On the micro-mechanisms of enhancing thermal conductivity of three-phase composites with bimodal particle size distribution

Particles with high thermal conductivity (TC) exhibiting a bimodal size distribution can substantially enhance the effective thermal conductivities (ETCs) of three-phase composites (TPCs). Traditionally, the mechanisms underlying this enhancement were explained through the concept of particle packing density, a principle commonly employed in granular mechanics. However, conventional packing density fails to adequately capture the reduction in void concentration that occurs when bimodal hybrid particle mixtures are introduced into TPCs. Moreover, a micromechanics-based framework specifically addressing this phenomenon has yet to be developed. The present study demonstrates that the fundamental mechanisms governing thermal conduction are more closely associated with the spatial arrangement of particles within the matrix rather than merely their compactness. Finite element method (FEM) simulations were performed to investigate particle contact probability and to assess the impact of particle size distribution on TCs. Numerical results revealed a strong positive correlation between changes in ETC and particle contact probability, which was further quantified by the volume fraction of contacting particle pairs and incorporated into the micromechanics-based model. Additionally, a double inclusion-based model was formulated to represent the effect of thermally conductive pathways formed through particle contacts. All numerical predictions were validated against experimental data to substantiate the proposed hypotheses. Consequently, the thermal conduction mechanisms in TPCs with bimodal size distributions can be interpreted from a novel perspective. The model presented herein provides a valuable theoretical approach for the design and optimization of such multiphase composite materials.