Geometry‐Programmable Heat Routing via Shear‐Aligned BNNT/Epoxy Composites: From Passive Spreading to Directed Guiding

ABSTRACT Miniaturization and rising power densities exacerbate localized hot spots on electrically insulating substrates, where heat spreads non‐selectively. A boron nitride nanotube (BNNT)/epoxy “ thermal guide ” is developed for selective, geometry‐programmable heat routing. A viscosity‐tuned BNNT/epoxy ink is processed by micro‐nozzle extrusion; confinement‐induced shear aligns BNNTs into an orientation‐defined architecture, as confirmed by small‐angle X‐ray scattering and supported by flow simulations identifying nozzle size as a key alignment control. Aligned bulk composites exhibit pronounced in‐plane anisotropy ( k y / k x ≈ 2.53; 2.96 vs 1.17 W m − 1 K − 1 , parallel vs transverse to the fiber direction), and infrared thermography visualizes alignment‐guided heat transport: at 5 cm from a 70°C source, the y‐axis‐oriented specimen reaches 55.8°C after 30 s, compared with 48.1°C for the x‐axis‐oriented specimen. Dielectric integrity is retained at network‐forming loadings, with volume resistivity of ∼10 13 Ω·m at 20 wt.% BNNT and low dielectric loss. Dispenser printing enables ∼200 µm‐wide guides; contacting a 70°C source at the guide terminus produces >20°C terminal contrast relative to the surrounding region outside the printed pattern. This method therefore enables electrically safe thermal routing to guide heat from a localized source to a target region while suppressing parasitic lateral diffusion into heat‐sensitive areas.