Tunable Low-Frequency Microwave Absorption and Thermal Management through In-Situ Confined Growth of Alloys within Aramid Nanofibers

Abstract

To address the electromagnetic pollution and thermal failure issues caused by satellite communications and radar detection equipment operating in the S and C bands (2-8 GHz). We propose novel strategy based on confined growth to construct the core-shell structure, consisting of magnetic alloy core@polymer shell, aiming to synergistically enhance tunable low-frequency microwave absorption and high thermal conductivity. Firstly, aramid nanofibers (ANFs) were prepared via the deprotonation method. Subsequently, CoNi magnetic particles were confined grown within the ANFs by the in-situ solvothermal reduction process, constructing CoNi@ANFs with a magnetic core@polymer shell structure. The CoNi@ANFs composites were then fabricated using vacuum filtration combined with hot-pressing techniques. The CoNi@ANFs composites exhibit a tunable effective absorption bandwidth covering 2-8 GHz, with a minimum reflection loss (RL_min) below −55 dB (absorption rate exceeding 99.999%), significantly surpassing that of commercial magnetic microwave absorbers/ANFs composites. When microwaves are vertically incident, the reduction values of radar cross section reduction exceed 40 dB m². Furthermore, CoNi@ANFs composites demonstrate excellent thermal conductivity (λ>2.8 W m⁻¹ K⁻¹) and electrical insulation (volume resistivity>10¹¹ Ω cm). This work provides a new insight into designing integrated composites with tunable low-frequency microwave absorption and high thermal conductivity, as well as their applications in electromagnetic wave devices.

Graphical abstract

Keywords

Tunable / Low-frequency microwave absorption / Thermal conductivity / ANFs composites

1 Introduction

The core operating frequency of electromagnetic wave devices in fields such as satellite communications and radar detection are concentrated in the low-frequency bands (S band: 2-4 GHz and C band: 4-8 GHz) [1-3]. Although these devices have made outstanding contributions to global information exchange, the accompanying electromagnetic wave pollution poses a serious threat to human health, military security, and the stable operation of high-precision electronic devices [4-6]. Microwave absorption materials (MAMs) become the key to addressing the aforementioned electromagnetic pollution issues due to effectively reduce the radar cross section (RCS) [7-9]. Currently, MAMs have gradually evolved towards low-frequency bands, tunability, and multi-functionality [10-12]. Furthermore, MAMs will convert the incident waves absorbed into heat for dissipation. Meanwhile, the significant Joule heat generated during the operation of the devices will cause local heat accumulation, leading to performance degradation and even thermal failure [13-15]. Therefore, it is urgent to develop composites that possess both efficient tunable low-frequency microwave absorption performance and excellent thermal conductivity. By constructing multiple loss mechanisms and efficient heat conduction pathways can jointly address the challenges of electromagnetic pollution and thermal management.

To meet the dynamic electromagnetic compatibility and stealth requirements of modern satellite and radar systems in the S and C bands under multi-tasking conditions. Tunable MAMs have attracted widespread attention due to respond to changes in working frequency bands in real time and achieve dynamic control of microwave absorption performance [16-18]. Wang et al. [19] fabricated a polyimide encapsulated carbon nanowire coil/carbon foam (PI@CNCs/CF) composites using the simple vacuum impregnation method. By adjusting compression ratio of the PI@CNCs/CF composites, the absorption frequency can be precisely tuned. When the initial thickness of the composites is 5 mm, the compression strain increases from 0% to 60%, enabling the effective absorption bandwidth (EAB) to be precisely adjusted from the X band (8-12 GHz) to the Ku band (12-18 GHz). The tunable microwave absorption performance of PI@CNCs/CF composites are attributed to fine-tuning electromagnetic parameter caused by compression and the change in pore structure. Liu et al. [20] fabricated PAN@MXene aerogels through electrospinning process, and further modified it in-situ using thermal annealing and atomic layer deposition (ALD) techniques to obtain N-CNF@MXene@MoS₂ aerogels (NCMMA). The thickness of MoS₂ grown layer by layer on N-CNF@MXene was controlled by adjusting the number of ALD cycles to obtain tunable microwave absorption performance. As the number of cycles increased from 200 to 1000, the adjustable frequency band gradually shifted from the C band to the Ku band. However, existing tunable MAMs generally suffer from unstable absorption rate during the tuning process, difficulty in balancing frequency tunability and efficient absorption performance [21-23]. Additionally, the tuning range is concentrated in the medium and high frequency bands, which fails to meet the demand for low-frequency tunability, severely limiting the application of MAMs in multi-band, adaptive scenarios covering the S and C bands.

