The Polymer Society of Korea

Abstract

This study reports a millimeter-thick, free-standing h-BN/Al₂O₃ hybrid composite film enabling dual-mode thermal management via passive radiative cooling and vertical heat dissipation, addressing the durability and scalability limits of ultrathin coatings. Hybrid dispersion of microscale h-BN and nanoscale Al₂O₃ in an epoxy matrix yields high solar reflectance of 80% (UV–Visible) and 92% (Near-infrared), together with strong mid-infrared emittance of 85% within the 8–13 µm atmospheric window. A vertically oriented h-BN network enhances out-of-plane thermal conductivity to 6.72 W m⁻¹K⁻¹. Outdoor field tests in Anseong, South Korea demonstrate a net cooling power of 141 Wm⁻² and a maximum sub-ambient temperature reduction of −12.4 °C, confirming the potential of thick, scalable h-BN/Al₂O₃ composites for sustainable thermal management.