Lightweight 3D Graphene Metamaterials with Tunable Negative Thermal Expansion

In this study, a 3D graphene metamaterial (GM) showing negative thermal expansion is prepared using a strategy of hyperbolically oriented freezing under a dual temperature gradient along orthogonal directions after the pi-pi stacking-derived assembly of 2D graphene sheets. As the fundamental construction element of the 3D GM, the graphene sheet displays anomalous shrinking deformation with a thermal expansion coefficient of (-6.12 +/- 0.28) x 10(-6) that is triggered by thermally induced out-of-plane vibrations of the C-C bonds. A combination of numerical simulations and experimental investigations validates 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 realize the desirable regulation of the NTE performance with the NTE coefficient ranging from negative ((-7.5 +/- 0.65) x 10(-6) K-1) to near-zero values ((-0.8 +/- 0.25) x 10(-6) K-1). This is attributed to the NTE-derived release regulation of the primary stress/strain of the microstructure, and the 3D GM exhibits 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.