Conduction mechanisms and dielectric relaxations in ternary ZnO/Zn2SiO4/SiO2 composites for energy storage systems

Abstract

In this paper, ZnO-based composite (ZnO/Zn2SiO4/SiO2) powders were prepared and investigated as a function of SiO2 added amount. Indeed, the X-ray diffraction patterns (XRD) confirm the formation of ternary ZnO/Zn2SiO4/SiO2 composites. The structural defects are shown to increase with the SiO2 amount. In addition, the SEM observations reveal agglomerated and heterogenic grain shape distribution. Furthermore, dielectric properties were investigated as a function of SiO2 added amount. The Nyquist plots are simulated by an electric circuit formed by a parallel distribution of resistance (R), capacitance (C), and a constant phase element (CPE), in all composites. Moreover, using the Jonscher's law, we demonstrate a change in conduction mechanism as a function of SiO2 content. Indeed, for the 10% SiO2 doping, the conduction process is ensured by defects such as Zn interstitial and the oxygen vacancies, consistent with the Correlated Barrier Hopping (CBH) model. Whereas, for the 20% SiO2 doping, the conduction is dominated by grain boundaries contribution, according to the Non-Overlapping Small Polaron Tunneling (NSPT) model. Besides, the 10% SiO2-doped composite exhibits localized relaxation process, while the 20% SiO2-doped sample reveals simultaneously long-range and localized relaxations. These variations, caused by the increase of SiO2 percent, can be explained by the influence of the growth of Zn2SiO4 phase, and subsequently the development of further interfacial defects. Accordingly, the composites display capacitive behavior, high permittivity, and low dielectric losses, which make them good candidates for energy storage systems.