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 κlat 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 κlat 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.
Highlights
Transition metal oxides (TMOs) are under debate for high-temperature thermoelectric (TE)applications since NaCo2 O4 was found to be a p-type material with a thermopower α as high as100 μV·K−1 at 300 K [1]
TMOs are subject of in-depth investigations [2,3,4] in particular because there is still a lack of appropriate n-type counterparts, lanthanum-doped SrTiO3 [5], ZnO [6] or doped TiO2−x [7,8,9] show promising results
In the three-phase region (2.82 ≤ x ≤ 2.88) of the system (Table 2), a significantly reduced κ lat is found for x = 2.84 up to T ≤ 700 K. This tendency is noticable from a plot of κ lat vs. the O/W-ratio x (Figure 10b inset). It indicates that at high temperatures, the lowest κ lat is achieved for WOx materials with multiple phases due to enhanced phonon scattering at the interfaces
Summary
Transition metal oxides (TMOs) are under debate for high-temperature thermoelectric (TE). Crystals 2017, 7, 271 metallic rutile-type WO2 , with varying coupling of the [WO6 ] octahedra (Figure 1b–e) Such structure modifications result in an increasing charge carrier concentration [12]. Microstrain-driven ordering of the CS crystals or under high pressure conditions The fact that they are notmechanism observed as bulk planes [19], which be sensitive to the synthesis conditions. Phases comprises high-temperature and WO2.90 (W20O58) were successfully prepared by spark plasma sintering (SPS) being promising heating of WOx powder mixtures over several days and weeks [14,15,21,22,23]. In this work we use the notation “WOx ” for sample compositions, and “Wa Ob ” for phases and their crystal structures
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