Directional fabricating of flexible and compressible cellulose nanofibril composite cryogel with excellent thermal insulation, flame-retardancy and radiative cooling for efficient thermal management

In order to reduce the thermal discomfort and fire accidents caused by overheating of electronics and high temperature of buildings, high-performance thermal insulation and fire-resistant materials are urgently desirable. Cellulose nanofiber cryogel derived from regenerative resources holds wide promise in the field of thermal insulation, but its inherent flammability, poor mechanical strength and water resistance make it face great challenges in practical application. Herein, a lightweight, porous, anisotropic cellulose nanofiber/hydroxylated boron nitride nanosheet/methyltrimethoxysilane composite cryogel (Si-CNF/BNNS) was prepared by unidirectional freeze-casting technique. Attributed to the hydrogen bond and silylation cross-linking of CNFs with BNNS and MTMS, as well as the aligned structure of cryogel, Si-CNF/BNNS cryogel was robust axially and compressible radially. The thermal conductivity of cryogel was 0.0621 W m(-1) K-1 in the axial direction and 0.0339 W m(-1) K-1 in the radial direction, demonstrating an anisotropic thermal insulation performance. In addition, the silylation modification and the addition of BNNS endowed cryogel with excellent flame retardant and hydrophobic properties, as evidenced by extremely low peak heat release rate of 14.05 kW M-2 and water contact angle of 136.6 degrees. Surprisingly, the high reflectivity of CNFs and BNNS with white appearance in the solar spectral range enabled the composite cryogel to have superior radiative cooling performance. This high-performance anisotropic Si-CNF/BNNS cryogel shows great potential for improving thermal comfort and environmental safety in the field of electronic packaging and energy efficient buildings. Moreover, this preparation procedure mainly includes liquid nitrogen orientation and freeze-drying, and can be further expanded for largescale production of cellulose-based cryogels. [GRAPHICS] .