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Abstract:
The solid-state reaction of WO3 with W was studied in order to clarify the phase formation in the binary system W-O around the composition WOx (2.50 <= x <= 3) during spark plasma sintering (SPS). A new phase "WO2.82" is observed in the range 2.72 x 2.90 which might have the composition W12O34. The influence of the composition on the thermoelectric properties was investigated for 2.72 <= x <= 3. The Seebeck coefficient, electrical conductivity and electronic thermal conductivity are continuously tunable with the oxygen-to-tungsten ratio. The phase formation mainly affects the lattice thermal conductivity kappa(lat) which is significantly reduced until 700 K for the sample with the composition x = 2.84, which contains the phases W18O49 and "WO2.82". In single-phase WO2.90 and multi-phase WOx materials (2.90 <= x <= 3), which contain crystallographic shear plane phases, a similar reduced kappa(lat) is observed only below 560 K and 550 K, respectively. Therefore, the composition range x < 2.90 in which the pentagonal column structural motif is formed might be more suitable for decreasing the lattice thermal conductivity at high temperatures.
