Precuring Matrix Viscosity Controls Thermal Conductivity of Elastomeric Composites with Compression‐Activated Liquid and Solid Metallic Filler Networks

Abstract

high-performance electronic devices, soft robotics, and flexible electronics. These materials ensure that devices reliably function by reducing thermal resistance between the heat-generating components and the cooling solutions. [1][2][3][4][5] Specifically, the inherent microroughness and imperfect contact at component interfaces leads to large thermal resistances. In order to function, TIMs need high thermal conductivity and good wettability to remove heat. TIMs also need to be mechanically compliant to overcome dynamic warpage of the mating surfaces. [6,7] This dynamic warpage arises due to the combined effects of thermal cycling and mismatches in thermal expansion of adjacent components. This temperature-induced dynamic warpage causes fluid-phase TIMs (e.g., silicone greases), to pump out of the interface, which in turn leads to a high thermal resistance and overheating. The inability to pump out of an interface is one of the advantages of solid-phase TIMs. Crosslinked polymers offer appropriate mechanical properties for solid-phase TIMs, but have a low thermal conductivity of 0.2-0.3 W m −1 K −1 . [8][9][10] To overcome this problem, high thermal conductivity rigid filler materials (alumina, copper, graphene, boron nitride, etc.) [11][12][13][14][15][16] are incorporated into the precursor polymer solution prior to crosslinking. A curing process is then used to crosslink the precursor polymer solution and create a solid polymer composite TIM pad. A high volume fraction of rigid filler particles substantially enhances the composite thermal conductivity by creating multiple percolation paths, but also causes detrimental stiffening of the polymer. [7] In addition, curing polymers with very high solid additive content results in pads that "crumble" and are not suitable for TIM applications. [6,17,18] In contrast, liquid fillers can be incorporated into polymer composites as microdroplets at high volume fractions without degrading the mechanical properties. [19][20][21] Liquid metals (LMs) based on Ga (e.g., elemental Ga, eutectic GaIn, and eutectic GaInSn) have sufficient thermal conductivity and mechanical compliance to function as TIMs, but present other challenges due to their reactive/corrosive properties. Consequently, LMs are typically used as fillers within polymer composites such that the polymer functions as a physical barrier that prevents corrosion between the TIM and Polymer matrix composites with liquid metal droplet and solid particle fillers are promising candidates for thermal interface materials (TIMs) used in electronics thermal management. To achieve good thermal transport, the particle and droplet fillers must be interconnected to form thermally conductive percolation pathways in the polymer matrix. This in turn requires displacement of the polymer between fillers as well as rupture of the oxide shell on the liquid metal droplets. This study demonstrates a multipronged strategy to achieve extensive filler bridging and a high thermal conductivity polymer TIM pad. The strategy synergistically e...

Liquid Metal + x: A Review of Multiphase Composites Containing Liquid Metal and Other (x) Fillers

“…Increased electrical conductivity; added electrical self-healing, magnetic properties, and shear hardening Ding et al [ 73] Carbon (CNT) nanotubes Sylgard 184 Increased electrical conductivity Guan et al [ 74] Carbon (CNT) nanotubes Sylgard 184 Increased electrical conductivity Li et al [ 75] Carbon (CNF) nanofibers Sylgard 184 Increased Young's modulus and tensile strength; added strain hardening and positive piezoconductivity; decreased electrical conductivity Zhang et al [ 76] Graphite flakes EcoFlex 00-50 Increased electrical and thermal conductivity Bilodeau et al [ 31] Graphite flakes EcoFlex 00-30 Increased electrical and thermal conductivity Li et al [ 77] Graphene nanoplatelets Sylgard 184 Increased thermal conductivity and Young's modulus Sargolzaeiaval et al [ 78] Graphene nanoplatelets Sylgard 184 Increased thermal conductivity Sargolzaeiaval et al [ 79] Graphene flakes Sylgard 184 Increased electrical and thermal conductivity; non-monotonic Young's modulus Saborio et al [ 80] Polydopamine-Coated Graphene Oxide flakes Sylgard 184 Increased dielectric permittivity; decreased breakdown strength Hu and Majidi [ 81] Ag particles Silicone oil Increased thermal conductivity Uppal et al [ 82] Ag particles Gelest PDMS Increased thermal conductivity Uppal et al [ 83] Ag particles Poly(styrene-bbutadiene-bstyrene) (SBS)…”

“…If solid inclusions with high thermal conductivities such as Fe, [40,68,71] graphite, [31,77] graphene, [78][79][80] Ag, [68,82,83,90] Ni, [40,68] or Cu [68,95,97] are also added to the elastomer, in addition to liquid metal, the thermal conductivity of the multiphase composite will increase relative to the liquid metal-only composite at the same loading percentage. However, the maximum possible volume fraction of the inclusions decreases when rigid inclusions are involved, due to the stiffness of the rigid inclusions, which prevents mixing and uniform formation of a multiphase composite.…”

Augmentation of Liquid Metal Particle Mechanics via Non‐Native Oxide Nanoshells

“…[24][25][26] Tensile, compressive, or photothermal forces provide mechanical stimuli needed to form continuous, conductive traces. [25,27,28] Often, the onset of this insulator-to-conductor transition for a set of particles is delegated by the mechanical behavior of the native oxide shell. As particles approach the nanoscale, the dominance of the oxide becomes even more significant and can lead to changes in surface composition or undercooling of particles.…”