Phase stability and chemical composition dependence of the thermoelectric properties of the type-I clathrate Ba_8Al_xSi_(46-x) (8≤x≤15)

摘要

Phase stability of the type-I clathrate compound Ba_8Al _xSi_(46-x) and the thermoelectric property dependence on chemical composition are presented. Polycrystalline samples were prepared by argon arc melting and annealing. Results of powder X-ray diffraction and electron microprobe analysis show that the type-I structure is formed without framework deficiency for 8≤x≤15. Lattice constant a increases linearly with the increase of x. Thermoelectric properties were measured for x=12, 14 and 15. The Seebeck coefficients are negative, with the absolute values increasing with x. The highest figure of merit zT=0.24 was observed for x=15 at T=1000 K, with carrier electron density n=3×10~(21) cm~(-3). A theoretical calculation based on the single parabolic band model reveals the optimum carrier concentration to be n～4×10~(20) cm ~(-3), where zT～0.7 at T=1000 K is predicted.