Study On Synthesis And Thermoelectric Properties Of Skutterudite Materials

Skutterudite compound has perfect electrical properties, including high carrier mobility and appropriate Seebeck coefficient. Though its thermal conductivity is higher than that of state-of-the-art thermoelectric materials such as Bi₂Te₃, fortunately, there are two large voids in a unit call of the crystal structure and undersized atoms can be filled into them to yield a remarkable reduction to the lattice thermal conductivity. Therefore it is of great importance for promising thermoelectric applications. A wide and deep investigation has been carried out on the synthesis process, microstructure, thermoelectric properties and transport mechanism of the Skutterudite compound. Mechanical alloying of Co-Sb and Co-Fe-Sb has been studied and applied to its preparation. Two new approaches, mechanical grinding-sintering and mechanical grinding-hot pressing-annealing, have been developed for synthesizing the substituted as well as the filled Skutterudite compounds. Structure analysis on the Fe substituted Skutterudite compounds Co_4-xFe_xSb₁₂ shows that there is a maximal solubility of Fe and is found at about x=0.68. Thermoelectric property measurement between 300⁷73K shows that both the electrical resistivity and Seebeck coefficient decrease with increasing Fe content due to an increase of the carrier concentration, and thermal conductivity is lowered considerably when Fe is substituted. Mass fluctuation and strain fluctuation scattering due to Fe substitution has been calculated in an effort to qualify the effect of Fe substitution on the lattice thermal conductivity, and it reveals that disorder scattering is not the main reason for decrease of the lattice thermal conductivity, while carrier scattering could not be ignored in these heavily-doped semiconductors, and some unknown mechanism may exist. The overall figure of metir (ZT) is improved with increasing Fe content except the Co₃FeSb₁₂ sample, therefore the maximal ZT value of 0.32 is achieved for Co^3.35Fe_0.65Sb₁₂ in 773K. Subsequently, La partially filled Skutterudite compounds LayCo4-xFexSb12 (x≤1,y≤1) have been prepared and their structure, optical and thermoelectric properties been studied. Investigation on Raman spectra of the Skutterudite compounds shows that, only Fe substitution makes little change, whereas La filling produces a shift and apparent broadening of the vibrational modes. Also abnormally large thermal parameter of the filling atom is observed from Rietveld refinement of the partially filled compound, revealing that the filling atom is rattling in the oversized voids. These local rattlers could scatter large numbers of phonons and decrease lattice thermal conductivity dramatically, which then is verified by the thermal conductivity measurement. Finally, preparation and interface analysis of a segmented Skutterudite/Bi2Te3 material have been performed. Strong bonding is acquired in sample with Bi2Te3-type powders as interface layer.