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Matthew Ralphs1 Emil Joseph1 Robert Wang1 Konrad Rykaczewski1

1, Arizona State University, Tempe, Arizona, United States

Following the evolution of traditional “hard” electronics, the power density of wearables and soft robotics is bound to increase with time. Dissipation of the heat produced by these stretchable electronics will necessitate the development of novel, stretchable thermal management methods. In fact, wearable electronics have much more stringent restrictions on maximum operating temperatures due to intimate contact with the user.1 Jeong et al.2 and Bartlett et al.3 recently demonstrated that incorporation of a high volumetric fraction of liquid metal (LM) droplets in an elastomer matrix results in a highly stretchable material with thermal conductivity of around 2 Wm-1K-1, which is an order of magnitude higher than the baseline polymer. Furthermore, stretching of such composites with LM micro-droplets induces their deformation and increases the directional thermal conductivity up to 10 Wm-1K-1.3 These composites were used to attach a high-power LED to a user as well as prevent it from overheating (which occurred with use of a silicone-only strap). 3 However, gallium-based liquid metals are expensive, have thermal properties significantly inferior to many common metals, and pose corrosion hazards.
With the goal of producing highly thermally conductive and stretchable materials that are cheaper and safer to use than high-LM content equivalents, here we describe the developement of several three phase elastomeric composites with hybrid liquid metal-solid fillers. The key concept here is to develop a hybrid particle with a solid metal core that provides superior thermal and electrical conductivity covered by a LM shell that enables deformability and potentially reduces particle-particle thermal resistance. We will discuss several approaches and issues that arise with fabrication of silicone composites with hybrid LM-Cu, LM-Al, LM-W, LM-Ni, and LM-SiC fillers. The selection of the core material provides opportunities for in situ alloying with the LM as well as magnetic alignment of the particles within the composite, as recently shown by Lu et al.4 Thermal and mechanical properties of these composites as well as potential for their use in thermal management of stretchable electronics will be covered.
1. Song et al. Nat. Sci. Rev. 3, 2016.
2. Jeong et al. Sci. Rep. 5, 2015.
3. Bartlett et al. PNAS, 114, 2017.
4. Lu et al. ACS App. Mater. Inter., 9, 2017.

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