Composition Dependence of Thermoelectric Properties in Polycrystalline Type-I Ba_8Ga_xSi_(46-x) (Nominal x=14-18) Clathrates Prepared by Combining Arc Melting and Spark Plasma Sintering Methods

摘要

The gallium composition dependence of thermoelectric properties in polycrystalline Ba_8Ga_xSi_(46-x) compounds with the type-I clathrate structure is presented. Polycrystalline n-type samples with various nominal gallium compositions (nominal x=14-18) were prepared by combining arc melting and spark plasma sintering (SPS) methods. Results of Rietveld refinement of powder x-ray diffraction data and electron probe microanalysis (EPMA) show that the type-I clathrate phase with the stoichiometric composition (x=16) is not formed even though the gallium rich compositions (nominal x=17, 18) and the solubility limit of gallium composition is about 15. The lattice constant a increases monotonically with increasing x in the nominal composition range of x=14-16 but is saturated in x>16. The Hall carrier concentration n at room temperature decreases from about 2×10~(21) cm~(-3) to about 4×10~(20) cm~(-3) with increasing x. The magnitude of Seebeck coefficient S increases and the electrical conductivity σ decreases with increasing x at room temperature due to the decrease in n. The room temperature Hall mobility μ increases with decreasing n. The thermal conductivity κ decreases with increasing x due to the decrease in the electronic contribution. The reduced Fermi energy, the effective mass m~*, and the Lorentz number L are determined from an analysis of experimental data on a single parabolic band model with the acoustic phonon scattering as the predominant scattering mechanism. The lattice thermal conductivity κ_L determined by κ_L=κ-LTσ relation is as low as 1.1 Wm~(-1)K~(-1) for nominal x=16. The dimensionless thermoelectric figure of merit ZT is estimated to be 0.55 at 900 K for nominal x=16. The calculation of ZT using the determined parameters at room temperature, μ=10 cm~2V~(-1)s~(-1), m~*/m_0=2, and κ_L=1.1 Wm~(-1)K~(-1), predicts the maximum ZT=0.8 (900 K) at the optimum carrier concentration of 2×10~(20) cm~(-3).