Magnetic loss-type MAMs represented by Fe, Co, Ni and their alloys can effectively excite low-frequency natural resonance due to inherent high saturation magnetization and significant magnetic crystal anisotropy [24-26]. This characteristic enables it to overcome the Snoek limit constraint faced by traditional ferrite materials and demonstrates significant potential for low-frequency microwave absorption [27-29]. However, alloys usually own high conductivity, which easily causes severe skin effect and leads to impedance mismatch. At the same time, the excessively fast magnetic response frequency in the GHz band also limits low-frequency absorption efficiency [30-32]. To overcome this challenge, the current strategy is to construct the core-shell structure (magnetic core@dielectric shell) to suppress the eddy current loss of the magnetic core, and to adjust the dielectric constant by controlling the thickness of the shell layer to improve impedance matching [33-36]. Wu et al. [37] synthesized CoFe@C microwave absorption materials by the hydrothermal method. By adjusting the ratio of Co and Fe elements in the precursor, the microstructure and particle size of the CoFe alloy particles can be effectively controlled. The results show that when the ratio of Co to Fe is 1:3, the minimum reflection loss (RL_min) of CoFe@C at the C band reaches -55 dB, and the low-frequency EAB is 2.2 GHz (5.4-7.6 GHz). However, when the magnetic core@dielectric shell microwave absorption material is compounded with the polymer matrix, a large number of dielectric shells occupy the volume of the composites, diluting the magnetic loss contribution of the high Ms core, thus presenting difficulties in breaking through the Snoek limit again [38]. The design concept of directly using the polymer matrix as the dielectric shell to encapsulate the magnetic core not only avoids the occupation of additional dielectric shell layers and significantly saves the effective magnetic volume in composites. It is also conducive to solving the agglomeration phenomenon among magnetic cores caused by high surface energy and strong magnetic coupling, creating conditions for achieving high filler content of magnetic cores. Furthermore, high filler content is conducive to shortening the transmission path of phonons in the composites, enabling a more thorough utilization of the inherent thermal conductivity of the magnetic core, and simultaneously enhancing the thermal conductivity and thermal management capabilities of the composites.

Aramid nanofibers (ANFs) are often used as the matrix of MAMs due to unique structural designability and lightweight yet high-strength properties [39-41]. Based on this, ANFs were first prepared by the deprotonation method. Then, CoNi was confined grown within the ANFs through in-situ solvothermal reduction to construct CoNi@ANFs with a magnetic core@polymer shell structure. Further, CoNi@ANFs composites were prepared by the filtration and hot-pressing processes. The composition, crystal structure, saturation magnetization, microscopic morphology and elemental distribution of CoNi@ANFs were characterized by using infrared spectrometer (FT-IR), X-ray diffraction instrument (XRD), vibrating sample magnetometer (VSM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Furthermore, the differences in electromagnetic parameters and low-frequency microwave absorption performance between the directly mixing commercial magnetic MAMs/ANFs composites and the CoNi@ANFs composites were compared. Also, the tunable low-frequency absorption performance of the CoNi@ANFs composites was explored and the absorption mechanisms were analyzed. The low-frequency microwave absorption loss capability of the CoNi@ANFs composites under different frequencies was revealed through CST simulation. At the same time, the influence of the CoNi mass fraction on the thermal conductivity, mechanical properties, electrical insulation properties, flame retardancy and hydrophobic properties of the CoNi@ANFs composites were analyzed and studied. This work provides a new insight into the design of tunable low-frequency absorption and high conductivity integrated composites, as well as their application in electromagnetic wave devices for satellite communication and radar detection fields.

2 Results and Discussion

2.1 Morphology and composition of CoNi@ANFs

Figure 1(a) illustrates the schematic diagram for the preparation of ANFs and CoNi@ANFs composites. Initially, Kevlar fibers with a smooth surface and diameter of 12 μm (Figure S1a, Supporting Information) are dissolved in a mixed alkaline solution of potassium hydroxide (KOH) and dimethyl sulfoxide (DMSO). KOH extracts protons from the amide groups on the poly(p-phenylene terephthalamide) (PPTA) molecular chains, leading to the breakage of N-H bonds. As deprotonation proceeds, the PPTA molecular chains accumulate negative charges, resulting in significant electrostatic repulsion between chains and disruption of hydrogen bonding interactions, thereby forming a deep brown ANFs/DMSO dispersion [42-43]. This dispersion is prepared by solvent exchange with deionized water to yield nanofibrillar network ANFs with an interconnected microstructure (Figure S1b, Supporting Information). Subsequently, the ANFs are uniformly mixed with salt solutions of Co and Ni, and subjected to in-situ solvothermal reduction in a high-pressure reactor, where the CoNi particles are confined to grow within the pores of the ANFs (Figure S2, Supporting Information). This process enables the preparation of CoNi@ANFs with mass fractions of 82 wt%, 84 wt%, and 86 wt%, with specific dosages listed in Table S1 (Supporting Information). The Figure 1(b1−b2, c1−c2, and d1−d2) display SEM images of the CoNi@ANFs dispersions with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt%, respectively, along with the locally magnified views. The CoNi particles in CoNi@ANFs exhibit uniform size. As the mass fraction of CoNi gradually increases, the ANFs coating on the CoNi particles progressively decreases.

