(Nanowerk Spotlight) The ability to absorb and dissipate impact energy is crucial for protective materials in transportation and aerospace. Traditional materials face a fundamental limitation: they can be either stiff to resist deformation or elastic to absorb impact, but not both. This constraint has restricted the development of lightweight protective materials that can maintain their protective properties under repeated impacts.
Cellular materials – those containing engineered internal spaces – appeared promising for overcoming this limitation. Their internal architecture can theoretically be designed to optimize both strength and flexibility. However, attempts to create such materials revealed a stubborn trade-off: thick cell walls provide strength but break under stress, while thin walls flex but lack structural integrity. Engineering approaches like arch-shaped structures and micro-lattices improved either stiffness or elasticity, but not both. No previous material successfully combined high stiffness for structural strength with the elasticity needed to repeatedly absorb impacts without degrading.
Researchers from Zhejiang University and other Chinese institutions have now created a graphene aerogel that achieves this elusive combination through precise control of its internal structure. Their material uses what they call a “topological cellular hierarchy” – essentially a honeycomb pattern where each wall consists of extremely thin, wrinkled sheets of graphene.
Fabrication of ultra-stiff and super-elastic graphene aerogels with a topological cellular hierarchy. A) Schematic illustration of the fabrication process for the topological cellular hierarchical structure by the 3D self-confined bubbling within the graphene honeycomb framework. B–E) SEM and HR-TEM images of the hierarchical structure of topological cellular graphene aerogel (TCGA). F) The ultra-stiff TCGA can support up to ≈3000 times of its weight without deformation. G) Comparison of recovery speed between our aerogel and previously reported carbon-based aerogels. Insets are real-time images during a steel ball rebound test. H) Ashby plot of mechanical stiffness versus recovery ratio of TCGA and reported carbon-based porous materials.
The key advance lies in transforming traditional thick cell walls into assemblies of corrugated nanowalls just 40 nanometers thick. These walls form a honeycomb framework that distributes force across the structure. When compressed, the nanowalls flex and buckle without breaking, much like how a corrugated cardboard box absorbs impact by flexing its ridged walls. This allows the material to recover its shape even after severe compression.
The performance metrics demonstrate a significant advance over existing materials. The aerogel achieves a stiffness of 12 megapascals – a measure of its resistance to deformation that is nearly double that of conventional graphene aerogels. Despite this rigidity, it can be repeatedly compressed to 40% of its original height and bounce back without damage. The material maintains this recovery capability even after 10,000 compression cycles.
In high-speed impact tests, the researchers sandwiched their aerogel between protective epoxy boards. This structure absorbed 2.47 kilojoules of energy per meter, compared to 0.59 kilojoules for epoxy alone, 0.63 kilojoules for low-density graphene aerogels, and 1.45 kilojoules for conventional graphene aerogels. The material maintained 90% of this energy absorption capacity over 100 repeated impacts, while conventional materials dropped below 5% of their initial performance after the same number of cycles.
Ballistic testing provided even more compelling evidence of the material’s protective capabilities. When faced with projectiles traveling at 200 meters per second, the aerogel-based protective structure absorbed 14 kilojoules of energy per meter – 2.6 times more than standard epoxy protective materials. While conventional protective boards were penetrated by the projectiles, which retained 75% of their initial velocity, the new aerogel structure completely stopped the projectiles through controlled deformation of its internal structure.
This performance stems from the material’s hierarchical structure spanning three size scales: nanometer-thick walls, micrometer-sized corrugated pores (2-20 µm), and millimeter-scale honeycomb patterns. This architecture allows the material to distribute and absorb impact energy through controlled deformation rather than structural failure.
The development establishes new principles for engineering lightweight protective materials that require both stiffness and elasticity. The approach is particularly relevant for applications in aerospace and vehicle armor, where weight reduction without compromising protective capabilities could significantly improve fuel efficiency and performance. The demonstrated combination of high stiffness (12 MPa), exceptional elastic recovery (90% after compression), and superior energy absorption (2.6 times current materials) sets new benchmarks for protective material performance.
Beyond immediate applications in impact protection, this research demonstrates how hierarchical design – building from the smallest to largest scales with precise control at each level – can yield materials with unprecedented combinations of properties. For protective equipment applications where both strength and flexibility are essential, the principles established in this work provide a clear pathway for developing the next generation of high-performance materials.
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