Characterization of ZnO thermoelectric ceramics and their microstructures consolidated by two-step spark plasma sintering

Abstract

Zinc oxide (ZnO) thermoelectric ceramics were consolidated by spark plasma sintering (SPS) along with one-step/two-step sintering (OS/TS) temperature profiles, the latter employing a strategy of heating to a higher temperature (T₁) followed by prolonged sintering at a lower temperature (T₂) for densification while preventing grain growth. In the first step, the temperatures and holding times were 1100 and 1150 °C for 15 and 5 min, respectively, and the second-step was maintained at 1050 °C for 30, 60, and 120 min. By combining TS and SPS processes (TS-SPS), high relative densities of 91.4–94.9 % were achieved without significant grain growth. Electron backscatter diffraction (EBSD) revealed a noticeable grain-growth suppression of 56 % for the TS-SPS sample with T₁ = 1100 °C. The highest power factor of 7.8 × 10⁻⁵ W K⁻² m⁻¹ was achieved for the TS-SPS ZnO, while the lowest thermal conductivity of 4.7 W K⁻¹ m⁻¹ was achieved for the OS-SPS ZnO, attaining a figure of merit (ZT) of 0.017 at 775 °C.