Hierarchically Porous PDMS/ZIF-8@h-BN Bilayer Composite Membrane Enabling Synergistic Passive Daytime Radiative Cooling and Efficient Thermal Conduction for Electronic Thermal Management

The advancement of 5G and emerging 6G communications drives the integration and miniaturization of outdoor electronics, triggering an exponential surge in chip heat flux and severe challenges for low-energy passive thermal management. Passive daytime radiative cooling (PDRC) has emerged as a novel zero-energy cooling technology. Herein, a bilayer PDRC composite (B-PDRC) integrating radiative cooling and in-plane thermal conduction is designed and fabricated. The top radiative cooling layer is a hierarchically porous Polydimethylsiloxane (PDMS)/zeolitic imidazolate framework-8 (ZIF-8) foam prepared via a template-sacrificial method combined with vacuum impregnation, while the bottom thermally conductive layer is a PDMS/hexagonal boron nitride (h-BN) dense film fabricated by solution casting. The multiscale scattering induced by micron-sized pores and 600-800 nm ZIF-8 nanoparticles delivers a high solar reflectance of 94.45%, and the C-N bonds of ZIF-8 boost the average atmospheric window emissivity to 93.22%, compensating for the emissivity valley of neat PDMS. The h-BN fillers construct continuous thermal conduction paths, elevating the in-plane thermal conductivity to 0.92 W m-1 K-1 at 20 wt % of h-BN, a 179% enhancement compared to neat PDMS. Indoor tests demonstrate a maximum temperature drop of 11.9 °C for a charging smartphone covered with B-PDRC. Outdoor measurements under sunny conditions achieve a maximum subambient cooling of 8.8 °C, and a 14.8 °C temperature reduction for an operating smartphone. Furthermore, B-PDRC exhibits excellent hydrophobic self-cleaning performance (water contact angle of 135.4°), flexibility, mechanical robustness, long-term durability under UV and humid conditions, and universal adhesion to various substrates. This work provides a feasible design strategy for high-performance flexible PDRC composites for thermal management of outdoor electronic devices.