In this paper, 2 atom % Al-doped ZnO (AZO) was prepared by the co-precipitation method together with sparking plasma sintering (SPS) treatment. The as-synthesized AZO powders show the morphology of hollow hexagonal towers, which result in a high porosity of 50.6% in the bulk sample consolidated by SPS sintering at 400 °C, and the porosity decreases gradually with increasing sintering temperature up to 1000 °C. Positron annihilation measurements reveal that even after sintering at 1000 °C, there are still a considerable number of small pores. A high electrical conductivity of 3 × 105 S m-1 is achieved at room temperature for the AZO sample sintered at 1000 °C, while the absolute values of Seebeck coefficient keep at relatively high values between 59 and 144 μV K-1 in the measurement temperature range of 27-500 °C, leading to a high power factor of 3.4 × 10-3 W m-1 K-2. On the other hand, the pores in AZO act as strong phonon scattering centers, and an extremely low thermal conductivity of 1.5 W m-1 K-1 measured at room temperature is obtained for AZO sintered at 400 °C. Due to the residual pores in the 1000 °C-sintered sample, the thermal conductivity is still relatively low. As a result, a maximum ZT of 0.275 measured at 500 °C is obtained in this sample, which is the highest ZT reported for ZnO around this temperature.
Keywords: Al-doped ZnO; porosity; positron annihilation; spark plasma sintering; thermoelectric.