Synergistic Dual-Mode Cooling Enabled by h-BN/Al2O3 Hybrid Composites for Efficient Thermal Management.

This study presents a high-performance, millimeter-thick, free-standing hybrid composite sheet for enhanced thermal management and energy efficiency. This material is developed by integrating microscale hexagonal boron nitride (h-BN) platelets and nanoscale alumina (Al₂O₃) nanoparticles within an epoxy matrix. This hybrid design achieves broadband solar reflectance via complementary scattering effects across the solar spectrum and ensures high mid-infrared emittance within the atmospheric transparency window (8-13 μm). The optimized composite sheet exhibits average reflectance values of approximately 80% in the ultraviolet-visible (UV-vis) region and 92% in the near-infrared (NIR) region, along with a mid-infrared emittance of 85%. Furthermore, the aligned h-BN network imparts an excellent out-of-plane thermal conductivity of 6.72 W m^-1 K^-1, thereby enabling efficient heat dispersion. Theoretical simulations indicate that the proposed composite has a significantly enhanced net cooling power of up to 141 W m^-2. Field tests conducted over 2 days in Anseong, South Korea, demonstrate a peak subambient temperature drop of -12.4 °C and a stable average daily cooling performance of -10.2 °C, thereby demonstrating an almost 2-fold increase in efficiency compared to the control samples. This confirms the effective dispersion of heat throughout the 2 mm thick film, which enables bulk cooling beyond a mere surface effect. These findings represent a significant advancement for durable, self-standing passive cooling materials, with great potential for sustainable thermal management in various applications. While buildings and stationary electronics are canonical PDRC targets, mobile platforms such as unmanned aerial vehicles (UAVs) face acute solar heating with minimal allowance for active cooling or parasitic mass. The present dual-mode sheet is directly relevant to UAV outer skins: broadband solar backscattering curbs solar gain on sunlit surfaces, while the aligned h-BN network spreads internally generated heat through the 2 mm thickness and into the airstream. The electrically insulating, corrosion-resistant ceramic/epoxy architecture further suits composite airframes.