Effect Of Composite And Doping On The Transport Properties Of CoSb₃-based Filled Skutterudites

CoSb₃-based filled skutterudites have been extensively studied worldwide over the past several years for their potential advanced thermoelectric application in their intermediate temperature range, due to their extremely high carrier mobility and heavy effective mass. One of the distinguishing characteristics of the filled skutterudites is the significant depression of the lattice thermal conductivity due to the "rattling" of the filler atoms positioned in the oversized Sb-dodecahedron voids. However, the lattice thermal conductivity of the filled skutterudites still remains high as compared to the state-of-the-art hermoelectric materials, such as Bi₂Te₃-based alloys. The lattice thermal conductivity of the filled skutterudites could be further depressed through both approaches of doping and introducing second phase into the matrix. In this dissertation, Eu₂O₃ phase have been introduced into Eu_yCo₄Sb₁₂ matix forming Eu_yCo₄Sb₁₂/Eu₂O₃ composite and the effect of Eu₂O₃ particle dispersion on the electrical and thermal tansport properties has been studied and effects of Fe content on the thermoelectric transport properties of Yb_yFe_xCo_4-xSb₁₂ compounds has been investigated.Eu-filled skutterudites with the same nominal filling fraction for different annealing time were prepared by melting method. The obtained materials were consolidated by Spark Plasma Sintering (SPS) technique. The constituent phases of the samples are different fore-and-aft annealing. CoSb₂ and Sb phases are determined by X-ray diffraction (XRD) before annealing. The main phase Co₄Sb₁₂ is formated after annealing 24h, impurity phases such as CoSb₂ and Sb are determined by XRD too. With the prolongation of annealing time, impurity phases decrease and the lattice parameter increases, due to the increase of the filling fraction of Eu in CoSb₃. According to the EPMA chemical composition and the relationship between lattice parameter and Eu filling fraction, the filling fraction of Eu is up to 0.43 when the annealing time reaches 96h, similar to the filling fraction of reported compound with the same nominal composition after annealing 168h. Both Absolute value of Seebeck coefficient and lattice thermol conductivity decrease with the prolongation of annealing time due to the increase of Eu filling fraction. The highest ZT value of 0.9 is achieved for Eu_0.24Co₄Sb₁₂ at 850K after annealing 24h, similar to the highest ZT value of reported compound with the same nominal composition after annealing 168h.Eu_yCo₄Sb₁₂/Eu₂O₃ composites without impurity phases were synthesized by twice-annealing and in-situ reaction methods. Some Eu₂O₃ defects locate at the grain boundaries of matrix and others form nano-scale inclusions distributed within Eu_yCo₄Sb₁₂ grains. The combination of the "rattling" of the Eu ions inside the Sb-icosahedron voids in CoSb₃ and the phonon scattering of the oxide defects results in a large reduction in lattice thermal conductivity. The lattice thermal conductivity of the Eu_0.27Co₄Sb₁₂/Eu₂O₃ composite is about 2.04Wm^-1K^-1 at room temperature and decreases to 0.52 Wm^-1K^-1at 850K, approaching the limit of the theoretical minimum thermal conductivity. A high power factor for the composites as high as 50μWcm^-1K^-2 for Eu_0.27Co₄Sb₁₂/ Eu₂O₃ is achieved at 850K. A significant enhancement in thermoelectric performance is observed, and the improved ZT value reaches 1.1 at 850K.Yb_yFe_xCo_4-xSb₁₂ compounds were synthesized by melting method. The small amount of impurity phases such as FeSb₂ and CoSb₂ are founded by electron probe microanalysis (EPMA) when the norminal Fe content is larger than 1.5. All samples showed p-type conduction. Experimental results show that electrical conductivity of the samples increases, and the lattice thermal conductivity decreases with increasing iron content due to the vacancy point defect scattering at Co site introduced by iron. With Fe/Co ratio is about 1.2/2.8, the lattice thermal conductivity reaches the minimum value. The lowest lattice thermal conductivity at room temperature is about 1.33 Wm^-1K^-1. The highest ZT value is 0.67 for Yb_0.29Fe_xCo_4-xSb₁₂ at 800K.