Synthesis And Thermoelectric Properties Of Double-atom-filled Skutterudite Compounds

Skutterudite compounds,as a kind of new thermoelectric material,have drawn great attentions owing to large carrier mobility,high electrical conductivity and relatively great Seebeck coefficient.However,high thermal conductivity limits their application.Therefore,it is key problem as to how to significantly decrease the thermal conductivity of Skutterudite compounds with maintaining good electrical transport properties.The important routes to achieve high-performance skutterudite compounds are through single-atom-filling and double-atom-filling to enhance phonon scattering and optimize electron and phonon transport properties.The theoretical researches indicate that co-filling of atoms with different chemical properties can introduce much more intensive phonon scattering than single-atom-filling at the same filling amount.As a result,lattice thermal conductivity will be significantly reduced by double-atom-filling.Furthermore,compared with single-atom-filling,double-atom-filling can adjust and control carriers'transport properties on a larger scale,resulting in optimization of carriers' transport properties. In this thesis,single phase double-atom-filled R_mM_nFe_xCo_4-xSb₁₂(R,M:Ce,Ca,Ba and Sm)compounds have been prepared by melting method combining with spark plasma sintering technique.The effects of two atoms'differences in valence states, atom radius,and atom mass on crystal structure and thermoelectric transport properties have been investigated.The Rietveld analysis show that R_mM_nFe_xCo_4-xSb₁₂compounds possess filled skutterudite structure,and Sb-Sb bond length of R_mM_nFe_xCo_4-xSb₁₂increases as compared with that of unfilled compound.It indicates that foreign atoms have filled into the Sb icosahedron cages in skutterudite structure.As to double-atom-filling R_mM_nFe_xCo_4-xSb₁₂,both average atomic displacement parameter(ADP)of R and M, and ADP of each filling atom are larger than those of individual filled by R or M. Furthermore,the ADPs of filling atoms are much larger than those of framework atoms Fe,Co and Sb.It shows that filling atoms introduce rattling effect in the Sb icosahedron cages,and rattling effect is more intensive especially for double-atomfilling.The Raman spectra analysis show that Sb₄ ring breathing modes in R_mM_nFe_xCo_4-xSb₁₂shift and widen as compared with unfilled skutterudite.The more filling atoms are,the wider full width at half maximum of Raman peaks are.It can be concluded that the effect of rattling induced by double-atom-filling on lattice vibration of skutterudite compounds is more significantly than that of single-atom-filling.Sb_3dfitted XPS spectra of R_mM_nFe_xCo_4-xSb₁₂compounds show that the chemical states of Sb atom are complicated.It bonds with filled atoms,Fe/Co,Sb and O, respectively.The narrow XPS spectrum of filled atom Ca_2pshows that Ca atoms have two chemical states in the cage.Most Ca atom bond with Sb atom,and the rest transform into Ca²⁺ions.It means that filling atom Ca is hard to form ions and mostly occupies the cages'corner.The effects of different valences,masses and radiuses of filling atoms on lattice thermal conductivity have been investigated.The results indicate that there are three kinds of scattering mechanisms in the R_mM_nFe_xCo_4-xSb₁₂compounds.Firstly,the phonon scattering caused by the rattling of filling atoms is the major factor for the decrease of lattice thermal conductivity;secondly,compared with the valence difference of R and M,the self-valence fluctuation of each filling atom(R or M)is more beneficial to the decrease of lattice thermal conductivity.Furthermore,the mass-fluctuation scattering is the minimum factor in all scattering mechanisms. Therefore,as to obtain p-type skutterudite compounds with low lattice thermal conductivity,the filling atoms should possess large mass,small ion radius and several valences.It has been investigated how the co-filling of Sm-Ce,Ba-Sm,and Ca-Sm affect the thermoelectric properties of skutterudites.With the increase of co-filling amount, the Seebeck coefficient increases,and electrical conductivity and thermal conductivity decrease.When the filling amount are similar,as to the double-atom-filled skutterudite,the electrical conductivity is smaller,seebeck coefficient is higher,and thermal conductivity and lattice thermal conductivity are much lower than those of single-atom-filled skutterudites.The optimism amount of double-atom-filling is wider and easier to adjust than that of single-atom-filling.A maximum dimensionless figure of merit ZT reaches 0.85 at 775 K for Ca_0.15Sm_0.24Fe_1.51Co_2.48Sb₁₂compound.