Sintering characteristics and thermoelectric properties of Mn-Al co-doped ZnO ceramics

Polycrystalline Mn-Al co-doped ZnO ceramics were prepared by sintering in air. Increasing Mn dopant concentration led to an increase of the ZnO lattice constants indicating substitution of Mn on the Zn sublattice. A Mn-rich spinel secondary phase was also detected within grains and at grain boundaries. The addition of the Al dopant led to an initial reduction in grain size while the addition of Mn led to an increase in grain size. Thermoelectric property measurements revealed that Al doping led to a large initial increase in electrical conductivity and increasing Mn dopant addition resulted in electrical conductivity reduction. The absolute value of the Seebeck coefficient was largely unaffected at low Mn doping levels; however, an increase was observed for high Mn doping levels. The thermal conductivity showed a marked decrease with increasing Mn concentration, indicating the effectiveness of Mn dopant substitution for Zn to reduce the thermal conductivity by point defect scattering. Overall, the maximum ZT was achieved for the Zn0.99Al0.01O sample (ZT = 0.07 at 750 degrees C) and increasing Mn doping led to a decrease of ZT due to the increased resistivity caused by alloy scattering and secondary phase formation. (C) 2016 The Ceramic Society of Japan. All rights reserved.