Thermal Conductivity Enhancement of Sandwich Multifunctional Films via a Dual-Template Strategy Inspired by Paper-Cut Patterns

Cellulose nanofiber (CNF)-based sandwich composite films with multifunctional properties have been extensively applied in the thermal management of electronic devices as thermal interface materials (TIMs). In this work, inspired by the Chinese traditional paper-cut pattern of a snake with a folding structure, a dual-template strategy was proposed to construct a three-dimensional (3D) dual-channel structure within the electrical insulating layer of sandwich composite films based on the volume exclusion effect under pressure. One-dimensional (1D) fibrous CNFs with an electropositive charge (CNF⁺) and zero-dimensional (0D) spherical thermoplastic polyurethane (TPU) microspheres were selected as organic supporting templates, which were utilized to construct CNF⁺@nanodiamond (CNF⁺@NDs) units and TPU@silver-nanoflower (TPU@AgFs) units with a core–shell structure via the electrostatic self-assembly and microcladding methods, respectively. Leveraging these dual templates with different shapes, a three-dimensional (3D) dual-channel architecture was successfully established within the electrically insulating CNF⁺@NDs/TPU@AgFs (CN/TA-X) layer through one-step vacuum-assisted filtration. These closely packed TPU@AgFs microspheres formed a primary AgF–AgF thermal conductive pathway, while the CNF⁺@NDs fibers in their interstitial spaces created an auxiliary ND–ND conductive pathway, driven by the volume exclusion effect. At a loading of merely 14.35 vol % NDs and 2.78 vol % AgFs, the CN/TA-40 monolayer can achieve an exceptional vertical thermal conductivity (TC) of 4.49 W/mK and an electrical resistivity of 2.3 × 10⁹ Ω·cm, representing a 1113.51% improvement over pure CNF⁺. The optimized CN/TA-40 monolayer was then employed as the electrical insulating layer in a sandwich multifunctional composite. This dual-template strategy can extremely enhance the through-plane TC of the sandwich composite films (4.19 W/mK) and meanwhile maintain their excellent electrical insulation (1.98 × 10⁹ Ω·cm) and specific shielding effectiveness (SSE) of 193.3 dB/mm. This structural design, imitating the Chinese paper-cut pattern of a snake, offers a highly effective approach to address the challenge of the limited through-plane TC in conventional CNF-based sandwich films serving as multifunctional TIMs.