Anti-leak, self-adaptive liquid metal-epoxy in-situ cured composites with ultra-low thermal resistance via flexible droplet inclusions

Abstract

Generally, the function of thermal interface material (TIM) is filling rough surfaces, eradicating air pockets, decreasing thermal resistance, and improving heat dissipating effects. On this basis, we provide the TIM with another identity, “adhesion”, rendering it not only play a role of heat dissipation, but also serve as an adhesive to bond contacting surfaces. In this work, nano- to microscopic liquid metal (LM) droplets are embedded into the epoxy matrix to fabricate a LM-epoxy in-situ cured composite. By adding the curing agent, the LM-epoxy composite is cured into an entity structure, transforming from an amorphous grease state to a self-adaptive pad state. When the average droplet size is in a reasonable range of approximately 5–25 μm, the composite's thermal resistance (Rtotal) decreases to 2.19 mm2K/W at 70% volume fraction with its adhesive strength of 0.497 MPa, and the thermal conductivity (kTIM) could increase to 10.05 W/(m·K) at 85%. Both Rtotal and kTIM change rapidly for droplet size approximately ranging from 102 nm to 101 μm and then tend to level off when it further increases to 102 μm. A conception of equivalent contact thickness, δ, is presented to elucidate the contacting thermal resistance Rc, and set up predictive models of Rtotal for different volume fractions by considering the microscopic interfacial thermal resistance Rb between filler and matrix or not. This composite simultaneously possesses characteristics of the low thermal resistance like thermal grease, the whole entity like thermal pad, and the anti-leak characteristics by in-situ cured process. Actual cooling tests show it has a considerable temperature difference of 4.75 °C lower than the commercial thermal grease at 650 W heating power. This in-situ cured strategy, via decreasing the thermal resistance and solving the problem of LM leakage, remarkably promotes the LM-TIMs achieving a broader application prospect in the future.