Microstructure Evolution and Material Migration Mechanism in Nano ZnO@SiO2 Core-Shell Structured Powders

ZnO@SiO 2 core-shell structured powder, as the raw material for preparing ZnO varistor, is conducive to obtaining a fine and uniform microstructure, which is of positive significance for acquiring high-performance resistors. Currently, limited research has been conducted on the microstructural evolution of core-shell structures during the sintering process, and the mechanism of material migration remains inadequately understood. This study systematically investigated the sintering behavior of nanostructured ZnO@SiO 2 core-shell compacts by varying both the sintering profiles and Si 4+ /Zn 2+ molar ratios. XRD, SEM, and FTEM characterization techniques were employed to track the microstructural evolution across critical sintering stages and to analyze the effect of the shell layer thickness on the microstructure evolution. To more clearly elucidate the sintering characteristics of the core-shell structure, a control group comprising directly mixed ZnO-SiO 2 powders was included. The dominant mechanisms of material transport were elucidated by EPMA through elemental mapping at critical sintering stages, coupled with microstructural evolution analysis. The core-shell structure, acting as a heterogeneous unit, demonstrated distinct sintering behavior compared to directly mixed ZnO-SiO 2 powders. This disparity arises from the modified interfacial contact between ZnO and SiO 2 in the core-shell architecture. Specifically, the phase transition temperature was reduced, the onset sintering temperature was elevated, and the densification temperature was lowered. During microstructural evolution, Si from the shell layer initially diffuses into the sintered neck regions via surface transport. Notably, the mid-stage sintering is characterized by extensive formation of a glassy liquid phase, which significantly accelerates densification through viscous flow-mediated mass transfer. During the final densification stage of sintering, grain boundary diffusion mechanisms dominate the process. Throughout the sintering process, the network structure formed by Si migration promotes microstructural homogenization while effectively suppressing grain growth.