Absorption Bandwidth Reflection Loss Research Articles

Reduced Graphene Oxide Modified Nitrogen-Doped Chitosan Carbon Fiber with Excellent Electromagnetic Wave Absorbing Performance.

Lightweight and low-cost one-dimensional carbon materials, especially biomass carbon fibers with multiple porous structures, have received wide attention in the field of electromagnetic wave absorption. In this paper, graphene-coated N-doped porous carbon nanofibers (PCNF) with excellent wave absorption properties were successfully synthesized via electrostatic spinning, electrostatic self-assembly, and high-temperature carbonization. The obtained results showed that the minimum reflection loss of the absorbing carbon fiber obtained under the carbonization condition of 800 °C is -51.047 dB, and the absorption bandwidth of reflection loss below -20 dB is 10.16 GHz. This work shows that carbonization temperature and filler content have a certain effect on the wave-absorbing properties of fiber, graphene with nanofiber, and the design and preparation of high-performance absorbing materials by combining the characteristics of graphene and nanofibers and multi-component coupling to provide new ideas for the research of absorbing materials.

Optimization of multicomponent microwave absorbents for improved electromagnetic wave absorption of layered foamed cementitious materials

The integrated design of materials and structures is essential for achieving superior electromagnetic wave absorption performance (EWAP) in cementitious materials. In this study, multicomponent microwave absorbents are designed to improve the EWAP of foamed cementitious materials. The effects of the type, content, and single/compound doping of the absorbents and material density on the EWAP of multilayer foamed cementitious materials are tested. The experimental results show that the multilayer structure can enhance the impedance-matching characteristics between cementitious materials and air. It also conduces more electromagnetic waves to penetrate deeper into the cementitious materials to be absorbed. The electromagnetic dissipation of cementitious materials can be significantly improved by combining the effects of interlayer reflection scattering, interface polarization, and resonance. Furthermore, the multilayer structure offers flexibility in adjusting the type and content of the absorbent in each layer and endows each layer with different electromagnetic dissipation mechanisms, resulting in a significantly improved EWAP. For four layers of foamed cementitious materials, the EWAP is optimized using three different iron ores (WMHL-135: 10 wt% magnetite, 30 wt% hematite, and 50 wt% limonite) in the absorbing layers. It achieves the optimum EWAP with a full-band effective absorption bandwidth of reflection loss (RL) < -10 dB, in the frequency range of 2–18 GHz, a minimum RL of −37.0 dB, and an average RL of –23.4 dB.

Construction of Co@C nanocapsules by one-step carbon reduction of single-crystal Co3O4 nanoparticles: Ultra-wideband microwave absorber verified via coaxial and arch methods

It is highly demanding and challenging to construct the nano-scale microwave absorber with wide-frequency responding feature. Herein, a series of Co@C nanocapsules (NCs) are fabricated via one-step carbon reduction of Co3O4 nanoparticles (NPs) of only 20 nm obtained by the nitrate pyrolysis method. The electromagnetic parameters of the samples can be effectively regulated by flexibly adjusting the carbon shell thickness. Surprisingly, all samples exhibit ultra-wide microwave absorption (MA) performance investigated by the coaxial method. For the Co@C NCs with a carbon shell of 25 nm, especially, the effective absorption bandwidth (EAB) for reflection loss (RL) below −10 dB reaches a record high of 15.2 GHz (2.8–18 GHz), which completely covers the whole C-, X-, and Ku-bands. More excitingly, the absorption bandwidth for RL ≤ −20 dB is up to 7.1 GHz at only 2.0 mm thickness. Such outstanding MA properties are attributed to nano-size effect, synergistic effects of strong dielectric/magnetic loss, and superior impedance matching characteristics. Notably, the polarization and magnetic coupling behaviors are clarified with the aid of electric field and magnetic field simulations using High-Frequency Structure Simulator (HFSS). The plate coating sample is further prepared and measured by the arch method, which also displays an ultrawide MA bandwidth. This work provides a new design strategy toward the facile synthesis of ultra-broadband microwave absorbers.

