(Invited) Engineered Particle-Particle Contacts for Improved Thermal Interface Materials

Abstract

I present my group’s ongoing work to use liquid metal to overcome thermal transport bottlenecks in polymer composites. The central premise behind this effort is that liquid metal-coated particle inclusions will have enlarged particle-particle contact regions relative to the point-like contacts between non-coated particle inclusions. In effect, the LM functions as a thermally conductive “solder” between particles. Polymer composites play an important role for thermal interface materials (TIMs). TIMs improve thermal transport by filling micro air gaps between the rough surfaces of adjacent components (e.g., between a computer chip and heat spreader). These applications require mechanically compliant materials such as polymers that unfortunately also suffer from low thermal conductivity. Consequently high thermal conductivity solid particles are typically added to the polymer to boost thermal transport. Despite these efforts, TIM conductivities still usually fall below 2 W m-1 K-1because the small point-like contacts between solid filler particles lead to large thermal contact resistances that bottleneck thermal transport. I first present our work on polymer composites with nickel-based particle inclusions. We explore bare nickel particles, nickel particles coated with silver, and nickel particles coated with liquid metal. These coatings impart mechanical compliance to the surface of the hard nickel particles and lead to a 3-fold increase in thermal conductivity of the polymer composite. These studies also reveal that a nanometer-scale native oxide coating on the liquid metal impedes thermal transport. In order to improve thermal transport for liquid-metal coated particles, we next focus on methods to physically break the native oxide. Rupturing this thin oxide allows liquid metal to flow and yields improved contact between particles. To address this challenge, we conduct a series of thermal-mechanical studies on liquid metal droplets. In order to focus on the key physics, we create beds of liquid metal droplets with their native oxides intact (i.e. no polymer matrix and no solid particle core). We then study thermal transport as a function of applied mechanical pressure. We observe an enhancement in thermal transport that accompanies the rupture of the native oxide. We also find that the oxide rupture follows theoretical predictions for failure of thin-walled pressure vessels. Lastly, we chemically modify the native oxide shell to reduces its strength, promote oxide rupture, and improve thermal transport.