Hydro-sensitive sandwich structures for self-tunable smart electromagnetic shielding
Graphical abstract
Carbon-based sandwich structures are constructed via highly conductive surface and porous spacers, which exhibit hydro-sensitive electromagnetic shielding performance via changing the water loadings in the spacers. The results highlighted hydro-sensitive electromagnetic shielding structures as smart electromagnetic response materials for self-tunable capability.
Introduction
Since smart materials and structures with the ability of shape transformation or functional variation automatically when environmental conditions vary have been massively used for developing advanced components for intelligent devices and equipment, including sensors, actuators, self-recovery and self-healing systems [1], [2], [3], [4], [5]. Generally, both physical and chemical approaches could be applied to realize shape transformation or functional changes. In principle, external stimuli could be known as stress, temperature, moisture, pH, electric or magnetic fields [6], [7], [8], [9], [10]. A smart material and structure can response in forms of either generation of stresses and strains or changes in chemical structures, resulting in shape transformation or intrinsic physical properties, respectively [11], [12], [13].
On the other hand, for addressing such issues induced by electromagnetic emission and interferences in the communicating apparatus, a variety of lightweight materials with electromagnetic interference (EMI) shielding capability have been massively investigated to meet the requirements in many industries [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. In addition to traditional metal materials (Cu, Al, and alloys), carbon materials (carbon nanotubes, graphene and carbon filers) have attracted great attention for their unique advantages in electrically conductive feature, which allow them for achieving considerably high EMI shielding performance [27], [28], [29], [30], [31], [32]. Fundamentally, the shielding mechanism suggests that of EMI shielding materials could be divided into reflection, absorption attenuation and multiple reflections. In the case of absorption dominant shielding materials, electric dipole and electrical conductance is critical for electric materials and meanwhile magnetic dipole is responsible for magnetic materials. In typical examples, 3D porous nickel framework was introduced for growing a 3D graphene framework, and the resulting composite foams delivered EMI shielding performance of 28 dB and 36 dB at thickness of 2 mm and 3 mm, respectively [17]. In the case of reflection loss dominant shielding materials, continuous electrically conductive paths and large interfacial impedance mismatching conditions are the critical conditions for creating massive electromagnetic reflection at the material-air interfaces [33], [34], [35], [36], [37]. For instance, Song and coworkers have used fabricate flexible polymeric graphene composite films, which deliver the sufficient EMI shielding performance (shielding effectiveness (SE) values greater than 20 dB) in the X band [36]. Furthermore, Shen and coworkers fabricated graphene thin films through thermal treatment in the inert environment, which enables to offer strong EM wave reflection based on the highly conductive fashion [39], [40]. On the other hand, Song and coworker recently presented a novel prototype of sandwich structures for generating multiple reflection among the conductive sandwich interfaces, which establishes a new stage for designing advanced EMI shielding structures with unique features [41].
Up to date, exploratory studies are still required on developing self-adaptive smart electromagnetic response materials and structures. In the early studies by Cao and coworkers [18], [21], silicon-based carbon composites were fabricated to investigate the temperature effects on the electromagnetic response capability, and thus temperature-sensitive self-adaptive EMI shielding composites were achieved based on the changes of intrinsic electrical properties at various temperatures. For further development, in this work, we construct a carbon-based sandwich structures based on highly conductive pyrolytic graphite papers and porous polymeric non-woven composites, which play the roles as the conductive sandwich surfaces and sandwich spacers, respectively. For releasing the smartness, hydro-stimulus (water as polar molecules) was utilized as the stimulus to tune the electromagnetic response properties of the porous spacers materials. With the presence of the water molecules, the EMI shielding performance of the spacers was found to be substantially improved. Meanwhile, the EMI shielding performance was observed to be linked with the intrinsic spacer features of the sandwich configuration and hydro-induced property variation in the various spacers. The results suggest two types of smart EMI shielding structures, i.e. function switchable EMI shielding materials and function enhanced EMI shielding materials, based on the initial EMI shielding performance and wet conditions. Therefore, the strategy and mechanism in this work highlights a significant plateau, on which self-adaptable and self-tunable smart electromagnetic response materials and structures would be envisaged.
Section snippets
NW spacer
Polyprolene non-woven (NW) spacer with thickness of ∼2 mm was provided from Aerosapce Research Institute of Materials & Processing Technology.
RGO/NW spacer
Graphene oxide (GO) solution was prepared based on the modified Hummers method [42]. A piece of NW (∼2 mm in thickness) was cleansed and dried as pre-treatment. The as-treated NW was positioned into a mixed solution of GO (5 mg/ml) and hydroquinone (GO:hydroquinone = 1:5 wt/wt), followed by sonication to reach homogeneous fashion. Then, the mixture was
Results and discussion
In the fabrication of the sandwich structures, various spacers were initially prepared. Briefly, various types of spacers were fabricated, as shown in Fig. 1a–c. Based on the as-fabricated spacers, two pieces of conductive PG papers (electrical conductivity on the order of 105 s/m) were used to integrate sandwich structures (Fig. 1d–f). For measuring the shielding performance at wet conditions, the spacers of the sandwich structures were injected with a portion of water (Fig. 1g–i). In the
Conclusions
In summary, novel hydro-sensitive EMI shielding sandwich structures with self-tunable capability were demonstrated via sandwiching electrical conductive PG papers into polymeric porous carbon-based spacers. With increasing the water loadings of the spacers, electromagnetic response capability would be substantially improved via modifying the polarization and loss in the spacers. As a result, the EMI shielding performance of the sandwich structures would be greatly changed upon different water
Acknowledgement
Financial support from National Natural Science Foundation of China (Grant 51302011) and Beijing Natural Science Foundation (Grant No. 16L00001 and 2182065) is gratefully acknowledged.
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These authors made equal contribution to this work.