Hierarchical Porous Hydroxyapatite Nanowire-Assisted Aramid Nanofiber/Nanocellulose Composite Aerogel for Enhanced Thermal Insulation and Flame Retardancy

Aerogels are considered the most ideal materials for thermal insulation. Among them, the comprehensive properties of aramid nanofiber (ANF)-based aerogels are particularly prominent. Notably, the fire and flame retardancy properties of ANF aerogels are particularly important in extreme environments. Here, a lightweight and hierarchical porous ANF/CNF/HAnw (ACHA) composite aerogel, comprising aramid nanofibers, cellulose nanofibers (CNFs), and hydroxyapatite nanowires (HAnws), is prepared through an improved bidirectional freezing method. During the bidirectional freezing process, ice crystals grow along a specific direction, forming macropores with aligned arrangements. Simultaneously, HAnws rich in hydroxyl groups can aggregate with aramid nanofibers in the gaps between ice crystals, creating mesoporous structures. Additionally, the nanoscale interactions between aramid nanofibers and nanocellulose led to the formation of micropores. The ACHA aerogel with a hierarchical porous structure formed through the auxiliary role of hydroxyapatite significantly enhances the thermal insulation performance by integrating macropores, mesopores, and micropores. Simultaneously, the multiscale pore distribution significantly improves the fire resistance and flame retardancy of the aerogel. Moreover, the hierarchical porous ACHA aerogel exhibits an excellent thermal insulation performance, with a radial thermal conductivity of only 22.89 mW·m–1·K–1. In addition, the ACHA aerogel exhibits an HRR of 75.7 W·g–1 and a THR of 5700 J·g–1, further demonstrating exceptional flame resistance with a limiting oxygen index (LOI) of 32.1%. Therefore, the hierarchical pore structure ACHA aerogel has broad application prospects in the field of thermal insulation and flame retardancy.