Effect of annealing in air on the microstructural and thermoelectric characteristics of ZnO-based ceramics synthesized in a reducing atmosphere

Sintering in a reducing atmosphere, in comparison with sintering in air, greatly improves the electrical performance of ZnO-based thermoelectric materials. However, it also raises a question concerning their chemical stability in air for applications at high temperatures. The influence of annealing in air at temperatures up to 1000°C on the characteristics of ZnO-based ceramics synthesized in a reducing atmosphere of N2 + H2 was studied. The annealing process primarily results in acceptor states at the grain boundaries with the formation of electrostatic Schottky barriers, leading to decreased charge-carrier mobility and a lowering of the electrical conductivity and consequently 4-times lower power factor in comparison with the ceramics before annealing. The annealing in air does not affect the enhanced solid solubility of the donor dopants (i.e., Al) in the ceramics by sintering in a reducing atmosphere; however, it lowers the concentration of intrinsic donor defects, i.e. zinc interstitials (Zni) and oxygen vacancies (VO), thus decreasing the concentration of charge carriers. Nevertheless, the power factor of air-annealed ceramics was still 8-times higher than for ZnO ceramics prepared only by sintering in air. These results provide guidelines for the development of ZnO ceramics with annealing in air at high temperatures up to 1000°C after sintering in a reducing atmosphere, which not only enhanced thermoelectric characteristics, but also improved stability for applications in air at high temperatures, compared to the samples synthesis in air only.