Lightweight and High-Performance Electromagnetic Wave Absorbers Based on Hollow Glass Microspheres and Carbon-Supported Ni-Co Composites

Abstract

With the continuous advancement of electromagnetic protection technologies, the development of lightweight electromagnetic wave-absorbing materials with excellent absorption performance has become a critical challenge in the field. In this study, commercially available hollow glass microspheres (HGMs) were employed as templates, and Ni²⁺/Co²⁺ metal ions were used to catalyze the polymerization of dopamine (PDA), forming HGM@Ni_xCo_y/PDA precursors. Subsequent high-temperature pyrolysis yielded lightweight composite absorbing materials, denoted as HGM@Ni_xCo_y/C. This material integrates dielectric loss, conductive loss, magnetic loss, and resonance absorption mechanisms, exhibiting outstanding electromagnetic wave absorption properties. The absorption performance can be effectively tuned by adjusting the Ni-to-Co ratio, with the optimal performance observed at an atomic ratio of 2:3. At a filler loading of 20 wt.%, HGM@Ni₂Co₃/C achieved an effective absorption bandwidth (EAB) of 6.83 GHz (ranging from 10.53 to 17.36 GHz) and a minimum reflection loss (RL_min) of −27.26 dB. These results demonstrate that the synergistic combination of hollow glass bubbles and carbon-based magnetic components not only significantly reduces the material density and required filler content but also enhances overall absorption performance, highlighting its great potential in the development of lightweight and high-efficiency electromagnetic wave absorbers.

1. Introduction

Electromagnetic (EM) wave pollution, induced by the proliferation of wireless communication devices, radar systems, and electronic equipment, has raised increasing concerns over its adverse effects on both human health and information security. Consequently, the development of efficient, lightweight EM wave-absorbing materials (EWAMs) has become a pressing focus in materials science and applied physics. Among various candidates, ferrite-based absorbers, such as manganese–zinc ferrite, have been extensively investigated due to their intrinsic magnetic loss capabilities. However, their high density (~5.0 g/cm³) [1,2,3] and the need for high filler content (often exceeding 50 wt.%) [4,5,6,7] in coating systems severely limit their practical applications, particularly in aerospace and mobile electronic systems, where weight reduction is critical.

To overcome these limitations, hybridization with carbonaceous materials has emerged as a promising strategy. Carbon materials, such as graphene, carbon nanotubes, and amorphous carbon, offer excellent electrical conductivity and dielectric loss properties, thereby complementing the magnetic loss of ferrites [8,9,10,11,12,13]. Despite the synergistic advantages, the intrinsic density of ferrite particles and their high loading requirements still pose a significant barrier to achieving truly lightweight, high-performance EWAMs. In this context, the integration of hollow microstructures with magnetic and dielectric components provides a compelling pathway for designing next-generation EWAMs. Hollow structures not only reduce the overall density of the material but also introduce multiple internal reflections, promoting energy absorption and enhancing EM wave absorption. Furthermore, by tailoring the size and structure of the internal cavity, the resonance behavior and impedance matching of the absorber can be significantly tuned, enabling broadband absorption across lower frequency ranges. Herein, we report a novel approach to fabricating ultralight, high-efficiency EM wave absorbers by using commercially available hollow glass microspheres (HGM, ~0.46 g/mL) as the structural scaffold [14,15,16]. A dual-metallic (Ni²⁺ and Co²⁺) polydopamine (PDA) layer was deposited onto the surface of HGM via coordination-assisted oxidative polymerization, forming HGM@Ni_xCo_y/PDA precursors [17,18]. Subsequent high-temperature pyrolysis transformed the surface coating into a magnetic carbonaceous shell, yielding HGM@Ni_xCo_y/C core–shell composites. By combining dielectric, conductive, magnetic, and resonant attenuation, this design leverages multiple loss mechanisms within a single hierarchical architecture.

