Thermoelectric properties and thermopower enhancement of Al-doped ZnO with nanosized pore structure

Thermoelectric properties of Al-doped ZnO (Zn_0.98Al _0.02O) with nanosized pore (nanovoid) structure were investigated. Nanovoids were formed by using monodisperse polymethylmethacrylate (PMMA) particles of 150, 425, and 1800nm in average diameter as a void forming agent (VFA). Whereas the thermal conductivity of the samples sintered with the 150nm PMMA particles of 510 wt% was efficiently suppressed, the magnitude of the suppression was almost the same as that of the electrical conductivity, revealing that the selective phonon scattering was failed in these samples. However, being helped by the contribution from an enhancement in the Seebeck coefficient, the samples sintered with 510 wt% of 150nm VFA successfully attained the ZT values of 0.55-0.57 at 1000°C. Furthermore, the reduction of the thermal conductivity with increasing porosity was much steeper than ordinary porous materials, suggesting significantly stronger phonon scattering by the nanosized pore structure.