Porous Carbon/ZnO Composites Synthesized Using an Environmentally Friendly Metal–Polyphenol Network for Electromagnetic Wave Absorption

Abstract

Composite materials made of transition metals/metallic oxides and porous carbon stemming from metal–organic frameworks (MOFs) have drawn tremendous attention within the field of electromagnetic wave absorption. However, the fabrication of most MOFs involves the use of toxic solvents and specially synthesized organic ligands; furthermore, high-pressure hydrothermal processes are commonly employed and are environmentally unfriendly and unsustainable. Herein, a Zn metal–polyphenol network (MPN) was developed by replacing organic ligands with biomass tannic acid at room temperature by using water and ethanol as solvents. The resulting MPN exhibited a microstructure and performance similar to those of MOFs. Subsequently, in situ pyrolysis of the Zn–tannic acid network was conducted to prepare porous carbon/ZnO (PC/ZnO) composites. The PC/ZnO composite pyrolyzed at 700 °C showed a strong reflection loss of −57.8 dB at a matching thickness of 3.5 mm, and a wide effective absorption bandwidth of 5.5 GHz was attained at 2.5 mm. The excellent electromagnetic wave absorption performance can be ascribed to the occurrence of multiple scattering events due to the porous structure, the moderate resistance loss of the porous carbon network, and the strong dipolar and interfacial polarization losses due to the ZnO nanoparticles and defect carbon. This study provides a convenient, environmentally friendly, sustainable, and tunable approach to prepare excellent broadband electromagnetic wave absorbers.

MWCNT-Reinforced Polymethacrylimide Foams for Broadband Electromagnetic Wave Absorption

“…According to Debye relaxation, as the frequency increases, the dipole inside the absorbing material under the external electromagnetic field cannot adjust its orientation in time to adapt to the frequency change, resulting in a decrease in ε’ at high frequencies . It can be obviously seen that the curve of ε’’ appears several prominent formant peaks at the same frequency, indicating that the multipolarization relaxation process has occurred. , The dielectric loss of the material in response to the electromagnetic wave can be characterized by the dielectric loss tangent (tan δ ε = ε’’/ε’ ), which mainly depends on the conductive loss and polarization relaxation loss. The value of tan δ ε increases with the increase of MWCNT content, mainly attributing to the conductive loss caused by the increase of electrical conductivity and the polarization loss caused by the increase of heterogeneous interface and microscopic defects, and therefore the comprehensive performance is attributed to the enhancement of dielectric loss. − However, μ’ , μ’’ and the permeability loss (tan δ μ =μ’’/μ’ ) of the composite foam are small (Figure S5).…”

“…The study of the relationship between them is helpful to further understand the microwave absorption mechanism of the foam. According to Debye relaxation theory, ,,, ε’ and ε’’ can be expressed as ε ′ = ε ∞ + ε s − ε ∞ 1 + false( 2 π f false) 2 τ 2 ε ″ = 2 π f τ false( ε s − ε ∞ false) 1 + false( 2 π f false) 2 τ 2 …”

“…where ε ∞ , ε s , and τ represent the relative permittivity, static permittivity, and polarization relaxation time, respectively. The correlation between ε’ and ε’’ can be derived from the above two Debye expressions: ,, false( ε ′ − ε s + ε ∞ 2 false) 2 + false( ε ″ false) 2 = false( ε s − ε ∞ 2 false) 2 …”

“…The study of the relationship between them is helpful to further understand the microwave absorption mechanism of the foam. According to Debye relaxation theory, ,,, ε’ and ε’’ can be expressed as …”