Figure 2(a1−a2) present the FT-IR spectra of ANFs and CoNi@ANFs with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt%. Characteristic peaks are observed at wavenumbers of 3315 cm⁻¹, 1645 cm⁻¹, and 1305 cm⁻¹ for both ANFs and CoNi@ANFs, corresponding to the stretching vibrations of N-H, C=O and C-N bonds in ANFs, respectively. As CoNi is an inorganic material, no distinct stretching vibration peaks are detected. Notably, the characteristic peak at 3315 cm⁻¹ in CoNi@ANFs decreases rapidly with increasing CoNi mass fraction, which is primarily attributed to the high filler content of CoNi and the reduced ANFs content. From the XRD patterns of CoNi@ANFs (Figure 2(b)), diffraction peaks at 44.2^o, 51.5^o, and 75.9^o as well as those at 44.5^o, 51.8^o, and 76.4^o correspond to the characteristic peaks of standard Co (PDF#15-0806) and Ni (PDF#87-0712) phases, respectively [44]. Compared to CoNi, the diffraction peaks of ANFs exhibit broadening and dispersion, attributable to ANFs being semi-crystalline polymers. Concurrently, due to the low ANFs content within CoNi@ANFs, the diffraction peaks of ANFs cannot be detected. The combined FT-IR and XRD characterizations confirm the successful preparation of CoNi@ANFs. Figure 2(c1−c2) display the magnetic hysteresis loops of ANFs and CoNi@ANFs. It is evident that ANFs exhibit no magnetism, while CoNi@ANFs exhibit typical S shaped hysteresis loops, indicating soft magnetic behavior. The saturation magnetization (Ms) values for CoNi@ANFs with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt% are 84.8 emu g⁻¹, 88.9 emu g⁻¹, and 90.9 emu g⁻¹, respectively, with corresponding coercivity (Hc) values of 184.5 Oe, 188.6 Oe, and 201.1 Oe. As the CoNi content decreases in CoNi@ANFs, Ms shows a slight reduction while Hc exhibits a minor increase, yet all samples maintain Ms higher than 80 emu g⁻¹ and Hc lower than 205 Oe. The high Ms and low Hc contribute to overcoming Snoek′s limit and achieving natural resonance through matching the intrinsic frequency of microwave, thereby enhancing low-frequency microwave absorption [45]. Figure 2(d1−d2) show TEM and HRTEM images, and elemental mapping of Co, Ni and N for CoNi@ANFs. The purple zones correspond to Co, green zones to Ni, and blue zones to N. It can be observed that ANFs form a uniform and continuous coating layer on the surface of the CoNi alloy. Meanwhile, the Co and Ni elements are mainly concentrated in the core region, while N elements are uniformly distributed in the shell region, confirming the successful construction of the CoNi@ANFs core-shell structure.

2.2 Microwave absorption performance of CoNi@ANFs composites

To highlight the superior performance of the CoNi@ANFs, six common commercial magnetic MAMs are selected as control samples, namely carbonyl iron (CIP), iron-silicon alloy (FeSi), molybdenum permalloy (FeNiMo), iron-cobalt-vanadium alloy (FeCoV), nanocrystalline soft magnetic alloy (NP), and amorphous soft magnetic alloy (AP). The SEM images and elemental mappings of these six materials are shown in Figures S3-S8 (Supporting Information). Vibration sample magnetometer test revealed high Ms and low Hc for CIP, FeSi, FeNiMo, FeCoV, NP and AP (Figure 3(a), Figure S9, and Table S2, Supporting Information), providing a foundation for overcoming the Snoek limit and achieving microwave absorption in S and C bands. The commercial magnetic commercial/ANFs composites are fabricated by directly mixing common commercial magnetic MAMs with ANFs. Their real and imaginary permittivity (ε′ and ε′′) and permeability (μ′ and μ′′) are measured via the coaxial method (Figure 3(b−c)), with reflection loss (RL) values calculated for 2-8 GHz at varying thicknesses and frequencies (Figure 3(d1−d6)). The composites exhibited effective low-frequency microwave absorption (Table S3, Supporting Information), primarily dominated by magnetic loss with synergistic dielectric contributions (Figure S10, Supporting Information). However, at high filler content, severe agglomeration and network formation occurred due to the inherent high surface energy and magnetic coupling interactions of commercial magnetic MAMs (Figure S11, Supporting Information). This inhomogeneous dispersion caused abnormal elevation of ε′ (Figure S12, Supporting Information), hindering effective synergy with μ′ and resulting in significant impedance mismatch, thereby compromising microwave absorption. Therefore, in this work, magnetic particles are directly synthesized in-situ on the ANFs matrix. During the nucleation and growth of the magnetic particles, their surfaces are real-time coated and modified by the active groups or polymer segments in the ANFs matrix. In-situ coating suppresses agglomeration tendencies from high surface energy and magnetic coupling, significantly improving dispersion homogeneity and interfacial compatibility (Figure S13, Supporting Information). It breaks through the filling bottleneck of traditional blending methods and is expected to achieve uniform dispersion under higher magnetic absorption agent loading, thereby improving electromagnetic parameters, optimizing impedance matching, and regulating low-frequency absorption performance.

Figure 4(a) displays the real permittivity (ε′) and imaginary permittivity (ε′′) of CoNi@ANFs composites with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt%. As the CoNi mass fraction increases, ε′ rises while ε′′ declines under the same frequency, eventually stabilizing. This trend indicates enhanced electrical energy storage capacity in the composites. The increase in ε′ is attributed to improved electron migration capability within CoNi@ANFs due to higher CoNi loading. Concurrently, the attenuation constant (α, Equation S1, Figure S14a, Supporting Information) gradually increases, signifying stronger microwave dissipation, enabling faster conversion of incident waves into thermal energy [46]. Two distinct relaxation peaks in the ε′′ curve indicate intense polarization relaxation, confirming that beyond conductive loss from CoNi, polarization loss is a key microwave absorption mechanism. To quantify contributions from conductive and polarization losses (Figure 4(f)), ε′′-ε′ curves were plotted based on Debye relaxation theory (Figure S15, Supporting Information). Semicircular arcs represent polarization loss, while linear segments represent conductive loss [47]. The semicircles near 3.0 GHz and 8.0 GHz correspond to multiple relaxation processes, aligning with ε′′ peaks. Furthermore, dense encapsulation of CoNi by ANFs generates abundant heterogeneous interfaces, amplifying space charge polarization phenomenon. Figure 4(b) displays the real permeability (μ′) and imaginary permeability (μ′′) of CoNi@ANFs composites with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt%. As the CoNi mass fraction increases, μ′′ rises while μ′ remains at high levels (μ′ is around 2 at 2 GHz) under identical electromagnetic wave frequencies. Two natural resonance peaks are observed in the μ′′ curves, spanning 2.0-3.5 GHz and 3.5-8.0 GHz, respectively, covering the entire S and C bands. This behavior results from the high Ms and low Hc of all CoNi@ANFs composites, which induce magnetic coupling effects (Figure 4(f)) and suppress eddy current loss (Figure S14b, Equation S2, Supporting Information). Consequently, these properties facilitate overcoming the Snoek limit to generate natural resonance in S and C bands, thereby enhancing low-frequency impedance matching (|Δ|, |Δ|<0.4 indicates optimal matching, Figure S16, Equation S9, Supporting Information) and low-frequency microwave absorption performance. Figure 4(c) displays the dielectric loss tangent (tanδ_ε, Equation S3, Supporting Information) and magnetic permeability loss tangent (tanδ_µ, Equation S4, Supporting Information) for CoNi@ANFs composites with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt%. It is observed that the tanδ_µ values of the CoNi@ANFs composites consistently exceed the tanδ_ε values, indicating that magnetic loss dominates, which encompasses natural resonance and magnetic coupling effects.

