Notes on Porous Carbon RAM and Related EMW Absorbers

Frequency Selective Surface Properties of Microwave New Absorbing Porous Carbon Materials Embedded in Epoxy Resin

• Objective and approach: Evaluate EM wave absorption of sustainable porous carbon (from black liquor of cellulose production) in epoxy RAM composites; study porosity-size and particle-size effects; examine how a frequency selective surface (FSS) structure can tune RL in the X-band (8.2–12.4 GHz).

• Porous carbon and sample classification: Two porosity ranges defined by PMMA spheres: 180 μm < Ø1 < 250 μm and 425 μm < Ø2 < 500 μm; two particle-size groups: ϕ1 < 250 μm and 250 μm < ϕ2 < 425 μm. Four porous carbon fillers (C1–C4) correspond to combinations of porosity and particle-size (C1: ϕ1, Ø1; C2: ϕ1, Ø2; C3: ϕ2, Ø1; C4: ϕ2, Ø2).

• Composite processing: 5 wt%, 10 wt%, 15 wt% carbon in epoxy; 2.0 mm thickness for comparison; FSS structures produced by drilling two arrays of holes in the polymer–filler plate: set A with two 2.0 mm holes, set B with two 5.0 mm holes; center-to-center spacing 11.5 mm.

• Measurement and analysis: Complex permittivity ε* = ε′ − jε″ obtained via Nicolson–Ross–Weir (NRW) from S-parameters (S11, S21, S12, S22) in the X-band; reflectivity RL defined as RL = 20 log10| Zin − Z0 | / | Zin + Z0 | with Zin derived from εr and μr and sample thickness d. Rectangular-waveguide measurements simulate a large-area FSS interaction on a dielectric substrate.

• Key results (no FSS): RL around −19 dB at ≈11.7 GHz for C3-15wt% (best RAM among no-FSS samples). Higher porosity/particle-size combination and higher carbon loading increase ε′ and ε″, aiding dissipation; RL below −10 dB is achievable across X-band for certain compositions.

• Key results (FSS effects): Two-hole (2.0 mm) FSS shifts RL maximum; some samples showRL improvements or shifts of the RL minimum to different frequencies, with C1-15wt% giving about −14.5 dB at some frequencies and C4-10wt% around −6.0 dB for the 2.0 mm hole set. For the 5.0 mm hole set, RL peaks shift and reach substantial absorption (e.g., ~−17 dB for C1-15wt% in some cases).

• Conclusions: Porous carbon fillers with controlled porosity and particle size allow tunable RAM behavior; adding FSS geometry enables frequency-selective tuning of RL maxima, enabling targeted absorption within X-band for stealth and EMI shielding applications. The approach leverages sustainable fillers and common epoxy matrices.

Dielectric characterization of white birch–activated biochar composites: A sustainable alternative to radar-absorbing materials

• Objective: Characterize dielectric and EM properties of activated biochars derived from white birch (BC-CO2 and BC-KOH) in epoxy or silicone matrices, and assess the impact of filler morphology (flakes vs powders) on RAM performance and reflectivity.

• Materials and synthesis: Two activated biochars (BC-CO2 via physical activation with CO2; BC-KOH via chemical activation with KOH) produced from birch biochar; composites prepared by incorporating 2 wt% biochar into a silicone rubber or related matrix; also compared biochar flakes vs powders.

• Characterization methods: XRD, Raman, XPS, SEM/EDS, mercury porosimetry, and VNA-based dielectric measurements in the X-band (NRW method used to extract εr and μr from S-parameters); reflectivity measured with a metallic plate behind the sample to obtain RL.

• Dielectric properties (comparison):

  • Flake fillers (BC-CO2 and BC-KOH flakes) show non-linear ε′ and ε″ across the X-band and display discrete peaks indicating polarization relaxations; BC-CO2 flakes exhibit RL features up to about −8 dB (top RL), with peaks around 10–12 GHz in some configurations. BC-KOH flakes show more pronounced porosity effects, yet their RL features are sample- and thickness-dependent.

  • Powder fillers show more linear dielectric behavior across the X-band; average permittivity values are lower: e′ ≈ 3.45–3.55 and e″ ≈ 0.12 for BC-CO2 and BC-KOH powders, indicating weaker dielectric loss than flakes due to the absence of extended conductive pathways formed by flakes.

• Permittivity and loss tangents: Dielectric loss tangent tan δε = ε″/ε′ is substantial for carbon-based composites; magnetic losses are not primary in these non-magnetic biochar systems, so ε″ dominates the loss behavior; Cole–Cole representations show multiple semicircles indicating several Debye relaxations and interfacial polarization processes at carbon–air and carbon–biochar interfaces plus possible mineral residues.

