Lightweight 3D Graphene Metamaterials with Tunable Negative Thermal Expansion.

Materials science involves trade-offs between mechanical deterioration and thermal instability. The concept of metamaterials has been proposed as a novel approach to overcome the exclusive responses among physical properties by artificially designing structures at multiple scales, beyond new component compositions or chemical modifications. In this study, a three-dimensional (3D) graphene metamaterial (GM) showing negative thermal expansion was prepared using a strategy of hyperbolically-oriented freezing under a dual temperature gradient along orthogonal directions after the π-π stacking-derived assembly of 2D graphene sheets. As the fundamental construction element of the 3D GM, the graphene sheet displayed anomalous shrinking deformation with a thermal expansion coefficient of (-6.12 ± 0.28) × 10 -6 that was triggered by thermally-induced out-of-plane vibrations of the C-C bonds. A combination of numerical simulations and experimental investigations validated that anomalous negative thermal expansion (NTE) behavior can be effectively delivered to scalable 3D GM candidates at larger dimensions beyond the basic 2D graphene sheets at the microscale. The multiscale design and optimization of the structural characterization of the 3D GM further realized the desirable regulation of the NTE performance with the NTE coefficient ranging from negative ((-7.5± 0.65) × 10 -6 K -1 ) to near-zero values ((-0.8 ± 0.25) × 10 -6 K -1 )). This was attributed to the NTE-derived release regulation of the primary stress/strain of the microstructure, and the 3D GM exhibited high thermal stability while preserving the desirable structural robustness and fatigue resistance under thermo-mechanical coupling conditions. Therefore, this 3D GM offers promising potential for applications as protective skin, thermal actuator, smart switcher, and packing filler . This article is protected by copyright. All rights reserved.