Pressure-Activated Thermal Transport via Oxide Shell Rupture in Liquid Metal Capsule Beds

Abstract

Liquid metal (LM)-based thermal interface materials (TIMs) have the potential to dissipate high heat loads in modern electronics and often consist of LM microcapsules embedded in a polymer matrix. The shells of these microcapsules consist of a thin LM oxide that forms spontaneously. Unfortunately, these oxide shells degrade heat transfer between LM capsules. Thus, rupturing these oxide shells to release their LM and effectively bridge the microcapsules is critical for achieving the full potential of LM-based TIMs. While this process has been studied from an electrical perspective, such results do not fully translate to thermal applications because electrical transport requires only a single percolation path. In this work, we introduce a novel method to study the rupture mechanics of beds composed solely of LM capsules. Specifically, by measuring the electrical and thermal resistances of capsule beds during compression, we can distinguish between the pressure at which capsule rupture initiates and the pressure at which widespread capsule rupture occurs. These pressures significantly differ, and we find that the pressure for widespread rupture corresponds to a peak in thermal conductivity during compression; hence, this pressure is more relevant to LM thermal applications. Next, we quantify the rupture pressure dependence on LM capsule age, size distribution, and oxide shell chemical treatment. Our results show that large freshly prepared capsules yield higher thermal conductivities and rupture more easily. We also show that chemically treating the oxide shell further facilitates rupture and increases thermal conductivity. We achieve a thermal conductivity of 16 W m −1 K −1 at a pressure below 0.2 MPa for capsules treated with dodecanethiol and hydrochloric acid. Importantly, this pressure is within the acceptable range for TIM applications.

High Thermal Conductivity in Multiphase Liquid Metal and Silicon Carbide Soft Composites

“…In Figure 4b,c, applied pressure is slowly and incrementally increased up to ≈1 MPa while monitoring thermal conductivity throughout. As demonstrated in our prior work, [ 59 ] the dynamics of this incrementally increasing pressure provide insight into the formation of thermal transport pathways via LM deformation and coalescence. Figure 4b illustrates the total thermal resistance during these measurements decreases with increasing pressure and increasing LM content, up to 20 vol%.…”

“…This mechanism is described in our prior work on pressure‐activated thermal transport in LM microdroplet beds. [ 59 ] Figure 4b–c illustrates the effects of a gradually increasing pressure on the thermal transport in silicone‐based greases. The main difference between the experiments in Figures 4a and 4b,c is the manner in which pressure is applied.…”

Multifunctional liquid metal polymer composites

“…However, while an electrical interconnection can be established and maintained via one pathway, thermal transmission throughout a material would theoretically require many pathways, as heat transport does not actively take the path of least resistance. This concept was found to be proven correct, as research found that the thermal conductivity increased in proportion to mechanical compression until hitting a peak in pathways and dipping due to an outflow of LM from the deformed material 36 . Similar research around thermal properties enhanced by compression has developed porous elastomer foams wetted by liquid metal via LM‐saturation 126 .…”

“…On one hand, this “native” oxide skin stabilizes the suspension of LM droplets and allows micro‐contact particle mixing and transfer printing 31–33 ; the protective oxide skin assists the processing of LM‐based novel materials. On the other hand, the oxide layer functions as an electron and thermal transport barrier due to its poor conductivity 34–36 …”

“…On the other hand, the oxide layer functions as an electron and thermal transport barrier due to its poor conductivity. [34][35][36] In different forms, LMs can serve different purposes and applications. When injected into microfluidic devices, the material can serve as a flexible and stretchable conduit for electrical applications.…”