High‑Performance CoFe/C Core–Shell Nanocomposites for Broadband Microwave Absorption

Abstract

CoFe/C core–shell nanocomposites (CoFe@C) were fabricated by thermal decomposition of acetylene using CoFe₂O₄ as precursor. XRD, XPS, Raman, TEM and thermogravimetric analysis confirmed an amorphous carbon shell 5–30 nm thick, comprising ~48.5 wt % of the composite. The synergy between intrinsic magnetic properties and high electrical conductivity yields exceptional microwave‑absorbing performance: a minimum reflection loss (RL) of –44 dB at 4.0 mm and a –10 dB bandwidth of 4.3 GHz centred at 2.5 mm.

Background

Electromagnetic (EM) interference poses a growing challenge to modern electronics. Microwave‑absorbing materials (MAMs) mitigate this by converting unwanted EM energy into heat or other harmless forms [1–5]. Ideal MAMs exhibit wide bandwidth, strong absorption, low density and robust stability [6–9]. Core–shell nanostructures, in particular, combine multiple loss mechanisms—dielectric, magnetic and interfacial—leading to superior performance [10–14]. For instance, Fe₃O₄/C nanorings achieved enhanced low‑frequency absorption [18], while CoFe@C composites demonstrated a 4.3 GHz effective bandwidth in previous reports [33]. However, scalable, uniform synthesis of such core–shell systems remains challenging.

Methods/Experimental

Synthesis of CoFe₂O₄

CoCl₂·6H₂O (2.5 g) and FeSO₄·7H₂O (5.6 g) were dissolved in 80 mL deionised water and stirred at 80 °C for 1 h. A 1 M oxalic acid solution (30 mL) was heated to boiling and slowly added under vigorous stirring, forming a black precipitate. After ice‑water cooling, the precipitate was collected, washed with water and ethanol, and dried at 60 °C under vacuum for 12 h. The powder was calcined at 600 °C for 1 h (1 °C min^–1) in a muffle furnace.

Synthesis of CoFe@C

The CoFe₂O₄ powder was placed in a porcelain boat, introduced into a tube furnace, evacuated, and then exposed to acetylene gas at atmospheric pressure. The reaction proceeded at 400 °C for 1 h (5 °C min^–1). After cooling, the CoFe@C nanocomposite was recovered.

Characterisation

TEM and HRTEM were performed on a JEOL JEM‑2100 microscope. XRD used Cu Kα radiation on a Bruker D8 Advance diffractometer. XPS employed an AXIS SUPRA spectrometer with monochromatic Al Kα (1486.6 eV). Thermogravimetric data were collected with a TA Q600 system (10 °C min^–1 in air). Raman spectra were recorded on a Renishaw inVia Reflex using a 532 nm laser. Magnetic measurements used a MicroMag 2900/3900 alternating‑gradient magnetometer.

Microwave Absorption Measurement

Samples were mixed (50 wt %) with paraffin and pressed into cylindrical shapes. The cylinders were cut into toroids (outer diameter 7.00 mm, inner diameter 3.04 mm). Reflection and transmission parameters (S₁₁ and S₂₁) were measured across 2–18 GHz with an Agilent N5230A VNA using the coaxial‑line method, from which complex permittivity and permeability were derived.

Results and Discussion

Structural Analysis

XRD patterns confirmed the inverse spinel structure of CoFe₂O₄ (JCPDS 03‑0864). CoFe@C exhibited Fe–Co alloy peaks (JCPDS 44‑1483) with no crystalline graphite, indicating an amorphous carbon shell. XPS survey spectra revealed C, O, Fe and Co, with a C 1s peak at 284.5 eV (sp² hybridisation). Raman spectra showed a D‑band at 1345 cm^–1 and a G‑band at 1604 cm^–1, further confirming disordered carbon. TEM images displayed mesoporous CoFe₂O₄ particles (average 40–70 nm) and CoFe cores surrounded by a 5–30 nm carbon shell. Thermogravimetric analysis indicated a carbon content of ~48.5 wt % in CoFe@C.

Magnetic Properties

CoFe₂O₄ exhibited a saturation magnetisation (M_s) of 61.7 emu g^–1 and coercivity (H_c) of 1536.8 Oe. CoFe@C showed M_s = 42.6 emu g^–1 and H_c = 729.2 Oe, consistent with literature for CoFe/C composites. The moderate M_s and lower H_c compared to bulk FeCo suggest that the carbon shell reduces magnetic domain pinning.

Microwave Absorption Performance

CoFe₂O₄ displayed weak absorption (minimum RL ≈ –7.1 dB at 2.5 mm). In contrast, CoFe@C achieved remarkable performance: RL minima of –15.5 dB (17.1 GHz) at 2.0 mm, –17.9 dB (13.3 GHz) at 2.5 mm, –20.8 dB (10.9 GHz) at 3.0 mm, –26.1 dB (9.3 GHz) at 3.5 mm, –44.0 dB (7.9 GHz) at 4.0 mm, –31.8 dB (7.0 GHz) at 4.5 mm and –24.4 dB (6.2 GHz) at 5.0 mm. Notably, RL < –10 dB persisted across 11.6–15.9 GHz (4.3 GHz bandwidth) at 2.5 mm, meeting the 90 % absorption benchmark for practical EM shielding.

Dielectric and Magnetic Loss Mechanisms

Complex permittivity of CoFe@C (ε′ = 5.5–9.1, ε″ = 2.0–5.4) exceeded that of CoFe₂O₄, attributable to high electrical conductivity and interfacial polarization. Magnetic permeability (μ′ = 0.98–1.2, μ″ = 0–0.23) indicated significant magnetic loss, especially near natural (≈3 GHz) and exchange (≈12.5 GHz) resonances, while eddy‑current losses were negligible (μ″μ′^–2f^–1 not constant). Dielectric loss tangent (tan δ_E ≈ 0.7) and magnetic loss tangent (tan δ_M ≈ 1.4) confirmed strong loss mechanisms. Overall, the synergy of conduction loss, interfacial dipole polarization and resonant magnetic loss underpins the superior absorption of CoFe@C.

Conclusions

We have introduced a scalable route to produce CoFe/C core–shell nanocomposites with an amorphous carbon shell (~48.5 wt %). The resulting CoFe@C exhibits record‑level microwave absorption: RL of –44 dB at 4.0 mm and a 4.3 GHz –10 dB bandwidth centred at 2.5 mm. These impressive metrics arise from a balanced combination of dielectric and magnetic loss mechanisms, positioning CoFe@C as a strong candidate for next‑generation EM shielding applications.