Enhancing Thermal Transport in Silicone Composites via Bridging Liquid Metal Fillers with Reactive Metal Co-Fillers and Matrix Viscosity Tuning.

Polymer matrix composites containing room temperature liquid metal (LM) microdroplets offer a unique set of thermo-mechanical characteristics that makes them attractive candidates for high performance thermal interface materials. However, to achieve the desired level of the composite thermal conductivity, effective bridging of such fillers into interconnected percolation networks needs to be induced. Thermal percolation of the LM microdroplets requires two physical barriers to be overcome. First, the LM microdroplets must directly contact each other through the polymer matrix. Second, the native oxide shell on the LM microdroplet must also be ruptured. In this work, we demonstrate that both physical barriers can be penetrated to induce ample bridging of the LM microdroplets and thereby achieve higher thermal conductivity composites. We accomplish this through a synergistic combination of solid silver and LM fillers, tuning of the silicone oil "matrix" viscosity, and sample compression. We selected silver as the solid additive because it rapidly alloys with gallium to form microscale needles that could act as additional paths that aid in connecting the LM droplets. We systematically explore the impact of the composition (filler type, volume fraction, and matrix oil viscosity) and applied pressure on the thermal conductivity and multiscale structure of these composites. We reveal the microscopic mechanism underlying the macroscopic experimental trends and also identify an optimal composition of the multiphase Ag-LM-Silicone oil composite for thermal applications. The identified design knobs offer path for developing tunable LM-based polymer composites for microelectronics cooling, biomedical applications, and flexible electronics.