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 WO_2.90 and WO_2.722. 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 WO_2.90. The figures of merit (ZT = 0.13 at 1100 K for WO_2.90 and 0.07 at 1100 K for WO_2.722) are relatively high for tungsten–oxygen compounds and metal oxides in general. The electrical resistivity of WO_2.722 shows a metallic behaviour with temperature, while WO_2.90 has the characteristics of a heavily doped semiconductor. The low thermopower of 80 μV K⁻¹ at 1100 K for WO_2.90 is attributed to its high charge carrier concentration. The enhanced thermoelectric performance for WO_2.90 compared to WO_2.722 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.