Figure 4(d1−d3) present the reflection loss (RL, Equation S5, Supporting Information) and effective absorption bandwidth (EAB, defined as the frequency range where RL is less than −10 dB) for CoNi@ANFs composites with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt% across varying thicknesses and frequencies within the 2-8 GHz. The results demonstrate that for CoNi@ANFs composites with the CoNi mass fraction of 82 wt% and thickness of 4.6 mm, the minimum reflection loss (RL_min) of −59.6 dB and the low-frequency effective absorption bandwidth (EAB) of 2.00 GHz (3.92-5.92 GHz) are achieved. When the CoNi mass fraction increases to 84 wt% with thickness of 5.4 mm, the composites exhibits the RL_min of −56.4 dB and EAB of 1.52 GHz (2.48-4.00 GHz). Furthermore, at the CoNi mass fraction of 86 wt% and thickness of 4.8 mm, its RL_min is −55.6 dB and EAB is 1.04 GHz (2.08-3.12 GHz). Furthermore, as the CoNi mass fraction increases progressively from 82 wt% to 84 wt% and finally to 86 wt%, the frequency corresponding to the RL_min shifts gradually towards lower frequency (moving from 4.80 GHz to 3.12 GHz, and further to 2.48 GHz), and the low-frequency absorption performance remains consistently high, exhibiting an absorption rate exceeding 99.999% (Figure S17, Supporting Information). This tunability arises because increasing the relative CoNi content within the CoNi@ANFs composites elevates their permittivity and induces a slight low-frequency shift in the natural resonance peak observed on the μ′′ curve. Consequently, both the RL and EAB shift towards lower frequency, accompanied by a corresponding shift in impedance matching characteristics (Figure S16, Supporting Information). Superior impedance matching facilitates the unimpeded penetration of electromagnetic waves into the composite interior, where they are efficiently attenuated through multiple loss mechanisms predominantly governed by magnetic loss. This synergistic effect endows the CoNi@ANFs composites with highly tunable low-frequency microwave absorption properties. The comprehensive bandwidth covered by the CoNi@ANFs composites (2.08-5.92 GHz) nearly encompasses the entire 2-6 GHz spectrum. Simultaneously, the composites achieve exceptionally high RL values of −59.6 dB at 4.80 GHz, −56.4 dB at 3.12 GHz, and −55.6 dB at 2.48 GHz. This exceptional performance profile enables precise matching to the core operational bands of satellite communication and radar detection systems, effectively resolving critical electromagnetic compatibility and stealth requirements for such equipment.

Compared to commercial magnetic MAMs/ANFs composites prepared by direct blending of commercial magnetic MAMs with ANFs, the CoNi@ANFs composites not only achieve tunable low-frequency microwave absorption performance in S and C band but also exhibit superior low-frequency reflection loss and bandwidth (Figure 4(e) and Table S3, Supporting Information). Furthermore, to elucidate the advantages of synthesizing high filler content CoNi@ANFs composites via the in-situ solvothermal reduction method, a directly mixing CoNi/ANFs composites with the CoNi mass fraction of 82 wt% are prepared. Both pure ANFs and the 82 wt% CoNi/ANFs composite lack microwave absorption performance (Figures S18 and S19, Supporting Information). This phenomenon occurs because at high filler content, CoNi particles readily agglomerate within the ANFs matrix, preventing ideal homogeneous dispersion. Such non-uniform distribution promotes the formation of localized high-density regions or continuous conductive networks, subsequently triggering significant localized charge accumulation effects and ultimately leading to impedance mismatch. As a result, the incident waves are reflected rather than absorbed. In contrast, the in-situ confined growth method prevents direct particle contact by coating each CoNi core with the ANFs shell, preserving impedance matching and enabling efficient absorption through multiple loss mechanisms.

