Experimental and numerical study of the effective thermal conductivity of nano composites with thermal boundary resistance

Abstract

The thermal interface resistance at the macro scale is mainly described by the physical gap between two (inter) faces and constriction resistance due to this gap. The small gaps and surface geometry mismatch between the two material faces makes up the majority of thermal interface resistance (Rc) at the macro scale. There are various models to predict Rc at macro scale. Although Rc represents thermal resistance accurately for macro size contacts between two metals, it is neither suitable nor accurate to describe interface resistance of a modern composite Thermal Interface Material (TIM) containing micron to nano-sized particles. The thermal discontinuity at a perfectly bonded interface of two dissimilar materials is termed as thermal boundary resistance (Rb) or Kapitza resistance. It is necessary to understand feasibility of using nanoparticles in composite TIM by having better understanding of thermal boundary resistance at that scale. The phenomenon of thermal boundary resistance is an inherent material property and arises due to fundamental mechanisms of thermal transport. For metal–matrix particulate composites, Rb plays a more important role than Rc. The free flowing nature of the polymer would eliminate most of the gaps between the two materials at their interface. This means almost all of the thermal resistance at particle/matrix interface would occur due to Rb. Here, the thermal boundary resistance for silica nanoparticles embedded inside epoxy resin is studied. The bulk conductivity of the sample is measured, and Rb is back calculated using the Hasselman–Johnson’s (H–J) equation. The numerical validation of the equation is also presented, including extrapolation study to predict effective conductivity of the nanocomposite TIM.