RESUMO
Lightweight and efficient electromagnetic interference (EMI) shielding materials play a vital role in protecting high-precision electronic devices and human health. Porous PVDF/CNTs/urchin-like Ni composites with different cell sizes from nanoscale to microscale were fabricated through one-step supercritical carbon dioxide (CO2) foaming. The electrical conductivity and electromagnetic interference (EMI) shielding performance of the composites with different cell sizes were examined in detail. The results indicated that the nanoscale cell structure diminishes the EMI shielding performance of the composite, whereas the microscale cell structure with an appropriate size is beneficial for improving the EMI shielding performance. A maximum EMI shielding effectiveness (SE) of 43.4 dB was achieved by the composite foams which is about twice that of the solid composite. Furthermore, as the supercritical CO2 foaming process reduces the density of the composite by 25-50%, the EMI SSE (specific shielding effectiveness)/t(thickness) of the composite reaches 402 dB/(g/cm2), which is the highest value of polymer foam obtained to the best of the authors' knowledge. Finally, compression tests were performed to show that the composites still maintained excellent mechanical properties after the supercritical CO2 foaming process.
RESUMO
Polymer composite foams are desirable materials for electromagnetic (EM) energy attenuation. However, a number of challenges limit improvement in the EM energy attenuation properties of foams. In this study, a simple microcellular injection molding method was used to fabricate highly compressible thermoplastic urethane (TPU)/carbon nanotube (CNTs) composite foams, which also had increased conductivity with an increase in CNT content. Compared to unfoamed composites, foamed composites exhibited higher conductivity and EM attenuation properties because of the presence of a microcellular structure. Moreover, the TPU/CNT foam with 4 wt % CNTs (F(4)) demonstrated strong EM dissipation and an optimal reflection loss (RL) value of -30.4 dB. Furthermore, stimulated by thermal heating and cyclic compression, EM attenuation was observed to increase because of the higher conductivity. Note that F(4) foam having a small thickness of 1.3 mm when treated at 333 K had the highest EM dissipation and the lowest RL value of -51.8 dB. Enhanced polarization and ohmic losses and multiscattering were responsible for the increased EM absorption. This behavior is attributed to the movement of CNTs within the TPU elastomer walls via thermal or compression stimulation. For designing stimulation-dependent multifunctional materials, composite foams with response to thermal heating were proved to be an alternative approach.
RESUMO
The facile fabrication of thin flexible electromagnetic interference (EMI) shielding materials with fast heat dissipation for adaptable tuning in both civil and military applications is in urgent demand. In our work, the flexible poly(vinylidene fluoride) (PVDF)/carbon nanotube (CNT) composite films decorated with anisotropy-shaped Co in flowers or chains were prepared and studied. The results showed that by increasing the Co filler contents, the EC (electrical conductivity), TC (thermal conductivity), and EMI shielding properties of such PVDF/CNT/Co (flowers or chains) flexible films were significantly improved. In contrast, the PVDF/CNT/Co-chain flexible films exhibit higher performance with respect to the EC, TC, and EMI shielding properties. Total shielding of 35.3 and 32.2 dB were, respectively, obtained by the PVDF/CNT/6 wt % Co-chain with an EC of 2.28 S/cm and the PVDF/CNT/6 wt % Co-flower with an EC of 1.94 S/cm at a film thickness of 0.3 mm. Possibly owing to the conductive dissipation, interfacial polarization, magnetic loss, multiple reflections, and scattering of EM waves, such flexible composite films possessed a remarkable absorption-dominated EMI shielding behavior. These new composite films with enhanced TC are easily able to transform microwave energy into Joule heating systems, making themselves greatly potential for effective EMI shielding as well as rapid heat dissipation.