The central goal of this study is to optimize both the chemical composition and structural configuration of the absorber to achieve efficient EM wave absorption at reduced filler loading. By systematically tuning the Ni/Co atomic ratio, we demonstrate that the absorber with a Ni:Co ratio of 2:3 exhibits superior performance, with an effective absorption bandwidth (EAB) of 6.83 GHz (10.53–17.36 GHz) and a minimum reflection loss (RL_min) of −27.26 dB at only 20 wt.% filler loading. These findings highlight the importance of synergistic interactions among dielectric and magnetic constituents and underscore the potential of hollow glass microsphere-based hybrids in designing next-generation lightweight EWAMs. This study not only provides a cost-effective and scalable strategy for synthesizing multifunctional absorbing materials but also offers critical insights into structure–property relationships in composite absorbers. The proposed methodology paves the way for the rational design of low-density, broadband, and high-efficiency materials tailored for modern EM shielding applications.

2. Experimental Section

2.1. Materials

Sodium hydroxide (NaOH), nickel chloride (NiCl₂), cobalt chloride (CoCl₂), Tris buffer, and dopamine hydrochloride were purchased from Aladdin Reagent (Shanghai) Co., Ltd. (Shanghai, China) and used without further purification. iM16-type hollow glass microspheres were obtained from 3M (Saint Paul, MN, USA).

2.2. Preparation of HGM@Ni₂Co₃/C Composite

Initially, commercial iM16-type hollow glass microspheres (HGMs) were dispersed in 20 mL of 1 mol/L NaOH solution and subjected to ultrasonic treatment for 10 min to remove surface impurities and introduce hydroxyl groups. The HGMs were then thoroughly washed with deionized water until a neutral pH was reached, followed by drying to yield hydroxyl-functionalized HGMs. Subsequently, 0.5 g of the treated microspheres was mixed with specific volumes of 10.5 mmol/L NiCl₂ and 10.5 mmol/L CoCl₂ aqueous solutions. After thorough mixing, the pH of the mixture was adjusted to 8.5 using 10 mmol/L Tris buffer. A dopamine solution (20 mL, 1 mmol/L) was then added, and the mixture was stirred under ambient air for 24 h to promote the oxidative polymerization of dopamine catalyzed by the transition metal ions, yielding HGM@ Ni_xCo_y/PDA precursors (see

Table 1. Preparation ratio of materials for HGM@Ni_xCo_y/PDA.

Sample	HGM	10.5 mmol/L NiCl2	10.5 mmol/L CoCl2
HGM@Ni1Co1/PDA	0.5 g	20 mL	20 mL
HGM@Ni1Co4/PDA	0.5 g	8 mL	32 mL
HGM@Ni2Co3/PDA	0.5 g	16 mL	24 mL
HGM@Ni3Co2/PDA	0.5 g	24 mL	16 mL
HGM@Ni4Co1/PDA	0.5 g	32 mL	8 mL

for specific compositions). The resulting solids were collected by centrifugation and freeze dried. Finally, the HGM@Ni_xCo_y/PDA powders were pyrolyzed at 800 °C for 3 h (heating rate: 5 °C/min) under an argon atmosphere (flow rate: 50 °C/min) in a tubular furnace to obtain the final HGM@ Ni_xCo_y/C composite materials. A schematic of the synthesis procedure is provided in Figure 1.

2.3. Testing Instruments and Testing Details

The morphology and microstructure of the samples were examined using Scanning Electron Microscopy (SEM, Carl Zeiss Sigma 500, Zeiss, Oberkochen, Germany) and Transmission Electron Microscopy (TEM, FEI-Tecnai G2 F-20, Thermo Fisher Scientific, Waltham, MA, USA). Elemental composition and surface distribution were analyzed via Energy-Dispersive Spectroscopy (EDS) coupled with these microscopes. Crystalline phase identification was conducted using Powder X-ray Diffraction (XRD) with Cu Kα radiation, scanning from 5° to 90° at a rate of 2° min⁻¹. Surface chemical states were investigated by X-ray Photoelectron Spectroscopy (XPS) employing a Thermo Scientific K-Alpha instrument with an Al Kα source (1486.6 eV). Functional groups were identified using Fourier Transform Infrared Spectroscopy (FTIR, Nicolet is50, Thermo Fisher Scientific, Waltham, MA, USA), and spectra were acquired between 500 and 4000 cm⁻¹ (32 scans) using the KBr pellet method. Magnetic hysteresis loops were measured up to a maximum field of 15 kOe utilizing a Vibrating Sample Magnetometer (VSM, LakeShore 8604, Carson, CA, USA). For electromagnetic absorption (EMA) property evaluation, the materials were uniformly blended with paraffin wax with homogeneity. These measurements were performed on toroidal samples (7 mm outer diameter, 3.04 mm inner diameter) within the 2–18 GHz frequency range using a Keysight (Santa Rosa, CA, USA) N5222B Vector Network Analyzer.

