Enhancement of Thermoelectric Properties of Zinc Oxide Composites via Doping with Active Metal Oxide

Active metals such as calcium (Ca)-doped zinc oxide (ZnO)-based thermoelectric (TE) materials are less studied in the literature, primarily due to the expected inherent instability caused by surface hydration. In this work, we synthesized Ca-doped ZnO-based composites (Zn_(1−x)Ca_xO) by doping ZnO with CaO nanoparticles using the spark plasma sintering method (0.05 to 0.2 mol. %) and subsequently investigated the TE properties of these Zn_(1−x)Ca_xO composites. Unlike well-known Al- or Ga-doped ZnO composites, Zn_(1−x)Ca_xO exhibited a different behavior from Al- or Ga-doped ZnO. The electrical conductivity of Zn_(1−x)Ca_xO was much improved from being nonconductive to being conductive, achieving an electrical conductivity of 650 S cm⁻¹ when doping with 0.1 mol. % CaO. This improvement is due to the surface hydration of hygroscopic CaO, which forms conductive Ca(OH)₂, enhancing the electrical conductivity. However, the electrical conductivity was remarkably decreased after Zn_(1−x)Ca_xO was subjected to annealing at 860 K. In contrast, the Seebeck coefficient, ranging from −90 to −160 μV K⁻¹, was not substantially affected by the high-temperature annealing process, indicating that part of Zn²⁺ ions were substituted with Ca²⁺ ions during annealing. Zn_(1−x)Ca_xO composites were studied by x-ray diffraction and x-ray photoelectron spectroscopy. It was found that Zn_(1−x)Ca_xO exhibited a crystal structure similar to that of ZnO. X-ray photoelectron spectroscopy results showed that there are 2 different types of Ca²⁺ ions in the Zn_(1−x)Ca_xO composites. One type is assigned to Ca²⁺ ions that substituted for Zn²⁺ ions in the ZnO lattice, while the other type is Ca²⁺ ions from nonelectrically conductive free CaO. The temperature-dependent thermal conductivity of ZnO_0.99CaO_0.01 before and after annealing was examined, revealing that thermal conductivity decreased with an increase in temperature consistently after annealing compared to that before annealing. A highest ZT of ~0.043 at 723 K was obtained, which is 8 times of that of undoped ZnO. Our study demonstrates the feasibility of CaO-doped ZnO as a potential TE material.