Study On The Synthesis And Thermoelectric Properties Of Narrow-Band-Gap Materials With Diamond-like Structure

Growing energy consumption and environmental problems have spurred research to find new green energy materials and more efficient ways to use energy,of which the photovoltaic materials have been put into use successfully.However,thermal energy plays a negative role in photovoltaic process.The solar energy would be used more efficiently if there was any other technology being used together with photovoltaic cells.Thermoelectric materials then bring hope for this because of its advantages such as small,noiselessness,ecofriendly,and so on.Diamond-like structured compound A12BⅣCⅥ3(AI=Cu,Ag;BIV=Ge,Sn,Pb;CVI=Se,Te)is exactly a king of promising thermoelectric material.Among AⅠ2BⅣCⅥ3 this thesis investigated Cu2GeSe3,which currently lacks further systematic research.Cu2GeSe3 belongs to orthorhombic system,Imm2 group,in which cations occupy half of the carbon sites and anions occupy the other half.This thesis found that anti-side defects existed in our p-type materials,influenced the electric properties as well as energy band structure of samples,and leaded prominent optimizing to TE properties.Cu2GeSe3 was investigated in three aspect as follows:In the first part,this thesis tracked out the effective-and-efficient synthesis of Cu2GeSe3,followed by adjusting the density of carriers by Cu deficiency.On the one hand the zT was optimized from 0.29@755K to 0.65@758K,on the other hand this paper studied TE phenomena of Cu2GeSe3 theoretically.For example,this paper explained the reduction of electric conductivity and density of carriers in Cu-deficient samples by defect(such as Cugc),explained the bending at 500K in Seebeck-Temperature curve of Cu-deficient samples by "resonant level" and"compensatory effect".In the second part,S atoms and Te atoms were dissolved in the place of Se atoms respectively.The former reduced the phonon mean free path and thermal conductivity of samples by point defects(mass fluctuation and strain field fluctuation);the later optimized the PF of samples and obtained higher mean zT.In the third part,this explores studied TE properties of Ga-doped samples based on Cu-deficient and S-dissolved samples respectively,and obtained zT of 0.79@765K and 0.66@756K.In addition,this thesis added an annealing step to the preparation of Cu-deficient samples according to pre-existing reports,and compared the TE properties with that in the first part.Systematicness in this paper were confirmed by this part of studies.Finally we optimized the zT value of Cu2GeSe3from 0.29@755K to 0.79@765K,which belongs to the result of Cu1.8Ge0.99Ga0.01Se3.