Thermoelectric Properties and Microstructure of Ba_8Al_(14)Si_(31) and EuBa_7Al_(13)Si_(33)

Abstract

Powder samples of the type I clathrate phases, with the proposed stoichiometry Ba_8Al_(16)Si_(30) and Eu_2Ba_6Al_(16)Si_(30), were synthesized using direct reaction of stoichiometric amounts of the elements. Rietveld refinement of powder X-ray diffraction data is consistent with the clathrate type I structure. Composition, microstructure, and thermoelectric property measurements were made on hot-pressed pellets. Stoichiometry of the Ba_8Al_(16)Si_(30) sample was determined from microprobe data to be Ba_8Al_(14)Si_(31), close to the deficient framework solid solution of the general formula, Ba_8Al_xSi_(42-3/4x[]4-1/4x) (x = 14; [], open square indicates lattice defect). In the case of the rare-earth-substituted compound, microprobe analysis of the microstructure indicates that it is not single phase, but contains multiple components, most of the general clathrate stoichiometry. The majority phase has the stoichiometry EuBa_7Al_(13)Si_(32). Both phases show conductivity typical of heavily doped semiconductors with negative Seebeck coefficients. The highest Seebeck coefficients are − 99 and −71 μV/K at 1192 K for Ba_8Al_(14)Si_(31) and EuBa_7Al_(13)Si_(33), respectively. The maximum zT is 0.34 at 1150 K and 0.22 at 1165 K for Ba_8Al_(14)Si_(31) and EuBa_7Al_(13)Si_(33), respectively.

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