Phase and morphology evolution of high dielectric CoO/Co3O4 particles with Co3O4 nanoneedles on surface for excellent microwave absorption application

Novel dual phased CoO/Co3O4 particles with Co3O4 nanoneedles on surface are obtained by annealing dual-phased Co (hexagonal and cubic phases) particles. Their phase and morphology evolution are clarified by XRD, SEM and TEM measurements. The early formed CoO and Co3O4 on surface from dual-phased Co promote the outdiffusion process of inside Co with kirkendall effect and restrict the growth of Co3O4 with a specific orientation, leading to the formation of Co3O4 nanoneedles on the surface. The microwave absorption measurement shows that the CoO/Co3O4 particles with annealing time of 1 h possess higher permittivity and better performances than pure Co and Co3O4 particles. An enhanced reflection loss (RL) value of −38.8 dB is obtained for the CoO/Co3O4-paraffin wax (PW) composite with a thickness of 2.8 mm. The absorption bandwidth for RL below −10 dB reaches 8.7 GHz. The Co3O4 nanoneedles on surface, porous structure and inner dual phased CoO/Co3O4 bring increased multiple reflection, interface/dipolar polarizations, conductive loss and better impedance matching. This clarification of phase and morphology evolution and novel CoO/Co3O4 material with nanoneedles on surface may provide new routes for designing high performance Co-based microwave absorbers with unique structures.

Electromagnetic wave absorption properties of Fe/MgO composites synthesized by a simple ultrasonic spray pyrolysis method

Electromagnetic (EM) wave absorbing materials consisting of iron (Fe) nanoparticles supported by magnesium oxide (MgO) matrix were synthesized by ultrasonic spray pyrolysis (USP) and subsequent hydrogen reduction. The Fe nanoparticles were formed by the selective reduction of iron oxide in the as-pyrolyzed powder, itself acting as a microreactor, and were successfully embedded in or at the MgO surface, allowing for magneto-dielectric structures without aggregation. Fe content in the composite particles was controlled only by the molar ratio of the precursor solution ([Fe3+]:[Mg2+]). The remarkable shielding effectiveness reached −65.6 dB at 12 GHz with a corresponding thickness of 1.5 mm and an absorption bandwidth for reflection loss (RL) of less than −20 dB is about 7.8 GHz. This design approach can be extended to realize magneto-dielectric materials with tailored absorption frequencies of high bandwidth due to the combination of the well-dispersed decoration of magnetic particles with controlled content and effective insulation. This type of material would be well suited for use in the research community and industry alike due to the reduction in energy required for synthesis and attendant oxidation losses.

Microporous Co@C Nanoparticles Prepared by Dealloying CoAl@C Precursors: Achieving Strong Wideband Microwave Absorption via Controlling Carbon Shell Thickness

Excellent magnetic features make Co-based materials promising candidates as high-performance microwave absorbers. However, it is still a significant challenge for Co-based absorbers to possess high-intensity and broadband absorption simultaneously, owing to the lack of dielectric loss and impedance matching. Herein, microporous Co@C nanoparticles (NPs) with carbon shell thicknesses ranging from 1.8-4.9 nm have been successfully synthesized by dealloying CoAl@C precursors. All of the samples exhibit high microwave absorption performance. The microporous Co@C sample possessing a carbon shell of 1.8 nm exhibits the highest absorption intensity among these samples with a minimum reflection loss (RL) of -141.1 dB, whose absorption bandwidth for RL ≤ -10 dB is 7.3 GHz. As the thickness of the carbon shell increases, the absorption bandwidth of the NPs becomes wider. For the sample with the carbon shell thickness of 4.9 nm, the absorption bandwidth for RL ≤ -10 dB reaches a record high of 13.2 GHz. The outstanding microwave attenuation properties are attributed to the dielectric loss of the carbon shell, the magnetic loss of the Co core, and the cooperation of the core-shell structure and microporous morphology. The strong wideband microwave absorption of the carbon-coated microporous Co NPs highlights their potential applications in microwave absorbing systems.

Excellent microwave absorption properties of Fe ion-doped SnO2/multi-walled carbon nanotube composites

Fe ions doped SnO2/MWCNTs composites with 48.8% Fe ions doping content showed the maximum reflection loss was −44.54 dB at 15.44 GHz, and the maximum absorption bandwidth of reflection loss below −10 dB was 4.5 GHz in the Ku band.