Study On Preparation, Structure And Properties Of La Filled Skutterudite Thermoelectric Materials

Recently CoSb₃ skutterudite compound attracts considerable attention due to its large Seebeck coefficient and appropriate electrical conductivity. However, its thermal conductivity is too high to suit thermoelectric application. There are two large cage-like voids in a unit cell of skutterudite structure, and some undersized rare earth or alkaline earth atoms could be filled into them to form the so-called filled skutterudite structure. These filling atoms, which are weakly bonded to the lattice,"rattle"about their equilibrium position substantially more than the other atoms in the structure, and results in a significant decrease in the lattice thermal conductivity due to the effective phonon scattering caused by the"rattling"of the filled atom without deteriorating the electrical transport properties of the compound significantly. Therefore it is of great importance for improving their thermoelectric performance. In this dissertation, p-type La_xFe_yCo_4-ySb₁₂ (y=1,2,3,4) and n-type La_xNi_0.2Co_3.8Sb₁₂ ( x = 0.1,0.3,0.5,0.7 ) filled Skutterudite compounds were prepared by mechanical alloying (MA) and subsequent hot pressing (MA-HP) method. Various analysis and measurement techniqiues, such as X-ray diffractometer (XRD), field emission scanning electron microscope (FE-SEM), energy-dispersive X-ray (EDX), and Seebeck coefficients and electrical and thermal conductivity measurement (TC-7000), were performed to study the microstructure, thermoelectric properties, thermal stability and oxidation resistance of the Skutterudite compounds.In the first charpter, a detailed review was present on the research and development status for several thermoelectric materials, especially for filled skutterudite based thermoelectric materials, and the research goal, task and content of this work was then put forward.Based on virtual instrument technique,a device, which was named as HGTE-I, was designed and developed for measuring both the Seebeck coefficient and electrical resistivity of thermoelectric materials. The device includes four parts: temperature controlling part, sample setting module,data acquisition module and measurement software. It can control temperature, collect and process data automaticly and measure in a wide temperature range (R.T.～800K) with a high precision. The deciation ranges within 6% on measuring Seebeck coefficient and within 5% on measuring electrical resistivity.Nominal La_0.4FeCo₃Sb₁₂ powders were consolidated by MA-HP, effect of processing conditions such as milling and hot pressing time, hot pressing temperature and annealing time and annealing temperature on microstructure and phase constitution was studied. Single phase skutterudite could be obtained when the hot pressing duration was more than 2h and the hot pressing temperature is higher than 650℃. Microstructure observation of the hot pressed samples was performed on a Sirion 200 field emission scanning electron microscope (FE-SEM). It shows there are two kinds of microstructures with evidently different sizes: one is the nanocrystalline matrix with an average size about 70 nm; the other is a coral-like microstructure which includes lots of radial branches or spokes and the grains are about 200–800nm in size. The coral-like microstructure could be eliminated by increasing the annealing temperature.P-type La_xFe_yCo_4-ySb₁₂ (y=1,2,3,4) and n-type La_xNi_0.2Co_3.8Sb1(2x=0.1, 0.3, 0.5, 0.7)filled Skutterudite compounds were prepared by MA-HP method. A maximum ZT value of 0.51 was achieved for the annealed p-type La_1.5Fe₄Sb₁₂ compound at about 750K. On the other hand, a maximum ZT value of 0.33 was achieved for the n-type La0.5Ni_0.2Co_3.8Sb₁₂ compound at about 750K.Rietveld refinements for the annealed samples of CoSb₃ and La_xFe_yCo_4-ySb₁₂ were carried out using a DBWS9807 program. It shows that La atom has anomalously large thermal parameter, revealing that the filling atoms rattle in the oversized voids. The bond distance and the bond angles and the void radius of the Skutterudite were calculated, and it is found that Sb-Sb bond increases gradually with increasing La filling and Fe substituting amount, especially the shorter Sb-Sb bond distance increasing more evidently. In addition, the void radius also increases with increasing La filling and Fe substituting amount.Thermal stability of CoSb₃ and La_xFe_yCo_4-ySb₁₂ annealed samples and oxidation resistance of CoSb₃ and La_1.5Fe₄Sb₁₂ annealed samples were investigated. The peritectic decomposition temperature of Skutterudite phase decreases and thermal stability degrades with increasing the amounts of La filling and Fe substitution. For the first time, the activation energies Q for oxidation of CoSb₃ and La_1.5Fe₄Sb₁₂ annealed samples calculated between 350℃-650℃according to the Arrhenius equation was 149.32±15.38kJ/mol and 126.95±11.81kJ/mol, respectively.