Study On The Properties Of Co-doped SnSe Based Polycrystalline Thermoelectric Materials

Thermoelectric conversion technology is a safe,reliable,green and environmental new energy technology,which can realize the direct mutual conversion of electric energy and thermal energy.It provides an all solid-state way of thermoelectric power generation and refrigeration,and has a very broad application prospect in the current situation of energy shortage in the world.Recently,the record ZT value of?2.6 at 923 K has been reported for SnSe single crystal along the b-axis due to the ultralow lattice thermal conductivity stemming from strong lattice anharmonicity and large power factor originating from the intrinsically high mobility,which has attracted intensive attention.However,due to the sophisticated crystal-growth techniques,prospective high production cost and undesirable mechanical properties,SnSe-based single crystals are not well-suited for thermoelectric devices.In contrast,polycrystalline SnSe overcomes these shortcomings,which leads to the fact that the improved thermoelectric performance of polycrystalline SnSe has become a research hotspot in recent years.In this paper,we consider polycrystalline SnSe thermoelectric material as the research object,and optimize thermoelectric properties of their p-type through monovalent elements doping in Sn sites.Three series of alloy samples,Sn_1-xNa_xSe,Sn_0.98Na_0.02-xAg_xSe,Sn_0.98Na_0.02-xM_xSe(M=K,Li),were synthesized by the combination traditional solid-state smelting and SPS sintering process.The effects of different doped elements on the phase,solid solubility,carrier concentration and thermoelectric properties for polycrystalline SnSe were systematically studied.The main research contents of this project are as follows:(1)From the viewpoint of optimizing carrier concentrations and thermoelectric properties of polycrystalline SnSe,Sn_1-xNa_xSe(x=0,0.01,0.02,0.03)samples were prepared by solid-phase melting and then SPS sintering process.It is discovered that the carrier concentrations remarkably increases from 5.21×10¹⁷cm^-3in the pure SnSe to5.22×10¹⁹cm^-3in 2at.%Na-doping,which results in the significant result more carrier pockets participate in the transport and further improve the density of state effective mass.Furthermore,in conjunction with reducing the lattice thermal conductivity owing to the enhanced phonon scattering by point defect,the thermoelectric performance of polycrystalline SnSe can be notably improved.(2)P-type co-doped polycrystalline Sn_0.98Na_0.02-xAg_xSe(x=0,0.004,0.008,0.012,0.016,0.02)samples were obtained by Na and Ag simultaneous substitute in Sn sites.The results show that the function of Na elements doping can effectively maintain the carrier concentration at a high level in these samples.In addition,the introduction of Ag elements will lead to the formation of more point defects and precipitation of the second phase to enhance the phonon scattering probability,thus reducing the lattice thermal conductivity of the system.As a result,the highest ZT value of 1.2 at 785 K is obtained in Sn_0.98Na_0.016Ag_0.004Se sample,and the maximum average ZT value of 0.74 is achieved in the range of 335-785 K.Moreover,its theoretically thermoelectric conversion efficiency is up to 11%,which is of great significance for the application of practical thermoelectric devices.(3)The influence of co-doped active alkali elements(K,Li)along with Na on the thermoelectric properties of polycrystalline SnSe was investigated.The results show that the thermoelectric properties of(Na,K)and(Na,Li)co-doped samples are not as good as those of(Na,Ag)co-doped polycrystalline SnSe samples,which is due to three main reasons by comparing with(Na,Ag)co-doped samples:(1)K and Li are relatively active in chemical properties,the atomic mass of Li is small and unstable,and K has strong chemical activity and low boiling point,which makes it easy to be heated and evaporated in the process of sample synthesis;(2)K and Li with strong chemical activity are easily to react with quartz tube,and form oxides at the grain boundary of sample,which suppresses the effect of doping on the optimization of carrier concentration;(3)precipitation of the second phase observed in(Na,Ag)co-doped samples were not found in(Na,K)and(Na,Li)co-doped samples.