Preparation And Thermoelectric Properties Of Typical Cubic Thermoelectric Chalcogenides

Thermoelectric(TE)materials,which can convert energy from heat to electricity or vice versa,hold promises in alleviating energy shortage and environmental pollution.Boosting the TE performance of traditional thermoelectric materials and exploring new thermoelectric materials are the research hotspots in the field of thermoelectricity.Cubic chalcogenides have attracted significant interests due to the features of wide source,being easy to be synthetic,convenience for large-scale production and excellent TE performance.Therefore,in this study,we focus on the optimization of thermoelectric properties for typical cubic chalcogenides:PbTe and Cu12Sb4S13.The main results achieved in this thesis are summarized as follows:(1)The thermoelectric properties of Pb1-xBixTe compounds and Pb0.995Bi0.0055Te/f wt.%Cu1.75Te composites were investigated in the temperature range of 300-773 K.The results indicate that Bi doping is useful to adjust the carrier concentration and increase the electronic density of states.The PF value of Pb0.995Bi0.005Te sample reaches～16.5 μW cm-1 K-2.In order to furtherly optimize the thermoelectric performance of ntype PbTe based materials,the compound Pb0.995Bi0.005Te is selected as the matrix to prepare composites,and coherent nanophase Cu1.75Te is in situ formed in the Pb0.995Bi0.005Te matrix,which can simultaneously optimize the thermal and electrical properties.As a result,the highest ZT value of 1.4 is obtained at 623 K for Pb0.99sBi0.005Te/0.86 wt.%Cu1.75Te sample and the ZT holds a higher value in broad temperature,which is beneficial to the average ZT value(ZTave～0.9).(2)The thermoelectric properties of Pb0.995Bi0.005Te(PBT)based composites incorporated with MgO nanoparticles were investigated in the temperature range from 300 to 773 K.The results show that the addition of appropriate amounts of MgO nanoparticles can give rise to obvious enhancement of PF and the certain reduction ofκL.The enhanced PF mainly comes from decreased electrical resistivity via increasing electron concentration.Meanwhile,the κL is reduced due to intensified phonon scattering by the interfaces between MgO inclusions and the PBT matrix.As a result,ZT～1.4 is achieved at 773 K in PBT/0.3 wt%MgO sample,indicating that incorporation of a proper amount of MgO in Pb0.995Bi0.005Te is an effective way to improve its thermoelectric performance.(3)Dually substituted tetrahedrite samples were prepared by fusion methods.The results indicate that through dual substitution of Se for S and Zn for Cu in the compound both the electrical conductivity and thermopower are enhanced,leading to increase of PF as high as～33%(at 723K).Furthermore,lattice thermal conductivity is reduced by as low as～30%(at 723K)upon dual substitution due to enhanced phonon scattering of the dopants Se and Zn.As a result,ZT=0.9(at 723 K)is achieved in Cu12yZnySb4S12.8Se0.2 samples with y=0.025 and 0.05,which is～41%higher than that of Cu12Sb4S13.(4)The electrical transport behavior and thermoelectric properties of Cu12xInxSb4S12.8Se0.2 samples were investigated.The electrical transport in Cu12xInxSb4S12.8Se0.2 samples is governed by small polaron hopping at T=300-623 K;at T>623 K the transport mechanism changes to band conduction dominated by acoustic phonon scattering.Dual substitution of In for Cu and Se for S can significantly reduce the lattice thermal conductivity κL of Cu12Sb4S13 as large as 58%(～300 K)through enhanced phonon scattering mainly by point defects.As a result,ZT～1.0 at 723 K is achieved in Cu11.95In0.05Sb4S12.8Se0.2 sample,which is～56%higher than that of Cu12Sb4S13,indicating that dual substitution is an effective way to improve the thermoelectric performance of tetrahedrite.(5)The effects of Gd substitution at the Cu sites on the microstructures,electronic structures and thermoelectric performance of Cu12Sb4S13 were investigated.Rietveld refinements of XRD results show that accompanying inhibition of second phase Cu3SbS4,the number of Cu vacancies rises dramatically with increasing x(x≤0.3),which leads to increased electrical conductivity σ and reduced κL.Our SRPES result shows a substantial increment of DOS near Fermi level upon Gd substitution;meanwhile our first-principle calculations reveals that the increased DOS comes from contribution of Gd 4f orbit,leading to enhanced S.Consequently,remarkable enhancement in thermoelectric performance is obtained with a highest ZT of～0.94 for the sample Cu11.7Gd0.3Sb4S13 at 749 K,which is～41%higher than that of undoped Cu12Sb4S13 sample.