Impacts of Ga doping on structural and thermoelectric properties of ZnO bulks sintered by solid-state reaction

Abstract

Introduction: ZnO-based bulks have been known as high-temperature thermoelectric materials due to their high thermal stability and large Seebeck coefficient. The biggest handicaps of pure ZnO are normally low electrical conductivity and weak thermoelectric power factor. This work aims to improve the electrical and thermoelectric properties of pure ZnO bulks by Ga doping. Methods: The pure ZnO and Zn0.98Ga0.02O bulks were fabricated by solid-state reaction in air at high temperature. The crystalline and microstructural properties of the samples were analyzed by using X-ray diffraction and field-emission scanning electron microscopy, respectively. The dependences on temperature of electrical conductivity, Seebeck coefficient, and power factor were recorded by using a commercial LSR-3 system. Results: The Ga doping increases carrier concentration and density-of-state effective mass, leading to simultaneous improvements in electrical conductivity and Seebeck coefficient of ZnO. At 500oC, the thermoelectric power factor of the Zn0.98Ga0.02O bulk is remarkably enhanced by 36% to 169.8 µV/mK2 as compared to the ZnO. Conclusion: The Ga doping shows not only the significant improvement in power factor, but also the potential for reducing thermal conductivity due to spinel phase segregation. Thus, this work provides a promising solution for controlling the thermoelectric performance of ZnO by doping Ga.