From Molecular Mobility to Phonon Transport: Mechanistic Insights into Thermally Conductive Epoxy–Alumina Adhesives

Abstract

This study investigates the mechanisms involved in designing thermally conductive adhesives based on epoxy resin and alumina microparticles. Both the presence of fillers and the chemical crosslink density (CLD) influence the final properties of the cured resins. Therefore, the contributions of these two factors are examined by studying the curing kinetics of the systems. The results indicate that while alumina particles can promote the curing reaction through the chemical contribution of surface hydroxyl groups, the interactions between polymer chains and microparticles can slow down the curing kinetics—particularly due to the formation of an interconnected network at high filler contents, which reduces the molecular mobility of the system and leads to lower final CLD values. Additionally, enhanced phonon diffusion—primarily due to the development of the filler network—significantly increases thermal conductivity (TC) beyond a filler content of 30 wt.%, consistent with the rheological percolation threshold. Finally, the high TC induced by conductive fillers, combined with their effect on reducing the chemical CLD, increases the thermal sensitivity of the composites. Further investigations reveal that the filler particles also influence the final strength by facilitating stress concentration.