• RL and RAM performance: Samples with flakes showed more prominent RL features (peaks and dips) than powders, consistent with enhanced interfacial polarization and conduction paths in the flaky morphology. Powders yielded smoother RL with less pronounced peaks, suggesting weaker localized resonance effects. RCS simulations indicated notable reflectivity reductions (RCS below about −20 dBsm in some cases), supporting RAM potential for biochar-based fillers.

• Conclusions: Activated white birch biochar fillers enable RAM when embedded in a polymer matrix; flakes can promote stronger dielectric loss via interfacial polarization and conduction networks, while powders yield more linear dielectric behavior. The sustainable, low-cost fillers offer RAM potential with tunable EM properties via morphology and activation route.

Porous carbon nanotube–activated carbon nanostructures: Feasible EMW absorbers with tunable dielectric/magnetic loss (FeNC nanocages)

• Objective: Develop Fe- and N-codoped carbon nanocages (FeNC) with encapsulated Fe nanoparticles that exhibit strong, thin-plate EMW absorption; elucidate the loss mechanisms via experiment and theory.

• Synthesis: FeNC nanocages prepared by pyrolysis of a Fe-coordinated bis(imino)pyridine polymer precursor (Fe–DAP–DAN) to yield spherical graphitic shells containing FeNx sites and encapsulated Fe nanoparticles; graphed as Fe1N1C, Fe1N2C, Fe1N4C, Fe1N8C (Fe-to-DAP–DAN molar ratios of 1:1, 1:2, 1:4, 1:8).

• EMW performance: Measured RL in X-band (2–18 GHz) at multiple thicknesses; RLmin up to −75.5 dB at 1.83 mm thickness with Fe1N2C; EAB ≈ 5.41 GHz; RCS reduction up to 29.2 dBm^2 at small incident angles. The impedance matching (Zin ≈ Z0) is achieved for Fe1N2C, enabling deep wave penetration and high dissipation; attenuation constant α reaches high values (≈166 at 18 GHz) for optimal samples.

• Mechanism and structure–property relationships:

  • Hierarchical porous structure with thin graphitic shells and Fe nanoparticles promotes multiple internal reflections and enhanced dielectric loss via interfacial polarization between FeNx sites and graphitic carbon, plus conductive loss from the carbon network.

  • Encapsulated Fe nanoparticles provide magnetic loss (natural/exchange resonance and eddy currents) that improves impedance matching and adds magnetic loss in the GHz range.

  • DFT shows FeN4 and FeN4/Fe junctions create charge transfer and spin polarization that reinforce dipole and magnetic responses, respectively; interfacial Fe–N coordination enhances both dielectric and magnetic losses.

• Equations and concepts:

  • RL(dB) = 20 log10 | (Zin − Z0) / (Zin + Z0) | with Zin derived from εr and μr via NRW theory.

  • Impedance matching condition Zin ≈ Z0 yields minimal reflection; thickness and frequency determine RL peaks via quarter-wavelength considerations.

• Conclusions: FeNC nanocages deliver exceptional EMW absorption at very thin thicknesses with high RLmin and broad EAB, owing to synergistic dielectric/magnetic loss from their unique architecture and Fe doping. The design provides a path toward ultrathin RAM and sensor applications.

Tea waste-derived microporous carbon (TWPC): Tunable low- and mid-frequency microwave absorption

• Objective: Use green tea waste to make TWPC with controllable porosity; tune absorption frequency from Ku-band toward C-band by adjusting carbonization temperature and N-doping; evaluate RAM performance in X-band.

• Synthesis and porosity control: TWPC fabricated by carbonization of tea waste at 650–950 °C in inert atmosphere; porosity tailored by carbonization temperature and subsequent exposure to activation-like conditions (implication of porosity increase with temperature); BET surface area increases with temperature (TWPC-650 ≈ 476 m^2/g; TWPC-750 ≈ 521 m^2/g; TWPC-850 ≈ 566 m^2/g; TWPC-950 ≈ 658 m^2/g) with pore sizes around 1.8–2.05 nm.

• EM measurements and results: RLmin and EAB depend on thermal treatment; TWPC-850 °C yields the best performance: RLmin ≈ −47.60 dB at 7.84 GHz with d ≈ 2.2 mm; EAB ≈ 4.88 GHz (at 2.2 mm); TWPC-650 and TWPC-750 show strong dielectric loss contributions but lower RLmin or narrower EAB. RCS simulations indicate the TWPC-850 °C-coated plate reduces radar scattering significantly (RCS < −20 dBsm over angles).

• Dielectric behavior and mechanism: Increased N content and graphitization with higher carbonization temperature raise ε′ and ε″, enhancing polarization loss (interfacial polarization) and conduction loss via improved conductive networks; the balance between impedance matching and loss governs RL and EAB. Cole–Cole plots reveal multiple Debye relaxations due to defects and heteroatom doping; the quarter-wavelength condition explains the shift of RL peaks with thickness.

• Conclusions: TWPC from tea waste provides tunable, broadband microwave absorption with excellent low-frequency performance (C-band) and strong high-frequency absorption, leveraging porosity, N-doping, and controlled graphitization; RCS simulations demonstrate practical RAM potential.

