Because of the disunity of resolution and exploration range in applied geophysics, the effective medium theory (EMT) should be developed to help us to understand the geological microstructure. We extended the EMT to complex permittivity in high frequency, and calculated the imaginary part of effective complex permittivity of composite as the effective conductivity using Finite-Difference Time-Domain (FDTD) numerical method. The result we obtained is the intrinsic property of the equivalent medium, which has explicit geological signification to satisfy the requirement of interpretation.
With the aggravation of electromagnetic radiation pollution, it is urgent to develop green, lightweight, ultra-thin and high-performance electromagnetic interference shielding materials to eliminate unnecessary electromagnetic interference; however, the construction of wood-based high-performance electromagnetic shielding materials by simple methods remains a challenge. Based on the layer-by-layer assembly strategy, a lightweight Ni/Wood/Ni composite (NWNC) with an interlayer structure was constructed by a simple electroless plating method using natural wood as a substrate for electromagnetic interference shielding. The synthesized NWNC has a smooth surface, and its minimum surface roughness is only 8.34 μm. After 15 min of electroless nickel plating, the contact angle (CA) of NWNC with an ultra-thin nickel layer (65 μm) was 118.3°. When the thickness of the nickel layer is only 0.102 mm, the conductivity can reach 1659.59 S/cm when the three electroless nickel plating time is 15 min. In the L-band, the electromagnetic shielding effectiveness can reach 94.1 dB after three times electroless nickel plating for 20 min. This is due to the conductive loss, magnetic loss and interface polarization loss generated by the electromagnetic network constructed by the nickel layer, which makes the composite material produce an electromagnetic shielding mechanism dominated by absorption. The L-band absorption efficiency can reach 39.01 dB, and due to the porous structure of the original wood, the multiple reflection and absorption inside the wood further lose the electromagnetic wave. This study provides a low-cost and simple method for the design of light, ultra-thin and efficient controllable wood-based electromagnetic shielding materials and has broad application prospects in the fields of construction and aerospace.
The changes of properties of wood-based Cu-Ni composites were studied via electroless Cu and Ni on wood surface to obtain Cu-Ni multilayer composites with excellent properties. The surface and interface morphology of the composite coatings were investigated via laser confocal microscopy and scanning electron microscope (SEM). The crystal structure was characterized by XRD. The hydrophobic properties of the composite coatings were tested via contact angle meter. The surface conductivity of composites was tested via four-probe. The results showed that the electrical conductivity of wood-based Cu-Ni composites was 2370.76 S/cm. The surface roughness was 9.99 μm and the thickness of uniform coating reached 157 μm via one time electroless Ni deposition and two times electroless Cu deposition. XRD analysis showed that the wood surface was uniformly covered with metal coating. The metal Cu and Ni were closely nested together to form a dense composite layer, and the composite material was light in weight. When the electroless Ni was 55 min, the contact angle could reach 123°, indicating that had best hydrophobicity. The average electromagnetic shielding effectiveness (EMSE) of Cu and Ni wood-based composites can reach 93.8 dB at L band ranging from 0.3×103 to 3.0×103 MHz) with a low thickness (157 μm), verified the multilayer composite materials can block over 99.99% of incident EM waves.
Prepare a sort of Wood-based EMI shielding materials can resist harsh electromagnetic wave environment had become a developing trend. The affect was differ from heat treatment temperatures on the wood-based composites was analyzed. The results showed that the surface roughness of metal coatings were 9.202 μm when the wood was electroless Ni and the heat treatment temperature was 120°C. When the wood was electroless Ni via 180°C heat treatment, the coatings thickness were 100.86 μm. EDS spectrums verifies the existence of Cu and Ni. The contact Angle of composite materials could reached 120.2°, and the hydrophobic property of wood was better. The average electromagnetic shielding efficiency of electroless 2Cu1Ni via 180°C heat treatment reached 91.40 dB in the frequency ranging from 300 KHz to 3.0 GHz, which verified that the composite material can shield 99.99% of the incident electromagnetic wave. The conductivity gradient structure can realize multi-dielectric interface loss and multiple reflection loss.
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