Sustainable cellulose foams for all-weather high-performance radiative cooling and building insulation.

Passive daytime radiative cooling (PDRC) as a zero-energy-consumption cooling technique offers rich opportunities in reducing global energy consumption and mitigating CO2 emissions. Developing high-performance PDRC coolers with practical applicability based on sustainable materials is of great significance, but remains a big challenge. Herein, polyvinyl alcohol (PVA) and esterified cellulose (EC) extracted from sawdust were used as raw materials to construct foams by using a dual-crosslinking assisted-unidirectional freeze-drying strategy followed by hydrophobic surface modification. The resultant PVA/EC (PEC) foams with ideal hierarchical macropore structure displayed various excellent features, such as low thermal conductivity (26.2 mW·m-1·K-1), high solar reflectance (95 %) and infrared emissivity (0.97), superhydrophobicity as well as high mechanical properties. The features allowed the PEC foams to be used as radiative coolers with excellent PDRC performance and thermal insulating materials. A maximum sub-ambient temperature drops of 10.2 °C could be achieved for optimal PEC foams. Building simulations indicated that PEC foams could save 55.8 % of the energy consumption for Xi'an. Our work would give inspiration for designing various types of PDRC coolers, including but certainly not limited to foams-based radiative coolers.

Flexible, Flame-Resistant, and Anisotropic Thermally Conductive Aerogel Films with Ionic Liquid Crystal-Armored Boron Nitride.

With the rapid development of miniaturization and high-power portable electronics, the accumulation of undesired heat can degrade the performance of electronic devices and even cause fires. Therefore, multifunctional thermal interface materials that combine high thermal conductivity and flame retardancy remain a challenge. Herein, an ILC (ionic liquids crystal)-armored boron nitride nanosheet (BNNS) with flame retardant functional groups was first developed. The high in-plane orientation structure aerogel film made of such an ILC-armored BNNS and aramid nanofiber and polyvinyl alcohol matrix through directional freeze-drying and mechanical pressing exhibits strong anisotropy thermal conductivity (λ// of 17.7 W m-1 K-1 and λ⊥ of 0.98 W m-1 K-1). In addition, the highly oriented IBAP aerogel films have excellent flame retardancy (peak heat release rate = 44.5 kW/m2 and heat release rate = 0.8 MJ/m2) due to the physical barrier effect and catalytic carbonization effect of ILC-armored BNNS. Meanwhile, IBAP aerogel films exhibit good flexibility and mechanical properties, even in harsh environments such as acids and bases. Further, IBAP aerogel films can also be used as a substrate for paraffin phase change composites. The ILC-armored BNNS provides a practical way to produce flame-resistant polymer composites with high thermal conductivity for TIMs in modern electronic devices.