Electronic Structure And Thermoelectric Transport Properties Of Barium And Indium Double-filled Skutterudite Thermoelectric Materials

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.