CST software is employed to simulate the Radar Cross Section (RCS) of CoNi@ANFs composites with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt% at frequencies of 4.80 GHz (Figure 5(a1−a3)), 3.12 GHz (Figure 5(b1−b3)), and 2.48 GHz (Figure 5(c1−c3)), respectively, thereby characterizing the electromagnetic wave absorption capabilities under actual far-field conditions [48,49]. Compared to Perfect Electric Conductor (PEC, Figure S20, Supporting Information), the CoNi@ANFs composites significantly suppressed electromagnetic wave scattering. The results demonstrate that composites with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt% exhibit minimized scattering signals at 4.80 GHz, 3.12 GHz, and 2.48 GHz, respectively. One-dimensional RCS curves for both the PEC plane and the CoNi@ANFs composites are extracted over an angular range form −90^o to 90^o, as shown in Figure 5(a4−c4). At the incident angle of 0^o, the reduction values of RCS with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt% are 43.1 dB m² (4.80 GHz), 42.5 dB m² (3.12 GHz), and 41.7 dB m² (2.48 GHz), respectively. This further confirms the tunable low-frequency microwave absorption capability of the CoNi@ANFs composites.

2.3 Thermal conductivity of CoNi@ANFs composites

Electromagnetic waves absorbed by CoNi@ANFs composites are efficiently converted into thermal energy [50]. Simultaneously, in application scenarios such as satellite communications and radar monitoring, significant Joule heat generated during equipment operations readily causes localized heat accumulation, leading to performance degradation or even failure of devices [51]. Therefore, CoNi@ANFs composites require not only excellent microwave absorption performance but also high thermal conductivity to achieve rapid heat dissipation and effective thermal management. Figure 6(a) presents the λ_∥ (in-plane thermal conductivity) of ANFs and CoNi@ANFs composites with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt%. It is evident that λ_∥ of CoNi@ANFs composites increases with higher CoNi loading. At the CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt%, the λ_∥ values of the composites reach to 2.82 W m⁻¹ K⁻¹, 3.28 W m⁻¹ K⁻¹, and 3.60 W m⁻¹ K⁻¹, representing increases of 66%, 109%, and 139% compared to pure ANFs (1.04 W m⁻¹ K⁻¹). This enhancement is attributed to the intrinsically higher λ of Co and Ni compared to the ANFs matrix [52]. As the CoNi loading increases, improved interconnection between CoNi particles facilitates the formation of thermal conduction pathways. Furthermore, the λ_∥ of both pure ANFs and CoNi@ANFs composites exhibits a slight increase with rising temperature (Figure 6(b)). The primary reason is that elevated temperatures accelerate phonon transport, thereby enhancing thermal conductivity. This self-reinforcing thermal mechanism is crucial for maintaining long-term reliable operation of satellite communication equipment in orbit, particularly during high-throughput communication tasks or sudden high-load scenarios. Figure 6(c) and Figure S21 (Supporting Information) show infrared thermal images and corresponding surface temperatures during heating for ANFs and CoNi@ANFs composites with 82 wt%, 84 wt%, and 86 wt% CoNi. Notably, composites with higher CoNi loading exhibit faster surface temperature rise under identical conditions. This occurs because of uniform dispersion of CoNi in ANFs and increased probability of thermal pathways formation at higher loadings improve heat transfer efficiency. Figure 6(d−e) display infrared thermal images and temperature-time curves for LED lamp with CoNi@ANFs composites having CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt% attached to its rear surface. It is worth noting that the heat source is the LED lamp, while the CoNi@ANFs composites serve as a heat spreader. Therefore, in the infrared thermal images, the main observation is the heat changes of the LED lamp. When air serves as the heat dissipation medium, the LED lamp temperature rises rapidly to 63.8 °C after 60 s. In contrast, using CoNi@ANFs composites significantly slows the heating rate, with surface temperatures reaching only 57.8 °C, 56.6 °C, and 54.8 °C after 60 s for the respective composites. After power interruption, the composites cool to 35.8 °C, 33.7 °C, and 32.1 °C within 60 s, significantly lower than the LED lamp self-cooling temperature (45.2 °C). This demonstrates superior heat dissipation capability, consistent with the λ test results. Applying CoNi@ANFs composites to thermal management in satellite communication and radar detection equipment offers advantages for instant heat dissipation in high-power devices and thermal recovery under extreme operating conditions.

2.4 Mechanical/insulating/flame-retardant/desiccant properties of CoNi@ANFs composites

Figure 7(a) presents the tensile strength (σ_t) and elongation at break (δ) of ANFs and CoNi@ANFs composites with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt%,with corresponding stress-strain curves detailed inFigure S22 (Supporting Information). The tensile strength and elongation at break of pure ANFs measure 145.1 MPa and 14.1%, respectively. As the CoNi mass fraction increases, both properties progressively decline. At CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt%, the tensile strengths of the CoNi@ANFs composites decrease to 58.1 MPa, 46.1 MPa, and 36.7 MPa, while their elongation at break values reduce to 5.8%, 5.1%, and 3.0%, respectively. This reduction occurs because introducing CoNi particles induces stress concentration within the CoNi@ANFs composites, resulting in a certain degree of decrease in mechanical properties. Figure 7(b) demonstrates the volume resistivity (ρ_v) for the same series. Pure ANFs exhibit ρ_v of 7.0×10¹³ Ω cm, while the CoNi@ANFs composites show declining resistivity with increasing CoNi content, specifically, ρ_v of 5.5×10¹² Ω cm, 1.1×10¹² Ω cm, and 1.8×10¹¹ Ω cm, satisfying the electrical insulation standard (>10⁹ Ω·cm) [53]. This trend is attributed to the thinning of ANFs interlayers between adjacent CoNi particles, which enhances carrier transport under applied voltage and consequently reduces volume resistivity. To quantitatively evaluate the flame retardancy of the CoNi@ANFs composites, the heat release rate curves of ANFs and the CoNi@ANFs composites are obtained through microcalorimetry (Figure 7(c)). The results reveal that while ANFs exhibit a single-stage heat release profile, the CoNi@ANFs composites demonstrate a two-stage heat release process. Pure ANFs exhibit a peak heat release rate (pHRR_MCC) of 263.0 W g⁻¹, whereas the CoNi@ANFs composites with CoNi mass fractions of 82 wt%, 84 wt%, and 86 wt% achieve significantly reduced pHRR_MCC values of 29.3 W g⁻¹, 27.6 W g⁻¹, and 24.2 W g⁻¹ representing reductions of 89%, 90%, and 91% compared to pure ANFs, respectively. This confirms that the introduction of CoNi effectively reduces the heat release rate for CoNi@ANFs composites. From water contact angle measurements (Figure 7(d)), the contact angle of pure ANFs is 98.3^o, indicating hydrophobic property. As the CoNi mass fraction increases, the CoNi@ANFs composites display enhanced hydrophobicity with contact angles rising to 101.8^o, 111.1^o, and 116.0^o. This phenomenon is primarily attributed to the markedly increased surface roughness induced by CoNi incorporation, which promotes a Cassie-Baxter contact state between water droplets and the material surface, substantially reducing solid-liquid contact area.

