Preparation And Electrical-thermal-mechanical Properties Of N-type In-filled Skutterudite-based Nanocomposite Thermoelectric Materials

Filled skutterudites have been considered as the most promising thermoelectric(TE)material due to the excellent TE transport performance in the middle temperature range.Nanocomposite was considered to be an optimization strategy that could further improve the TE performance of n-type filled skutterudite.However,there are still facing two challenging problems in the research of nanocomposite TE materials.One is how to synergistically decouple electrical and thermal transport properties in nanocomposite TE materials.The other is how to effectively improve the mechanical properties of nanocomposite TE materials.In view of these two challenging problems,in this thesis,high-energy ball milling and liquid-solid sedimentation separation methods were used to obtain different types of nanoparticle suspensions.The n-type filled skutterudite-based nanocomposite TE materials were successfully prepared by ultrasonically mixing different types of nanometer suspensions and n-type In-filled skutterudite matrix combined with SPS sintering.The effects of different types and contents of nanoparticles on the phase composition,microstructure,electronic structure,electrical and thermal transport properties and mechanical properties of In-filled skutterudite TE matrix materials were systematically studied.The main research results are given as following:The magnetic nanocomposite TE materials x Fe₃O₄/In_0.25Co₄Sb₁₂(Fe₃O₄/In_0.25Co₄Sb₁₂mass percentage x=0,0.05%,0.10%,0.15%,0.25%)were successfully prepared with nano-sized Fe₃O₄ particles(Fe₃O₄-NPs)as the second phase and In_0.25Co₄Sb₁₂ as the matrix material.Phase and microstructure analyses show that the addition of Fe₃O₄-NPs did not change the phase composition of the matrix,and effectively suppress grain growth of matrix during the SPS process,resulting in multi-scale matrix grains.This multi-scale grain structure could significantly reduce the lattice thermal conductivity of the nanocomposite materials.The magnetoelectric interaction between ferromagnetic Fe₃O₄-NPs and the matrix,leading to reduction in the carrier concentration.The Seebeck coefficient was significantly increased by the electron multiple scattering of superparamagnetic Fe₃O₄-NPs.Therefore,Fe₃O₄-NPs can optimize synergistically the electrical and thermal transport properties of n-type filled skutterudite.The maximum dimensionless TE figure of merit(ZT value)of 1.12 at 675 K was obtained for the sample with x=0.10%,which is about 4.3%higher than that of the matrix.At the same time,the introduction of Fe₃O₄-NPs can significantly optimize the mechanical properties of nanocomposite TE materials.ThemixedmagneticnanocompositeTEmaterials0.45%(x Co+y Ba Fe₁₂O₁₉)/Ba_0.3In_0.3Co₄Sb₁₂(x:y=0,1:1,2:1,3:1,4:1)were successfully prepared by using Co nanoparticles(Co-NPs)and Ba Fe₁₂O₁₉ nanoparticles(Ba M-NPs)as the second phase,Ba_0.3In_0.3Co₄Sb₁₂ double filled skutterudite as a matrix material.Phase and microstructure analysis show that the introduction of mixed magnetic nanoparticles did not change the phase composition of the matrix,and effectively suppress grain growth of matrix during the SPS process,resulting in multi-scale matrix grains.The ferromagnetic-paramagnetic transition of Ba M-NPs can suppress the deterioration of the TE properties in the high temperature region.The electrical and thermal properties were synergistically controlled by the effect of charge transfer and interface effect between Co-NPs and the matrix.The maximum ZT value was obtained 1.31 at 850 K for the sample with x:y=2:1,which is about 17%higher than that of the matrix.And the average ZT value is 1.2 in the temperature range of 675-850 K.In addition,the introduction of mixed magnetic nanoparticles can also optimize significantly the mechanical properties of nanocomposite TE materials.The nanocomposite TE materials x BST/In_0.25Co₄Sb₁₂(BST/In_0.25Co₄Sb₁₂ mass percentage x=0,0.1%,0.2%,0.3%,0.4%)were prepared by using p-type bismuth telluride Bi_0.5Sb_1.5Te₃(BST)as the second phase,In_0.25Co₄Sb₁₂ as a matrix material.Phase and microstructure characterization presented that Te nanoparticles(Te-NPs)were precipitated from BST during the SPS sintering process.BST and Te nanoparticles were randomly distributed on the crystal grains and grain boundaries of the matrix without changing the phase composition and crystal structure of the matrix.The introduction of BST nanoparticles can also suppress effectively the growth of the matrix grains,resulting in multi-scale matrix grains.This multi-scale grain structure and increased dislocations could significantly reduce the lattice thermal conductivity of the nanocomposite materials.The analysis results of work function show that the charge transfer from Te-NPs to the matrix can increase the carrier concentration and electrical conductivity of nanocomposite TE materials.The increase of Seebeck coefficient was attributed to the selected electron scattering induced by the interface band bending and the increase of scattering factor.Due to the synergistic optimization of the electrical and thermal transport properties,a maximum ZT of 1.22 was obtained at 650 K for the sample with x=0.1%,which is about 13%higher than that of the matrix.The average ZT value of BTS01 is close to 1 in the range of 300-700 K.At the same time,the introduction of BST-NPs can significantly optimize the mechanical properties of nanocomposite TE materials.