3. Results and Discussion

The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of HGM@Ni_xCo_y/C are presented in Figure 2. SEM images reveal that the hollow glass microspheres (HGMs) are uniformly coated with a surface layer exhibiting distinct protrusions reminiscent of bayberry-like structures. Both Ni²⁺ and Co²⁺ ions can chelate with dopamine, initiating its polymerization, with Co²⁺ showing a higher catalytic efficiency in this process [1]. As the Ni/Co ratio varies, notable differences in the morphology and density of the surface protrusions are observed, attributable to the distinct catalytic activities of the transition metal ions. A higher Co content accelerates dopamine polymerization, promoting localized aggregation of polydopamine, which in turn affects the homogeneity of the carbon layer after high-temperature pyrolysis. Further structural analysis of HGM@Ni₂Co₃/C and HGM@Ni₁Co₁/C was conducted using TEM and high-resolution TEM (HRTEM). The TEM images indicate that the carbon layer deposited on the HGM surface is approximately 100 nm thick. HRTEM images reveal three primary types of lattice fringes: one corresponding to either the NiO (200) or CoO (111) plane with an interplanar spacing of 0.21 nm; another associated with the C (002) plane (0.34 nm); and the third assigned to the Ni (111) plane with a spacing of 0.20 nm.

The XRD patterns of HGM@NixCoy/C are presented in Figure S1. The diffraction peak at 22.0° corresponds to SiO₂ (PDF#00-029-0085). Peaks observed at 44.5°, 51.9°, and 76.5° can be indexed to the (111), (200), and (220) planes of metallic Ni (PDF#04-001-0091), respectively. The peak at 26.2° is assigned to the (002) plane of graphene (PDF#01-071-4630). Diffraction peaks at 36.5°, 42.4°, and 61.5° are attributed to the (111), (200), and (220) planes of CoO (PDF#00-009-0402), while those at 37.3°, 43.3°, and 62.9° correspond to the (111), (200), and (220) planes of NiO (PDF#00-004-0835), respectively. These results suggest that, following high-temperature pyrolysis, both metallic and oxide phases of Ni and Co coexist in the HGM@NixCoy/C composites.

The pyrolysis products of metal–organic precursors under an inert atmosphere—whether elemental metals, alloys, or metal oxides—are largely determined by the reduction potentials of the constituent metal ions. Ni²⁺ (−0.25 V) and Co²⁺ (−0.28 V), possessing relatively high reduction potentials, tend to form metallic phases upon pyrolysis [19,20]. In contrast, metal ions, such as Mn²⁺ (−1.185 V), Mg²⁺ (−2.372 V), Zr⁴⁺ (−1.45 V), and Zn²⁺ (−1.199 V), typically yield metal oxides under similar conditions [21,22,23]. Both the metallic and oxide forms of Ni and Co are known for their magnetic properties, thereby contributing to the magnetic loss behavior essential for effective electromagnetic wave absorption. Notably, the XRD pattern of HGM@Ni₄Co₁/C exhibits a broad hump around 2θ ≈ 20°, indicative of an amorphous structure [6,7]. This suggests the presence of amorphous carbon coating the surface of the hollow microspheres. Given that amorphous carbon generally possesses lower electrical conductivity than graphitized carbon, the real and imaginary parts of the dielectric constant for HGM@Ni₄Co₁/C are, consequently, lower than those of other compositions.

