To enhance the sensitivity of graphene aerogel‐based piezoresistive sensors by weakening their compressive strength while keeping their elasticity, lightweight and lamellar graphene aerogels (LGAs) with high elasticity and satisfactory electrical conductance networks are fabricated by bidirectional‐freezing of aqueous suspensions of graphene oxide in the presence of small amounts of organic solvents, followed by lyophilizing and thermal annealing. Because of the lamellar structure of the LGA, its compressive strength along the direction perpendicular to the lamellar surface is much lower than those of both isotropic and unidirectionally aligned graphene aerogels with similar apparent densities, leading to an ultrasensitive LGA‐based piezoresistive sensor with a high sensitivity of −3.69 kPa−1 and a low detection limit of 0.15 Pa. The ultrahigh sensitivity and low detection limit of LGA‐based piezoresistive sensor contribute to detecting subtle pressure at room temperature and in liquid nitrogen with ability to detect dynamic force frequency and sound vibration. Besides, thanks to the fewer junction points between the graphene lamellae, LGAs slices can be integrated as a wide‐range and sensitive bending sensor, which can detect arbitrary bending angles from 0° to 180° with a low detection limit of 0.29°, and is efficient in detecting biosignals of wrist pulse and finger bending.
Recently, there has been growing interest in using silicon-based integrated circuits at high microwave and millimeter-wave frequencies. The high level of integration offered by silicon enables numerous new topologies and architectures for low-cost reliable SoC applications at microwave and millimeter-wave bands, such as broadband wireless access (e.g., WiMax), vehicular radars at 24GHz and 77GHz [1], short range communications at 24GHz and 60GHz, and ultra narrow pulse generation for UWB radar.
Recently, increasing attention has been concentrated on negative permittivity with the development of the emerging metamaterials composed of periodic array structures. However, taking facile preparation into consideration, it is important to achieve negative permittivity behavior based on materials' intrinsic properties rather than their artificially periodic structures. In this paper, we proposed to fabricate the percolating copper/epoxy resin (Cu/EP) composites by a polymerization method to realize the negative permittivity behavior. When Cu content in the composites reached to 80 wt.%, the conductivity abruptly went up by three orders of magnitudes, suggesting a percolation behavior. Below the percolation threshold, the conductivity spectra conform to Jonscher's power law; when the Cu/EP composites reached to percolating state, the conductivity gradually reduced in 2 high frequency region due to the skin effect. It is indicated that the conductive mechanism changed from hopping conduction to electron conduction. In addition, the permittivity did not increase monotonously with the increase of Cu content in the vicinity of percolation threshold, due to the presence of leakage current. Meanwhile, the negative permittivity conforming to Drude model was observed above the percolation threshold. Further investigation revealed that there was a constitutive relationship between the permittivity and the reactance. When conductive fillers are slightly above the percolation threshold, the inductive characteristic derived from conductive percolating network leads to the negative permittivity. Such epsilon-negative materials can potentially be applied in novel electrical devices, such as high-power microwave filters, stacked capacitors, negative capacitance field effect transistors and coil-free resonators. In addition, the design strategy based on percolating composites provides an approach to epsilon-negative materials.
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