Nanocomposite foam with a large expansion ratio and thin cell walls is promising for electromagnetic interference (EMI) shielding materials, due to the low electromagnetic (EM) reflection and high EM absorption. To overcome the dimensional limitation from two-dimension (2D) thin walls on the construction of conductive network, a strategy combining hybrid conductive nanofillers in semi-crystalline matrix together with supercritical CO2 (scCO2) foaming was applied: (1) one-dimension (1D) CNTs with moderate aspect ratio was used to minimize the dimensional confinement from 2D thin walls while constructing the main EM absorbing network; (2) zero-dimension (0D) carbon black (CB) with no dimensional confinement was used to connect the separated CNTs in thin walls and to expand the EM absorbing network; (3) scCO2 foaming was applied to obtain a cellular structure with multi-layer thin walls and a large amount of air cells to reduce the reflected EM; (4) semi-crystalline polymer was selected so that the rheological behavior could be adjusted by optimizing crystallization and filler content to regulate the cellular structure. Consequently, an advanced material featured as lightweight, high EM absorption and low EM reflection was obtained at 0.48 vol.% hybrid nanofillers and a density of 0.067 g/cm3, whose specific EMI shielding performance was 183 dB cm3/g.
A simple, feasible, and environmentally friendly method for supercritical fluid-assisted construction of a two-dimensional (2D) response network in three-dimensional (3D) composites is proposed to improve sensor performance so as to meet the requirement of new-generation strain sensors including high sensitivity, large strain range, and other auxiliary properties, such as light weight and thermal insulation. The mechanism is that cell wall stretching in the supercritical fluid foaming process changes the conductive fiber dispersive status by eliminating agglomeration and facilitating orientation in 3D cells. For the thermoplastic polyurethane (TPU)/carbon nanofiber (CNF) foam strain sensors manufactured in this work, there is an optimal cellular structure, that is, an optimal 2D response network in 3D composites to maximize sensitivity, and the gauge factor (GF) of the foamed TPU/CNF strain sensor is increased by 89 times compared to that of the unfoamed composite, together with increased thermal insulation and lightweight performance than the unfoamed composite. The obtained TPU/CNF foam strain sensor shows good stability and repeatability in human motion detection. To enlighten the structural design, a 2D response network model in a 3D foam for a foam strain sensor is established for the first time. The model reveals that filler content and aspect ratio are the key points to regulate the response network and there is an optimal value for these two factors to maximize the sensitivity in a foam strain sensor. Therefore, the knowledge on foam strain sensors based on the experimental and theoretical results in this work provides a new structural design strategy for sensing materials and guides the optimization of the sensing structure to improve sensor sensitivity.
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