Role of Interface on the Thermal Conductivity of Highly Filled Dielectric Epoxy/AlN Composites

The interface between filler and matrix has long been a critical problem that affects the thermal conductivity of polymer composites. The effects of the interface on the thermal conductivity of the composite with low filler loading are well documented, whereas the role of the interface in highly filled polymer composites is not clear. Here we report on a systematic study of the effects of interface on the thermal conductivity of highly filled epoxy composites. Six kinds of surface treated and as received AlN particles are used as fillers. Three kinds of treated AlN are functionalized by silanes, i.e., amino, epoxy, and mercapto group terminated silanes. Others are functionalized by three kinds of materials, i.e., polyhedral oligomeric silsesquioxane (POSS), hyperbranched polymer, and graphene oxide (GO). An intensive study was made to clarify how the variation of the modifier would affect the microstructure, density, interfacial adhesion, and thus the final thermal conductivity of the composites. It was found that the thermal conductivity enhancement of the composites is not only dependent on the type and physicochemical nature of the modifiers but also dependent on the filler loading. In addition, some unexpected results were found in the composites with particle loading higher than the percolation threshold. For instance, the composites with AlN treated by the silane uncapable of reacting with the epoxy resin show the most effective enhancement of the thermal conductivity. Finally, dielectric spectroscopy was used to evaluate the insulating properties of the composites. This work sets the way toward the choice of a proper modifier for enhancing the thermal conductivity of highly filled dielectric polymer composites.