Using crystallographic shear to reduce lattice thermal conductivity: high temperature thermoelectric characterization of the spark plasma sintered Magnéli phases WO2.90 and WO2.722

Abstract

Engineering of nanoscale structures is a requisite for controlling the electrical and thermal transport in solids, in particular for thermoelectric applications that require a conflicting combination of low thermal conductivity and low electrical resistivity. We report the thermoelectric properties of spark plasma sintered Magnéli phases WO2.90 and WO2.722. The crystallographic shear planes, which are a typical feature of the crystal structures of Magnéli-type metal oxides, lead to a remarkably low thermal conductivity for WO2.90. The figures of merit (ZT = 0.13 at 1100 K for WO2.90 and 0.07 at 1100 K for WO2.722) are relatively high for tungsten-oxygen compounds and metal oxides in general. The electrical resistivity of WO2.722 shows a metallic behaviour with temperature, while WO2.90 has the characteristics of a heavily doped semiconductor. The low thermopower of 80 μV K(-1) at 1100 K for WO2.90 is attributed to its high charge carrier concentration. The enhanced thermoelectric performance for WO2.90 compared to WO2.722 originates from its much lower thermal conductivity, due to the presence of crystallographic shear and dislocations in the crystal structure. Our study is a proof of principle for the development of efficient and low-cost thermoelectric materials based on the use of intrinsically nanostructured materials rather than artificially structured layered systems to reduce lattice thermal conductivity.

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

“…A local maximum at 635 K is in good accordance with previous records [11,12,24]. The electrical conductivity of WO2 and WO2.72 indicates typical metallic behaviour with σ(T) ∝ 1/T dependency ( Figure S1b).…”

“…However, for WO 2.72 the approximation of κ lat shows uncertainty of maximum 45% and a significant difference regarding WO 3 cannot be determined. WO 2.90 shows κ tot ≈ 3.5-4.5 W·m −1 ·K −1 over the whole temperature range (Figure 6a), which is close to previously published values [11,24]. With respect to errors its lattice contribution κ lat equals that of WO 3 for T > 560 K (Figure 6b).…”

“…Recently WO 2.72 (W 18 O 49 ) and WO 2.90 (W 20 O 58 ) were successfully prepared by spark plasma sintering (SPS) being promising bulk TE materials [11,12,24]. SPS combines the solid-state reaction of powdered precursor mixtures with simultaneous shaping, and provides an effective manufacturing route for TE materials due to low temperatures and short reaction times [8].…”

“…An interesting approach is the exploitation of crystallographic shear (CS) for the reduction of the lattice thermal conductivity κ lat due to increased phonon scattering [4,10,11]. CS plane structures occur when a transition metal is partially reduced and oxygen layers are removed from the structure, e.g., of TiO 2 , VO 2 , MoO 3 and WO 3 .…”

“…In the tungsten-oxygen system ( Figure 1a) with decreasing O/W-ratio x the observed phases are the insulating ReO 3 -type WO 3 , the semiconducting phase W 20 O 58 with CS planes, the metallic phase W 18 O 49 with pentagonal columns (PC) and the The vapor-transport preparation route to obtain these phases comprises a high-temperature heating of WOx powder mixtures over several days and weeks [14,15,[21][22][23]. Recently WO2.72 (W18O49) and WO2.90 (W20O58) were successfully prepared by spark plasma sintering (SPS) being promising bulk TE materials [11,12,24]. SPS combines the solid-state reaction of powdered precursor mixtures with simultaneous shaping, and provides an effective manufacturing route for TE materials due to low temperatures and short reaction times [8].…”