As shown in Figure 3a, the Raman spectra exhibit two prominent peaks at 1349 cm⁻¹ and 1590 cm⁻¹, corresponding to the D and G bands of carbon, respectively [24,25]. The degree of graphitization in carbon-based materials is typically evaluated using the intensity ratio of the D to G bands (I_D/I_G), which in this case ranges from 0.93 to 1.12, as shown in Table S1. A higher I_D/I_G ratio indicates a greater density of structural defects, and values around 1.0 suggest the presence of substantial disorder within the carbon framework. Furthermore, the presence of a broadened 2D band in the spectra provides evidence of a layered architecture, confirming the formation of nitrogen-doped, defect-rich multilayer graphene structures on the surface of the hollow glass microspheres.

The magnetic properties of the synthesized nanocomposites were investigated using a vibrating sample magnetometer (VSM). As shown in the hysteresis loops in Figure 3b, all HGM@Ni_xCo_y/C samples exhibit typical ferromagnetic behavior, with the corresponding magnetic parameters summarized in Table S2. The saturation magnetization (M_s) and coercivity (H_c) values for HGM@Ni₁Co₂/C, HGM@Ni₃Co₂/C, HGM@Ni₁Co₁/C, HGM@Ni₂Co₃/C, and HGM@Ni₁Co₄/C are 0.64 emu/g & 32.0 Oe, 2.01 emu/g & 248.5 Oe, 1.14 emu/g & 248.9 Oe, 2.28 emu/g & 310 Oe, and 2.63 emu/g & 332 Oe, respectively. Compared to previously reported values, the slightly lower M_s observed here is likely due to the relatively thin carbon layer coated on the hollow glass microspheres, which reduces the overall proportion of magnetic components in the composite. The observed magnetism of the HGM@Ni_xCo_y/C composites primarily arises from nanoscale Ni and Co particles embedded within the carbon matrix. With increasing Co content, the coercivity (H_c) increases significantly, in agreement with the findings by Gong et al. [26], which showed that Co particles possess higher coercivity than Ni counterparts. Enhanced coercivity contributes to greater magnetic anisotropy (see Equation (1)), thereby shifting the natural resonance frequency to higher values and improving the magnetic loss performance of the material [27,28].

$$f_{\gamma }=\gamma H/2\pi$$

(1)

where $f_{\gamma }$ is the natural resonance frequency, γ is the gyromagnetic ratio, and H denotes the magnetic anisotropy field.

X-ray photoelectron spectroscopy (XPS) analysis of the HGM@Ni_xCo_y/C series, presented in Figure 4, confirms the presence of Ni, Co, O, C, and N elements. As the atomic ratio of Ni to Co (x:y) increases, the relative intensity of the Ni peak gradually increases, while that of Co shows a slight decrease in the survey spectra. High-resolution Co 2p spectra reveal that Co is primarily present in the form of metallic Co and CoO [29], consistent with the XRD findings. Similarly, the Ni 2p spectra confirm the coexistence of metallic Ni and NiO species [30,31,32], further corroborating the XRD results. The N 1s spectrum indicates the presence of multiple nitrogen bonding configurations, including pyridinic N, pyrrolic N, graphitic N, and oxidized N. These nitrogen species originate predominantly from the decomposition of polydopamine, which facilitates their incorporation into the carbon matrix during high-temperature treatment, in agreement with previous studies [33].

The microwave absorption capability of a material is commonly assessed by its minimum reflection loss (RL_min). An RL_min value below −10 dB typically indicates effective microwave absorption, while values below −20 dB suggest that the material is capable of absorbing over 99% of incident electromagnetic waves. Based on transmission line theory, the RL_min can be quantitatively described by Equations (2) and (3) [34,35].

$$Z_{in}=Z_0\sqrt{{\displaystyle \frac{{\mu }_r}{{\epsilon }_r}}}\mathit{tan}h\left[j\left({\displaystyle \frac{2f\pi d}{c}}\right)\sqrt{{\mu }_r{\epsilon }_r}\right]$$

(2)

$$RL=20\mathit{log}\left|{\displaystyle \frac{Z_{in}-Z_0}{Z_{in}+Z_0}}\right|$$

(3)

According to the generalized transmission line theory, the reflection loss (RL) of HGM@Ni_xCo_y/C composites mixed with 20 wt.% paraffin is illustrated in Figure 5. The effective absorption bandwidth (EAB), defined as the frequency range where RL < −10 dB, corresponds to an electromagnetic wave absorption efficiency exceeding 90%. The detailed absorption parameters are summarized in

Table 2. Summary of the absorbing properties of HGM@Ni_xCo_y/C samples.

