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

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 ∼10¹³ 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.