction.This is an almost 6 times larger value than measured on the undoped composite.This study provides a new way of thinking and solution for optimizing the thermoelectric properties of materials through interface regulation.