Synergistic Enhancement of Thermal Conductivity and Reduction of Thermal Expansion in Multi‐Sized Al ₂ O ₃ /Epoxy Composites

The high‐power density of electronic devices imposes urgent demands on packaging materials with high thermal conductivity and low thermal expansion. Epoxy resin (EP), a widely used packaging material, suffers from intrinsically low thermal conductivity and high coefficient of thermal expansion (CTE). In this work, Al 2 O 3 nanoparticles were incorporated into EP to improve its performance, and a model was established to predict the CTE. For mono‐sized particle composites, smaller particles could yield higher thermal conductivity due to denser filler networks. This phenomenon differs from the behavior observed in sparse nanoparticle fillings, in which smaller particles lead to lower thermal conductivity when the filling ratio is below the threshold for forming thermal conduction networks. The smaller the nanoparticles, the lower the CTE of the composites, due to the lower CTE of the nanoparticles. For mono‐sized particle composites, the 10nm‐Al 2 O 3 composite achieves the highest thermal conductivity of 0.569 W·m −1 ·K −1 (2.42 times that of EP) and simultaneously the lowest CTE of 37.2 × 10 −6 /°C (48% lower than that of EP). Although smaller nanoparticles are suggested for improving the performance, they are prone to aggregation, thus jeopardizing their performance. To suppress nanoparticle aggregation at high filler loadings, multi‐sized Al 2 O 3 /EP composites were applied. Small particles could fill the interstices between large particles, enabling the filler loading to reach 60 wt.% in this work. The multi‐sized composite could achieve a thermal conductivity of 0.825 W·m −1 ·K −1 (3.5 times that of EP), and the CTE could be reduced to 39.7 × 10 −6 /°C (44% lower than that of EP). This work provides a strategy for designing high‑performance packaging composites.