Preparation And Enhanced Thermoelectric Performance Of Polycrystalline SnSe Material

SnSe is a layered structure semiconductor material with simple chemical composition and non-toxic,rich component elements.SnSe single crystals have attracted extensive attention for their excellent ZT value.However,considering the mechanical properties and the time cost of growth techniques for single crystals,more researches have focused on polycrystalline SnSe materials.In this paper,the influence of non-stoichiometric ratio on P-type and N-type transition of polycrystalline SnSe materials has been studied,which based on the preparation process of melting combined with SPS.On the basis of this study results,the strategy of optimizing the TE properties of polycrystalline P-type SnSe by introducing second phase Cu₂Se and the trial to improve the TE properties of polycrystalline N-type SnSe by doping BiI₃were investigated to explore the new ways to enhance the TE properties of polycrystalline SnSe materials.The main results are as follows:?1?Pure phase polycrystalline P-type SnSe bulk thermoelectric material was successfully prepared by melt quenching combined with SPS process.The maximum power factor reached 3.28?W·K^-2·cm^-1 at 773K,while the maximum ZT value 0.41was obtained.When the starting material Se was excessive,the impurity phase SnSe₂would be formed in the SnSe material,which would increased the thermal conductivity of the material;when the Se was deficient,elemental Sn would appeard in the SnSe material,which would increased the thermal conductivity and resulted a transition from P-type to N-type.?2?A series of P-type xCu₂Se/SnSe?x=0,0.10%,0.15%,0.20%,and 0.25%?composite TE materials were prepared by the combination of ultrasonic dispersion and SPS.The secondary phase Cu₂Se was uniformly distributed in the grain boundary of the matrix.Properly addition of Cu₂Se could simultaneously improved the electrical and thermal transport properties of the matrix materials:the sample with x=0.15%gained the maximum power factor of 3.84?W·cm^-1·K^-2 at 773 K,while obtained the lowest thermal conductivity of 0.56 W·m^-1·K^-1 at 673K.In general,the ZT value of the xCu₂Se/SnSe composites increased first and then decreased with the increasing x.The largest ZT value of 0.51 at 773 K was achieved for the sample with x=0.15%,increased by 24%compared with the matrix.In addition,the TE properties of polycrystalline P-type SnSe with Cu₂Se second phase stayed stable after repeated measurements.?3?On the basis of the preparation of stable polycrystalline N-type SnSe materials,a series of N-type polycrystalline bulk material with a nominal composition of SnSe_0.9+xmol%BiI₃?x=0,0.1,0.2,0.3,0.4?were prepared by adjusted melting combined with SPS process.When the doping exceed the limit,the excess Bi element would be mutually soluble in the Sn element while I element would be enriched in the grain boundary.The electrical transport properties of N-type polycrystalline SnSe materials was greatly improved by doping BiI₃,and the power factor have increased from 0.79?W·cm^-1·K^-2 to 2.87?W·cm^-1·K^-2.At the same time,more defects were introduced in the matrix material which enhanced the scattering of phonon and reduced the thermal conductivity.The maximum ZT value of 0.28 was obtained when the doping amount x=0.4,which was greatly improved compared to the N-type SnSe matrix with the maximum ZT value of 0.06.After repeated cycles of measurement,the electrical and thermal conductivity decreased while Seebeck coefficients increased,and the chang of ZT value was not significantly.