High-temperature thermoelectric properties of polycrystalline Zn1−x−yAlxTiyO ceramics

The as-sintered Zn1−xAlxO (0 ≤ x ≤ 0.05) samples crystallized in the ZnO with a wurtzite structure, along with a small amount of the cubic spinel ZnAl2O4. The addition of Al2O3 to ZnO gave rise to a decrease in grain size, ranging from 7.3 to 2.7 μm and in relative density, ranging from 99.2 to 90.1% of the theoretical density. In the Zn0.97Al0.03−yTiyO samples, as the amount of TiO2 increased, the grain size of ZnO grains and second phases, such as Zn2TiO4 and ZnAl2O4, as well as density increased. The co-doping of Al and Ti led to a significant increase in both the electrical conductivity and the absolute value of the Seebeck coefficient, resulting in an increase in the power factor. The highest value of power factor (3.8 × 10−4 W m−1 K−2) was attained for Zn0.97Al0.02Ti0.01O at 800 °C. It is demonstrated that the Al and Ti co-doping is fairly effective for enhancing thermoelectric properties.