3 Conclusions

This work proposes a novel strategy based on confined growth to construct the core-shell, consisting of magnetic alloy core@polymer shell, aiming to synergistically enhance tunable low-frequency microwave absorption and high thermal conductivity. Firstly, ANFs were prepared via the deprotonation method. Subsequently, CoNi magnetic particles were confined grown in-situ within the ANFs through the solvothermal reduction process, constructing CoNi@ANFs with a magnetic core@polymer shell structure. The CoNi@ANFs composites were then fabricated using vacuum filtration combined with hot-pressing techniques. The CoNi@ANFs composites exhibit a tunable effective absorption bandwidth covering 2-8 GHz, with the RL_min below −55 dB (absorption rate exceeding 99.999%). When microwaves are vertically incident, the reduction values of RCS exceed 40 dB m², significantly surpassing that of commercial magnetic MAMs/ANFs composites. Furthermore, the CoNi@ANFs composites demonstrate exceptional thermal conductivity (λ>2.8 W m⁻¹ K⁻¹) and electrical insulation (volume resistivity>10¹¹ Ω cm). This work provides new insight for designing integrated composites with tunable low-frequency microwave absorption and high thermal conductivity, as well as their applications in electromagnetic wave devices.

4 Experimental Section

Preparation of ANFs: Kevlar fibers were cut into short fibers (1-2 cm), followed by ultrasonic cleaning with acetone for 2 hrs. Then, the short fibers were washed with deionized water, followed by vacuum drying for 6 hrs. Cleaned short fibers (2 g) and KOH (2 g) were immersed in DMSO (500 mL), which was magnetically stirred in an oil bath at 30 °C for 7 days until complete dissolution to prepare reddish-brown aramid nanofibers/DMSO (ANFs/DMSO) dispersion. Deionized water was further added for suction filtration and washing until neutral, after which deionized water was added to adjust the volume to 1000 mL. The mixture was uniformly dispersed using a homogenizer to obtain the ANFs aqueous dispersion (2 mg/mL).

Preparation of CoNi@ANFs composites: CoCl₂·6H₂O and NiCl₂·6H₂O were dissolved in 36 mL ethylene glycol (EG) solvent at a 1:1 molar ratio, followed by adding 0.6 g polyvinylpyrrolidone (PVP). The mixture was magnetically stirred with ultrasonic oscillation at room temperature for 2 hrs. Then, 15 mL, 17.5 mL and 20 mL ANFs aqueous dispersion were respectively added into the solution with magnetic stirring for 30 min. After thorough mixing, 3 mL 80% hydrazine hydrate (N₂H₄·H₂O) was introduced drop by drop into the mixed solution and stirred for 10 min. Further the solution was heated at 120 °C for 2 hrs in an oven. The products were collected, washed several times with deionized water to obtain CoNi@ANFs aqueous dispersion. CoNi@ANFs composites with mass fractions of 82 wt%, 84 wt% and 86 wt% were fabricated via vacuum-assisted filtration and hot-pressing (60 °C/0.1 MPa/24 hrs), with specific quantities listed in Table S1 (Supporting Information) and schematic diagram shown in Figure 1(a). As comparisons, composites containing mass fraction of 82 wt% conventional magnetic microwave absorption materials (CIP, FeSi, FeNiMo, FeCoV, NP, AP) and CoNi particles were prepared by direct blending with ANFs to obtain different composites.

References

[1] Rao L J , Wang L , Yang C D . et al. Confined diffusion strategy for customizing magnetic coupling spaces to enhance low-frequency electromagnetic wave absorption. Adv Funct Mater, 2023, 33: 2213258

[2] Zhong X , Gu J W . Core-shell Co-C@CNT engineered to enable concurrent thermal conductivity and microwave absorption for Poly(4, 4′-dihydroxybiphenyl isophthalate). Transactions of Materials Research, 2026, 2: 100184

[3] Shan B , Wang Y , Ji X Y . et al. Enhancing low-frequency microwave absorption through structural polarization modulation of MXenes. Nano-Micro Lett, 2024, 16: 212

[4] Cai B , Zhou L , Zhao P Y . et al. Interface-induced dual-pinning mechanism enhances low-frequency electromagnetic wave loss. Nat Commun, 2024, 15: 3299

