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 x O) 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 x O composites. Unlike well-known Al- or Ga-doped ZnO composites, Zn (1− x ) Ca x O exhibited a different behavior from Al- or Ga-doped ZnO. The electrical conductivity of Zn (1− x ) Ca x O 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 x O 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 Zn 2+ ions were substituted with Ca 2+ ions during annealing. Zn (1− x ) Ca x O composites were studied by x-ray diffraction and x-ray photoelectron spectroscopy. It was found that Zn (1− x ) Ca x O exhibited a crystal structure similar to that of ZnO. X-ray photoelectron spectroscopy results showed that there are 2 different types of Ca 2+ ions in the Zn (1− x ) Ca x O composites. One type is assigned to Ca 2+ ions that substituted for Zn 2+ ions in the ZnO lattice, while the other type is Ca 2+ ions from nonelectrically conductive free CaO. The temperature-dependent thermal conductivity of ZnO 0.99 CaO 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.