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)CaxO) by doping ZnO wit...
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| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
American Association for the Advancement of Science (AAAS)
2025-01-01
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| Series: | Energy Material Advances |
| Online Access: | https://spj.science.org/doi/10.34133/energymatadv.0140 |
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| Summary: | 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)CaxO) 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)CaxO composites. Unlike well-known Al- or Ga-doped ZnO composites, Zn(1−x)CaxO exhibited a different behavior from Al- or Ga-doped ZnO. The electrical conductivity of Zn(1−x)CaxO 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)CaxO 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)CaxO composites were studied by x-ray diffraction and x-ray photoelectron spectroscopy. It was found that Zn(1−x)CaxO 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)CaxO 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 ZnO0.99CaO0.01 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. |
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| ISSN: | 2692-7640 |