Rectorite–chitin–derived carbon aerogels (cRTS) with hierarchical porosity: Ultra-broadband EMW absorption

• Objective: Build composite aerogels by integrating rectorite lamellae (REC) with regenerated chitin fibers (CT) to create a porous carbon aerogel with abundant heterogeneous interfaces for broad EMW absorption; optimize via heat treatment to induce defects and charge-transfer interfaces.

• Structure and fabrication: CT–REC lamellae assembled via hydrogen-bond/electrostatic interactions; directional freezing creates aligned pores (~20 μm) with REC lamellae forming cell walls; carbonization transforms the composite into cRTS with hierarchical cellular architecture while preserving pores. The REC lamellae act as functional interfaces that promote interfacial polarization in the dielectric matrix.

• EM performance: RLmin ≈ −26 dB at ~15.3 GHz with d ≈ 3.1 mm; EAB up to 8.3 GHz across 9.7–18 GHz at 3.5 mm; with thickness tuning RL shifts to lower frequencies following quarter-wavelength behavior. RCS simulations indicate significant reduction (e.g., >44 dBsm at 0°) for patching cRTS onto a PEC, showing practical radar stealth potential.

• Dielectric/magnetic response: μ′ ≈ 1 and μ″ ≈ 0 (non-magnetic); ε′ and ε″ are enhanced by REC addition and interfacial polarization at REC–CT/graphitic carbon interfaces; Cole–Cole analysis shows multiple Debye relaxations from heterogeneous interfaces and defects, signifying strong polarization loss; conduction loss is reduced relative to non-reinforced CT–TS skeleton due to insulating REC lamellae, while polarization loss is enhanced by interfaces.

• Mechanical and processing notes: cRTS shows good mechanical resilience (low plastic deformation after repeated compression); the lamellae alignment is critical for maintaining oriented pores and high packing stability.

• Conclusions: The cRTS aerogel platform achieves ultra-broadband EMW absorption through hierarchical porosity and REC–CT interfaces, enabling effective RL across X–Ku bands and strong RCS suppression in far-field simulations, suitable for lightweight RAM and stealth coatings.

Quick reference: common EMW parameters and design concepts used in these works

• Reflection loss (RL):
  $ext{RL} = 20 \log<em>{10} \left| \frac{Z</em> ext{in} - Z<em>0}{Z</em> ext{in} + Z_0} \right|$

• Input impedance (NRW-related):
  $Z_ ext{in} = Z<em>0 \sqrt{ \frac{\mu</em>r}{\varepsilon<em>r} } \ \tanh\left( j \frac{2 \pi f d}{c} \sqrt{ \mu</em>r \varepsilon_r } \right)$

• Complex permittivity and permeability: $\varepsilon<em>r = \varepsilon' - j\varepsilon'' , \quad \mu</em>r = \mu' - j\mu''$

• Quarter-wavelength condition (approximate matching thickness for a peak RL): roughly tm ≈ n (c/ (4 fm sqrt{ μr εr })) with n = 1,3,5, …

• Cole–Cole representation (dielectric relaxation): semicircles in the (ε′, ε″) or (ε′, ε″/f) plots indicate multiple Debye relaxations and interfacial polarization processes; linear tails indicate conduction loss.

• RCS (Radar Cross Section) modeling (example):
  $\sigma(\mathrm{dB m^2}) = 10 \log<em>{10} \left( 4 \pi S \lambda^2 \left| \frac{E</em>S}{E_i} \right|^2 \right)$
  or CST/NRC-based expressions, depending on the setup; used to quantify far-field radar detectability reductions when RAM coatings are applied.

Takeaways for quick recall

• FSS can tune the RL peak frequency of RAM composites with porous carbon fillers embedded in epoxy, enabling targeted absorption in X-band.

• Activated biochar fillers (BC-CO2 vs BC-KOH) in silicone or epoxy matrices provide RAM via interfacial polarization and dielectric loss; flakes tend to enhance polarization more than powders due to extended interfaces.

• FeNC carbon nanocages demonstrate exceptional RAM at very thin thicknesses due to synergistic dielectric and magnetic loss from FeNx dopants and encapsulated Fe nanoparticles; DFT supports strong interfacial dipoles and magnetic moments.

• TWPC derived from tea waste achieves strong low-frequency RAM (C-band) by leveraging high porosity, nitrogen-doping, and controlled graphitization; EAB can be broad with thickness optimization.

• cRTS aerogels combine CT fibers and REC lamellae to yield ultra-broadband RAM with hierarchical porosity and many interfacial polarizations; RCS simulations confirm potential radar stealth performance.

If you’d like, I can tailor these notes to a particular exam focus (e.g., more equations, or more emphasis on material synthesis vs. EM characterization) or condense each section further into a compact one-page reference card.