Flexible and biocompatible polyurethane/Co@C composite films with weakly negative permittivity

Epsilon-negative materials (ENM) have become a research hotspot because of their unique physical properties and novel applications in electromagnetic stealth, wireless power transmission, etc. In this work, Co nanoparticles (NPs) encapsulated into carbon particles (Co@C) were synthesized by carbonizing the metal–organic frameworks, and the negative permittivity with a small absolute value was achieved by dispersing Co@C into polyurethane (PU) matrix to prepare PU/Co@C composites. When the content of Co@C reached 20 wt.%, a continuous network of Co@C was set up, resulting in induced electric dipole resonance of carbon and plasma oscillation of Co NPs, and thereby the weakly negative permittivity (< -3300). Besides, alternating current conductivity induced percolation phenomenon was observed when the content of Co@C increased from 15 wt.% to 20 wt.%. The remarkable biocompatibility and flexibility of the PU/Co@C metamaterial reveal the potential application of PU/Co@C films in diverse scenarios, including underwater detection and wearable devices. etc.