【作者】 魏平；

【导师】 张清杰； 赵文俞；

【作者基本信息】 武汉理工大学 ， 新能源材料， 2012， 博士

【摘要】 方钴矿热电材料在200-500℃中温区具有优异的热电性能和结构稳定性,被认为是太阳能热电-光电复合发电和汽车尾气余热发电的关键材料。本论文以(Ba,In)双原子填充方钴矿材料为研究对象,围绕该材料的热电性能优化、电热输运物理机制及其应用可行性问题,研究该材料的电热输运性能；测量该材料的X射线光电子能谱(XPS)和同步辐射X射线吸收近边结构(XANES)和扩展X射线吸收精细结构(EXAFS)并结合多重散射理论和第一性原理计算,系统研究In在方钴矿中的存在形式和(Ba,In)双原子填充方钻矿的化学成键与电子结构；在此基础上,研究该材料在循环热载荷作用下结构和热电性能的热稳定性。采用熔融-淬火-退火和放电等离子烧结工艺制备了名义组成为Ba0.3InxCo4Sb12(0≤0.3,Δx=0.05)的(Ba,In)双原子填充方钴矿块体材料,研究了In填充对(Ba,In)双原子填充方钻矿材料的物相组成、显微结构和热电性能的影响。结果表明,材料的实际组成可表示为BarInsCo4Sb12(0.14≤r≤0.25,0≤s≤0.23)。随x增大,r逐渐减小,s逐渐增大,总填充分数r+s逐渐增大。In填充对材料显微结构的影响很小。随r减小和s增大,材料的载流子浓度逐渐升高,迁移率逐渐降低。与Ba单原子填充方钴矿材料相比,(Ba,In)双原子填充方钻矿材料的晶格热导率明显降低,功率因子显著增大,材料ZT值大幅度提高。Ba0.15In0.16Co4Sb12和Ba0.14In0.23Co4Sb12材料的ZT值在850K分别达到1.33和1.34。采用XANES和EXAFS技术并结合多重散射理论计算,深入研究了In掺杂方钴矿InxCo4Sbi2(0≤x≤0.25)材料中In的存在形式。InxCo4Sbi2的In K边XANES实测谱的特征结构指示In存在于方钴矿晶格内。采用多重散射理论计算了In填充方钴矿中Sb12二十面体空隙位和取代方钴矿中Sb位和Co位三种情况下的In K边XANES理论谱,发现In填充时其理论谱与实测谱最吻合,两种取代情况下的In K边XANES理论谱均与实测谱相差较大,这为In能够填充方钴矿中Sb12二十面体空隙提供了直接证据。In0.2Co4Sb12的In K边EXAFS谱分析进一步证实In能够填充方钴矿中Sb12二十面体空隙。In填充方钴矿的第一性原理计算表明,In填充原子与邻近Sb原子以弱共价键结合,In在价带中央和费米能级附近形成超深缺陷态和深缺陷态,分别属于In-Sb弱共价键的成键态和反键态。建立了方钻矿CoSb3中Sb4矩形四元环的不等性sp电子轨道杂化模型,采用XPS和EXAFS技术研究了(Ba,In)双原子填充方钴矿BarInsCo4Sb12的化学成键和局域结构。提出CoSb3结构中两个Sb原子通过不等性sp电子轨道杂化方式形成能量不等、相互垂直的ββσ键和ppσ键,分别构成Sb4矩形四元环的Sb-Sb短键和长键。BarInsCo4Sb12的Sb3d5/2XPS芯级谱定量分析表明,In与Sb之间的电子轨道杂化导致Sb4矩形四元环增大和更方,Ba与Sb之间的电荷转移导致Sb4矩形四元环缩小和更方。BarInsCo4Sb12的Sb K边EXAFS实测谱拟合分析进一步证实(Ba,In)双原子填充可导致Sb4四元环由矩形向正方形转变。Sb4四元环形状变化引起的晶格畸变可合理解释(Ba,In)双原子填充方钻矿材料的晶格热导率大幅度降低现象。采用XPS和XANES技术结合第一性原理计算,研究了(Ba,In)双原子填充方钴矿BarInsCo4Sb12的价带和导带电子结构。理论计算表明,未填充和填充方钴矿的价带电子结构均可用8个精细电子态模型描述,Ba和In填充引起Sb5p成键态附近产生局域电子共振态。BarInsCo4Sb12的XPS价带电子谱定量分析表明,用8个精细电子态模型可合理描述其价带结构,观察到Sb4四元环的Sb5p成键态在价带中央产生简并现象,这与BarInsCo4Sb12中Sb4四元环变方引起的对称性升高有关。BarInsCo4Sb12的XANES谱分析表明,未填充方钴矿CoSb3导带底的电子态密度源于Co3d和Sb5p未占据态的贡献,In填充原子在费米能级附近形成局域电子共振态,(Ba,In)双原子填充导致Sb K边XANES谱的吸收边强度增加,指示位于导带底的Sb4四元环Sb5p反键态的能带结构发生了简并,这与Sb4四元环对称性升高有关。导带底部能带简并和In填充原子在费米能级附近引起的局域电子共振态是(Ba,In)双原子填充方钻矿材料具有优异电输运性能的物理基础。(Ba,In)双原子填充方钴矿块体材料在室温-450。C-室温的循环热载荷作用下其结构和热电性能的热稳定性研究表明,循环热载荷作用会导致材料晶界处产生次生沉积物、出现Ba富集和Sb缺失现象,材料内部析出Ba5Sb3；循环热载荷作用初期,材料电导率逐渐降低和Seebeck系数绝对值逐渐增大,这源于晶界处次生沉积物引起的电子能量过滤效应；晶格热导率随循环热载荷作用次数增多而逐渐增大,这与Ba填充原子从Sb12二十面体空隙中析出有关；循环热载荷作用后,材料的ZT值变化幅度较小,800K时ZT值为1.20的初始材料经2000次循环热载荷作用后仍为1.14,仅下降了5.0%。材料的热电性能在升温和降温过程中与温度的关系曲线表明可以不考虑循环热载荷作用引起的微结构弛豫对热电性能的影响。这些实验结果为(Ba,In)双原子填充方钴矿热电材料具有优异的热稳定性和可以应用于工作温度呈周期性变化的太阳能热电-光电复合发电系统提供了依据。更多还原

【Abstract】 Skutterudite thermoelectric materials exhibit superior thermoelectric transport properties and robust structural stability in the moderate temperature range (200-500℃), have been considered as critical materials in solar thermoelectric-photovoltaic hybrid power generation and vehicle exhaust waste heat power generation. In this dissertation,(Ba,In) double-filled skutterudite materials were focused on as research object with the aim to explore the relevant issues about thermoelectric properties optimization, physical mechanism of thermoelectric transport properties, and feasibility of application. The thermoelectrical properties of (Ba,In) double-filled skutterudite materials were investigated. X-ray photoelectron spectroscopy (XPS) and synchrotron radiation X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques, along with multiple-scattering and first principle theoretical calculations, were used to investigate the existence form of In in skutteruidite and the chemical bond and electronic structure of (Ba,In) double-filled skutterudite compounds. On these bases, the thermal stability of structure and thermoelectric properties of these materials during the periodically thermal loading were studied.