Spark Plasma Sintering of Tungsten Oxides WOx (2.50 ≤ x ≤ 3): Phase Analysis and Thermoelectric Properties

Abstract

Abstract:The solid-state reaction of WO 3 with W was studied in order to clarify the phase formation in the binary system W-O around the composition WO x (2.50 ≤ x ≤ 3) during spark plasma sintering (SPS). A new phase "WO 2.82 " is observed in the range 2.72 ≤ x ≤ 2.90 which might have the composition W 12 O 34 . 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 κ lat which is significantly reduced until 700 K for the sample with the composition x = 2.84, which contains the phases W 18 O 49 and "WO 2.82 ". In single-phase WO 2.90 and multi-phase WO x materials (2.90 ≤ x ≤ 3), which contain crystallographic shear plane phases, a similar reduced κ 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.

Electrical, Optical, and Thermal Transport Properties of Oxygen-Deficient Amorphous WOx (2.5 < x < 3) Films

“…We plotted the reported κ of c-WO x (white square) for comparison. Although c-WO x shows an inverse linear dependency of the observed κ obsd on x , i.e., 1.3 W m –1 K –1 ( x = 2.98) to 12 W m –1 K –1 ( x = 2.72), a-WO x films did not show such tendency; κ obsd is in the range from ∼0.9 W m –1 K –1 ( x = 2.982) to ∼1.6 W m –1 K –1 ( x = 2.642), which is far lower than that of c-WO x (Figure a). Note that the porous structure of the a-WO x films ( x > 2.93) deposited at higher P O 2 (>2.5 Pa) would show a slightly lower κ obsd .…”

Rapid Synthesis of W_18O_49 via Reactive Spark Plasma Sintering with Controlled Anisotropic Thermoelectric Properties

“…It suggested that single phase W18O49 can be straightforwardly synthesized by reaction of WO3 and WO2 during the reactive SPS. This is remarkable, since other reports regarding the synthesis of monolithic ceramic W18O49 would involve multiple complicated steps 9,28,29) . In order to elucidate the anisotropic orientations, XRD patterns are shown in expanded 2θ sections, from 22° to 27° (Fig.…”

Environmentally Friendly Thermoelectric Materials: High Performance from Inorganic Components with Low Toxicity and Abundance in the Earth

“…Tungsten oxide is also n-type, and there is little work dealing with its thermoelectric performance. Non-stoichiometric tungsten oxide, prepared by SPS, showed moderate thermoelectrical properties, with a maximum zT = 0.061 for WO 2.90 at 963 K. [140] Something similar is happening with vanadium, where low values for relevant thermoelectric applications have also been reported for vanadium oxide, without relevant results for this material so far. [141] Finally, non-stoichiometric cobalt oxides are among the metal oxides with the highest zT values have been reported.…”

“…Vapor deposition techniques such as atomic layer deposition, [152] sputtering, [127] molecular beam epitaxy, [153] or pulsed laser deposition [133,154] can also be used to form highly crystalline oxides. Finally, sintering methods such as SPS [140] or solid-state reactions [138] have also been used to form certain metal oxides…”