Introduction

Material science has always been the cornerstone of major technological developments of humankind. Achieving commercial flight capability, micro chips, space flight and deep sea exploration are in one way or the other interlinked with development of high grade advanced materials. Development of alloys helped us produce lighter, durable and effective metals. Production of silicon based semiconductors ruled the world of electronics and helped yield faster and better processors. Another one is the development of graphene aerogel metamaterial which shows a unique set of proprieties, i.e., being ultralight, ultrastrong, and supertough.

Even exhibiting a grea optenital for military and  aerospace applicaitons,  the lack of satisfactory mechanical properties and multiscale structural regulations are acting as major drawbacks in bringing graphene aerogels into practical use.  Absence of multiscale structural regulation restricts graphene aerogel applications in wearable electrical circuit boards, soft robots and stimulus responsive devices. To tackle these issues, the study in discussion is proposing a laser engraving strategy toward graphene meta-aerogels. The achieved nanofiber-reinforced networks show the ability to transform the graphene walls deformation from microscopic buckling to bulk deformation under compression. This novelty leads to a high elasticity (around 5400% reversible elongation), stiffness, light weightness, and robustness into the material. Graphene aerogels exhibit an excellent balance of important mechanical material properties like mechanical strength, ultralight weight and fatigue resistance. Attaining these qualities for a single material is a holy grail in material science.

Fabrication techniques

Freeze drying is a well known method to obtain graphene aerogel’s original appearance of molds; this method has the ability to regulate structural arrangement of graphene sheets resulting in porous cellular hyperbolic networks. FDM 3D printing is an alternative extrusion technique that enables to produce graphene aerogels. This method also has the advantage of producing customizable structures. However, the need of using graphene oxide dispersion to obtain sufficient viscosity is on of the downsides of this techinque. The addition of  high viscosity makes graphene oxide strenuous to adjust the microstructure and leads to a weaker sheet to sheet interaction and, ultimately, impacts its macroscopic mechanical properties.

Failure behavior and mechanical strength improvement

Three different precedural patterns can be identified when defroming graphene aerogels. This is in a sequence of linear elasticity, collapse, and densification throughout the compressions process. These deformation failures are accelerated by microscopic hinges, which take place at the regions where the walls experience maximum bending moment and eventually collapse. The probability of the occurrence of this phenomenon is also highly influenced by the elastic modulus of graphene aerogels. Improving the bending stiffness of these materials will considerably increase their mechanical strength. Hydrothermal treatment, molecule crosslinking, and polymer reinforcement have proven to show significant improvement in terms of bending stiffness.

Fig1. Daily use materials that are typically subjected to laser engravings.

Laser engraving strategy

Graphene meta-aerogels (GmAs) are produced following a laser engraving technique to produce multi-functional macroscopic structures and high ordered micro-networks. The versatility of producing line, plane, 3D lattices and hole bulks is available for GmAs which end up in enhanced features like 5400% elongation, low specific weight and ultrawide poisson’s ratio. The bending stiffness of graphene walls is observed to increase significantly because of the 1D nanofiber reinforced 2D sheet structure. During compression, stable bulk deformation is observed in the internal deformation mode of GmAs instead of microscopic bulking, which is a significant mechanical strength improvement. This process improvement enables better mechanical  properties in comparison to its counterparts, carbon aerogels.

Structuring GAs has always been a concern as it demands a high level of precision due to its soft and fragile material attributes. By using a high micron-sized laser beam that has the ability to induce thermal effects locally and to break covalent bonds with accuracy, laser engravingis makes this fabrication possible. Even with this sophisticated technology, pristine GAs are currently hard to produce. Mostly because of the brittleness of the graphene sheets.

Mechanical performance of GmAs

At the end of the day, the sophistication of manufacturing a meta material will be rewarding based on its mechanical behavior. Compression experiments are performed on the prepared GAs. When subjected to uniaxial plane compression, even with 90% loading and unloading the GmA is able to recover its original state. This proved the elastic behavior of the material at large deformations.

The fatigue behavior of the GmA is observed with the help of cyclic loads. This material is ws shown to retain 95% after 1000 cycles of 50% compression. At 80% compression the ability to retain 82% stress checked off the durability box for this material. In addition to this, with a  precompressed treatment, the stress retention is enhanced to 99% and 90% at 50% and 80% compression respectively.

Fig2. Superior mechanical properties of GmA’s

Conclusion

This research focused on the advantages of fabricating graphene aerogel based metamaterial over carbon based aerogels. The mechanical performance, in terms of strength and durability, of this material is observed to be superior when compared to its counterparts. A closer look on the mechanism investigation and design principle of GmAs is presented in this research. Additionally, the possible roadblocks of fabricating such complex structures with current technology are reviewed.