(Ba,In) double-filled skutterudite bulk materials with nominal composition Ba0.3InxCo4Sb12(0≤x≤0.3, Δx=0.05) were fabricated by melting-annealing-quenching and spark plasma sintering approaches. The effect of In filling on the composition, microstructure, and thermoelectric properties of (Ba,In) double-filled skutterudite materials were investigated. The results indicated that the actual compositions of these materials can be expressed as BarInsCo4Sb12(0.14≤t≤0.25,0≤s≤0.23). With increasing x, r decreased and s increased, and the total filling fraction r+s increased. In filling has less impact on the microstructure of BarInsCo4Sb12materials. With decreasing r and increasing s, carrier concentration gradually increased while mobility decreased. Compared with Ba single-filled skutterudite, lattice thermal conductivity of (Ba,In) double-filled skutterudites dramatically depressed and power factor significantly improved, leading to the remarkable enhancement in ZT. ZT values of1.33and1.34at850K were achieved for Ba0.15In0.16Co4Sb12and Ba0.14In0.23Co4Sb12materials.The existence form of In in In doped skutterudites materials InxCo4Sb12(0≤x<≤0.25) was investigated in depth by using XANES and EXAFS techniques associating with multiple-scattering theoretical calculations. The characteristic absorption structures of In K-edge XANES experimental spectra of InxCo4Sb12indicated that In has been incorporated into the lattice of skutterudite. The XANES theoretical spectra of three different cases, corresponding to In filling Sb12icosahedron void, substituting Sb and Co sites of skutterudite, were calculated by multiple-scattering theory. The best agreement between experimental and theoretical XANES spectra were observed when In filling Sb12icosahedron void, while significant differences between experimental and theoretical XANES spectra were found for the rest two cases, which provided direct evidence that In can fill Sb12icosahedron void of skutterudite. The analysis of EXAFS spectrum of Ino.2Co4Sb12compound further confirmed that In has filled Sb12icosahedron void. First principle calculations demonstrated the weak bond between In and neighboring Sb and that In filler forms a hyper-deep defect state in the middle of valence-band and a deep defect state near Fermi level, which are derived from bonding state and antibonding state of In-Sb bond, respectively.An inhomogeneous sp electron orbital hybridization model of Sb4rectangle ring was proposed to understand the formation mechanism of Sb4rectangle ring of skutterudite CoSb3. The chemical bond and local structure of (Ba,In) double-filled skutterudites BarInsCo4Sb12were investigated by XPS and EXAFS techniques. It was found that the orthogonal ββσbond and ppσ bond with different energy, corresponding to the Sb-Sb short bond and Sb-Sb long bond, are formed by inhomogeneous sp orbital hybridization of two Sb atoms in Sb4ring. The quantitative analysis of Sb3d5/2core-level XPS spectra indicated that the orbital hybridization between In and Sb makes Sb4ring bigger and squarer, and the charge transfer from Ba to Sb makes Sb4ring smaller and squarer. The fitness of Sb K-edge EXAFS spectra