Samples	Thickness/mm	EAB/GHz	RLmin/−dB
HGM@Ni4Co1/C	4.0	2.00 (16.00–18.00)	−21.47 (18.00)
HGM@Ni3Co2/C	2.5	3.85 (13.16–17.01)	−13.16 (14.73)
HGM@Ni1Co1/C	2.5	2.84 (15.16–18.00)	−35.22 (16.56)
HGM@Ni2Co3/C	2.5	6.83 (10.53–17.36)	−27.26 (11.90)
HGM@Ni1Co4/C	2.0	3.45 (14.55–18.00)	−19.73 (16.51)

. Both HGM@Ni₄Co₁/C and HGM@Ni₂Co₃/C demonstrate superior microwave absorption performance (Figure 5). Specifically, HGM@Ni₁Co₄/C exhibits an EAB of 3.45 GHz (14.55–18.00 GHz) and a minimum reflection loss (RL_min) of −19.73 dB at a thickness of 2.0 mm. When the thickness increases to 2.5 mm, the EAB expands to 3.94 GHz (11.37–15.31 GHz), with an RL_min of −15.62 dB (12.95 GHz). Notably, HGM@Ni₂Co₃/C achieves the optimal performance at a thickness of 2.5 mm, delivering a broad EAB of 6.83 GHz (10.53–17.36 GHz) and a deep RL_min of −27.26 dB.

At a higher filler loading of 30 wt.%, the HGM@NixCoy/C absorbers continue to exhibit remarkable microwave absorption performance, as illustrated in Figure S2 and summarized in Table S3. Notably, HGM@Ni₁Co₄/C, HGM@Ni₂Co₃/C, HGM@Ni₁Co₁/C, and HGM@Ni₃Co₂/C demonstrate outstanding reflection loss characteristics across a range of thicknesses. Specifically, HGM@Ni₁Co₄/C achieves an effective absorption bandwidth (EAB) of 3.42 GHz (9.34–12.76 GHz) and a minimum reflection loss (RL_min) of −29.87 dB at 10.56 GHz with a thickness of 2.5 mm. HGM@Ni₂Co₃/C reaches an RL_min of −48.90 dB at 7.23 GHz with a 3.5 mm thickness and an EAB of 2.13 GHz (6.33–8.46 GHz). HGM@Ni₁Co₁/C exhibits an EAB of 5.48 GHz (10.15–15.63 GHz) and an RL_min of −30.31 dB (12.21 GHz) at 3.0 mm thickness, while HGM@Ni₃Co₂/C presents an EAB of 2.80 GHz (7.91–10.71 GHz) with an RL_min of −39.45 dB (9.00 GHz) at the same thickness.

In general, a higher attenuation constant (α) corresponds to a stronger capability to dissipate electromagnetic energy. The attenuation constant can be calculated using the following equation (Equation (4)):

$$\mathrm{\alpha }=\frac{\sqrt{2}\pi f}{c}\times [({\mu }^{″}{\epsilon }^{″}-{\mu }^{\prime }{\epsilon }^{\prime })+\sqrt{({\mu }^{″}{\epsilon }^{″}-{\mu }^{\prime }{\epsilon }^{\prime }{)}^2+{\left({\mu }^{\prime }{\epsilon }^{″}+{\mu }^{″}{\epsilon }^{\prime }\right)}^2}{]}^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}$$

(4)

As shown in Figure 5f, the attenuation constants of the HGM@NixCoy/C composites follow the order: α(HGM@Ni₂Co₃/C) > α(HGM@Ni₁Co₄/C) > α(HGM@Ni₃Co₂/C) > α(HGM@Ni₁Co₁/C) > α(HGM@Ni₄Co₁/C). This trend is consistent with the reflection loss behavior observed in Figure 5a–e, indicating that higher attenuation constants play a critical role in enhancing microwave absorption performance.

