Hierarchical Assembly of Monodisperse Hydroxyapatite Nanowires and Construction of High-Strength Fire-Resistant Inorganic Paper with High-Temperature Flexibility

Graphical A strong flexible fire-resistant hydroxyapatite (HAP) paper based on network-structured HAP nanowires is reported. A new low-cost method is also developed for the large-scale synthesis of HAP nanowires using water as the only solvent. High-strength flexible inorganic paper with fire-resistant and adiabatic properties is highly demanded in various high-temperature applications. However, constructing inorganic paper that not only has high strength and high flexibility at room temperature but also can prevent the high-temperature-induced friability is still a great challenge. Inspired by the hierarchical structure and excellent mechanical properties of the tooth enamel, we have developed a systematic approach for the bottom-up fabrication of multi-hierarchical fire-resistant hydroxyapatite (HAP) nanowire paper with balanced tensile strength and flexibility that includes four steps: (1) the synthesis of monodisperse HAP nanowires from the molecular level to the nanoscale; (2) the self-assembly of HAP nanowires into long fibers and two-dimensional (2D) nanowire networks from the nanoscale to the mesoscale; (3) the layered assembly of 2D nanowire networks into the highly flexible high-strength fire-resistant paper from the mesoscale to the macroscale; (4) reinforcing the HAP nanowire paper with inorganic additives to enhance the tensile strength and to overcome the high-temperature-induced pulverization. By adopting this strategy, the mechanical properties of the fire-resistant HAP nanowire paper are greatly improved. The experimental results show that the tensile strength of the as-prepared HAP nanowires-based inorganic paper is greatly enhanced to ≈15 MPa, which is close to that of the commercial copying paper, and the A4-sized HAP nanowires-based inorganic paper is highly flexible and can be directly printed using a commercial printer. Owing to the synergistic effect of all components and the unique hierarchical structure, the as-prepared fire-resistant HAP nanowire paper can also preserve well its high flexibility even under high-temperature conditions.