Carbothermal reduction reaction as a strategy for boosting thermoelectric performance of binary metal oxides: a case study of ZnO ceramics

As a powerful concept, grain boundary (GB) engineering has been increasingly applied to enhance the energy conversion efficiency in thermoelectric materials through directly controlling the electrical and thermal transport characteristics. However, most efforts thus far have been focused solely on introducing extrinsic defects or impurity phases into the system without considering the role of intrinsic defects as critical factors. Thus, herein, we provide a holistic discussion on the correlation between the intrinsic defects and thermoelectric properties of materials using the carbothermal reduction (CTR) process. Taking ZnO as an example material, we experimentally show that vacancy-assisted migration of Zn interstitials lead to a decrease in the Schottky barrier height at grain boundaries. In addition, increasing the carrier concentration and band gap narrowing caused by the CTR reaction can lead to power factors up to 246 μW/mK2, which is comparable to the excellent values of transition metal-doped ZnO compounds. By minimizing the specific contact resistance of the interfaces down to 12.7 mΩ cm2 at a clamping pressure of 20 MPa, the output power of a single-leg module reaches an outstanding value of 292 μW at ΔT = 65 K, which is extremely higher than those of the state-of-the-art ZnO based materials, almost by a factor of three. We believe that the current study manifests a cost-effective avenue toward the development of efficient and practical thermoelectric devices.