The electromagnetic parameters of HGM@Ni_xCo_y/C with a filler loading of 20 wt.% are presented in Figure 6. As the frequency increases, the real part of the permittivity (ε′) exhibits a decreasing trend, which is consistent with the typical frequency-dispersion behavior of carbon-based materials. Interestingly, unlike conventional absorbers, the imaginary part of the permittivity (ε″) does not follow the expected downward trend with increasing frequency. Instead, several resonance peaks emerge, which can be attributed to the unique hollow structure of the glass microspheres, enabling energy storage and facilitating multiple relaxation processes.

Regarding the complex permeability, both the real part (μ′) and imaginary part (μ″) show mild fluctuations with frequency. The μ′ value remains close to 1.0, while μ″ stabilizes around 0.1. The dielectric loss tangent (tanδ_ε) is slightly higher than the magnetic loss tangent (tanδ_μ), indicating that both dielectric and magnetic losses contribute to the microwave absorption of HGM@Ni_xCo_y/C, with dielectric loss being the dominant mechanism.

The incorporation of Ni and Co nanoparticles endows the HGM@Ni_xCo_y/C composites with abundant magnetic nanophases, making magnetic loss a crucial factor in determining their microwave absorption performance. Typically, magnetic losses originate from hysteresis loss, domain wall resonance, eddy current loss, natural resonance, and exchange resonance. In the microwave frequency range, hysteresis loss and domain wall resonance are generally negligible. To assess whether eddy current loss is the dominant mechanism, the C₀ parameter (C₀ = μ″(μ′)⁻²f⁻¹) is employed [36,37]. If the magnetic loss arises solely from eddy currents, C₀ should remain constant across the frequency spectrum. Deviations from constancy suggest the involvement of natural and exchange resonance mechanisms. As depicted in Figure 7, C₀ values fluctuate significantly within the 2–6 GHz range, indicating that natural and exchange resonances dominate in this region. In contrast, from 6 to 16 GHz, C₀ remains nearly constant, confirming that eddy current loss governs the magnetic dissipation. Beyond 16 GHz, renewed fluctuations in C₀ suggest a reemergence of resonance-driven magnetic losses. Furthermore, the unique layered carbon network structure of HGM@Ni_xCo_y/C promotes multiple internal reflections and interfacial scattering events upon electromagnetic wave entry. This extended propagation path enhances wave–matter interactions, thereby improving the overall microwave absorption efficiency.

Figure 8 schematically illustrates the propagation and absorption pathways of electromagnetic (EM) waves within the HGM@Ni_xCo_y/C composite system. Compared with most previously reported absorbers, this material exhibits superior performance, as detailed in

Table 3. Comparison of absorbing performance parameters of samples.

Samples	Thickness mm	${\mathit{R}}_{\mathit{L}}$/-dB (Corresponding Frequency GHz)	Frequency Range/GHz	EAB/GHz	Ref.
MMT/Fe3O4/PPy	3.5	41.35 (11.52)	6.47–15.39	8.92	[6]
Mn0.5Zn0.5Fe2O4	3.0	32.46 (15.72)	14.81–17.39	2.59	[17]
C-Mn0.5Zn0.5Fe2O4@PDA	2.0	17.57 (13.17)	11.70–17.06	5.36	[17]
Ni/NC@Ti3C2Tx-2	1.5	20.60 (15.20)	13.20–17.85	4.65	[18]
RGO/ZnFe2O4	2.0	~35.0 (12.0)	10.80–14.00	3.20	[41]
Fe3C/N-Doped Carbon Fibers	2.16	~18.0 (14.8)	12.72–18.00	5.28	[42]
C/ZnO composites	1.5	24.83 (16.72)	14.39–18.00	3.61	[43]
N/ZnO0.064-900	3.0	43.15 (9.68)	8.61–11.40	3.51	[44]
Fe3O4/N-doped carbon/carbon fiber	3.0	31.38 (15.50)	9.36–18.00	8.64	[45]
Fe-3SM-9AN	2.0	36.70 (11.41)	9.93–18.00	8.07	[46]
HGM@Ni2Co3/C	2.5	27.26 (11.90)	10.53–17.36	6.83	This work

