Preparation of monophasic titanium sub-oxides of Magneli phase with enhanced thermoelectric performance

Titanium sub-oxides of Magneli phase are considered as earth-abundant, non-toxic materials for thermoelectric application at high temperature, but very limited investigation has been reported mainly owing to the difficulty lying in obtaining monophasic material. Herein we describe a facile strategy to prepare monophasic Ti9O17 and Ti4O7 powder, in which the single Ti9O17 phase is obtained for the first time. After spark plasma sintering, the thermoelectric properties of monophasic Ti9O17 and Ti4O7 ceramics are studied comparatively. Despite the one magnitude lower electrical conductivity, Ti9O17 shows higher power factor compared with Ti4O7 due to the large Seebeck coefficient. Moreover, with the low electronic thermal conductivity, sub-micron scaled grain size and nano-scaled crystallographic shear plane, the Ti9O17 ceramic exhibits even lower thermal conductivity compared to Ti4O7, leading to a ZT of 0.16 at 764 K, which is the best thermoelectric performance among reported Magneli phases.