[5] Wang J Q , Liu L , Jiao S L . et al. Hierarchical carbon fiber@MXene@MoS₂ core‐sheath synergistic microstructure for tunable and efficient microwave absorption. Adv Funct Mater, 2020, 30: 2002595

[6] Xiang L L , Qi X S , Rao Y C . et al. A simple strategy to develop heterostructured carbon paper/Co nanoparticles composites with lightweight, tunable and broadband microwave absorption. Mater Today Phys, 2023, 34: 101030

[7] Ai Y S , Xing R Z , Ren N . et al. Biomass-derived hierarchical carbon frameworks enable robust microwave absorption. Matter, 2025, 8: 102289

[8] Liu P B , He Z Z , Li X C . et al. Multifunctional hollow carbon microspheres enable superior electromagnetic wave response and corrosion barrier. Adv Mater, 2025, 37: 2500646

[9] Hou Y L , Liu Z Y , Wang Q . et al. Magnetic graphene composite aerogel for highly efficient electromagnetic wave absorption and anti-corrosion. J Mater Sci Technol, 2026, 251: 1–10

[10] Ding C Y , Shao C S , Wang Z . et al. Leaf vein micronetwork engineering enhanced energy conversion strategy for C-band ultralight yet tunable microwave absorption. Rare Metals, 2025, 44: 6513–6530

[11] Lim D D , Ibarra A , Lee J W . et al. A tunable metamaterial microwave absorber inspired by chameleon’s color-changing mechanism. Sci Adv, 2025, 11: eads3499

[12] He M K , Hu J W , Yan H . et al. Shape anisotropic chain‐like CoNi/polydimethylsiloxane composite films with excellent low‐frequency microwave absorption and high thermal conductivity. Adv Funct Mater, 2025, 35: 2316691

[13] Tang Z M , Xu L , Xie C . et al. Synthesis of CuCo₂S₄@Expanded Graphite with crystal/amorphous heterointerface and defects for electromagnetic wave absorption. Nat Commun, 2023, 14: 5951

[14] Rao L , Huang M , Wang X . et al. Atomic infusion induced reconstruction enhances multifunctional thermally conductive films for robust low‐frequency electromagnetic absorption. Angew Chem Int Ed, 2025, 64: e202418338

[15] Hao B , Chai Z H , Li M . et al. Design of mesoscopic metacomposites for electromagnetic wave absorption: enhancing performance and gaining mechanistic insights. Soft Science, 2025, 5: 39

[16] Wu L H , Shi S H , Wang G Z . et al. Carbon nanocoils/carbon foam as the dynamically frequency‐tunable microwave absorbers with an ultrawide tuning range and absorption bandwidth. Adv Funct Mater, 2022, 32: 2209898

[17] Miao P , Zhou R , Chen K J . et al. Tunable electromagnetic wave absorption of supramolecular isomer‐derived nanocomposites with different morphology. Adv Mater Interfaces, 2020, 7: 1901820

[18] Zhang Y , Huang Y , Zhang T F . et al. Broadband and tunable high‐performance microwave absorption of an ultralight and highly compressible graphene foam. Adv Mater, 2015, 27: 2049–2053

[19] Liu X , Wu L H , Liu J . et al. Dynamically frequency‐tunable and environmentally stable microwave absorbers. Carbon Energy, 2024, 6: e589

[20] Sun X T , Wu Z , Tan X L . et al. High-efficiency tunable microwave absorption in N-CNF@MXene@MoS₂ aerogel with island-chain structured MXene and multiple heterogeneous interfaces. Carbon, 2025, 233: 119909

[21] Yu G Y , Shao G F , Xu R P . et al. Metal-organic framework‐manipulated dielectric genes inside silicon carbonitride toward tunable electromagnetic wave absorption. Small, 2023, 19: 2304694

[22] Chen P , He S B , Zou Z K . et al. Intelligent hydrogels enabled large‐scale variability in continuously tunable microwave absorption. Adv Funct Mater, 2025, 35: 2506308

[23] Du L F , Zhang J S , Zhou Q . et al. Hierarchical CNTs/PyC/SiBCN foam with tunable microwave absorption properties conspired by melamine-derived pyrolyzed carbon and carbon nanotubes. Carbon, 2025, 234: 120030

[24] Chen G , Zhang R , Yuan M . et al. Visualizing nanoscale interlayer magnetic interactions and unconventional low‐frequency behaviors in ferromagnetic multishelled structures. Adv Mater, 2024, 36: 2313411

[25] Liu X J , Duan Y P , Guo Y . et al. In situ construction of complex spinel ferrimagnet in multi-elemental alloy for modulating natural resonance and highly efficient electromagnetic absorption. Chem Eng J, 2023, 462: 142200

[26] Sun R , Lv H W , Lian G J . et al. Dielectric shell regulation in synergy FeCoNi@ZnIn₂S₄ microspheres with broadband electromagnetic wave absorption. Soft Science, 2025, 5: 35

[27] Zhao R Z , Gao T , Li Y X . et al. Highly anisotropic Fe₃C microflakes constructed by solid-state phase transformation for efficient microwave absorption. Nat Commun, 2024, 15: 1497

[28] Han Y , Guo H , Qiu H . et al. Multimechanism decoupling for low‐frequency microwave absorption hierarchical Fe‐doped Co magnetic microchains. Adv Funct Mater, 2025, 35: 2506803

[29] Zhang H , Sun C , Jiang Y . et al. Construction of chiral‐magnetic‐dielectric trinity structures with different magnetic systems for efficient low‐frequency microwave absorption. Small, 2024, 20: 2407176

