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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.

Abstract Thermoelectric materials are highly promising for waste heat harvesting. Although thermoelectric materials research has expanded over the years, bismuth telluride‐based alloys are still the best for near‐room‐temperature applications. In this work, a ≈38% enhancement of the average ZT (300−473 K) to 1.21 is achieved by mixing Bi0.4Sb1.6Te3 with an emerging thermoelectric material Sb2Si2Te6, which is significantly higher than that of most BiySb2−yTe3‐based composites. This enhancement is facilitated by the unique interface region between the Bi0.4Sb1.6Te3 matrix and Sb2Si2Te6‐based precipitates with an orderly atomic arrangement, which promotes the transport of charge carriers with minimal scattering, overcoming a common factor that is limiting ZT enhancement in such composites. At the same time, high‐density dislocations in the same region can effectively scatter the phonons, decoupling the electron‐phonon transport. This results in a ≈56% enhancement of the thermoelectric quality factor at 373 K, from 0.41 for the pristine sample to 0.64 for the composite sample. A single‐leg device is fabricated with a high efficiency of 5.4% at ΔT = 164 K further demonstrating the efficacy of the Sb2Si2Te6 compositing strategy and the importance of the precipitate‐matrix interface microstructure in improving the performance of materials for relatively low‐temperature applications.