. The material adopts a hierarchical core–shell architecture, comprising hollow glass microspheres (HGMs) as the inner core and nitrogen-doped carbon encapsulating Ni_xCo_y bimetallic nanoparticles as the shell. This multiscale design enables incident EM energy to undergo a cascade of synergistic dissipation processes—including multiple reflections/refractions, conductive loss, polarization relaxation, resonance trapping, and magnetic loss. First, the shell–cavity architecture of the hollow microspheres provides a labyrinth-like propagation path. Incident waves undergo repeated reflection and scattering between the porous shell and the inner void (Figure 8a), significantly extending the transmission distance and reducing the transmitted power P_out, thereby laying the geometric foundation for efficient energy absorption [38]. Second, the carbon shell spontaneously forms a three-dimensional conductive network during pyrolysis, facilitating electron hopping, migration, and contact transfer under alternating EM fields (Figure 8b). The resulting Joule heating constitutes the primary conductive loss channel. Simultaneously, the uniformly embedded Ni_xCo_y nanoparticles contribute to natural resonance and eddy current loss, with their magnetic dissipation synergistically amplified through coupling with the conductive matrix. Moreover, nitrogen functionalities within the carbon matrix—such as pyridinic-N, graphitic-N, and structural defects—act as strong dipole centers, inducing localized charge accumulation and bound polarization, leading to significant dipolar relaxation losses (Figure 8c). At the same time, heterogeneous interfaces—such as carbon/metal, metal/glass, carbon/glass, and carbon/air—undergo Maxwell–Wagner interfacial polarization under EM stimulation. The rapid accumulation and release of interfacial charges on the microsecond timescale provide additional dielectric dissipation channels (Figure 8d). Furthermore, the internal cavity of HGM acts as a natural resonator in the centimeter-to-millimeter wave regime. Resonant standing waves formed within the cavity walls contribute to enhanced energy trapping and absorption (Figure 8e). Owing to the integration of these multiple absorption mechanisms, HGM@Ni_xCo_y/C exhibits excellent impedance matching and broadband absorption characteristics. At high frequencies (>12 GHz), dielectric, conductive, and magnetic losses dominate; in the intermediate range (8–12 GHz), interfacial polarization and magnetic resonance play leading roles; and at lower frequencies (<8 GHz), cavity resonance and magnetic hysteresis are primarily responsible for energy absorption. Overall, five major dissipation pathways—conductive loss, interfacial polarization, dipolar polarization, resonance loss, and magnetic loss—govern the absorption behavior of the composite. The multifunctional integration of core–shell–cavity structures enables efficient radar wave absorption across the 2–18 GHz spectrum, underscoring the synergistic potential of bimetallic/carbon hybrids and hollow microsphere frameworks for high-performance microwave absorption applications [39,40].

4. Conclusions

In this study, a series of lightweight carbon-based magnetic composite hollow microsphere absorbers, denoted as HGM@Ni_xCo_y/C, were successfully synthesized and systematically investigated. By finely tuning the atomic ratio of Ni to Co, the electromagnetic parameters and microwave absorption performance of the composites were effectively optimized. The results reveal that HGM@Ni_xCo_y/C exhibits outstanding microwave absorption capabilities at notably low filler loadings, thereby underscoring its intrinsic lightweight advantage. The enhanced absorption performance is primarily attributed to the synergistic effects of dielectric loss, magnetic loss, conductive loss, and multiple interfacial polarization mechanisms. In addition, the hollow microsphere architecture contributes significantly to improved impedance matching and facilitates multiple internal reflections and scattering of incident electromagnetic waves. The optimal absorption properties were achieved with a Ni:Co atomic ratio of 2:3 (HGM@Ni₂Co₃/C), which, at a low filler content of 20 wt.% and a thickness of 2.5 mm, delivered a minimum reflection loss (RL_min) of −27.26 dB and an effective absorption bandwidth (EAB, RL < −10 dB) of 6.83 GHz, covering a wide frequency range from 10.53 to 17.36 GHz. Collectively, these findings demonstrate that hollow glass microsphere-supported Ni–Co/carbon composites represent a highly promising class of materials for high-efficiency, lightweight, and broadband microwave absorption applications.