of BarInsCo4Sb12further confrimed that Ba and In double-filling results in the shape transition of Sb4ring from rectangle to square. The lattice distortion caused by shape transition of Sb4ring can reasonably explain the dramatic depression of lattice thermal conductivity of (Ba,In) double-filled skutterudite materials.The valence-band and conduction-band electronic structure of (Ba,In) double-filled skutterudites BarInsCo4Sb12were studied by XPS and XANES techniques associating with the first principle calculation. Theoretical calculations indicated that the valence-band can be described by an eight electronic states model for unfilled and filled skutterudites. Ba and In filling resulted in localized resonant states near Sb5p bonding state. The quantitative analysis of valence-band XPS spectra of BarInCo4Sb12revealed that their valence-band structure can be reasonably described by the eight electronic states model. The degeneracy of Sb5p bonding state of Sb4ring in the middle of valence-band was observed, which was related to the enhanced symmetry of Sb4ring due to the shape transition of Sb4ring from rectangle to square in BarInsCo4Sb12. The XANES spectra analysis of Ba,InrCo4Sb12revealed that the electronic density of states at conduction-band bottom are mainly contributed from Co3d and Sb5p unoccupied states, In forms localized resonant states in the vicinity of Fermi level. The enhancement of absorption edge intensity of Sb K-edge XANES suggested the degeneracy of Sb5p antibonding states of Sb4ring at conduction-band bottom, which was due to the enhanced symmetry of Sb4ring caused by Ba and In double-filling. The band degeneracy behavior of conduction-band bottom and the localized resonant states near the Fermi level derived from In filler were found to be the physical mechanism of the excellent power factor of (Ba,In) double-filled skutterudites.The thermal stability of structure and thermoelectric properties of (Ba,In) double-filled skutterudite bulk materials during periodically thermal loading at RT-450℃-RT were investigated. The results indicated that thermal loading led to the secondary precipitation, enrichment of Ba and loss of Sb and Co on the grain boundaries, separation of BasSb3from interior. In the early of periodically thermal loading, electrical conductivity decreased and the absolute Seebeck coefficient increased, which should be attributed to energy filtering effect caused by the secondary precipitates along the boundary. Lattice thermal conductivity increased with the cycles of thermal loading, which should be related to the separation of Ba filler from Sb12icosahedron voids. The variations of ZT values were unremarkable after thermal loading, ZT value of1.14at800K was remained after2000cycles for as-prepared material with initial ZT value of1.20, showing a degradation of only5.0%. The temperature dependence of thermoelectric properties measured during the process of increasing and decreasing temperature indicated that the micro structure relaxation caused by thermal loading has neglected effect on the thermoelectric properties. These experimental results confirmed that (Ba,In) double-filled skutterudite materials have excellent performance stability and are suitable for the application in the solar thermoelectric-photovoltaic hybrid power generation system with periodical temperature fluctuation working environment.更多还原