The role of silicon-based nanofillers and polymer crystallization on the breakdown behaviors of polyethylene blend nanocomposites

Good breakdown strength is an important feature for the selection of dielectric materials, especially in high-voltage engineering. Although nanocomposites have been shown to possess many promising dielectric properties, the breakdown strength of nanocomposites is often found to be negatively affected. Recently, imposing nonisothermal crystallization processes on polyethylene blends has been demonstrated to be favorable for breakdown strength improvements of dielectric materials. In an attempt to increase nanocomposites' voltage rating, this work reports on the effects of nonisothermal crystallization (fast, moderate and slow crystallizations) on the structure and dielectric properties of a polyethylene blend (PE) composed of 80% low density polyethylene and 20% high density polyethylene, added with silicon dioxide (SiO2) and silicon nitride (Si3N4) nanofillers. Through breakdown testing, the breakdown performance of Si3N4-based nanocomposites was better than SiO2-based nanocomposites. Since nanofiller dispersion within both nanocomposite systems was comparable, the enhanced breakdown performance of Si3N4-based nanocomposites is attributed to the surface chemistry of Si3N4 containing less hydroxyl groups than SiO2. Furthermore, the breakdown strength of SiO2-based nanocomposites and Si3N4-based nanocomposites improved, with the DC breakdown strength increasing by at least 12% when both the nanocomposites were subjected to moderate crystallization rather than fast and slow crystallizations. This is attributed to changes in the underlying molecular conformation of PE in addition to water-related effects. These results suggest that apart from changes in the nanofiller surface chemistry, changes in the underlying molecular conformation of polymers are also important to improve the breakdown performance of nanocomposites