Fig. 2. Mechanical properties of the 3DGraphene foam at cryogenic temperature of 4 K.

(A) Schematic of measurement and optical images demonstrating reversible super-compressive elasticity of the 3DGraphene foam at 4 K. Dotted boxes mark the cylindrical sample with both 15 mm diameter and height; yellow and green arrows indicate the moving directions of the test head. The brightness and contrast of the optical images are enhanced for clarity. Scale bars, 1 cm. (B) The single-cycle stress-strain curve at 4 K is almost completely the same with the curve at RT (both along the axial direction at a rate of 0.1% strain s⁻¹). (C) Strain versus time curve of a stepwise compress-release measurement with increasing maximum strains but a constant rate (0.1 strain s⁻¹) and the corresponding stress-strain curves of such measurements at 4 K along both axial and radial directions. (D) Young’s modulus and Poisson’s ratio plots versus applied engineering strain (and the relative density of the compressed 3DGraphene foam) at 4 and 298 K, showing almost identical Young’s modulus variation trend and constantly near-zero Poisson’s ratio. (E) Stress-time curves of 100 compress-release cycles along the axial direction at 4 and 298 K. Each 2-s cycle was performed between 0 and 90% strain at a rate of 90% strain s⁻¹, as shown in the inset. The stress values at 0 and 90% strains of each cycle were emphasized by labeling symbols, and the dashed and dotted lines correspond to least-squares fittings of stress at 0 and 90% strains of each temperature, respectively. Almost identical and overlapping stress-time curves indicate the great cycle stability of the material maintained even at cryogenic temperature. (F and G) The Young’s moduli (F) and near-zero Poisson’s ratios (G) both remain unchanged during the cycling test at 4 and 298 K, showing the great cycle stability down to deep cryogenic temperatures. Error bars in (D), (F), and (G) represent SDs for repeated measurements.