Ambient Drying Route to Aramid Nanofiber Aerogels with High Mechanical Properties for Low-k Dielectrics

Abstract

Polymeric aerogels with low dielectric constant (low-k) are an important material solution for applications in high-frequency terminal electronic devices. However, most aerogels are prepared using high-cost supercritical drying or freeze-drying methods and typically exhibit poor mechanical properties and low thermal stability, which limits their use in practical applications. Here, we report the fabrication of aerogels with layered structure and hierarchical nanopores based on aramid nanofibers (ANFs), using a vacuum-assisted filtration method followed by low-cost ambient drying. The porosity of the ANF aerogels can be controlled from 38 to 79% by rationally selecting solvents with different surface tension for solvent exchange, tuning the affinity between solvents and ANF skeletons as well as controlling the solvent evaporation rate during the ambient drying process. The ANF aerogels have an ultralow and tunable dielectric constant as low as 1.56 while exhibiting a low dielectric loss between 0.0040 to 0.0055 at 1 MHz. The advantageous low-k property can be preserved at high temperatures up to 300 °C, and an inherent flame retardancy is achieved due to the rigid and all-aromatic backbone structure of poly(p-phenylene terephthalamide). Moreover, the nanoporous ANF layers with relatively lower porosity compared with the overall porosity of the aerogel provide strong mechanical strength and modulus, while the presence of larger nanopores from the interspacing of ANF layers ensures a high porosity for the entire aerogel, which endows the aerogel film with superior tensile strength and modulus compared with their conventional counterparts containing homogeneous porous structures. Collectively, these ANF aerogels exhibit low-k, outstanding mechanical properties, high thermal stability, and inherent flame retardancy, enabling them to become very promising next-generation dielectric materials.