Large seebeck coefficients of protonated titanate nanotubes for high-temperature thermoelectric conversion

Titanate nanotubes Na_2-xH_xTi₃O₇ produced by alkali hydrothermally treated ground TiO₂ aerogels are investigated as possible materials for high-temperature thermoelectric conversion by measuring their thermoelectric properties. Strikingly, the Seebeck coefficients increased sharply in the temperature range 745 to 1032 K, reaching a maximum of 302 μV/K. The electrical resistivity of the TNNTs ranged from 325 to 525 Ωm, which is lower than that of bulk TiO₂, and thermal conductivities at room temperature were also very low, ranging from 0.55 to 0.75 Wm^-1 K^-1. The hollow structure of the titanate nanotubes, with small, uniform diameters, is thought to be responsible for the ultralow thermal conductivity. The large thermoelectric power and ultralow thermal conductivity suggest that titanate nanotubes represent a new kind of p-type oxide thermoelectric material.