【作者】 刘桃香；

【导师】 唐新峰；

【作者基本信息】 武汉理工大学 ， 材料学， 2008， 博士

【摘要】 Skutterudite化合物由于具有大的载流子迁移率、高的电导率和较大的Seebeck系数,而作为一种新型的热电材料引起了人们极大的兴趣,但该化合物的热导率较高。如何在保持该化合物好的电传输特性的同时,大幅度降低其热导率、提高其热电性能是国内外关注的重要课题。通过单原子填充和双原子复合填充来增强对声子的散射、调整和优化电热输运特性是提高其热电性能的重要途径。理论研究表明:几种不同性质的原子复合填充时,填充原子对声子的散射作用可能比相同填充分数的单一原子所产生的散射作用更强,晶格热导率可能会降低得更多;此外多种原子复合填充时,与单一原子相比,可以在更宽的范围内调整和控制载流子的特性,从而可以在更宽的组成范围内调整和优化化合物的电传输特性。本论文以双原子复合填充的CoSb₃基化合物为研究对象,用熔融法结合放电等离子快速烧结技术制备出双原子复合填充的单相R_mM_nFe_xCo_4-xSb₁₂（R和M为填充元素Ce、Ca、Ba、Sm等）化合物,研究了不同化合价、原子半径、原子质量的两种原子复合填充对其晶体结构和热电性能的影响。Rietveld结构解析表明:合成的R_mM_nFe_xCo_4-xSb₁₂化合物具有填充式Skutterudite结构,与未填充的Skutterudite化合物相比,R_mM_nFe_xCo_4-xSb₁₂化合物的Sb-Sb键长增加,说明外来原子填充进了Skutterudite化合物结构中的Sb二十面体空洞;两种原子复合填充时,无论是平均原子位移参数还是各自的位移参数都比它们单独填充时要大,而且远远大于框架原子Fe、Co、Sb的位移参数,说明填充原子在Sb二十面体空洞中具有扰动效应,并且两种原子复合填充时扰动效果更明显。拉曼光谱分析表明:与未填充Skutterudite化合物相比,R_mM_nFe_xCo_4-xSb₁₂化合物的Sb原子四边形环的拉曼呼吸振动模峰位偏移并宽化。比较各试样拉曼峰的半高宽,可以发现填充Skutterudite化合物的谱线比未填充时宽、双原子填充又比单原子填充时宽,说明双原子填充比单原子填充的扰动对Skutterudite化合物晶格振动的影响更大。R_mM_nFe_xCo_4-xSb₁₂化合物表面Sb_3d电子的窄区XPS谱线分析表明Sb原子化学状态复杂,分别与填充原子、Fe/Co、Sb、O成键;填充原子Ca_2p的窄区XPS谱线分析表明Ca在空洞中有两种化学状态,大部分Ca与Sb成键,少部分Ca离子化,这说明填充原子Ca离子化程度较小且主要占据空洞的角落。研究了两种填充原子化合价差、质量差、半径差对R_mM_nFe_xCo_4-xSb₁₂化合物晶格热导率的影响,结果表明在R_mM_nFe_xCo_4-xSb₁₂化合物中存在以下几种散射机制,并且对声子的散射作用依次减小。首先,填充原子的扰动对声子的散射是降低其晶格热导率的主要原因;其次,与两种原子的化合价差相比,每种填充原子自己本身的化合价波动更有利于降低化合物的晶格热导率;质量波动散射对晶格热导率的降低所起的作用比较小。据此,对于p型Skutterudite化合物来说,要想得到较低的晶格热导率,所选择填充原子必须同时具有以下特点:原子质量大、离子半径小、并且有几种变价。分别研究了Sm和Ce、Ba和Sm、Ca和Sm双原子复合填充对Skutterudite化合物热电性能的影响,结果表明:随着复合填充总量的增加,R_mM_nFe_xCo_4-xSb₁₂化合物Seebeck系数增加、电导率和热导率降低;在填充总量相近的情况下,和单原子填充相比,双原子复合填充的Skutterudite化合物电导率较低、Seebeck系数较高、总热导率和晶格热导率更低;最佳双原子复合填充总量的范围较宽,比最佳单原子填充分数容易调控;在本研究中Ca和Sm复合填充的Ca_0.15Sm_0.24Fe_1.51Co_2.48Sb₁₂化合物的最大热电性能指数ZT_max值在775K时达0.85。更多还原

【Abstract】 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.更多还原