[30] Jia X S , Wang Z W , Zhang H . et al. Structural design of FeCo/C magnetic nanocomposites: implications toward C-band electromagnetic wave absorption. ACS Appl Nano Mater, 2024, 7: 19602–19618

[31] Fan K , Gu H , Wang C . et al. Magnetic-field induced vertically oriented structure engineering for significantly enhanced electromagnetic absorption. Adv Compos Hybrid Ma, 2025, 8: 292

[32] Wang J Y , Zhou J T , Van Zalinge H . et al. Hollow spherical SiC@Ni composites towards the tunable wideband electromagnetic wave absorption. Compos Part B-Eng, 2024, 276: 111361

[33] Zhou W Y , Cao G Z , Yuan M X . et al. Core-shell engineering of conductive fillers toward enhanced dielectric properties: a universal polarization mechanism in polymer conductor composites. Adv Mater, 2023, 35: 2207829

[34] Chen X L , Zhou W Y , Wang F . et al. Meliorative dielectric properties in core@double-shell structured Al@Al₂O₃@PDA/PVDF nanocomposites via decoupling the intra-particle polarization and inter-particle polarization. Mater Today Energy, 2024, 41: 101543

[35] Deng Z Y , Wang L , Peng B Y . et al. Optimization of electromagnetic wave absorption properties by formation of magnetoelectric synergistic effect of COF-derived carbon composite Fe/Fe₃C. Chem Eng J, 2025, 505: 159457

[36] Sibt u H , Yang Y , Qamar T H . et al. Core-shell designed high entropy alloy CoNiFeCuV-C nanoparticles for enhanced microwave absorption. Appl Phys Lett, 2024, 124: 121902

[37] Du J W , Zhang M Y , Tang P . et al. Enhanced low-frequency electromagnetic wave absorption by CoFe alloy/C microspheres synthesized from metal-organic frameworks. J Alloys Compd, 2025, 1010: 177936

[38] Feng S X , Wang H W , Ma J . et al. Fabrication of hollow Ni/NiO/C/MnO₂@polypyrrole core-shell structures for high-performance electromagnetic wave absorption. Compos Part B-Eng, 2024, 275: 111344

[39] Sun H , Yang B , Zhang M Y . Functional-structural integrated aramid nanofiber-based honeycomb materials with ultrahigh strength and multi-functionalities. Adv Fiber Mater, 2024, 6: 1122–1137

[40] Liu S , Zhou D , Huang F . et al. Heterointerface engineering of polymer-based electromagnetic wave absorbing materials. Soft Science, 2025, 5: 7

[41] Wang C , Zhu Z J , Han L J . et al. Poly(vinyl alcohol) hydrogels enhanced with Ti₃C₂T_x MXene nanosheets and aramid nanofibers for electromagnetic shielding and motion detection. ACS Appl Nano Mater, 2024, 7: 24925–24937

[42] Han Z M , Hou Y Z , Liu H C . et al. Fast and massive production of aramid nanofibers via molecule intercalation. J Am Chem Soc, 2025, 147: 7939–7949

[43] Han Y , Ruan K , He X . et al. Highly thermally conductive aramid nanofiber composite films with synchronous visible/infrared camouflages and information encryption. Angew Chem Int Ed, 2024, 136: e202401538

[44] Peng Y M , Gong K J , Liu A . et al. Ultralight and rigid PBO nanofiber aerogel with superior electromagnetic wave absorption properties. J Mater Sci Technol, 2025, 211: 320–329

[45] Liu X J , Duan Y P , Wu N . et al. Modulating electromagnetic genes through bi-phase high-entropy engineering toward temperature-stable ultra-broadband megahertz electromagnetic wave absorption. Nano-Micro Lett, 2025, 17: 164

[46] Wu P , Kong X , Feng Y . et al. Phase engineering on amorphous/crystalline γ‐Fe₂O₃ nanosheets for boosting dielectric loss and high‐performance microwave absorption. Adv Funct Mater, 2024, 34: 2311983

[47] Han H , Lou Z C , Wang Q Y . et al. Introducing rich heterojunction surfaces to enhance the high-frequency electromagnetic attenuation response of flexible fiber-based wearable absorbers. Adv Fiber Mater, 2024, 6: 739–757

[48] Tan D L , Wang Q , Li M R . et al. Magnetic media synergistic carbon fiber@Ni/NiO composites for high-efficiency electromagnetic wave absorption. Chem Eng J, 2024, 492: 152245

[49] Zuo X Q , Zhang H , Zhou C . et al. Hierarchical and porous structures of carbon nanotubes‐anchored MOF derivatives bridged by carbon nanocoils as lightweight and broadband microwave absorbers. Small, 2023, 19: 2301992

[50] Zhong X , He M , Zhang C . et al. Heterostructured BN@Co‐C@C endowing polyester composites excellent thermal conductivity and microwave absorption at C band. Adv Funct Mater, 2024, 34: 2313544

[51] Xiao H , Si L M , Li Y J . et al. A thermal management hybrid membrane with both high thermal conductivity and electrical insulation. Adv Funct Mater, 2025, 35: e08737

[52] Wen S Y , Liu Y L , Kaptay G . et al. A new model to describe composition and temperature dependence of thermal conductivity for solution phases in binary alloys. J Mater Sci Technol, 2020, 59: 72–82

[53] Zhou J W , Yu Z X , Mohideen M M . et al. Constructing hierarchical polymer nanocomposites with strongly enhanced thermal conductivity. ACS Applied Materials and Interfaces, 2023, 15: 42900–42911

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