image: The aerogel contains a unique nano–scaled core–sheath structure in ceramic aerogel featuring a nanofibrous core framework for ultra–flexible deformation and a nanoporous aerogel sheath for thermal superinsulation.
Credit: ©Science China Press
Nanoporous aerogels, characterized by their ultra–low density, high specific surface area, and low thermal conductivity (κ < 24 mW·m−1·K−1), have been widely utilized as “thermal superinsulator” in the aerospace, military, energy, electronics and personal protection fields. However, owing to the inherently weak necklace connection pattern between the nanoparticles and restricted deformation space within the nanopores, conventional nanoporous aerogels are typically highly brittle and therefore unsuitable for practical applications. To this end, achieving favorable mechanical and thermal properties in nanoporous aerogels while simultaneously maintaining their thermal superinsulation performance is highly challenging.
Microstructural design of aerogels
To achieve the desired mechanical properties and thermal stability without any impairments to their inherently superior thermal insulation performance, researchers design a unique nano–scaled core–sheath structure in ceramic aerogel featuring a nanofibrous core framework for ultra–flexible deformation and a nanoporous aerogel sheath for thermal superinsulation, more advanced than conventional micron–scaled core–shell structured aerogels.
Specifically, the nanoporous aerogel constituting the sheath structure effectively introduces a large number of nanopores to suppress air conduction, resulting in a κ less than 24 mW·m−1·K−1. Concurrently, the nanofibrous aerogel, serving as the inner core, provide flexible and robust support to prevent stress concentration and facilitate compatible deformation of the outer sheath. Consequently, all nanoporous aerogels can be assembled onto the surface of the fibers in an orderly manner, which yields a structure much more stable than that in the case of randomly filled patterns in fiber–reinforced aerogels.
Mechanical flexibility of aerogels
The resulting aerogel demonstrates remarkable mechanical flexibility with a compressive strain of up to 80%, a fracture strain of up to 21.9% and a bending strain of up to 100%. Moreover, our core–sheath structure design resulted in a near–zero Poisson's ratio (ν = 2.5×10−4) behavior in the aerogel materials, owing to the synergistic effect. Such a near–zero ν resulted in effective reductions in the plastic deformation through restrictions of the local and global tension–induced fracture, ultimately enhancing the deformation compatibility.
The remarkable flexibility is attributed to the multi–dimensional core–sheath structure and multi–component crystal phase. Owing to the robust chemical bonding of the interfaces, the nanoporous sheath can be assembled in an orderly manner on the surface of the nanofibrous core, and the entangled core framework possesses sufficient deformation space and thereby facilitates compatible deformation of the sheath. Concurrently, the unique solid solution structure of the La2Y0.4TiZr2O9.6 ceramic can trigger an enhanced lattice distortion.
Thermal superinsulation of aerogels
Owing to the lower density and extremely high porosity, the aerogel exhibits no evident change in morphology and only a small strength degradation of less than 1.5% after being annealed at 1,000 °C for 1 h. Under various conditions, the aerogel exhibits remarkable deformability as well as a high working temperature of 1,300 °C while simultaneously retaining satisfactory thermal superinsulation performance (κ < 24 mW·m−1·K−1 at 26 °C).
As for experimentation with aerogel applications, researchers evaluate its practical thermal superinsulation performance as a typical thermal protective layer (TPL) via large–scale production. For daily life scenarios, the body temperature of the aerogel—with a thickness of 2 mm—can be maintained within a favorable range (36.2–37.3 °C) under cold (−80 °C), strong sunlight (50 °C), and hot (150 °C) conditions. For deep space explorations, the aerogel—with a thickness of 2 mm—can function as a TPL of a spacesuit; in this scenario, the body temperature can be maintained within a bearable range during the night (−150 to −80 °C) and day (100 to 150 °C), even in a short–term emergency situations (extreme high temperatures around 1,300 °C).
The team says further studies are needed, but this report represents the first step toward describing advanced aerogels in novel materials R&D. “Our nano–scaled core-sheath ceramic aerogel demonstrates a unique combination of favorable thermomechanical properties and thermal superinsulation performance, which can guide and accelerate the development and application of high–performance aerogel materials in the future.” says Wenshuai Chen, a professor at the Northeast Forestry University and co-author of the study.