Molybdenum oxides MoO x in the composition range 2 ≤ x ≤ 3 were synthesized and compacted by the solid-state reaction of powdered α-MoO 3 with Mo in the spark-plasma synthesis (SPS) process at temperatures up to 973 K. From six known compounds in the Mo−O system, Mo 18 O 52 (x = 2.889), Mo 17 O 47 (x = 2.760), and γ-Mo 4 O 11 (x = 2.750) were synthesized as practically single-phase products despite their narrow composition ranges (≈0.1 at %) with a yield (0.5−1 g per synthesis) sufficient for thermoelectric property measurements on the polycrystalline bulk material from room temperature to 763 K. For Mo 18 O 52 , which appears to be an intrinsic narrow-gap semiconductor (activation energy ≈ 0.3 eV), exceptionally low thermal conductivity over the full temperature range (0.5−0.9 W m −1 K −1 ), high p-type Seebeck coefficient at room temperature (+140 μV K −1 ), and a p−n transition at 440 K were found. The phases MoO 2 , γ-Mo 4 O 11 , and Mo 17 O 47 show poor metallic conductivity. The highest power factor is achieved by Mo 17 O 47 (30 Wm −1 K −2 for 440 K ≤ T ≤ 610 K). In γ-Mo 4 O 11 , a hitherto unknown reversible transition is found at 450 K from specific heat measurements.

By applying the melt-centrifugation, porosity and dislocations are introduced in the microstructure of the n-type Zintl phase Mg3+xSb1.5Bi0.49Te0.01leading to significantly low κl (∼ 0.33 W m−1 K−1 at 723 K) and enhanced zT (∼ 1.64 at 723 K).

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.

Pt-based intermetallic compounds M2Pt with anti-CaF2 type of crystal structure and counterpart elements (M) from groups 13 (Al, Ga, In) and 14 (Sn) were studied as electrocatalysts for the oxygen...

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