Anisotropic Thermoelectric Properties Of Layered Sn-Se Compounds

As one of the most promising clean energy materials,thermoelectric(TE)materials can directly convert heat into electricity based on Seebeck and Peltier effects.In recent years,many layered compounds,such as BiCuSeO and SnSe,are reported to possess good TE properties.Similar to high performance SnSe thermoelectrics,SnSe2 is also a layered structured semiconductor,which is reported to have very high theoretical zT.However,its anisotropic thermoelectric properties are less experimentally investigated.In this work,the layered SnSe2 and SnSe materials are carefully studied.The polycrystalline bulk samples of SnSe2 and SnSe are successfully prepared by vacuum melting and hot pressing.The anisotropic transport properties of the samples,together with its thermal stability,are carefully studied by elements doping in order to optimize the carrier concentration.The main results are listed as below:(1)It is obvious that in this work the phase structure of all bulk materials prepared by is the right phase of SnSe2.The carrier concentration of SnSe2-based materials has been increased to 6.46 × 1019 cm-3 from 1018 cm-3.Hence,the electrical transport properties are greatly improved.So,its thermoelectric potential becomes well-known.Besides,acoustic phonon scattering of carriers may be dominant in the SnSe2-xClx samples.It is found that,for the same sample,the out-of-plane zTc value is unexpectedly higher than in-plane zTa value due to the higher ka/kc than ?a/?c.A maximum out-of-plane zTc of 0.4 and in-plane zTa of 0.3 are achieved at 673 K for SnSe1.88Cl0.12 sample.(2)The effect of thermoelectric performance by Ag dopping is studied in SnSe2 materials.The conduction behavior of all the Sn1-xAgxSe2 samples is not similar to that of the p-type but n-type semiconductor,inconsistent to our theoretical forecast.To some extent,their electrical transport properties after 473 K become better than the pure SnSe2.And it's easier to prepare than Cl-doped sample.Therefore,the zT values of all samples are near 0 at room temperature,but it rises up with temperature.Finally,the highest zTc of out-of-plane is 0.21 for the sample of x = 0.05,about several orders of magnitude higher than the pure sample.(3)Combining the previous results,anisotropic thermoelectric properties of Ag and Cl co-doping SnSe2-based materials are explored for developing optimized SnSe2-based materials with high carrier concentration.As a result,the relative density and the chance of success for preparing are both promoted as expected.The electrical conductivity in high temperature range is also optimized.Finally,the zTa of Sn0.99Ag0.01Se1.92Cl0.08 is up to 0.31,and zTc is 0.52 at 673 K.(4)The anisotropic thermoelectric properties are studied in SnSe-based materials with Ag,Cl,Al,Sb doped and SnSe2 composited.It turns out that the thermoelectric properties of all the samples are bad except for Ag-doped sample.Although the anisotropy of electrical and thermal transport properties is evident due to the existence of strong texture,the anisotropy of zT is not obvious for SnSe-based materials,because of the close degree of strength of anisotropic electrical and thermal transport properties.