Surface‐Energy‐Matched Interfaces as Key Design Rule for Ultra‐Low‐Resistance Thermal Interface Materials

ABSTRACT Thermal interface materials (TIMs) for high‐power, highly integrated electronics must deliver high intrinsic thermal conductivity, thin bond lines, and excellent interfacial contact. Yet the high filler loadings required for percolated heat paths typically stiffen the composite, thicken the bond line, and raise contact resistance. Here, we show that the alkyl chain length of self‐assembled monolayers (SAMs) on Al 2 O 3 fillers is a powerful molecular lever to co‐optimize bulk and contact contributions to the effective thermal resistance R EFF in silicone‐based TIMs. Using a library of chemically identical silanes (C1–C18) grafted onto hydroxylated Al 2 O 3 , we map how chain length controls filler surface energy, dispersion, rheology, and bond‐line thickness ( BLT ). Surface energy analysis identifies an optimal surface energy match between filler and matrix at C4, which minimizes re‐agglomeration. This matching reduces yield stress and enhances shear thinning, collapsing BLT from ∼608 µm (unmodified) to ∼20 µm at C4 under identical pressure. Atomic force microscopy measurements reveal a soft, highly dissipative “molecular spring” at C4 that combines low interfacial modulus with maximal adhesion, suppressing contact resistance. The C4 formulation thus attains an ultra‐low R EFF of 0.142 K cm 2 W −1 , establishing a general molecular‐to‐macro design rule for oxide‐filled TIMs.