Organic‐Solvent‐Free Fabrication of Highly Thermally Conductive and Electrically Insulating Films via Magnetic Alignment of Large h‐ <scp>BN</scp> Platelets in Waterborne Polyurethane

ABSTRACT The escalating power density of modern electronic devices has created an urgent demand for thermal interface materials (TIMs) delivering exceptional through‐plane thermal dissipation and robust electrical insulation. However, conventional processing methods (e.g., doctor‐blading) inevitably induce an in‐plane orientation of two‐dimensional fillers, creating barriers to vertical heat flux. Herein, we report an organic‐solvent‐free, magnetic field‐assisted realignment strategy to construct vertically aligned phonon expressway, using large‐sized hexagonal boron nitride (h‐BN) platelets within a waterborne polyurethane (WPU) matrix. Through kinetic and rheological analyzes, we elucidate the alignment dynamics in highly concentrated fluids. By balancing magnetic torque against matrix viscosity, we demonstrate that the applied magnetic field overcomes shear‐induced horizontal orientation, while revealing steric hindrance from solid‐like networks at ultra‐high loadings. At optimal loading of 50 wt% (under 75 mT), the composite exhibits through‐plane thermal conductivity of ~6.4 W/m·K, outperforming its random and horizontally aligned counterparts. Despite the vertical alignment, the material maintains high dielectric breakdown strength (~17.17 kV/mm), ensuring safety in high‐voltage environments. Cooling tests on high‐power LEDs show that this vertically aligned TIM reduces the junction temperature by 7.31°C compared to a high‐performance commercial benchmark, underscoring its potential for advanced thermal management solutions.