RESUMEN
Typically, solid materials exhibit transverse contraction in response to stretching in the orthogonal direction and transverse expansion under compression conditions. However, when flexible graphene nanosheets are assembled into a 3D porous architecture, the orientation-arrangement-delivered directional deformation of micro-nanosheets may induce anomalous mechanical properties. In this study, a 3D hierarchical graphene metamaterial (GTM) with twin-structured morphologies is assembled by manipulating the temperature gradient for ice growth during in situ freeze-casting procedures. GTM demonstrates anomalous anisotropic compression performance with programable Poisson's ratios (PRs) and improved mechanical properties (e.g., elasticity, strength, modulus, and fatigue resistance) along different directions. Owing to the designed three-phase deformation of 2D graphene sheets as basic microelements, the twin-structure GTM delivers distinctive characteristics of compressive curves with an apparent stress plateau, and follows a strengthening tendency. This multiscale deformation behavior facilitates the enhancement of energy loss coefficient. In addition, a finite element theory based numerical model is established to optimize the structural design, and validate the multiscale tunable PR mechanism and oriented structural evolution. The mechanical and thermal applications of GTM indicate that the rational manipulation-driven design of meta-structures paves the way for exploring graphene-based multifunctional materials with anomalous properties.
RESUMEN
Assembling Ti3C2Tx MXene nanosheets into three-dimensional (3D) architecture with controllable alignment is of great importance for electromagnetic wave absorption (EMA) application. However, it is a great challenge to realize it due to the weak van der Waals interconnection between MXene nanosheets. Herein, we propose to introduce gelatin molecules as a "chemical glue" to fabricate the 3D Mxene@gelatin (M@G) nanocomposite aerogel using a unidirectional freeze casting method. The Ti3C2Tx MXene nanosheets are well aligned in the M@G nanocomposite aerogel, yielding much enhanced yet anisotropic mechanical properties. Due to the unidirectional aligned microstructure, the M@G nanocomposite aerogel shows significantly anisotropic EMA properties. M@G-45 shows a -59.5 dB minimum reflection loss (RLmin) at 14.04 GHz together with a 6.24 GHz effective absorption bandwidth in the parallel direction (relative to the direction of unidirectional freeze casting). However, in the vertical direction of the same M@G aerogel, RLmin is shifted to a much lower frequency (4.08 GHz) and the effective absorption bandwidth decreases to 0.86 GHz. The anisotropic electromagnetic energy dissipation mechanism was deeply investigated, and the impendence match plays a critical role for electromagnetic wave penetration. Our lightweight M@G nanocomposite aerogel with controllable MXene alignment is very promising in EMA application.